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70 Posts
Discussion Starter #1
I am considering using solid state relays in a battery balancer system. The
DC models seem to be very expensive. I wonder why? Don't they consist of
just a pair of high power mosfets and an optoisolator/driver?
Could I build my own?

Al
 

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70 Posts
Discussion Starter #2
Hello Al,

You can get solid state relays from www.allelectrolnics.com for less than
$10.00 that may work for you. Yes, the ITE solid state relays I have been
running for 22 years every day cost me about $80.00 for a 40 amp at 100 VDC
back in 1985.

Roland


----- Original Message -----
From: "Al" <[email protected]>
To: <[email protected]>
Sent: Sunday, July 29, 2007 10:39 PM
Subject: solid state relays


> I am considering using solid state relays in a battery balancer system.
> The
> DC models seem to be very expensive. I wonder why? Don't they consist of
> just a pair of high power mosfets and an optoisolator/driver?
> Could I build my own?
>
> Al
>
>
 

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Registered
Joined
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70 Posts
Discussion Starter #3
Al wrote:
> I am considering using solid state relays in a battery balancer system.
> The DC models seem to be very expensive. I wonder why? Don't they
> consist of just a pair of high power mosfets and an optoisolator/driver?
> Could I build my own?

A normal DC SSR (Solid State Relay) uses a single transistor. It might
be a MOSFET, IGBT, darlington, or bipolar transistor. But all these
devices have a + and - polarity. The voltage must be of one polarity,
and they only conduct current in one direction.

A normal AC SSR uses a single triac, or back-to-back SCRs. They don't
care about the applied polarity or direction of current flow; but they
won't turn off until the current momentarily goes to zero.

Neither type is suitable for building a normal battery balancer. The
current in each switch in a balancer generally has to withstand both
polarities of voltage when off, and needs to carry both polarities of
current when on; but you must be able to shut it off even with current
flowing.

You can use *two* DC SSRs, wired in series in anti-parallel, with a
diode across each. Some DC SSRs are even built with way. But this
doubles their cost and complexity.

DC SSRs cost more than AC SSRs because a) they need more parts, b)
transistors cost more than SCRs or triacs, and c) AC SSRs are built in
far larger quantities.

There are ways to get around some of this. For example, use AC SSRs, but
put a capacitor in series with every battery lead. The capacitor passes
a pulse of current, but once charged up it forces the current to go to
zero so the relay turns off.
--
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in -- Leonard Cohen
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net
 

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Discussion Starter #4
AC SSRs are Triac driven. You cannot use them on DC. Get a MOSFET or IGBT
based SSR designed for DC.

The "trick" would be the same thing used in the SCR controllers, you have to
drop the current to zero or reverse it to get it to stop conducting. This
would require more parts than it's worth.

Either that, or you can add a conventional relay in series... =)

-Phil
----- Original Message -----
From: "Roger Daisley" <[email protected]>
To: <[email protected]>
Sent: Monday, July 30, 2007 9:36 AM
Subject: FW: solid state relays


>I have a related question that I have been meaning to write about:
>
> I have a box of new Crydom (CSD2450) SSR's that I bought on eBay. The
> input
> is 3.5-15VDC and the switched output is 240 VAC @ 50A.
>
> I had planned on using them in my conversion for switching low power DC
> circuits, such as horns, lights, radio's, etc. The problem, I have
> discovered, is that AC switching loads will not work on DC. (I'm sure most
> people on this board already that ... And are chuckling at my ignorance!)
>
> Through experimentation, using a 12-v headlight, I find that the relay
> will
> switch the light ON, but when the input is removed ... The light stays ON.
> So much for that idea!
>
> The purpose of this message is to see if there is some "trick" that I can
> use to shut the SSR down, when switching 12-VDC loads ... Or should I
> start
> looking for different SSR's?
>
> Thanks,
> Roger Daisley
> Pullman, WA
> http://www.96-volt.com
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~!//!~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
>
> You can get solid state relays from www.allelectrolnics.com for less than
> $10.00 that may work for you. Yes, the ITE solid state relays I have been
> running for 22 years every day cost me about $80.00 for a 40 amp at 100
> VDC
> back in 1985.
>
> Roland
>
>
> ----- Original Message -----
> From: "Al" <[email protected]>
> To: <[email protected]>
> Sent: Sunday, July 29, 2007 10:39 PM
> Subject: solid state relays
>
>
>> I am considering using solid state relays in a battery balancer system.
>> The
>> DC models seem to be very expensive. I wonder why? Don't they consist of
>> just a pair of high power mosfets and an optoisolator/driver?
>> Could I build my own?
>>
>> Al
>>
>>
>
>
 
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