So I currently have a drive run by a PSU. I have a thermal controller that will cut power to the drive when it runs over temperature but it doesn't latch off, so as soon as the drive cools a little it just turns back on. So I am trying to make a latching reset but I keep running into problems with the latch because the drive voltage needs to be 0 to 20 V. I have a crude diagram below.

I have looked up multiple latching switches using BJTs tried a relay with a push button and a 12 volt rectifier leeching off the thermal controller. Most fail when using the drive at low voltage sub 3 volts and burnt a few parts when going over the 20v.

Again I currently have it cutting the output to the PSU when it goes over temperature but I want it to stay off until manually reset in case I am not watching it or so it wont restart if I try to change something after it over-temperatures.

I am using a temperature controller model SlB4848-R2 and TDK-Lambda model UP36-6 power supply I would prefer to use those since I have. I also have access to a wide array of other common parts.

Thank you in advance for any of your advice I can post previous attempts I had if desired.

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So I found a higher output op amp used as a compactor as the latch with a push button to trigger the relay. Would not have figured this out without user Transistor showing me CircuitLab made so much easier. The voltage divider is meant to both limit current and stop the input voltage from exceeding the op amp output.

  • \$\begingroup\$ Look up motor start stop circuits and note that your reset button becomes the start switch. \$\endgroup\$
    – Andy aka
    Mar 6, 2020 at 19:56
  • \$\begingroup\$ Whats your budget? A shunt trip circuit breaker is a COTS solution for this. Send it 24VAC and it trips the breaker for you. \$\endgroup\$ Mar 6, 2020 at 19:58
  • \$\begingroup\$ Any chance you can rotate your sketch? Try to avoid slang such as "over temps" for the sake of non-native English speakers of which there are many on the site. \$\endgroup\$
    – Transistor
    Mar 6, 2020 at 20:06
  • \$\begingroup\$ To Andy aka will do. My I am flexible if its works well will look into the shunt trip circuit breaker. Also will fix and keep in mind Transistor. \$\endgroup\$ Mar 6, 2020 at 20:16
  • \$\begingroup\$ An SCR in series with the power will "remember" the loss of current and not re-conduct until current is applied to gate, and will lock on with a short gate pulse \$\endgroup\$
    – dandavis
    Mar 6, 2020 at 21:40

2 Answers 2



simulate this circuit – Schematic created using CircuitLab

Figure 1. Latched thermal monitoring.

How it works:

  • The thermostat, T-STAT, has a normally closed contact. It opens when the drive goes over-temperature.
  • Pressing RESET energises RLY1 which latches itself on via RLY1-b.
  • Contact RLY1-a powers the drive. Using a separate contact for the drive power means that the RESET button doesn't have to carry the full drive current. The RESET button can be lightweight.
  • On over-temperature T-STAT's contact will open, the relay will unlatch. The drive will not be powered again until RESET is pressed.

Note that with this arrangement a reset is required on power-up.

  • \$\begingroup\$ This is a great solution aside from 2 things the T-STAT directly controls the PSU output with a TTL signal so can't get in before it and the second is my Line out of the PSU operates at 0v - 20v. so at low ends you can't reasonably trigger a relay \$\endgroup\$ Mar 9, 2020 at 12:17
  • \$\begingroup\$ You need to improve the question then and draw a proper schematic. \$\endgroup\$
    – Transistor
    Mar 9, 2020 at 12:24
  • \$\begingroup\$ I am giving you the answer since its would probably help most people in a similar situation and CurcuitLab let me work on it much better then with my horrible handwriting. \$\endgroup\$ Mar 9, 2020 at 14:36
  • \$\begingroup\$ Tip: when you use the schematic button on the editor toolbar you get free access to CircuitLab. When you hit Save and Insert an editable schematic is saved in your post. We can then copy and paste into our answers. No account, no screengrab, no image upload, no gridlines. Thanks for accepting my answer but I don't think you've got your solution yet. You can unaccept to encourage others to respond. \$\endgroup\$
    – Transistor
    Mar 9, 2020 at 14:52

I've done similar things for a consumer products in a plastic case. My solution used a Silego Greenpak programmable device, triggered by a remote NTC sensor to turn off downstream power and blink an LED indicating a problem. It would stay tripped until the user removed and restored wall power. You could use a small micro (e.g., ATTiny, PIC, etc.) to do something similar.

The main thing is, this logic needed its own power from the upstream supply to do its job, which doesn't seem to be a problem as you have this in your diagram. The logic, be it Silego or a microcontoller, could be adapted to drive a relay through which you'd feed power to your drive.


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