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I am trying to design a circuit with 2 different contours:

  1. Controlling contour: a STM microcontroller powered by a 3V battery.

  2. Power contour - a variable voltage power supply (1.23V - 20V, up to 5A) powered by a AC-DC laptop converter.

The microcontroller should be able to close and open the power circuit. It should be relatively fast (up to dozens/a few hundred milliseconds) and should not drain the battery too fast.

I assume I have to use a combination of an optocoupler (which has a low power consumption) with a transistor: I came up with a simple design which, however, would not work, due to the variable voltage in the power contour.

enter image description here

What would be a better design for such a power key?

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  • \$\begingroup\$ Is galvanic isolation a requirement for this application?? \$\endgroup\$
    – Kvegaoro
    Mar 10 '17 at 17:32
  • \$\begingroup\$ Would be nice to have, but not absolutely necessary \$\endgroup\$
    – xboborx
    Mar 10 '17 at 17:52
  • \$\begingroup\$ You have a fixed laptop supply and you want to make a variable lab supply out of it using a Buck design from 1.23 to Vin @5A with an output enable, right? Are you doing this to get experience or save money? \$\endgroup\$ Mar 10 '17 at 17:58
  • \$\begingroup\$ When you say "contour" do you mean "circuit"? \$\endgroup\$
    – The Photon
    Mar 10 '17 at 17:58
  • \$\begingroup\$ Yes, by contour I mean circuit \$\endgroup\$
    – xboborx
    Mar 10 '17 at 18:19
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You can try a device similar to the TLP190B that is an optocoupler device but its output its a photo-voltaic device that can be used to drive a MOSFET. You might just have find one that will suit your switching time requirements and output characteristic that can still give you low power consumption from the If of the optocoupler diode and enough Vout to drive a logic level MOSFET.

Additionally if the isolation is not required you can just do the swithcing with a suitable logic level MOSFET.

Another option is to use a relay and some external circuitry to drive the relay hard for a short amount of time and then just drive it with enough current to keep the contact closed

And finally a suitable latching relay could be another option

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