FW: solid state relays

Roger Daisley wrote:
> 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?

The easy answer is to start looking for a true DC solid state relay instead.

But if you are desperate (or cheap) you can use *two* of these AC
solid state relays to switch a DC load. The circuit looks like this
(view with a fixed width font):

+DC___________________
_|_ |
any | | >
DC | | R1 >
load |___| >
|____||____|
"on" | || | "off"
SSR / C1 / SSR
-DC________|__________|

C1 is an AC-rated motor "run" capacitor (not an electrolytic). Its
capacitance should be about C = Iload x 50 / Vdc (where C is in
microfarads). For example, if Vdc=120v and the load current is 12 amps,
then C = 12 x 50 / 120v = 5 uF.

R1 should be chosen to draw about 1 ma at the DC voltage being switched.
For example, 12k at 12vdc, or 120k at 120vdc.

Operation: Both the "on" and "off" SSRs should be pulsed with a
momentary switching function, such as a pushbutton. Pulse the "on" SSR's
input to turn the load on. The SSR will turn on, and *stay* on because
SCRs and triacs won't turn off until the current through them stops flowing.

While the "on" SSR is on, capacitor C1 will be charged up to the full DC
voltage by R1; + on the right side, - on the left. To turn off the load,
pulse the input of the "off" SSR momentarily on. The "off" SSR turns
on, shorting the right side of C1 to -DC, which pulls the left side of
C1 negative. This causes the "on" SSR to turn off.

C1 and the "off" SSR will carry the load current, until C1 discharges
and then re-charges in the opposite direction from the load current.
When the current in C1 falls below that required to keep the "off" SSR
on, the SSR turns off. Now both SSRs are off, and the load is off. C1
will slowly recharger via R1, to be ready for the next cycle.

This works best with plain old SCRs instead of SSRs. Fancy SSRs with
zero crossing detectors, undervoltage cutouts, etc. won't work on DC at all.
--
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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