I want to do a latching circuit with following requirements :

  • do not use any relay
  • on circuit output, load is powered with 36V DC
  • on circuit input, we have a 0/5V signal coming from an RC filter
  • 36V come from a battery, so power consumption must be taken into account
  • load is Input of a 36V to 5V DC/DC switching regulator, that powers an MCU (that will later power some audio components).

I've seen some latching circuit like this one (more explanations can be found here ) :

enter image description here

I need to replace Push Switch with an equivalent component controlled from 0/5V input (I do not want to use any relay). How can I do it?

May be some simpler circuits exits, I'm interested by your suggestions.

  • \$\begingroup\$ Be careful with the blocking diode, you don't want to put a diode in series with a motor/inductive load, unless they have a diode in parallel. The current can't be suddenly stopped (voltage = L*di/dt) so voltage will rise until current continues, this would reverse bias the diode until something brakes. \$\endgroup\$
    – Andrés
    Commented Jul 10, 2017 at 15:36
  • \$\begingroup\$ Do you have any requirements? Do you still want to keep that circuit? Can you use a low-side switch instead of a high-side? \$\endgroup\$
    – Andrés
    Commented Jul 10, 2017 at 15:38
  • \$\begingroup\$ Any specific reason you don't want to use a relay? Using a transistor to replace a switch to power or ground is normally fairly straight forward. Replacing a switch which is floating at some mid-rail voltage like this is trickier. \$\endgroup\$
    – Andrew
    Commented Jul 10, 2017 at 15:39
  • \$\begingroup\$ @Andrés this circuit is just an example, I'm not sure it is well suited for my case. \$\endgroup\$
    – rem
    Commented Jul 10, 2017 at 15:58
  • \$\begingroup\$ @rem what is it that you want to power? The easiest way would be to use a simple latching/inverter, so everytime you press you change the state. This can be done with flipflops, gates, etc. And the output of the circuit is fed to a low-side nmos switch. Much simpler and easier IMHO. \$\endgroup\$
    – Andrés
    Commented Jul 10, 2017 at 16:03

1 Answer 1


You have an incomplete Spec; Solid state only, toggle input power switch, 5V in, 36V out.

So far you show an example of a negative edge input toggle switch with a high side power switch of unknown surge current load and an unsafe series blocking diode rather than a safe reverse shunt diode, normally used.

Re-write your specs for Vin, +Vout, Io-pk, Io-avg. ( often DC motors are 10x ESD protection, EMC requirements ( long cables CM noise environment for crosstalk etc)

A D flip flop clk input with feedback from Qbar to D input is a T FF for example. THere are other methods like dual NPN bi-stable designs.

THere exist automotive Hi and lo side power "smart" switches with thermal and short circuit protection or you can add your own current limiter or PTC for protection and you also need kickback BEMF snubber, twisted pair outputs and other EMC considerations (electromagnetic compatibility)

But ALWAYS start a good question with clear specs. ( as much as you know)


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