# 12V 12A resettable crowbar circuit

The idea is to protect the input stage of a control card from improper power supply connection. The power supply is a standard desktop AC-DC power adapter (Meanwell GST160A12) with inbuilt shortcut, overload and overvoltage protection. Since other models are used (Meanwell GST160A15) the risk for improper connection and operation is high. The control card needs an input of 12V and draws circa 12 A. How should a circuit be design in respect with efficiency and simplicity? There is no problem in brutally shorting the power supply adapter. I was thinking in just placing a Zener diode of correct rating that can turn on when the voltage is above 12V and short out the PSU. Would this be a very bad design?

simulate this circuit – Schematic created using CircuitLab

• If you put 15V across that you will end up with 30A through the zener and resistor easily enough to destroy both. – ratchet freak May 30 '18 at 14:12
• A zener is not a crowbar - it doesn't "short" the supply when some voltage is exceeded - it just begins to conduct. – brhans May 30 '18 at 14:13
• The standard trick with a crowbar is to use a thyristor across the power supply. They are big brutes, usually with a big pulse rating as well. Trigger it on with some form of voltage detector. If you simply want to protect against reverse connection, then use a standard silicon diode. They can have brutish pulse rating as well. 1N540x series is 3A continuous, but 200A for 8.3mS. Make sure your fuse is small enough to blow before your crowbar does. – Neil_UK May 30 '18 at 14:14
• "shortcut" Do you mean short circuit? – winny May 30 '18 at 14:47
• tl431 datasheet has a crowbar example using a triac. Triacs seem to be more commonly available than SCR. – Indraneel May 30 '18 at 14:53

Try this as an example of using an SCR and a zener: -

If the voltage exceeds Vz + Vgt then the SCR fires and shorts out the supply. Only when the power supply is removed or dropped in voltage to a low level does the short circuit reset.