On 17 Dec 2007 05:18:06 GMT, "DoN. Nichols"
wrote:

On 2007-12-17, Grant Erwin wrote:

[ ... ]

I have several identical SSRTs in my junkbox. They are Potter & Brumfield part
no. SSRT-120D10. Here is a data sheet:
http://www.tinyisland.com/images/tem..._SSRT_0303.pdf

The control terminals are expecting between 8.5mA and 14mA current at between
3-32VDC. The output can carry 10 amps rms at 110VAC.

O.K.

What I want to know is if I can use this in the following way:

run 12VDC to the control inputs, with 1k ohm current limiting resistor in series
switch 12VDC 2-3 amps load current

Why the current limiting resistor? The relays can accept any
voltage between 3 to 32 VDC and will draw no more than 14 mA from that --
without an external limiting resistor.

Oops -- never mind. Looking at the data sheet, this is
apparently a style intended for constant current input -- and you can
hook several of them in series to control them all with a single
current. (The voltage input ones, which are what I am more accustomed
to, would be hooked in parallel instead.

But your more serious problem is trying to switch DC as the load
current.

I'm certain that there is another part better suited to my requirement. My
question is whether I can make this one work.

If the output can handle 110VAC why couldn't it handle 12-14VDC?

It can happily switch *on* the DC. The problem is that it is a
thyristor or triac controlled through an opto-isolator from the 3 to 32
VDC control signal.

A thyristor or triac switches on when it gets enough current
through the gate.

It switches back *off* only when the current through the load
drops to zero -- which happens once every 120th of a second with AC, and
never with DC -- unless you provide something else to interrupt the
current -- in which case you don't need this relay. :-)

Is a car battery through a 1kohm resistor sufficiently like a 12mA current
source?

It is not picky. Notice the 8.5 to 14 mA range. So -- this
will probably work. You can tell by looking at the LED on the relay.
The only problem might be that the input will drop a full 3V at the
maximum current. This would leave you with 11 VDC to drop through the
resistor -- so 11 mA which should be enough. (Note that automobile
batteries, while nominally 12 VDC are usually around 14V or a bit higher
when being charged. So set your resistor a bit on the high side and
then see whether the LED comes on.

And really -- don't bother, because it can't switch your DC load
off anyway.

Good Luck,
DoN.

You need a solid state relay with a FET output stage. Crouset GF.
Or transistor output. Crouzet GT.

Crouzet is one of the Schneider group.

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