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I want to trigger a timer (Omron H3DK-M2) using a circuit based on a MOSFET as a switch. The data sheet for the timer states "When using a 24 VDC power supply voltage there will be an inrush current of approximately 0.25 A. Allow for this inrush current when turning ON and OFF the power supply of the timer with device with a solid state output, such as a sensor."

Could I handle this problem effectively with a thermistor? If so where to place it in the circuit?

Is there anything more I should do to protect the gate of the MOSFET from over voltage? According to the data sheet it is rated as + - 20 VDC. At the moment I am using a simple voltage divider circuit.

I have a simple NO push button switch to trigger to switch the gate voltage of the MOSFET because this is a learning process for me.

Other things to note:

The 24 VDC supply is from a 240 VAC to 24 VDC regulated switching power supply.

The time controlled voltage is from another power supply therefore not shown.

The minimum voltage required by the timer control circuit is 20.4 VDC.

Terminals A1 and B1 when circuit is closed trigger the timer.

Terminal B1 is (+).

Kind regards Alasdair

MOSFET_trigger_circuit

ajr_chm
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    I think they are just letting you know what they've done to help limit the inrush current and that you need to make sure that any switch device you use (switchgear, so to speak) can support such a peak current. I don't think they are asking you to further limit it. But I could be wrong about that as I'm just reading what you wrote and guessing from there. – jonk May 17 '22 at 04:40
  • It would be nice if you drew your schematic following current [best practices](https://electronics.stackexchange.com/questions/28251/rules-and-guidelines-for-drawing-good-schematics). That upside down generator, ground placed at the top and that criss-cross of source and drain makes even such a simple schematic hard to read, especially for a newbie browsing the site. – LorenzoDonati4Ukraine-OnStrike May 17 '22 at 07:29
  • Welcome! EE top tip: please draw your schematics with positive voltages pointing up and negative (or ground) pointing down. Currently, your schematic is upside down. – winny May 17 '22 at 08:23
  • @winny thanks for the advice schematic revised hope this meets the standard – ajr_chm May 17 '22 at 10:20
  • Much better! Thank you. – winny May 17 '22 at 12:05

1 Answers1

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  1. You have a MOSFET IRFZ44N capable of pulsed drain current of 160A. I don't think 0.25A inrush is of any concern. In any case, you're switching the START input, not the power supply, which draws something like 100uA, with negligible inrush I should think. So the 0.25A inrush is not important to the MOSFET. It must leak less than 10uA, which might be okay in reality, but the datasheet only guarantees 25uA and only at room temperature (leakage will increase greatly with increasing temperature) so it's not great design practice.

  2. "240VAC to 24VDC step down transformer" presumably you mean a regulated switching power supply. A transformer would produce AC and if rectified and filtered would be more than 24VDC.

  3. That MOSFET requires only 10V Vgs to turn on quite well. There is no particular advantage in giving it 18V and you're much closer to the absolute maximum value than you need to be. In fact, I see no need for a transistor at all, the switch can directly trigger the timer relay. You can add a resistor to ensure a minimum switch current for reliability (see your switch datasheet for minimum current spec).

Reference: timer relay manual

Spehro Pefhany
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  • You are correct the power supply is as you describe I'll correct the post. I realised that it could be done without the transistor when I was writing the post but this is also a learning process for me as to the uses of MOSFETs . Thanks for pointing that out because for my uses a circuit without a transistor is probably a more robust and simple solution. – ajr_chm May 17 '22 at 07:22