Testing Diodes Connected to the Rectifier COLUMN BY NATHAN UNGER When testing diodes which are still con- nected to the rectifier, a good visual inspection is the first step. Look for solder joints that have a dull, crystallized [lr > appearance or seem to have been re-melted. Look for burned stator phases or coils, broken connection welds, any signs of heat discoloration, melted plastic, loose diodes or other parts, and excessive corrosion. Clean away foreign material that might cause false leakage readings. If you see any of these, you will have to separate the sta- tor and rectifier anyway. Testing diodes for leakage The simplest way to test diodes for leakage is to use an inexpensive ohm- meter with a X10K scale which uses a 9-volt battery to power it. Radio Shack used to make a tester like this. I think Triplett or Simpson do too. The 9-volt source gives you a better test than the typical 3-volt (two AA’s) which most meters use. The diode should show low resist- ance on the X1 scale in one direction, but the needle should not move on the X10K scale with the leads reversed. To test diodes for leakage while they are still connected to the stator, you would simply connect your ohmmeter from any stator-diode connection to either the positive heatsink or after reversing the leads, to the negative heatsink. This would give you the combined leakage of all the diodes on that heatsink. The needle should still not move. In my opinion, most diodes that have been stressed will have increased leak- age. If the stator lamination stack is touching the negative heatsink during this test (like the Ford 3G), be sure you are not condemning the diodes when the stator is the one with the leakage. A more sophisticated leakage test would be to apply 16 volts and measure the leakage current with a micro- ammeter. I choose 16 volts because some diodes will ‘zener’ (avalanche) at as low as 18 volts (more on zener diodes later). Again, you can measure leakage current with the diodes still connected to the stator. The combined leakage will be three times as high, assuming a three diode heatsink. Everyone wants to know what an acceptable leakage amount is, but the fact is that the fussier you are, the fewer failures you will encounter. Why not compare it to a new diode? Bosch has a new diode out, the ZR1480, rated at 80A. Its max reverse current rating is 100 microamps max at 16 volts. The stator itself can be checked in the normal way for current capacity and balance because (non-shorted) diodes will isolate the phases electrically. When checking the stator for insulation leakage, you would have to ensure that the stator lamination stack is not touch- ing the negative heatsink (like Ford 3G). Either style of diode leakage test, along with a good visual inspection, will eliminate most of the rectifier- stator combinations that will need to be desoldered/disconnected before going any further. Testing the forward voltage drop You need to run a substantial current through a diode (not too long) while applying a voltmeter (separately con- nected; remember Kelvin) across each diode. This will give you a forward volt- age drop reading. You can use a 12-volt car battery and a 20A fixed load (0.6 ohm 250W) in series with it. To test diodes while still connected to the sta- tor, connect the load side to the negative heatsink and the battery ground side to the positive heatsink. The 20A load will limit the current, and you are now turn- ing all the diodes on at once. Measure each diode with the volt- meter. If a diode was open, you would read at its terminal the voltage of the nearest diode plus the drop across coils of the stator. So although the diode wouldn’t read open, its forward voltage would be substantially higher. To check the trio diodes while still connected to the stator, you have to be more precise in reading the voltage. This is because the current should be limited to 3A (1A diode X 3). If one of the trio diodes was open, the low current would not generate much of a voltage drop across that phase of the stator, and so you would have to be more accurate in order to catch it. Please note that the forward voltage reading varies somewhat from one diode to another. If they have different amper- age ratings, are tested at different amper- ages, and reach different temperatures, you could expect even more variation. This is because of the manufacturing process. This last factor becomes signifi- cant when pairing diodes to increase load carrying capacity, because the diode with the slightly lower forward voltage drop will take most of the load, possibly over- stressing it. Also, in aircraft applications, diodes that are not exactly matched can cause beat frequencies which may inter- fere with nav/com equipment. Please note that the diodes with the higher voltage drop will get hotter than the others, and that is a question of reliability. For our ZR1480 80A Bosch diode, the forward bias voltage is 1.17 volts, tested at 200A. The maxim