I needed a rectifier for a new transformer I bought second hand. All I know is that it outputs 13.7 Volts. And I'm not very good at telling how many Amps it has by the size of it (~ 3 Inch each side), so I whould say that it is at least 2 Amps... but just in case, I want to use a rectifier that is more than 4 Amps...

I went to buy a rectifier to a local electronics store, and I basically had a choice between 4A 100V and 8A 400V.

Since I don't know the output Amps for the transformer, I want to use the 8A 400V rectifier.

But, is there a performance difference between the 100V and 400V when it will be used for only 13.7V ?

Also, if I use 2, 4 Amps rectifires in parallel, does that mean that together they are 8 Amps ?

 

No performance hit from higher than needed voltage rating in the diodes. Hint - it is often as cheap to buy 400 volt 35 amp bridges as several diodes rated at less current.

Paralleling rectifiers just means that one will get warmer than the other, hog current, die, and cause the other one to fail. Always get diodes with some extra current capacity.

 

OK, thank you very much

 

The higher-voltage rectifying diodes typically have a higher forward voltage drop than a low-voltage rectifier. If the forward voltage drop of the 400V rectifier is 1.3V, your DC output will drop about 1.2V compared to the low-voltage rectifier (in a bridge configuration).

 

So, are you saying that the diference between them is ~ 0.1V in the voltage drop ?

 

The higher-voltage rectifying diodes typically have a higher forward voltage drop than a low-voltage rectifier. If the forward voltage drop of the 400V rectifier is 1.3V, your DC output will drop about 1.2V compared to the low-voltage rectifier (in a bridge configuration).

uhh, no. A silicon diode drops the same amount of voltage. 400V isn't that much to solid state electronics. Maybe a 4KV diode would show some difference, never gone there. Differences between families may vary by millivolts, but not usually. It has to do with the PN junction drop, and the resistance of the connecting wires within the diode.

 

Bill, high reverse voltage specs in rectifying diodes usually come at the expense of forward voltage (VF). I think this is because manufacturers stack junctions to get the higher voltage ratings. The limit for silicon rectifying diodes seems to be about 200V. Above that, VF usually increases. Here are some specs for typical silicon rectifier diodes in the range that CVMichael is looking at.
Notice how VF jumps to from .88V to 1.25V @3A @25 degC in the MUR series as voltage doubles from 200V to 400V. This is pretty typical of silicon rectifiers.

CVMichael, be mindful of the VF spec of the two diodes you're considering. If the VF specs are similar to the link I posted, the 400V diode will dissipate ~45% more heat than the 100V diode, and your DC output in a full-wave rectifier circuit would be ~.74 volts lower.

 

Interesting. I looked up about 7 datasheets on the 1N4001 to 1N4007 family (which covers 50 V PIV to 1000 V PIV) and none of them made a distinction between PIVs, but all of them claimed 1.1 V instantaneous forward voltage drop.

The Motorola datasheet did add an additional value, average forward voltage drop, which I assume is RMS for AC.

 

At first I asked a general question, but it seems I have to be more specific.

These are the 2 rectifiers I was talking about:
KBU8G pdf - 8A 400V
KBL01 pdf - 4A 100V

 

The 8 ampere is a better choise in your case. Its forward voltage drop is 1.0V at 8A.
The 4 ampere rectifier has a forward voltage drop of 1.1V at 4A.

If you use the 8A rectifier at 4A its voltage drop will be lower than at 8A. Lower current in a diod results in a lower voltages drop.

 

The 8 ampere is a better choise in your case. Its forward voltage drop is 1.0V at 8A.
The 4 ampere rectifier has a forward voltage drop of 1.1V at 4A.

Agreed.

 

OK, thank you everyone

 

Actually, if you look at fig 3 for the KBU8G on page 2, you'll see that the typical instantaneous forward voltage for the bridge is about 0.84 @ 4A.

Looking at a similar chart in the KBLx datasheet, it's about 0.95 @ 4A.

So going by the typical values, the 8A bridge is about 0.11v better (roughly an 11.6% improvement.)