I have a safety circuit that will require some relays that can handle continuous operation with coil voltages from 3 to 25VDC. The contacts only have to have enough capacity to drive the coil of and "ice-cube" general purpose relay.

Being a safety circuit, solid state relays are not allowed. I would prefer a relay with a DIN mount socket (not pcboard solder in)

 

If you can find a mechanical relay that will actuate at 3.0 VDC, put a voltage regulator between the 3-25V signal and the relay coil (or a 3V zener and appropriate size dropping resistor). Double check your solution for adequate power dissipation at 25 VDC input. Power dissipated could be significant depending on the current requirements of the coil (you can help this situation by choosing the lowest power relay you can find that will switch the load ice cube relay). Also make sure to provide good surge suppression for the relay coil.

 

You should NOT be using ordinary relays for a SAFETY circuit unless you want OSHA to shut you down and maybe have to pay an injured party a LARGE settlement!

You should use SAFETY RELAYS like those available though Allen-Bradley or STI and other manufacturers. They have redundent contacts and an internal circuit that prevents activation if a failure was detected on the last circle.

 

There are some solid state relays that can be used with the specifications that you are requiring and permit DIN rail mount socket. This kind of relays are used to work with PLCS to protect their outputs...

 

You could use a Murr 6652050 spdt 12v@20ma coil 250v/6a contacts, DIN rail mount relay (about 4mm wide - mounts like a terminal). Look at a Murr catalog for other options. BUT, that said, being a "safety circuit", even standard relays may not be acceptable. Do you need redundancy, anti-weld contacts, etc? Have you checked out Safety Relays (Pilz, A-B, etc.)?

 

Are actual safety circuits allowed to use this type of relay? Or are you using the word safety a little loosely?

 

I was simplifying my terminology a little. The relay(s) will be used for equipment protection, not human protection, but will trip a machine shutdown.

The equipment in question utilizes a voltage signaling system with a range 4.3 to 18V zeners and a 24vdc supply. The other week the connection broke, and the umpteen years old system didn't detect it. By utilizing 2 relays, 1 in series with the signalling and one to ground, I should be able to provide a quick-and-dirty trip of the system with and open or ground. The customer is (hard to believe!) against solid state devices.

BTW, I do know about safety circuits as I have for a number of years designed control systems for Ski Lifts and Trams i.e. huge liability.

 

David,

Just a direction - DC reed relays apparently have a broad range of coil voltage that activates them - you may want to look into their specs. Plain DC relays are specified to pull as of something like 25% of nominal coil voltage - but not 12% or so as in your case. Without solid state devices, you may also want to look into a relay (e.g., 5
VDC coil) with a resistor in series with its coil - so that the current is largely determined by the resistor rather than the coil resistance (rudimentary "constant current source") - so any voltage between 3 and 25 VDC causes the minimum "pull current" to flow through the coil one hand, and 25 VDC do not burn the coil on the other hand.

I hope a resistor is not "solid state" for your customer.

PS - the name of the constant voltage diode inventor was Zenner.

Meir