DC relay latching voltage

Not safe. If an external magnetic field is present (e.g. from nearby relays) or a mechanical shock occurs it can activate and hold. Been there, had that.

Arie

That roughly fits my observations but I don’t think it can be trusted , aging, vibration or temperatures outside room temp could make it pull in earlier. You might need to characterise the relays you have in mind.

Thanks. The immediate application I have in mind is highly unlikely to be subjected to such external factors. and it won't have disastrous consequences if it did happen. But it's good to know for future consideration.

The application *is* likely to cause 20-25% of the nominal coil voltage to appear across the terminals for some 50 milliseconds though.

Thanks for the reply. Actually, my calculations indicate that less than

I'd suggest using a power supply at the maximum decay voltage you expect then bang the relay a bit to see if it will engage. I assume you have the normal back diode across the coil for clamping the ringing voltage. You could use the NO contacts in the relay to keep it energized (Hold) if it momentarily closes during testing.

John :-#)#

Most data sheets have a must-operate voltage and a must-release voltage, but typicals are well, typically, very different.

Test a few! I'd expect that it would be very unlikely for an em relay to operate at 40%.

We test parts and document whatever we learn.

FTR-B3 relay non-latch 12 volts JL June 2015

Coil measures 1.0K

Typical pickup = 7.7 volts spec is 9 dropout = 2.1 spec is 1.2

The 5 volt version pulls in at about 2.5 and drops at 1.4.

One non-energized relay measured 47 and 44 mohms on its NC contacts, measured at both 1.3 amps and 100 mA.

JL Jan 2024

That would give a good indication but wouldn't guarantee the same result with another sample.

I can't give too much detail but this is broadly what the circuit has to do: The relay and associated circuits are to be a part of a multi-section project. The relay switches another section on at power-up and then quickly disengage that section soon after power is switched off.

Normal power supply decay will not release the relay quickly enough. So I've added a section that turns the 12V relay off when the decaying PS drops below 7.5V. Works fine in simulation.

However, due to interaction with other sections, the gate voltage of the transistor driving the relay rises again briefly to about 2.5V before the shutdown process is complete. But by this time, the 12V supply has dropped to less than 3V.

I don't think any normal 12V relay will re-engage at 3V, but I wanted to be sure. When I cited 50% and 40% of the rated coil voltage, I was being conservative and night have misled readers. Sorry about that.

Hi Don,

Don't you mean that one could use a zener diode in place of the back-EMF diode - or parallel with it?

We've been using 1N400X (and 3A in some cases)diodes since the 70s for protecting pinball driver transistors - not too worried about the decay time so haven't really looked deeper into it before.

Thanks,

John :-#)#

Pimpom snipped-for-privacy@invalid.invalid wrote: [...]

Add a few diode drops in the supply to the Base of the transistor and a pull-down resistor from Base to Earth. You could even use a 7.5v Zener so the transistor switched directly off the PS and no other active components were needed.

or use a fet that is avalanche rated or has build in clamping

Now that sounds like a good idea. Thanks. I'll check it out to see if it suits my purpose. For one thing, switching the transistor will not be a snap action, but that shouldn't matter as long as the relay contacts snap open. The relay is likely to be a 12V 400Ω model so that the slow turn off wouldn't unduly stress the transistor.

No offense, but this is one of those "Why didn't I think of that" moments.But then no one else seems to have done so either. :-)

A 2-resistor voltage divider can reduce mosfet gate swing.

Or an R-C in his case of a transient gate drive spike.

Or both.

If the C is gate to drain (or base to collector) then the miller effect slows rate of flux collapse enough to remove need for any additional components across the coil.

Good reading! It appears counterintuitive, but if I warp (!) my head around it I should be able to sort it out.

Thanks, that is very helpful.

John :-#)#

An even simpler coil catcher is a resistor. Many times the extra power consumption is not an issue and resistors are even cheaper and more reliable than zener + diode.

At half-voltage and half-current, the most stressful condition, the dissipation of the transistor would be 90 mW.

Is there a community of pinball machine people?

At my new office, the next-door neighbor's husband has pinball machines, and I invited him over if he needs help with electronics.

The downside is that he plays drums too. I don't understand why anyone would do that. Lotta noise, both cases.

Essentially all modern discrete mosfets avalanche safely at ballpark

The damage limit becomes average power dissipated, which would be low in most cases of driving a small relay.

I recently designed a board brickwalled with about 130 power relays. Each has a sot-23 logic-level mosfet on the bottom of the board, snuggled between the thru-hole relay pins. No catch diodes.

Baking what? I'm getting whining that we are out of both biscuits and bread pudding, so I won't get much electronics done today.

Is there an index to these useful Application Notes? I only found this when I searched for "application notes":

John :-#)#