> 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
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