# [EVDL] Mosfet relays



## EVDL List (Jul 27, 2007)

Have you tried hooking these up to a series string of batteries? Seems to
me if you do a Lee Hart style battery ballancer you will end up shorting
out your pack through the intrinsic diodes in the 'FETs

> I have built up a bi-directional mosfet relay out of discreet components
> to
> use in a battery managment scheme.
>
> Its pretty simple. Two 200v 20a power mosfets are hooked source to source
> and the drains become the SPST NO "contacts". The gates are driven with an
> optoisolated gate driver chip powered by a very small isolated dc-dc
> converter. The on voltage drop is very low, so one of the two mosfets must
> be conducting "backwards". I even hooked up a 120VAC lamp and switched it
> on
> and off.
>
> The dc-dc converter is the only thing that is somewhat costly.
> Unfortunately
> you would not be able to use just one converter to power all the "relays".
> I
> cant figure out how they are obtaining gate drive on the ready made
> bi-directional mosfet relays. There is no such thing as a mosfet with an
> optically sensitive gate is there?
>
> Anyway, I plan to arrange the system similar to Lee Harts battery
> balancer.
> I will be able to read the voltage of each battery very quickly, so I can
> record the actual AH's used on a per battery basis and, of course, be able
> to "charge" the low batteries at a decent amperage.
>
> Any comments? Thanks, Al
>
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## EVDL List (Jul 27, 2007)

> Peter VanDerWal <[email protected]> wrote:
> > Have you tried hooking these up to a series string of batteries? Seems to
> > me if you do a Lee Hart style battery ballancer you will end up shorting
> > out your pack through the intrinsic diodes in the 'FETs
> ...


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## EVDL List (Jul 27, 2007)

>> I have built up a bi-directional MOSFET relay out of discreet 
>> components to use in a battery management scheme... Two 200v 20a 
>> power MOSFETs are hooked source to source and the drains become the
>> SPST NO "contacts". The gates are driven with an optoisolated gate
>> driver chip powered by a very small isolated dc-dc converter. The
>> on voltage drop is very low, so one of the two mosfets must be
>> conducting "backwards". I even hooked up a 120VAC lamp and switched
>> it on and off.



> Peter VanDerWal wrote:
> > Have you tried hooking these up to a series string of batteries?
> > Seems to me if you do a Lee Hart style battery balancer you will
> > end up shorting out your pack through the intrinsic diodes in the
> ...


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## EVDL List (Jul 27, 2007)

Hi Al, all,



> Al wrote:
> > The dc-dc converter is the only thing that is somewhat costly. Unfortunately
> > you would not be able to use just one converter to power all the "relays". I
> > cant figure out how they are obtaining gate drive on the ready made
> ...


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## EVDL List (Jul 27, 2007)

Nope, he is not going to short out through the internal diodes. He
connected source to source. That makes the diodes pointing to each other.
Current doesnt conduct through the diodes either since he turns on both
the FETs at the same time. I wouldn't have thought to hook them up like
that! Good thinking!

As for the DC-DC converter for an isolated supply, that is a very
expensive way to do it. Look up "charge pump". You just need an
oscillating signal, a diode and a capacitor. Or use the oscillating signal
and a voltage doubler (a few diodes and caps).

-Jon Glauser
http://www.evalbum.com/555

<quote who="Peter VanDerWal">
> Have you tried hooking these up to a series string of batteries? Seems to
> me if you do a Lee Hart style battery ballancer you will end up shorting
> out your pack through the intrinsic diodes in the 'FETs
>
>> I have built up a bi-directional mosfet relay out of discreet components
>> to use in a battery managment scheme.
>>
>> Its pretty simple. Two 200v 20a power mosfets are hooked source to
>> source and the drains become the SPST NO "contacts". The gates are driven
>>with an optoisolated gate driver chip powered by a very small isolated
>>dc-dc converter. The on voltage drop is very low, so one of the two
>>mosfets
>>must be conducting "backwards".

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