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# Diodes In parallel increased current capacity?

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#### terrapinlogo

##### Member
i am currently building a charge controller for a wind turbine and need 40 amp blocking diode so the batterys dont turn the turbine when there is no wind. the turbine produces about 40 amps and the largest diode i can get ahold of is 20 amps so i was woundering if two in parallel would give me 40 amps? any help is appreciated

Only if the diodes are exactly the same, which they never are. THe one with a lower forward voltage drop will take more current (and with ideal diodes it would take all the current).

so ill just have to find a larger diode?

so ill just have to find a larger diode?

hi
You could add a very low value resistor in series with each diode.

Say a 5milliohm, at 20Amp that would give a voltage drop across the resistor of

Vdp = 0.005 * 20 = 0.1V

Resistor Wattage = 0.1V * 20A = 2W, say at least 5W

By following the V/I curve of the diode you can calculate its own apparent resistance. It might help equalize the current or at least counteract the temperature coefficient.

You might find that three or four diodes in parallel would serve the purpose. They should be very tightly coupled thermally.

At these current levels I would probably consider active control using a big MOSFET.

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Although you 'should' use current balancing resistors, it's pretty common for commercial equipment to just stick them in parallel - and it's VERY rare for this to ever cause a problem.

However, if you feed the diodes via separate wires from the turbine, then the resistance of the wires will balance them anyway (and you can use thinner/cheaper wire).

From what I read from the OP's question I assume he's using a DC motor as a wind genny. OK now what size is the DC motor spec's please, what size is your batterybank and what voltage are you talking about. Unless these questions are answered everyone is just guessing at the vague question you posted.

May I add recently this forum changed the AE to Renewable Energy so this post really should go in there.

Regards Bryan1

From what I read from the OP's question I assume he's using a DC motor as a wind genny. OK now what size is the DC motor spec's please, what size is your batterybank and what voltage are you talking about. Unless these questions are answered everyone is just guessing at the vague question you posted.

May I add recently this forum changed the AE to Renewable Energy so this post really should go in there.

Regards Bryan1

G'day Bryan,
I would say my suggestion isnt a 'guess', it would be applicable to any application where two similar diodes are connected in parallel.

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Can you use a MOSFET, low resistance, high current

If the diodes are both in the same package then they should be reasonably matched so you can connect them in parallel to increase the current rating.

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we used to use banks and banks of high current diodes in our power supplies, all screwed into maaaaaaaaaaaaaassive heatshinks, giving 12,000A DC

we checked the mV on the diode tails, to make sure there were all sharing fairly evenly.

of course if the two devices have to share exactly, due to rating, then you need to be more careful

From what I read from the OP's question I assume he's using a DC motor as a wind genny. OK now what size is the DC motor spec's please, what size is your batterybank and what voltage are you talking about. Unless these questions are answered everyone is just guessing at the vague question you posted.

May I add recently this forum changed the AE to Renewable Energy so this post really should go in there.

Regards Bryan1

ok seems i havent given enough information i am using a three phase washing machine motor which has been re wired for 42v at maximum rpm it is being rectifyed and fed through a charge controller to a 12v battery bank.

Do what Eric Gibbs sais and it should work fine.

I have used balancing low ohm resistors with power diodes in parrallel and never had any problems with diodes failing.
Or make a small 10 turn 1cm Ø 1.5mm² Cu coil in series with each diode, the wire resistance will cause a few mV drop to balance the diode out.

It will probably work fine without these as well.

Hi,

I would not use two diodes rated 1 amps each in parallel to make a diode that
is rated for 2 amps. If either diode has any mismatch, then one diode draws more
current and heats up more. As that diode heats up it draws even more current
and heats up even more. The thermal difference is the big factor, and it might
be hard to eliminate thermal gradients completely with some diodes.
It might be wise, if you really do have to do this, is to use a factor of 1.5 for
each diode. In other words, use two 1.5 amps diodes to handle 2 amps.
Even this is a bit of a gamble unless you take measurements.

The best of course is to buy one diode that can handle the full current, plus
some extra headroom. For 40 amps a 50 amp diode would be a good idea.

Silicon rectifier diodes have very soft forward conduction curves, they can be 0.6v at start of conduction and well over 1v at their max current.
You can parallel them anytime assuming you use the same type of diode and have checked they have similar Vf (just use the diode test on your multimeter). This is fairly common in commercial equipment.

But schottky diodes have negative temp coefficient, ie their Vf gets LOWER as they get hot, so you can't parallel them safely, except of course when they are a dual schottky in a single TO-220 pack etc where the 2 diodes are very closely matched and bonded together on the same silicon die (ie the 2 diodes will always operate at the same temp). This paralleling is also common in PC PSUs.

The solution has already been suggested, use indivdual wires from the genrator to the power source. The resistance of the wire from the wind gen to the rectifier will equalize the currents. Use 3x 20 amp diodes to be safe. With 14 AWG copper wire 100' of wire is .258ohms. Even 50' (0.129 ohms) of wire is enough to equalize the currents.

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