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I'm using this circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

The circuit is powered by "24V" , "24V ENABLE" is the trigger. The circuit should act like a one-shot signal (or single impulse) that is OPEN for about 100us (between NODE1 and NODE2) when a transition from 0V to 24V occurs on ENABLE. It should then stay closed for the rest of the time.

When thr trigger was generated by a push button on the bench all worked well. Now that I've moved to the real application that uses an opto-relay to trigger the circuit, the circuit doesn't work for values lower than 100nF, this mean impulse time longer than 500us.

This happens because the voltage on the opto-relay rises very slowly and this rising speed is comparable to C1 charging speed. What happens is that M1 never triggers. With higher values of C1 the charge slows, but also the impulse time that should not exceed 100us.

Do you have suggestions on how to avoid this applying changes to this circuit?

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  • \$\begingroup\$ Can you show the relay part of the circuit also. Perhaps that can be sped up in some way; e.g. Schmitt trigger, positive feedback... \$\endgroup\$
    – AJN
    Commented May 25, 2021 at 11:32
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    \$\begingroup\$ Avoid this design and use a proper one that is not slew rate sensitive and is threshold sensitive and does not exceed absolute maximum Vgs \$\endgroup\$
    – D.A.S.
    Commented May 25, 2021 at 12:00

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