Originally Posted by SWF
Thanks for the input Cruisin. Would you be able to suggest to me here in this discussion which relay I should use? I would have preferred a mechanical relay, but could not find a good one that was rated to handle the voltage and current.
I agree with Crusin; these are not appropriate applications for an SSR (Solid State Relay). First, because they fail *on*, which is a bad or even dangerous condition. Second, because of the nature of the loads.
The KSI input is very low current; you would need a relay with a very low off-state leakage, and a rated carry current similar to the load you are actually switching, which is 50ma or less.
The DC/DC input has the problem of a huge peak inrush current. Most of them have large input filter capacitors, whose peak charging current can be 100 amps or more. SSRs are not good at handling such high peak currents.
I would consider something like a Schrack PT52A012B. This is a 4PDT relay with a 12vdc coil and contacts rated for 6a at 250vac or 30vdc, 40vdc at 1amp, 100vdc at 0.4a, and 300vdc at 0.25a. One source is www.digikey.com
, PB1157-ND, $8.86 each.
The voltage rating of relay contacts in series add. This relay has 4 poles. Each by itself can only break 30vdc at 6 amps; but by wiring all 4 poles in series, you have a relay that can safely switch 30x4=120vdc at 6 amps. For most DC/DC converters, a 6amp rating is adequate.
If your DC/DC draws a huge current spike at turn-on (witnessed by a big spark when the relay contacts close), then you need to add an inrush limiter. Without one, no relay (mechanical or SSR) would last long. Inrush limiters are a little $1 part, also available from vendors like www.digikey.com
For the KSI input, even a single pole of such a relay will work, because it is rated to switch low currents even at very high voltages. However, use a 2-pole relay anyway (such as a PT22A012B, the 2-pole version of the above). Connect KSI to the Common of both poles. Connect NC of one pole to B-, and NO of the other pole to to a fuse to whatever power source turns on the controller. Done this way, if the NO relay contact fails closed, you get a *short* when the relay turns off and the other pole shorts KSI to B-. This blows the fuse, guaranteeing that the power is off if the relay fails.