# Diodes in parallel

#### Edwin Fitzpatrick

Dec 26, 2012
23
To get high current, you can connect two diodes in parallel,They say to put a small resistor in series with each diode to get a .2 to .6 volt drop. At 80 amps, this is a very small amount of resistance. You can not buy this small a resistor. ??????
maybe just the circuit resistance would work. The diode ,that i had in mind ,is a pair of Fairchild RURG8060, 80 amp diodes Mounted, side by side, on the heat sink to keep the temp. the same.If I use a resistor,What size and wattage ? These diodes are to control inductive kick back from a PWM DC motor.Speed controller.
TNx,Fitz

#### davenn

Moderator
Sep 5, 2009
14,011
They say to put a small resistor in series with each diode to get a .2 to .6 volt drop. At 80 amps, this is a very small amount of resistance. You can not buy this small a resistor. ??????

who is THEY and didnt they suggest a value for the resistors ?

Dave

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
You cannot buy this small a resistor. ??????
Nonsense! I have a heap of 2 milliohm resistors. (you need between 3 and 7 milliohms). These are fine for up to 22A, I'm sure you can find higher power resistors.

But you could just use a length of wire.

You also need to keep the diodes in good thermal contact (and you have mentioned that) and ensure that your connections have very close to the same resistance (a few milliohms difference is very easy to achieve by accident).

At these currents, inductance may be a major issue, so you also need to ensure that you don't accidentally introduce more stray inductance than you can get away with.

use ohms law to determine the value and power for the resistor.

R = V/I

P = I^2.R

#### Edwin Fitzpatrick

Dec 26, 2012
23
Hi Steve,
That was what I had in mind. In the copper wire tables #14 wire has 2.525 ohms per 1000 feet. This is .002525 for one foot.If i took one foot of wire for each leg of the diode ,I could get .
.005 "5 milliohms".At 80 amps through the diode there would be .4 volts drop across the resistor.In the ball park.Now if you interwind the two coils together you can cancel the inductance as the current in each coil is in the opposite direction.

PS: The word "they is Normal knowledge.

#### CocaCola

Apr 7, 2012
3,635
That is what they said, when I asked them about it... They were pretty adiment that they were right and thus they were wrong... At the end of the day I think they were both acting a little silly as they didn't specify who they were just that they had an opinion on the matter... They said don't worry they is "normal knowledge" so no one will question why they said it, or who they were...

Don't blame me I'm just the messenger telling you what they said...

Last edited:

#### davenn

Moderator
Sep 5, 2009
14,011

I have moved the other posts into here

Dave

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