I recently re-homed several cheap 13.8V linear PSUs from rallies, ranging from 3 to 10A. My intention is to power smaller items of equipment; TNCs, FT817s and similar. However, after inspection it became apparent that all lacked any form over over voltage protection.
I at least expected a simple crowbar circuit!
Looking at several crowbar schematics online and considering modes of failure, I thought of another option. Instead of shorting the supply on an over-voltage condition, why not simply disable output with a voltage controlled switch? If the supply voltage exceeds a safe limit (for example 14.5V) kill the output.
The problem here is how to do this without a power transistor constantly sinking current. In the case of a 10A supply this could require a significant heatsink; Somewhat inefficient too!
So, welcome back, my old clunky friend: the relay.
The quick schematic above (subject to revision!) is controlled by Zener D1 (in this case a 14V Zener). When the voltage exceeds the avalanche voltage of D1 the base of Q1 starts to climb to 0.6v, becoming forward biased and hence switching on. This in turn activates the relay, switching the main V OUT off and instead illuminating an ERROR LED.
Both the relay and error LED are both protected from over-voltage (within reason) buy a cheap 12V regulator.
A graph showing V IN (top) plotted against the two relay outputs can be seen below. Note the current with just the LEDs (no load) is not equal as the ‘error’ LED is held at 12V.
It is shown that the switching point is between 14.7 and 14.8V. For a supply set at 13.8V I’m happy for this to occur anywhere up to 15V.
Dependent on the equipment connected, you should however be aware that the time taken for a relay to energise may still be enough time for damage to occur. In many cases, a power transistor or mosfet either passing current or acting as a crowbar may be the preferable option.
Also, these are just my thoughts coupled with a quick simulation in Circuit Lab; this is untested in practice 🙂