Pulse signal for latching relay

Hi all,

I want to drive latching relay for a protection circuit. Reset coil of the relay needs 50ms and 250mA pulse signal for the reset operation. I will drive it with a comparator circuit using an opamp. But when a protection state occurs, comparator drives continuously the reset coil and the system wasting power unnecessarily. How to convert continous comparator ouput signal to a one shot (50ms) signal? After a reset condition, i will not set the relay anymore and the relay will remain in the reset state permanatly. Thanks for advices.

Best regards,
Bragas

Hi,

from safety aspekt you should not do this.

Why not use an identical circuit than the activate signal?

Look for a monostable multivibrator.

Klaus

Dear Klaus,

Thanks for the reply. Could you explain the issues from the safety aspects? I need to reset relay when the temperature of the system exceeds the critical level. Also, what do you mean by the "identical circuit"? Thanks for the support.

Best regards,
Serkan

Hi,

Safety:
When in RESET, then it most probably should stay in RESET state. But if there is just a short RESTE pulse, then it can be SET immediately after the pulse.

"identical circuit":
My fault. I thought you already have a SET pulse.
Now I have to ask: How do you currently SET the relay?

Klaus

Hi,

If you're interested, this timer circuit does just that. Adding a mosfet/bjt is needed for the 250mA pulse.

The timer circuit alone is the first image. The second circuit is only simulated, so beware and check it's real/you trust the simulation results if you use it. You'll have to decipher the second schematic: only one comparator and 555 and so on are needed, the CD4017 and CD4071 are not needed for a one-shot and off circuit. The waveforms are for a circuit that theoretically tries three times then latches off at an e.g. overtemperature event - trigger signal is emulating a fluctuating e.g. temperature. I would put an AND gate at the 555 output (between it and the MOSFET) as a buffer, like the one on the timer input.







If it's of use, great. If not, hope you find a simple solution soon.

I think you could use a comparator and a flip flip, too, I suppose that version would need a mosfet/bjt to generate the 250mA.

Hi,

There are ICs for almost any application.
Read about Fairchild / ONSemi FAN3240 and FAN3241.

Klaus

Dear Klaus,

I will set the relay with a push button and reset with a comparator circuit. I have also checked the ICs in the market but I couldn't find a suitable product and I want to build the circuit with discrete components. The single solution seems to be the usage of a monostable multivibrator according to your and d123's comments and google search. I tought i could solve the problem with some basic RC circuits. Unfortunately, i didnt find such a circuit. I would appreciate if you have any other suggestions.

Best Regards,
Bragas

Hi,

There are enough monostable oscillator circuits in the internet. No need to reinvent the wheel.
I assume every circuit will work, independent, if it's made with a comparator, NE555 or any other IC.

We don't see a circuit from your side, we don't see timing diagrams, no simulation results, no effort from your side.
--> show what you have done so far and we will help you to make it work.

Klaus