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I can't twist my head around the proper use of my latching switch problem. In detail, the latching (or maintained) switch should switch a 48V DC source power supply for a cisco router. The 48VDC power supply is hooked up to 230V AC.

Ideally, you would push the button -> it should be "ON"; providing 48VDC to the device. you push the button again -> it should switch "OFF"; no power to the device.

now comes the logic: if the power supply on the 230V side doesn't have power, the maintained switch should be "reset" to off; meaning that if it's in "ON" position but you unplug the electricity cable; the maintained switch should get back to its "OFF" position.

I guess this problem has been solved many times, but I am not sure what kind of schematic I am looking for, i. e. what's the name of this circuit or is there a device capable of doing exactly that?

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    \$\begingroup\$ It's easier if you have separate on and off switches. Is that okay? \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Apr 13, 2015 at 19:43
  • \$\begingroup\$ In the shop-tool world, this is referred to as "magnetics" or "magnetic switch." Basically you arrange a relay so that it can be forced into the ON state with a momentary button. One of the switches controlled by the relay is in parallel with the momentary button and thus holds the relay on. Like Spehro said, it is easier (and more common) to have a separate off button. Not sure how you would do it if there is only one button. \$\endgroup\$
    – user57037
    Commented Apr 13, 2015 at 20:10
  • \$\begingroup\$ Of course, the seperate off button is obvious. Anyhow, for the ease of use (certainly in our application) it would really make much sense if we could control the state when power goes off to reset the button. \$\endgroup\$
    – user2901103
    Commented Apr 13, 2015 at 20:46

2 Answers 2

Reset to default
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You want 'relay toggle switch'. Bill Bowden's website http://www.bowdenshobbycircuits.info/ is great for simple circuits like that. Search for 'Relay Toggle Circuit'. Here is an example I am using in one of my projects:

Single Transistor Relay Toggle Circuit

(note this is for 12V; for 48V you will have to use a 48V relay, use transistor which is rated at least 100V V_CE, and increase base resistor from 560 ohms to 2-3K)

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  • \$\begingroup\$ Bill Bowden's website is gone. Apparently the domain was not kept up and now spammers have taken it and use it for serving ads. You can get to his original site on the Internet Archive. I found a discussion or two and there's no idea what happened - whether Bowden gave up on it or died or what. web.archive.org/web/20230410001107/http://… \$\endgroup\$
    – Tango
    Commented Dec 2 at 7:20
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From a digital standpoint, this behavior can be accomplished with a D Flip-Flop. A debounced momentary switch would act as the clock for the flip flop. The output would control the power to the router. If your 48V power supply goes down then you clear the flip flop, thus resetting router power to off. See simplified schematic below:

schematic

simulate this circuit – Schematic created using CircuitLab

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