So, I am building the soft-on/off circuit shown below and described here on a breadboard to test it out before I actually put it into a design.

schematic given by source

However, to better fit the parts on hand, I decided to alter a couple of the R and C values slightly, and use a pair of TO-220 FETs instead of the SO-8 dual FET, resulting in this:


simulate this circuit – Schematic created using CircuitLab

which yields the simulation results below when turned on without RL connected (NFG tracks Vout in both plots, so it's not shown):

simulation plot, no-load

and with RL connected for a 10mA load at full Vout (PFG, Vin, and PB track each other here):

simulation plot, 10mA load

However, when I prototype the same circuit that's in the simulation, without a load, I get this gem of a result on power-on:

oscillogram of the misbehaving circuit

Along with that, Vout comes up to a steady 12V, tracking Vin. What in the world could I have done wrong here? Is the circuit given really that sensitive to part selection? Or how could I have miswired it to produce the results I'm receiving? And where on Earth is that 370us pulse on PFG at startup coming from?

  • \$\begingroup\$ Is it really ZERO load? Because your Vout will go to Vin with zero current. \$\endgroup\$
    – Daniel
    Dec 3 '15 at 4:53
  • \$\begingroup\$ I have no load connected to the circuit whatsoever -- should I just hang a resistor off the output for testing purposes? \$\endgroup\$ Dec 3 '15 at 4:55
  • \$\begingroup\$ Please do that! \$\endgroup\$
    – Daniel
    Dec 3 '15 at 4:57
  • \$\begingroup\$ IMO, the transient is almost certainly the N-FET turning on. \$\endgroup\$
    – Daniel
    Dec 3 '15 at 5:00
  • \$\begingroup\$ Hey, I don't really want to be a dick about this, but I helped you solve your problem in under half an hour. Typically that kind of help is expen$ive. Could you at least credit me with the solution? \$\endgroup\$
    – Daniel
    Dec 3 '15 at 16:06

Summary from comments: I think you need a load for this circuit to work.

Shamelessly taking credit for this, here is an answer for you to mark :)

You should also take a look at Linear Technology's line of push button controllers. They are very low power and have worked well for me in the past.

  • \$\begingroup\$ The LTC pushbutton controllers are indeed nice-looking parts -- if I was going for a design where ease of hand assembly wasn't a prime concern, I'd have sprung for them in a heartbeat. \$\endgroup\$ Dec 3 '15 at 12:49
  • \$\begingroup\$ If you need to cut off battery power completely (down to a few uA) they are perfect, and they allow your microcontroller to turn the device off, which is really nice. \$\endgroup\$
    – Daniel
    Dec 3 '15 at 13:58

It turns out that it is indeed the no-load behavior of the circuit that is causing my issue! (Why does the sim show it coming up to ~3.75V?) I put a 1KΩ, >=1W resistor from the output to ground, and I get no voltage at the output and no stray pulse on the PFET gate, as seen in the oscillogram below:

oscillogram of the circuit with a 1kΩ load resistor in place

  • \$\begingroup\$ I think the load (at 10mA) is just high enough that you're getting 8V of drop across the PFET. You just happened to pick a very marginal number. \$\endgroup\$
    – Daniel
    Dec 3 '15 at 5:17
  • \$\begingroup\$ And it looks like, at start-up, there is some high-frequency ringing or oscillation happening, especially on the first image. Stick a ferrite bead on the gate pins and see if that gets rid of it... might impact the rest of the circuit behavior as well. \$\endgroup\$
    – rdtsc
    Dec 3 '15 at 14:26

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