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I've got a simple circuit that acts as a 230V softstart:

![enter image description here

It works, but there's one issue with it - after turning it on a 100uF capacitor is charged, and after turning off it is discharged by a 22k resistor. This 22k resistor is doing it too slow. It takes some 4 - 5 seconds. When circuit has to be switched on/off/on quickly it basically stops working,

A lower resistance instead of 22k messes up the ramp up time even on the first start, also voltage on out.

Is there is a way to discharge this 100uF (itgets charged up to ~6V) after turning power off by some transistor and resistor? If so, what transistor could work and how to put it into this circuit?

Here's a wider view:

enter image description here

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  • \$\begingroup\$ What exactly are you trying to soft start? What is your load? Do you have access to a neutral wire (or the other phase)? Or does it have to be an in-line soft starter? \$\endgroup\$
    – MOSFET
    Commented Feb 29 at 18:24
  • \$\begingroup\$ BTW, I believe your circuit is drawn incorrectly. Or it's not working the way you think it does... \$\endgroup\$
    – MOSFET
    Commented Feb 29 at 18:25
  • \$\begingroup\$ It works with normal single phase motors, of course there is acces to neutral, maybe triac on diagram is incorrectly, I'm not sure. \$\endgroup\$
    – fexr3i
    Commented Feb 29 at 18:40
  • \$\begingroup\$ This certainly can be done. But with more "active" circuitry. Also, is there anyway you can post the voltage waveform across the motor during start up? \$\endgroup\$
    – MOSFET
    Commented Feb 29 at 18:54
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    \$\begingroup\$ There are ICs for discharging X capacitors after power unplug to comply with relevant safety standards. <60 V after 1 s IIRC. Doesn’t burn power when energized yet allows low resistor values for quick discharge. Have you looked into those? \$\endgroup\$
    – winny
    Commented Mar 2 at 12:35

1 Answer 1

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A simple way can be done with a 230 Vac relay, with a normally close contact to discharge capacitor when power is off. Simulation does not agree with your voltage in 100uF capacitor, I get about 90 V in C1, so better use a resistor to limit current through the contact.

enter image description here

An electronic solution could be with a rectifier, optocoupler and transistors. This is an example with capacitive impedance (C7) and filter (C9) to get a minimum of 6 mA and peak of 24 mA in opto emitter. Transistors should support more than 100 V, according simulation.

enter image description here

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  • \$\begingroup\$ I'm sure this circuit works differently depending on load (its power), also I'm using bt138-600e, voltage on this 100uF was measured right after switching of, not during run, but this is 100uF 25V and he's ok, thanks for these 2 versions \$\endgroup\$
    – fexr3i
    Commented Mar 2 at 20:28

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