On another thread we're discussing replacing stud mounted diodes (SMd) with a bridge rectifier. While in discussion I remembered some SMd's I have. Not knowing anything about them I decided to test them. Using a power supply (13.82V) I ran the voltage through the diode. The voltage on the other side of the diode was 13.72V. That would suggest a 0.1Vf, and that's unbelievable. So I tested the diode using my meter set on diode test. One way the meter read 170mV and the other way it read "OL". So on diode check it said it was good. BUT I then connected the power supply backwards and checked the through voltage. Again, verified 13.82V but on the other end I got 13.77V. So I'm really confused.

I don't have any numbers on this diode, it's big. High amperage and out of an old welder. Perhaps my meter can't provide enough current through the diode to cause it to block one way? Going to test the diode again using an automotive lamp. Let you know the results.

 

Using an automotive bulb the SMd's all conduct one way but not in reverse. So they're good. The question remains, however, am I really seeing a Vf of only 100mV?

 

What was the Vf with the automotive bulb load?

 

Further testing: With a load running through the diode the voltage reads from 666mV to 716mV across the diode, depending on the load. If I light the parking filament it get 666mV. If I light the brake light I get 712mV. Through both filaments it's 716mV. That seems to make sense; a typical 0.7Vf.

 

I expected high power diodes to have a higher Vf.

 

Further testing: With a load running through the diode the voltage reads from 666mV to 716mV depending on the load. If I light the parking filament it get 666mV. If I light the brake light I get 712mV. Through both filaments it's 716mV.

Seems about normal for a Silicon PN diode voltage drop.


Diode leakage current likely affected the no or low current Vf measurements.

 

Further testing: With a load running through the diode the voltage reads from 666mV to 716mV across the diode, depending on the load. If I light the parking filament it get 666mV. If I light the brake light I get 712mV. Through both filaments it's 716mV. That seems to make sense; a typical 0.7Vf.

0.7V is a typical forward voltage for high-current silicon junction diodes at a few hundred milliamps.

Below is the LTspice sim showing the log current-voltage relation for a 3A diode over a wide current range:

 

Seems about normal for a Silicon PN diode voltage drop.

It might not be Silicon. There are other options. Germanium?

 

I expected high power diodes to have a higher Vf.

Why? Think of a high power diode as a lot of low power diodes in parallel. At the same current, I would expect lower Vf.

 

It might not be Silicon. There are other options. Germanium?

Possible but his under load Vf readings (666mV to 716mV) look normal for a Silicon diode.
That's why I posted a chart with several types and configurations.

His initial Vf of 0.1 was out of the range for old Germanium power diodes so I thought it was a leakage measurement artifact value of a likely Silicon diode.