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quick question about diodes

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weegee

New Member
quick question about diodes.

i have diodes here (1n5402) if it makes any difference, rated at 3A.

i need to pass 6A (4.58A but could get slightly higher if the voltage drops)

can i just run 2 diodes in parallel ??

thanks

graham
 

audioguru

Well-Known Member
Most Helpful Member
Two diodes will probably have slightly different forward voltages so won't perfectly share the current. Then one might fail which would cause the other to fail.
Use 6A diodes.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
To do so you should put low value resistors in series with each diode, to ensure equal current sharing - but in practice it will probably be perfectly fine.

As you have the diodes, you may as well give it a try? - make sure you space them well off the board, they are likely to run quite 'warm' :p
 

mneary

New Member
I would not do this in a commercial product. Sharing won't likely be graceful since typical power diodes have a negative temperature coefficient.

If this is in a rectifier, beware power diode failures tend to be short circuit.
 

weegee

New Member
to be honest its in my car, i built a 'police' style strobe kit (left/right/left/right) for my full beam headlights using an old flasher relay, and couldnt figure out why it wouldnt work - turns out that cos my car is quite high spec, it has lamp failure modules ect, and what was happening was the current was flowing back through the failure module and out the other side, so both lamps were on at all times. i added two diodes, to prevent the current running back to the failure module, but i know they are under rated and heat up a bit - but it was what i had at the time ;) but i think i will get higher rated diodes, and leave the doubling up for emergencys

thanks

graham
 

Nigel Goodwin

Super Moderator
Most Helpful Member
mneary said:
I would not do this in a commercial product. Sharing won't likely be graceful since typical power diodes have a negative temperature coefficient.

It's commonplace in commercial products, you see it all the time - personally I wouldn't do it in a commercial product without current sharing resistors (which is then perfectly OK) - but commercial products never do that, they just bang the diodes in parallel.

If this is in a rectifier, beware power diode failures tend to be short circuit.

Yes, failure mode is usually S/C - but I can't say I've seen any more fail in parallel than on their own?.
 

Dean Huster

Well-Known Member
If the output of your supply is electronically regulated, the series resistor trick on the paralleled rectifiers will be fine. In many/most cases it will be fine. However, if the supply has no post-rectification electronic regulation, be advised that resistors in series with the rectifiers will "soften" the supply such that heavier load demands will lower the output voltage more than the demands would with "non-resistored" rectifiers. That's the trade-off.

Small value resistors in series with rectifiers that are NOT in parallel are not the least bit uncommon in even the finest commercial and military designs. In those cases, they're used to limit inrush current when the supply is first turned on so that the diode's maximum current rating is not exceeded by massive filter capacitance in its initial charging from zero to max voltage. These resistor values are small enough so as to not contribute greatly to the "softness" of the supply. In most of those cases, there's a post-rectifier regulator anyway.

Beware of the power rating of any series resistor you introduce. Make sure that the I²R power dissipated by that resistance is covered by the power rating of the resistor. It'll handle the initial surge of inrush since that's momentary, but be sure that the normal running current is covered.

Dean
 
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