# 2 Bridge Rectifiers - Different forward voltage results

Discussion in 'The Projects Forum' started by kiwijeff, Oct 31, 2012.

1. ### kiwijeff Thread Starter New Member

Oct 4, 2012
11
1
I've got 2 bridge rectifiers that I've tested from each AC pole to each DC pole and verified conductivity in one direction, open in reverse direction. Forward voltage reading for rectifier#1 is around 0.420v, #2 is around 0.450v. This would lead me to believe both are ok.

I also tested between the DC terminals + and - and verified conductivity in one direction. however here's where it differs. As I understand it the test is passing through two diodes when reading across the DC terminals.

Forward voltage reading was around 0.840v on rectifier #1, but stayed around 450 on rectifier #2.

I can't seem to find anywhere that tells me if this forward voltage value should be doubled (#1) or not (#2) ? Or are they both ok ?

2. ### wayneh Expert

Sep 9, 2010
15,014
5,458
You may have a shorted diode on #2. Are these single piece bridges, or discrete diodes? If it's the former, I wouldn't put #2 in service until I did more testing.

Your forward voltage drops are quite low; is this a schotky rectifier?

Oct 4, 2012
11
1
4. ### wayneh Expert

Sep 9, 2010
15,014
5,458
Hmmm, those are both normal diode rectifiers, meaning the forward voltage drop should never be less than about 0.6V per diode at low current, up to 1.1V at high current. When you measure from the negative DC pole to the positive pole, you would expect 1.2 to 2.1V.

How exactly are you measuring the voltage drops?

5. ### kiwijeff Thread Starter New Member

Oct 4, 2012
11
1
I've got a diode test function on the multi meter. Tested out of circuit.

Would this be sufficient to warrant replacement?

6. ### wayneh Expert

Sep 9, 2010
15,014
5,458
Do you have a battery and a light bulb? Since you've removed these from the circuit I'd be tempted to just replace both of them, but you could test them to verify you can light the bulb in one direction and not the other. I suspect the shorted one will light the bulb both ways, which of course is very bad. If the rectifier passes the bulb test, I wouldn't hesitate to put it back in the circuit.

7. ### kiwijeff Thread Starter New Member

Oct 4, 2012
11
1
Ok, this is kind of weird. The KBU1506 lit the bulb in the "correct" direction.

I then reversed + and - on the bridge rectifier, and the bulb lit, for about 3-4 seconds and then went out. I disconnected and reconnected and the bulb was not lighting - as it shouldn't. But why did it light for 3-4 seconds when it was put in the reverse direction.

I've never quite seen anything like that. And I couldn't repeat it. Seems to be some sort of intermittent issue with this particular bridge rectifier.