4.11 Testing and troubleshooting previous
4.11.1 Introduction index page

People who have understood the previous parts well and built things carefully, could skip these paragraphs because their charger should work right away. That is convenient for me since I feel unable to write an accurate, concise manual on testing and troubleshooting yet. I haven't seen enough trouble to know what parts are likely to get damaged, what errors builders, operators and users are most likely to make etc. So to people who have to refer to these paragraphs because their charger does not function properly: Please write me about what was wrong, what mistakes you have made, how you found out, how you solved it etc. I have learned so much from designing the firefly system that I myself wouldn't make many of the errors other people might make. So I need to learn from your errors if I ever want to write a useful manual on testing and troubleshooting.

The tests that are given in the subsequent paragraphs are meant to get the charger to work while in the field. So the troubleshooting guide is geared towards solving those problems that can be solved in the field. More serious problems are only identified.

The indicator on the switchboard is one of the more complicated parts of the system and if there is any fault in the system, it could very well be in the indicator part. For the moment, the indicator is less important since it is not essential for having the charger to produce electricity. So if it does not work, there is little sense in trying hard to repair it. As long as the indicator is not checked, it can not be assumed to be reliable, so it can not be used to measure whether other parts work properly. Use a tester instead.

After going through all the effort to build a charger, find a test site, bring all components there, maybe even build a weir and dig a canal, one must be quite eager to see it work for real. Very understandable, but there is a risc that once the charger, switchboard and battery are connected, one little component blows up because of a wrong connection. This could mean a lot of trouble for finding out what happened, what part was destroyed and get a spare one. That is why it is better to check everything very carefully according to the procedure of par. 4.11.2:

The charger is designed to charge batteries but when it is for testing and demonstrating only, also car bulbs can be used to dissipate the electricity generated by the charger. Instead of a battery, a make-shift piece of wiring with enough car bulbs to take up the output power of the charger (100 - 165 W) can be connected to the battery cable of the switchboard. Each bulb should have its own switch. There is no need to include a fuse (comparable to the battery fuse) in this circuit. To reduce the number of bulbs needed, 3 headlight bulbs and 3 bulbs of 15 W can be used (headlight bulbs of which the `low beam' filament has burnt out, are perfectly suitable). The `high beam' of headlight bulbs is ca. 55 W each so the total power of all 6 bulbs would be approximately 210 W. Using car bulbs instead of a battery has the following advantages:

However, there are also additional difficulties to overcome: Without a battery, the charger probably does not start on its own (although with a mechanical regulator and a high head so that it runs fast, it might just start). For generating electricity, it needs a field current that is normally taken from the electricity that it produces itself: A chicken-and-egg situation.

To overcome this, dry cell batteries (`penlight', size `AA' will do) can be used to provide the initial field current to get it started. These starting batteries could be connected at many points in the electrical circuit. A very reliable way is:

Now to start, one only has to hold the wires to the block of 4 batteries for a moment. The `E' wire should go to the `-' end of the battery block but if one has the polarity reversed, nothing will happen since the diode will block any current.

When a number of lamps are switched on, the above trick might not work, at least not when only one or two batteries in series are used. So always switch off all lamps before trying to start the alternator.

When using lamps as a load, the alternator will lose its voltage if:

So switch on lamps one by one and switch on the smaller, 15 W lamps once it seems that the charger is reaching its full capacity.

Probably still the alternator will lose its voltage many times when testing with lamps as a load. That is why an easy way of starting the alternator is very handy.

When charging a battery, there are no such problems because the battery will deliver the initial field current to get the alternator started.

In textbooks on alternators, there are strong warnings against having an alternator run without a battery being connected. In a firefly charger, there are no such problems because the alternator can never get a higher speed than say 4,000 RPM, while in a car it could be driven up to a speed of 15,000 RPM.


4.11.2 Final check

First check the charger itself:

Then some fast electrical check on the alternator:

Then one could check the electrical connections. If you are sure that all wiring has been made and checked properly, there is no need for this. But anyway it is handy to know how it could be done in case the charger doesn't work later and one has to check these.

Have the electrical circuit of fig. 4.25 at hand. For this, the charger and battery should be connected to the switchboard, but only after the battery fuse has been removed (or the wire to the battery + pole has been taken loose). This makes that the battery is connected to the switchboard only by the negative wire and it can not destroy any part.

First check the polarity of the battery: Measure battery voltage (with tester on the right voltage range, see annex 0) on the battery poles and pay special attention to the polarity:

Check the negative connections with a tester on `diode' range. If a connection exists, the tester should read 0 or near 0, and most testers produce a bleeb. Keep one test lead on the battery - pole and check with the other test lead whether the connections were made correctly:

Do not just follow a wire and then measure on the connection where it ends, read the connection codes on each component and measure on this connection. If these are O.K., check whether no wrong connections (short circuits!) were made inadvertently. So now the tester may read anything exept 0 or near 0:

Check the positive connections. Keep one tester lead on the lip of the fuse holder that is, via the connector and battery cable, connected to the switchboard and measure:

Check whether the following connection does not exists:

Check the field connections. Keep one tester lead on the `DF' connection on the regulator and measure:

That's it for the `dry' checking. Switch `off' the battery switch and replace the battery fuse (or fit the positive wire to the battery + pole).


4.11.3 Trying it out

When testing with a battery as a load, one could check whether it supplies the field while the alternator is not running yet. Switch `on' the battery switch and check whether the field current lamps that are on, light up dimly. If they do not, rotate the alternator shaft to see whether the brushes made poor contact on the conductor rings at some points. If the lamps still don't burn dimly, either the battery was completely discharged and probably useless by now (measure battery voltage). Or there is something seriously wrong: Better switch `off' the battery switch. Look out for parts that were going up in smoke and check the fuses. Try to locate the error using the electrical circuit of fig. 4.25 and the checks of this paragraph.

If the lamps work O.K. it is worthwhile to see whether the indicator works because it is handy with further tests. Switch it to `Voltage' and check whether the voltage matches with the voltage of the battery as measured with the tester. Then switch it to `Current' and see whether it indicates a small, negative current (the battery is not being charged, but is discharged because the alternator only consumes field current). See whether this current becomes more negative if more lamps are switched on. If the indicator does not work properly, it needs to be checked and calibrated again, see par. 4.9.3, and this is a job that is best be done at ease in a workshop. For the moment, switch `off' the battery to conserve energy.

Then everything is ready for the real, wet test.

When it is sure that the charger will not be powerful enough to blow fuses and it is connected to a battery, things are easy:

  1. Connect the water supply by closing the valve in the forebay tank (see fig. 4.32) or by building up a temporary wall in a canal so that the screen becomes submerged.
  2. See whether enough water comes through the charger (`enough' means `a lot of', if it's the first time to see it run). If not, see next paragraph.
  3. By now, the charger should start to run quite fast: About 1.7 times the optimum speed for the head given in par. 4.1. Consequently the blades of the runner produce a high-pitched sound.
  4. Switch off the field current control lamps (the one that has no switch will remain on) and then switch on the battery. If the head is high, the sound will change immediately as it already begins to charge the battery.
  5. Switch on more field current lamps. Once the pitch of the charger noise has dropped to about 0.6 times the original, high-pitched sound (corresponding with the free running speed), it should run close to its optimum speed and the charger should produce its maximum power.
    If the sound does not react to more field current lamps being switched on, probably the battery is nearly or fully charged and the regulator is limiting the voltage, see par. 4.9.4. You will have to discharge the battery a little before the charger can charge it at its full capacity.
    Whenever the charger suddenly starts running free, check the battery fuse first (and take care not to burn your fingers on it).

When the charger could be too powerful for the fuses, step 1 to 4 are the same as above, but step 5 should read:

  1. Measure the charging current using one of the methods described further on in this paragraph. Switch on more field current lamps until the sound drops to 0.6 times its original sound, but make sure that the current does not increase above 12 A.

When the charger is connected to a set of car bulbs (instead of a battery), there should be no risc of blowing fuses since there is only the 25 A switchboard fuse in the circuit. Then step 4 and 5 should read:

  1. Switch off all car bulbs and switch on all field current lamps. Then provide the initial field current using penlight batteries as described in par. 4.11.1. This should make the charger start generating electricity.
  2. Measure the voltage (with the switchboard indicator or with a tester connected to `batt.-' and `batt.+' on the connector block), it should be 14 V or above since the regulator is limiting the voltage (it won't be exactly 14.7 V since the regulator can not work accurately without the dampening effect of a battery). Carefully switch on car bulbs one by one until the voltage drops to 13 V or below. The sound will drop in pitch as the alternator has to generate more power. When the pitch drops below 0.6 times that of the original sound, try switching off a field current lamp. In this way, the optimum number of car bulbs and the optimum number of field current lamps should be found at the same time.
    Switching on one 15 W car bulb too many or switching off one field current lamp too many will make the voltage collapse and the charger run free again. Nothing has been damaged, it just needs starting again following step 4.
    The car bulbs could be blown up if the charger is producing more than 14.2 V for a long time, so avoid this by switching on more car bulbs.

Hopefully everything went well and by now, the charger is running either at (nearly) maximum power or at a power that is limited to approximately 165 W (because of the 12 A limit in order to protect the fuses). If it does not, see par. 4.11.5.

While it is running for the first time, it is adviseable to keep a close watch:

  1. Check whether any connections get exeptionally hot. If there is a funny, burning plastic-like smell, switch the battery switch off and locate where it comes from. Feel whether the battery connector, battery fuse holder, fuse holder on the switchboard and the connections on the alternator get hot. If one of them does, it means there is a bad connection. If this happens with the connections inside a fuse holder, the fuse will get too hot and it will blow at a current well below its rated current. Try to improve bad connections by cleaning, tightening, using `electric contact spray' or grease etc.
  2. Check whether the seal around the shaft works properly: Is the alternator compartment completely dry or is there a fine mist of water coming from the fan.
  3. Check the temperature of the alternator, especially of the ring of iron in the middle (this is the stator iron). As long as it remains below say 60 °, there is no problem.


4.11.4 Measuring its performance

The next thing to do is to make some measurements. Make notes of how the charger was running at the time (no. of field current lamps etc), how you made measurements (e.g.: With the tester or with the switchboard indicator) and the results. There is no need to do all measurements, or do them in the same order as they are described below. Just measure what you would like to know.

Then for people who aren't that experienced with testers: Testers have to be switched to the right range. A range is defined by:

  1. What you want to measure: a voltage, current, resistance, a voltage drop (with the `diode' range).
  2. With voltage and current ranges: Whether it is a `DC' (decent current)' value or `AC' (alternating current) value. For the charger, almost always DC measurements have to be made.
  3. The maximum value for this range, say 2 V, 20 V or 200 V. Digital testers can stand tremendous overvoltages on all ranges exept the current ones. (for analogue testers, with a needle, an overvoltage of more than say 5 times the maximum value set by the range, could become dangerous). Professional digital testers have `autorange' feature: The tester itself selects the right range with the right maximum value once it is set to what you want to measure, a DC voltage, resistance or whatever.

If with a voltage or current measurement, the tester displays a value at the moment it is connected, but the reading goes to 0 again within a few tenths of a second, most likely you are using the `AC' (alternating current) range instead of the `DC' (decent current). So then put it on the right `DC' range and try again.

Voltage measurements: These are needed for the following purposes:

Current measurements: These can be used for:

The easiest way to measure the charging current is by just using the switchboard indicator. But as long as it has not been checked, it can not be relied upon. Then there are the following ways to measure the current using a tester.

For checking the switchboard indicator, the direct method is more reliable. If the tester can only measure up to 5 or 10 A, the charging current can be reduced to below this maximum value by switching off field current lamps or car bulbs (roughly check this with the current shunt method). Then measure the current directly and compare it with the reading of the switchboard indicator. If the switchboard indicator is considerably wrong, write down both current readings (from the indicator and from the tester) so that you can calculate later how much longer or shorter the current shunt should be. how much longer or shorter.

The head: This is needed for comparing the power output with the expected output.

The simplest way of measuring the head is by measuring vertical distances with a spring rule or a folding rule. It does not matter whether you start at the top (at the water level around the screen) or at the bottom (at the height of the nozzle). When starting at the top, hold the top of the spring rule at the height of the water level, but so far away from the valley side that it hangs down vertically freely. Make a mark on the soil at 1 or 2 meters below this level. Climb down, hold the top at the heigth of the mark and let it hang down furter, make a new mark until you reach the nozzle. The inaccuracy of this method is in holding the top level with the mark on eyesight. It would be more accurate if the top of the rule was fixed on a straight board and the board was kept level with a carpenters level. Also the marks can be made more precise by placing sticks with the top at the correct height.

Another way is by fixing a light, tight wire from a point at the height of the nozzle to a point at the height of the water level at the top. It is best to choose such points that the thread will be quite steep. Measure accurately the angle a the thread makes with the horizontal direction. Use a protractor (a graduated arc) and a carpenters level for the horizontal. Then mark on the wire where it was fixed, remove it and measure its length l between the marks. Then the head H can be found by multiplying length l with the sine of angle a.

H = l * sin (a)

Alternatively, one could measure how much the thread climbes over 1 m of its length, and multiply this figure with the total length l of the wire.

Flow measurement: It would be interesting to know the flow also because then it is possible to calculate the overall efficiency of the charger. However, it would take quite some time to get everything set up for making flow measurements while it is not essential for getting the charger running properly. See annex 0 for how to make such a setup.

The results of the above measurements can be used for:

In par. 4.1, overall efficiency of the charger was estimated at 0.30. So if a charger produces say 15% more power than predicted in par. 4.1, its overall efficiency must be 15% higher than 0.30, so 0.345 (assuming that it consumes exactly the flow as predicted in par. 4.1).

In par. 4.1, friction losses in the penstock pipe were not taken into account. The part of gross, hydraulical power that is lost as pipe friction losses can be estimated by comparing the actual flow with the maximum allowable flow for that pipe diameter, length and roughness, see par. 4.10.8. So assuming the charger produces exactly the power predicted in par. 4.1 but pipe losses are 15 %, the net overall efficiency of the charger itself must be 0.30 / 0.85 = 0.353

If the measured power is higher than the expected power, the charger must be functioning quite well. Apparently the alternator has a reasonably good efficiency and the turbine part was built neatly, congratulations!

When overal efficiency ends up below 0.25, better try to find out why:


4.11.5 Troubleshooting

Troubleshooting must be quite difficult for someone with limited technical experience and without ever having seen such a thing run before. Especially when things are not going well, one is tempted to try many different ways to get it running and the results could become more and more confusing. Therefor the following advice:

The first step is observing what is going on. At least this should tell whether the problem is in the turbine part or with the alternator part. So check or measure:

Variables the turbine needs for functioning properly:

Things the alternator needs:

Output of the charger:

And in general:

The charger can operate basically in two different ways depending on whether the regulator is limiting the output voltage or not. It is important that one is able to recognise in what way it is working (see par. 4.9.4 for more details).

When the regulator is not limiting the voltage:

When the regulator is limiting the voltage:

Then there are some things that hold the middle between observing and repairing:

Table 4.4: Trouble-shooting guide.
Problem: Possible causes and ways to find out: Solution:
Once connected to the water supply, the charger gets kicked away and the pipe moves violently. The top end of the pipe is inclined downwards too much, see par. 4.10.7. Reduce the inclination of the top end of the pipe to less than 15°. For the moment, the problem can be avoided by having the flow to the charger increase only slowly when starting the charger
Actual head is too low. The flow in the canal is not enough for the charger. The screen is not completely submerged and the upper part of the pipe contains air instead of water, which makes that the charger experiences too low a head. Increase the capacity of the inlet and the canal, repair leakages.
Decrease the flow needed by the charger by fitting a larger blocking timber.
There is a large bubble of air trapped in the upper part of the pipe. Make sure that the top portion of the pipe leaves the forebay tank inclining at least 5° downwards, see par. 4.10.7.
The screen is completely blocked with weeds. Clean the screen.
Actual flow is too low (this could be caused by the head being too low, so also see there). The nozzle is partially blocked by a piece of wood, leaves, a pebble etc. Then often the water does not leave the runner in a regular way: Its speed is too low and one end of the runner gets more water than the other end. Remove the charger from the pipe and check.
Apparently the charger has run without a screen or with a damaged one, so check this also.
Shaft doesn't rotate at all Try to rotate it by hand:  
  • If it rotates easily, the head and flow must be very low.
See with low head and low flow.
  • If it is stuck completely, the runner is blocked by a piece of wood, pebble etc.
Remove charger from pipe and check.
  • If it can be moved but needs quite some force, there is too much friction in the seal or the runner gets stuck against the nozzle.
Check where the seal drags. Maybe the alternator can be moved a little by grinding a bit off the supports on the frame or by fitting thin rings (e.g. from a tin) in between. Or push some sandpaper in between the parts that drag and grind them off by rotating the runner by hand.
If it is the runner that gets stuck, try to get it better aligned on the shaft (see par. 4.4) or loosen the bolts that fix the alternator to the frame and fix them again while pushing the alternator away from the nozzle. File out the holes in the frame a little bit if necessary.
Speed is too low. The head and flow are very low. See with low head and low flow.
Too many field current lamps are on. Well, switch some off.
Speed is too high. Battery is nearly charged (or when testing with car bulbs, only a few of them are switched on). Measure output voltage, if it is nearly 14.5 V, the regulator is limiting the voltage, the power output is reduced and the speed goes up. Get a discharged battery, or switch on more car bulbs.
Too few field current lamps are switched on. Try whether switching on more field current lamps makes any difference.
Speed is much too high: The charger is running free. Usually, this is caused by the alternator not working at all and there could be many different causes for this. Probably it is an electrical problem. Check the electrical things mentioned in par. 4.11.2. If it still does not work, either the alternator or the voltage regulator must be broke and needs repair in a workshop.
There is no field current (the field current lamps that are on, do not burn at all). When testing with car bulbs, this could be a starting problem, see par. 4.11.1. Measure the initial field current supplied by the batteries with the tester on `DC current' range. With two penlight batteries, it should be ca. 0.4 A. If it is lower:
  • Check whether the batteries are empty (do they light a torch brightly)
  • Measure resistance between `F' and `E', see par. 4.11.2. If it is more than 6 Ohm, check whether the brushes could be wet. If so, check the seal around the shaft. If not, smoothen the conductor rings with sandpaper.
When testing with a battery, the regulator might not get any voltage. Measure the voltage over `D-' and `D+' on the regulator (= voltage Vr in fig. 4.25). If there is no voltage over the regulator:
  • Check whether the battery switch is `on'.
  • Check the connections between the battery and the switchboard.
  • Measure battery voltage itself. A completely dead battery can not be charged in this way.
The brushes might make bad contact on the conductor rings. Measure the resistance between `F' and `E', see par. 4.11.2. If resistance is above 6 Ohm: Check whether the brushes could be wet. If not, smoothen the conductor rings, see par. 4.11.2. If it doesn't make sense at all, maybe the `F' wire was connected to the wrong connection of the alternator.
The connections between the switchboard and the alternator could be faulty. Check the `F' wire and the `E' wire (and their connections) of the alternator cable with a tester on `diode' range.
The battery fuse blows instantly once you switch a battery `on'. Either the battery or the charger has been connected with the wrong polarity. Check the connections and the polarity of the battery, see par. 4.11.2.
If polarity was wrong, some alternator diodes might have been destroyed already. Take this into account when the alternator performs poorly afterwards.
Something is wrong inside the alternator, maybe several diodes have blown and now act as short-circuits. This can only be repaired in a workshop, see par. 4.11.6.
The battery fuse has much too low a rating. Find a 16 A fuse and fit this one.
The battery fuse blows after a few minutes to a few hours. Maybe the fuse got overheated because of lousy connections in the fuse holder. Check whether the fuse holder becomes hot during charging. Clean contact surfaces, use contact spray or grease them.
Maybe glass fuses were used. These are designed for 220 V and the heavy, 16 A types overheat easily by the current passing through them. Probably this is why in practice, glass fuses eventually blow at a current that is little more than half their rated current. Replace the fuse and fuse holder for types that are meant for cars, see par. 4.9.6.
For the moment, you could reduce charging current a little by switching off a field current lamp.
The switchboard fuse blows.
  • Maybe it got overheated because of lousy connections.
  • Maybe it had too low a rating
  • When connected to car bulbs, there is no battery fuse. So when the charger is producing a much higher output power than expected, this fuse will blow.
  • Clean contact surfaces and fit a new one.
  • Find a 25 A fuse.
  • Reduce output power by fitting the correct blocking timber, by reducing the number of field current lamps or by reducing the number of car bulbs switched on, see par. 4.11.4.
Charging current is negative, (-1 to -3 A). There is a field current and the charger runs very fast. This can only happen when connected to a battery. It supplies the field current (the -1 to -3 A that appears as the negative charging current) but it is not being charged. Measure the voltage directly on the `E' and `A' connection of the alternator:
  • If it is much higher than the output voltage, there must be something wrong with the `A' connection or the switchboard fuse
  • If it is equal to the output voltage, something inside the alternator must be wrong, see par. 4.11.6. Probably it needs repair in a workshop.
Check the `A' wire and check the switchboard fuse.
Charging current is lower than normal and the speed is higher than normal. Output voltage is 14.5 to 14.7 V. The most likely explanation is that the battery is nearly charged and therefor the regulator starts limiting the voltage. Check battery voltage (with tester leads directly on battery poles) to be sure: If this is well above 13.9 V, the battery is nearly charged. There is nothing wrong with the charger (there might be something wrong with the battery though, see annex C: More about batteries).
If battery voltage is below 13.9 V, there must be too high a voltage drop over the battery cable, the connector or the battery fuse. Check these connections, see fig. 4.25. By measuring voltage drops (see par. 4.11.4) you can find out where the problem lies. Fit a shorter or thicker battery cable if this was causing the problem.
Charging current is lower than normal and speed is higher than normal. Output voltage is below 14.5 V. Check whether the regulator is limiting the voltage (see at the beginning of this paragraph). If so, probably the regulator is adjusted too low. Measure voltage Vr over `D-' and `D+' on the regulator to be sure. Once the regulator is limiting the voltage, it should be 14.7 V. Readjust the regulator to 14.7 V if necessary, see par. 4.9.4.
If the voltage over `D-' and `D+' is 14.7 V while the output voltage is below 14.5 V, there must be a too high voltage drop over the battery switch or the current shunt. Measure the voltage drop over the battery switch. Maybe it is oxidized or, when too small a type was fitted, it could be partially destroyed. Measure the voltage drop over the current shunt, it should be 0.1 V or less, check shunt calculation, see par. 4.9.3.
If the regulator is not limiting the voltage, maybe resistance of the field and brushes (between `F' and `E') has become too high, for instance because the brushes are wet. Then field current will be lower than usual for this number of field current lamps that are switched on, and with that the charging current.
. Measure this resistance, see par. 4.11.2.
Check whether there is a mist of fine water droplets in the alternator compartment when it is running free (with the battery switched off). If so, the seal should be improved.
If the seal can not be repaired soon, avoid having the charger run free as much as possible. At normal operating speed, it is much less likely that water will pass the seal.
Don't bother about the water that is inside already, it will dry up in a few minutes running or after drying a few hours in the sun.
If the brushes are not wet while the resistance has increased, smoothen the conductor rings, see par. 4.11.2.
Maybe one or more diodes inside the alternator are destroyed. See par. 4.11.6. Probably the alternator needs repair in a workshop.
Output voltage increases above 14.7 V once battery is nearly charged. Voltage regulator is adjusted too high. Readjust voltage regulator to 14.7 V, see par. 4.9.4.
Charging current is abnormally low. Output voltage is way below 14.5 V. Speed is lower than normal for the number of field current lamps that are on. The turbine part is not producing its normal power. Maybe:
  • The canal doesn't supply enough water.
  • Air is trapped inside the pipe.
  • The screen is clogged.
  • The nozzle is partially blocked.
  • Too wide a blocking timber is fitted.
See with `low actual head' and `low actual flow' at the beginning of this table. Clean screen if necessary. Fit blocking timber of the right width for this head.
Charger runs perfectly normal (with normal speed), but the current reading is exeptionally low. Probably the contact surfaces inside the indicator range switch are oxidized because some water came in. This could also affect the voltage range but the effect is far greater on the current reading so it is more likely that you will notice this first. Operate the indicator range switch a few times to see whether that makes any difference. Protect the switchboard better against water if this was the problem.

In this guide, problems concerning the battery being charged are not included. It is assumed that for testing, car bulbs will be used or a new, functioning battery. See annex C for how to check batteries.


4.11.6 Checking diodes of an alternator

Checking alternator diodes could be difficult for someone without experience and proper instruments. Instead of checking the diodes themselves as described below, one could also check whether it is likely that diodes are destroyed:

Another reason for not trying too hard to figure out whether the diodes are O.K. is that, once it becomes clear that one or more diodes are destroyed, one will probably have to bring the alternator to a workshop in town anyway. For replacing a diode, a spare diode and a heavy soldering iron are needed so it is unlikely that can be done in the field. So when there is clearly something wrong with the alternator, one might as well bring it to a workshop and have it repaired there.

A diode is an electronic device that allows current to flow in one direction: The conducting direction (this is the direction of the arrow that can be seen in its electronic symbol). Then for a wide range of currents, there will be a nearly constant voltage drop of some 0.6 to 0.7 V over it (however, alternator diodes show a voltage drop of only 0.4 to 0.5 V when measured with a tester on `diode' range since the testing current provided by the tester is too low compared to their large capacity).

In blocking direction, a diode allows only a negligible current to flow. Then the voltage over it will be as set by the voltage it was connected to.

Inside the alternator, there is a rectifier circuit with 6 diodes, see the alternator circuit in fig. 4.25. The top row of 3 diodes are called `positive' diodes so clearly the bottom row are the `negative' ones. Together, these 6 diodes transform the A.C. (alternating current) electricity from the 3 stator phases to D.C. (decent current) electricity that can be used to charge a battery. Mind that the diodes are drawn `upside down': They are not usually conducting. Only when the voltage of a stator phase is above the voltage on `A', its positive diode will conduct. And when its voltage is below that on `E', its negative diode will conduct.

So the two connections of a diode are not interchangeable:

Finding out which end is which is difficult with alternator diodes because the positive diodes have their kathode connected to the housing, while the negative ones have their anode connected to the housing (but even this might be reversed in some large capacity alternators). In this way, all positive diodes can be fitted into one cooling element, and the negative ones to another one.

To check this, use a tester on `diode' range. If it displays a value of 0.4 to 0.5 V (or 0.6 to 0.7 for smaller size diodes), the diode must be connected in conducting direction. Then the diode end that is connected to the red tester lead must be the anode. Connected the other way round, the tester will display `OV' (overload), or its high, maximum value (the same value as with tester leads not connected to anything. Then the red tester lead must be connected to the cathode of the diode. But:

Diodes can be destroyed by:

A destroyed diode can react in two, opposite ways:

So once a diode is isolated from the circuit it is in, it can be checked with a tester on `diode' range. Then the tester will either displays `overload' if the diode is connected in blocking direction, or display the forward voltage drop in V if the diode is connected in conducting way.

The above way of checking the diodes is simple and effective, but it involves a lot of work to isolate them. Of course only one of its two connections has to be taken loose to make sure that a measurement is not influenced by the surrounding circuit. But still, the alternator probably has to be opened up to get to the diodes. The connections to the diodes are soldered and heavy, so for taking them loose, a large soldering iron is needed. Once you know that one or more of the diodes must be faulty, this is not a problem since you will have to open it up anyway and replace at least one. But for just checking whether the diodes are allright, there is a need for less time-consuming methods.

One possible method is by measuring the voltage drops over the complete set of diodes. For this, disconnect the alternator from the switchboard. Measure the voltage drop in conducting direction (red tester lead on `E' and black tester lead on `A'):

For measuring in blocking direction, change the tester leads (red on `A' and black on `E').

With these measurements, destroyed diodes that have a very low voltage drop will be found, but not diodes that do not conduct in either direction.

Japanese brands and Fiat alternators have an `N' connection to the star point of the stator coils. By measuring between `E' and `N', and between `N' and `A' in both directions, some more information can be gathered. Now the forward voltage drop should be 0.4 to 0.5 V because it is measured over one diode only.

On such alternators with an `N' connection, it is easy to check all diodes in one go while the alternator is running. Make sure that the regulator is not limiting the voltage and that the right number of field current lamps are switched on. Keep the black tester lead on `E', measure the voltage on `N' and compare this with the voltage on `A':

When the voltage on `N' is nearly half the voltage on `A', there is still a change that a negative and a positive diode are destroyed and that the effects even out one another. To find out more, one could measure the `AC' voltage on the `N' connection (so with the tester switched to an AC voltage range, have the black tester lead on `E' again). If all diodes are O.K. and the charger is producing some 12 A charging current, this should read no more than say 4 V. Every thinkable fault in the diodes will make this AC voltage increase to a much higher value.

There is no fixed value of what is normal. When the charger is hardly producing any charging current, it should be considerably less than this 4 V, while for an alternator that is running at full capacity, it could go up to maybe 8 V (this could never happen in a firefly charger since the turbine is not strong and fast enough). But still this AC voltage on the `N' connection will be somewhat higher if a relatively small alternator is producing a relatively large charging current.

It makes sense to measure this AC voltage on `N' connection once with an alternator that functions well (also note the conditions under which it was operating). Then this value can be used for a normal value with all diodes functioning.

On alternators without an `N' connection, it might be possible to remove a cover so that the diodes and phase coils become accessible. Maybe the star point can be found (three equally thick copper wires connected to one another and to nothing else). Then this can be used instead of the `N' connection and the alternator can be checked in the way described above. When only the diodes are accessible, these can be used. Measure the DC voltage of each stator phase (the thick copper wires that are connected to two diodes each). Like with the `N' connection, this voltage should be very close to half the voltage on `A'. An `AC voltage' measurement on stator coils gives no sensible information.

There are many more tricks to find out whether the diodes are functioning. But for most of them, a special measuring instrument or test device is needed, or experience in interpreting the results.

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