Crydom SSR for switching 12 volt/no volt

[pbronson]

Jon,

Can a Crydom SSR be used for no voltage switching (as in making continuity in a unpowered circuit) and /or 12 volt switching? The specs mention a 24V min operating range.

Pete

[JonnyMac]

The Crydom relays that we supply are for switching AC only -- if you try to switch DC you will have problems (if it works at all).  The reason is that SSRs use an electronic component called a triac.  A triac is a solid-state AC switch that is gated.  Here's the rub: once a triac is gated it will not shut off until power is removed.  In an AC system this happens 120 times per second at what we call the zero-cross (this is the point where the voltage transitions between the positive and negative phases).  If you attempt to switch DC with that relay it may work, but it won't shut off until you remove the DC power.

Crydom makes a wide array of relays; you can find Crydom relays with the same footprint that will switch DC, however, then tend to be a little pricey.

If your "no voltage" circuit can take an open-collector control input then you can use one of the OUTx terminals from the Prop-1; the ULN output floats (is not connected) when off, and switches to ground when it is on.  Keep in mind the current limitations through the ULN.

[pbronson]

Got it.

What does ULN stand for?

Pete

[JonnyMac]

I don't know.  The stock chip we use in our controllers is the ULN2803A which has eight channels; the 7-channel version is the ULN2003A.

[menehune]

The ULN chip is the ULN2803 chip in the socket next to the green screw terminal strips.  It buffers the processor and the terminal strip so the devices don't load down the processor output lines, but if you are using the processor as a serial output (driving a SSR) it can load down the serial output line (the triple pin connector) and cause problems with communications.

You can check the Serial communications thread for more information.

I think the ULN designation is the manufacturer(s) code.
ChipDir

[JonnyMac]

Thanks for the follow-up; Mike, I sometimes take for granted that everyone knows what the ULN2x03A is and does. 

To Mike's point, the ULN2x03A takes a small current signal from the BS1 microcontroller (the Prop-1 core) and allows it to switch a high-current DC signal on the OUTx terminals.  The maximum current is determined by simultaneous outputs; with just one output running you can safely switch up to 500 mA (half an amp) from the associated OUTx terminal.  If you run more than one output, the current you can switch per output is reduced.  The reason for all of this is that current running though the ULN2x03A can cause it to heat; too much current leads to too much heat and can cause it to fail.