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A wire has two states - 20A DC flowing or no current.

Is there a simple device/relay that can let 20A flow through it and close a switch?

enter image description here

The device must keep a path open when no current flows.
It must close the path when current (DC 20A) flows through it.

I used this ACS712 current sensor with a comparator and managed to detect when the current flows but it is too complicated and I feel there must be a more simpler way.


  • Over current protection is already taken care of externally. The wire has either 20A or none.
  • Hysteresis is not needed as we already have a big window (0-20A). If the switch closes when more than 1A flows through the wire, it is good.
  • Switch controls a 5V supply and only 10-20 mA current flows.
  • We're allowed to break the wire carrying 20A and put the device in series.

  • By simpler I mean minimum number of parts. Is there a device like a relay or something that lets 20A flow through it and closes a switch? It seems normal relays are controlled by 5V-12V DC voltage. I'm not able to find any relay that lets 20A control current flow through it and closes a switch

  • I want a high-side switch to control a 5V 10mA supply. The switch must close when DC 20A current flows through the wire.

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    \$\begingroup\$ There are many more specs than given which adds complexity. isolated rating, over-current protection, threshold, hysteresis, heat rise, switch rating for V,I \$\endgroup\$ Commented Jul 4, 2022 at 17:27
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    \$\begingroup\$ How many turns of AWG 14 magnet wire @ 20A is needed to switch a Reed switch? \$\endgroup\$ Commented Jul 4, 2022 at 18:11
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    \$\begingroup\$ @TonyStewartEE75 interesting, the wire is AWG 13. I'm not familiar with Reed switches, let me do some reading and get back.. \$\endgroup\$
    – across
    Commented Jul 4, 2022 at 18:26
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    \$\begingroup\$ @TonyStewartEE75 I bought 14mm reed switch and it closed with out any turns at 20A! Thank you so so much! I'm super excited as I can now replace the entire ACS712 current sensor and comparator PCB with just a single reed switch. This also makes quiescent current of my circuit 0! Here is a pic of my work if interested... With 6 turns, I could make the switch close at just 1.5A: prnt.sc/wg550LUEOUu3 \$\endgroup\$
    – across
    Commented Jul 9, 2022 at 16:07
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    \$\begingroup\$ Btw I'm using 13AWG wire with PTFE insulation so the thickness of insulation is less than 1mm. \$\endgroup\$
    – across
    Commented Jul 9, 2022 at 16:11

3 Answers 3

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You can wind 4-5 turns of the wire around a glass body reed contact. This contact will close at 20A.

I just tried reed contact ORD324-1015. One singe turn is enough to turn on at around 8A.

Update: sharing the picture to document the solution to the original problem

enter image description here

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    \$\begingroup\$ I used this exact setup as part of the deactivation circuit for a car alarm. Wrapped a couple turns of the cigarette lighter lead around a reed relay and used that contact to extend the entry delay. Push the lighter in and you get an extra 30 seconds. \$\endgroup\$ Commented Jul 5, 2022 at 18:29
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    \$\begingroup\$ This worked like a charm without any turns at 20A! With 6 turns, my 14mm reed switch is closing at as low as 1.5A current. Thank you so much! you all are awesome!! \$\endgroup\$
    – across
    Commented Jul 9, 2022 at 16:03
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    \$\begingroup\$ prnt.sc/wg550LUEOUu3 \$\endgroup\$
    – across
    Commented Jul 9, 2022 at 16:04
  • \$\begingroup\$ @across Thanks for sharing. \$\endgroup\$
    – devnull
    Commented Jul 9, 2022 at 18:30
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I think the current sensor is as simple as its going to get. You can then put the output signal into a comparator (or opamp, like LM741, acting as comparator). The other input of the comparator is coming from a potentiometer voltage divider where you can set the threshold between 0 and 5V, the output of the comparator then goes to a transistor to drive a relay coil. Modules like this exist already, but you have specific requirements that I don't think fit any of them, so you'll have to build your own module like this. https://www.aliexpress.com/item/4000089725866.html

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  • \$\begingroup\$ Ohk thank you so there are no simple relay/electromagnet like devices that just close a switch when 20A current flows. I'm already doing your idea: ACS712 current sensor - LM358 opamp - BC548 bjt. It's working fine but the current sensor also requires power supply and comparator requires a pot and BC548 requires two biasing resistors. I ended up with soldering 7 components on a small pcb board just for detecting current! Thanks again! \$\endgroup\$
    – across
    Commented Jul 4, 2022 at 17:53
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    \$\begingroup\$ A Hall effect sensor (the magnet sensing switching kind) would do, if a logic level rather than switch closure output is acceptable. \$\endgroup\$ Commented Jul 4, 2022 at 21:26
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Is there a simple device/relay that can let 20A flow through it and close a switch?

A range of current relays, meant for refrigerator / freezer applications, are available.

Typical operating currents in the range are as shown.

enter image description here

The one that suits your application may be selected from the range.

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