# [EVDL] solid state relay on bms/bad idea



## EVDL List (Jul 27, 2007)

If you are having a problem with SS relays, I would suggest you take a look
at what you are doing. There is something wrong with your design. You are
probably using AC SS relays which are a no no on DC.

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## EVDL List (Jul 27, 2007)

> Cruisin wrote:
> > If you are having a problem with SS relays, I would suggest you take a look
> > at what you are doing. There is something wrong with your design. You are
> > probably using AC SS relays which are a no no on DC.
> ...


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## EVDL List (Jul 27, 2007)

> Willie McKemie wrote:
> > It is a 240vac relay rated for 40 amps. Application is BMS control of
> > ac current going to chargers. It was not used at more than 20 amps
> > and was getting hot at that current. Steve put a computer CPU cooler
> ...


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## EVDL List (Jul 27, 2007)

Silly question, but a SS relay needs a heat sink and needs thermal 
paste coupling it to that heat sink. They are very short-lived 
without thermal paste.

At 06:35 PM 6/27/2011, you wrote:
>


> Cruisin wrote:
> > > If you are having a problem with SS relays, I would suggest you take a look
> > > at what you are doing. There is something wrong with your design. You are
> > > probably using AC SS relays which are a no no on DC.
> ...


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## EVDL List (Jul 27, 2007)

"Or, my favorite light bulb trick. Measure the current that the relay
coil draws on 120vac. Suppose it's 50ma; then it's a 120v x 0.05a = 6
watt coil. Connect a 6 watt 120v light bulb in series. Now you can power
the relay+bulb at either 120vac or 240vac! On 120vac, the bulb will stay
cold; it barely glows and its resistance is so low that the coil gets
almost all of the 120vac. On 240vac, the bulb lights at normal
intensity, drops half the voltage, and the coil gets 120vac."

That brought a smile. Clever by half!

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