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I have a circuit board that, among many things, drives a few 24 VDC relays on another board located 5-10 feet away.

Everything usually works just fine, but I recently had an incident where the relay +/- connections were shorted together, sending high current through my switching circuit and destroying a transistor.

So I'm looking now at putting short-circuit protection into the next revision of these boards to avoid having such a simple issue kill a whole board (not worth my time to fix a single tiny surface mount transistor on a small board -- we just replace the whole board). Normally I would implement something like this, but in this case, the high side is common among multiple relay coils (read one wire going over to the other board) and the low side is broken out for each relay. I rearranged some transistors in the standard short-circuit protection circuit and came up with this:

Low Side Short Circuit Protection Circuit Diagram

Is this the right way to go about it? Are there better configurations for the current-limiting resistors? Is it worth throwing in a TVS in parallel with the load? It seems to simulate fine in Spice, but I'd like any feedback here. Load is shown as 60 Ω as the approximate point at which the short circuit protection seems to kick in in my simulation. The actual normal load will be a ~2.5 kΩ, 10 mA relay coil. In practice, an N-channel switching transistor/FET is also at the bottom of the circuit to turn the relay on and off. Transistors picked in the Spice simulation are there just to get a basic model in and don't represent the actual transistors used in production.

ocrdu
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mech-eng_doing_ee
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    There are fully protected load switches like BD1LB500FVM, ZXMS6004DN8 – Jens Oct 05 '22 at 00:04
  • The voltage drop required by your ground return switching circuit isn't critical, it seems to me. You can afford a diode drop... or even two, I think. You might consider something like [this](https://electronics.stackexchange.com/a/481317/38098) as a means to an end. It will saturate out and work like a switch for lighter loads. But it will kick in hard and hold the line if the wires are shorted. – jonk Oct 05 '22 at 07:41

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You might be able to use a PTC thermistor for your application which would be simpler and might work better since the current involved is so low and a direct short would be amps of current.

enter image description here Source: https://article.murata.com/en-eu/article/ptc-over-current-protection-for-fa-device

Voltage Spike
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  • The downside here is that I need to be OK with a voltage drop due to the resistance of the PTC in normal, 25°C operation, correct? – mech-eng_doing_ee Oct 04 '22 at 17:49
  • Also, are these fast enough to save a transistor in the case of a hard short? – mech-eng_doing_ee Oct 04 '22 at 17:56
  • I didn't look at the murata ones, the page just gives an overview. But this one might be a good candidate: https://media.digikey.com/pdf/Data%20Sheets/Littelfuse%20PDFs/PPTC_zeptoSMDC_DS_v1.1.pdf It has a 20ms trip time which would most likely protect the transistor – Voltage Spike Oct 04 '22 at 18:27