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sustained signal to latching relay?

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sixspeedshifter

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I know that latching relays typically operate on pulses. Is there any problem with giving a mechanically latching relay a sustained signal? Are they rated for continuous current? (I realize that this situation lends itself better to a standard non-latching relay, however this is all I have.) Thanks!
 
You would have to look at the relay specification to determine that. It many only be rated for a momentary pulse. You could use a 555 one-shot timer to generate a pulse if you need that.
 
I know that latching relays typically operate on pulses. Is there any problem with giving a mechanically latching relay a sustained signal? Are they rated for continuous current? (I realize that this situation lends itself better to a standard non-latching relay, however this is all I have.) Thanks!
Welcome, sixspeedshifter!

You can use a standard DPDT relay as a latching relay that can tolerate sustained trigger signal(s) of any length you choose.

Once the trigger pulse hits the relay (thereby causing the latching action) that particular contact, by being pulled in, is no longer connected to a load.

I'll post a link to an ETO thread with a schematic, as soon as I find it.

Here ya go: https://www.electro-tech-online.com/threads/2-float-switch-tank-filling-circuit.133374/
 
The coil temperature would be higher than normal and so shorten the life of the coil wire insulation. Each 10C increase halves the insul life.
 
Most relays are rated for continuous operation. That's not to say they don't experience a rise in temperature, but generally that rise is of no consequence.
 
If you have a thermistor or thermocouple or even a small silicon diode (-2.2 mV/degree C at constant current) you could power up the relay and see what final temperature the coil reaches. It might take a half hour to level off at this temp. You don't need absolute accuracy, just accuracy in measuring the difference between final and intial temperatures.
Then you could use a formula to figure the coil life difference from room temperature (this assumes the latching pulse does not raise the coil temp) to your elevated measured temperature. I used to have this formula, if I can find it again, but the derivation is not too bad.

Lemme' know how you want to proceed.
 
Could you just apply the 'sustained signal' via a capacitor to the relay coil? That would effectively convert it to a pulse and avoid coil over-heating.
 
Agreed. That's a great idea!

I have often driven a relay from a large capacitor, which is charged through a resistor. The cap easily supplies the pull-in current pulse for the relay, then the resistor (which is then in series with the relay coil and PSU) supplies enough hold current to keep the relay closed.

Since this is a latching relay you could use a high value resistor, as long as the resistor supplies enough current to charge the cap between relay firings.
 
Alternately, one could throw your pulse through a large value capacitor, in series with the relay coil. For all intent and purpose, caps only pass AC and will block the DC component. So even if you use steady high/low signals to power your relay, the relay would only see that initial pulse. You just need to make sure the cap is high enough value and low enough ESR that the required amount of switching current is sustained for the required amount of time.

This is a passing thought/answer, the values required may be to large to be practical. I haven't really done the math. More experimentation may be required.

Edit: Looks like that's probably actually what alec_t is already saying, so... ignore me?
 
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