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Where to put a Diode?

Mike Cushway

Another question as I install the "Little Kilowatt" generator and wire it into the switchbox for the charge controller. I want to put a diode in the wiring to keep the generator from motoring if for some reason it stalls. It is a 110vdc/450 watt unit. My question is, how to size the diode. The generator output is listed. The current coming back from the battery bank is 12v nominal. I would appreciate if someone could elaborate the hows and why's to do this correctly. Thanx!


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Re: Where to put a diode?

Not quite certain of your problem. If just keeping power from flowing back to generator diode can be inserted in either side at any point. As to sizing, voltage ratings on diodes is specified in PIV, or Peak Inverse Voltage. The basic rule of thumb is to size diode at least 20% higher than possible back voltage. The problem will be the expected WATTAGE. 450 Watts at 110 VDC suggests a maximum current of 4.1 Amps. Voltage drop across diode will be about 0.7 VDC. 0.7x4.1=2.87 Watts (expected heat generated by diode under full load.) That will require a stud type diode with heat sink. Given the expected voltage heatsink will have to be issolated and protected from probing fingers.

I would use at least a 200PIV voltage rating (400 would be better) 5 to 10 Amp current rated stud diode with 4 to 10 square inches of aluminum heatsink.

Mike Cushway

Re: Where to put a diode?

Thanx Doc!
I ordered a set(one spare) of 1000v 40A case stud mount positive diodes for $14. Ill make a nice aluminum heatsink and put it in the engine cooling airstream.


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Re: Where to put a diode?

The 1kv rating is certainly a good idea. When putting a diode on ANYTHING that has windings (generators and motors, solenoids, ignition coils... ) keep in mind that the inductive kickback and RF voltages that occur can be mind-boggling, and P-N junction punishing. Put a capacitor across the diode, too- it'll 'snub' the owies substantially. ;)