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

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    \$\begingroup\$ If you put 15V across that you will end up with 30A through the zener and resistor easily enough to destroy both. \$\endgroup\$ – ratchet freak May 30 '18 at 14:12
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    \$\begingroup\$ A zener is not a crowbar - it doesn't "short" the supply when some voltage is exceeded - it just begins to conduct. \$\endgroup\$ – brhans May 30 '18 at 14:13
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    \$\begingroup\$ 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. \$\endgroup\$ – Neil_UK May 30 '18 at 14:14
  • \$\begingroup\$ "shortcut" Do you mean short circuit? \$\endgroup\$ – winny May 30 '18 at 14:47
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    \$\begingroup\$ tl431 datasheet has a crowbar example using a triac. Triacs seem to be more commonly available than SCR. \$\endgroup\$ – Indraneel May 30 '18 at 14:53

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

enter image description here

Picture source.

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.

Added section

If you need a tighter tolerance for crowbar activation than that offered by zener-sensing then use a controller: -

enter image description here

Vz in the circuit can be a precision device that is better than 1% in tolerance quite easily.

  • \$\begingroup\$ Thank you. Do you think we need an SCR controller IC or we can just be safe with a Zener? My idea would be to use a 13V zener diode with 5% tolerance, which should safely fire between 12,4 and 13,6V. We would like indeed to protect anything which is greater than 12V but we can accept 13,6V. \$\endgroup\$ – Francesco May 30 '18 at 15:27
  • \$\begingroup\$ @Francesco I think you need to read the comments below your question. A zener does not fire and it doesn't go into a short circuit unless you burn it. A zener IS NOT a crow bar circuit but it can be part of a crowbar circuit (as per my answer). A crow bar circuit responds to an over voltage by turning into a short circuit that can be reset by removing power. A zener on its own or in series with a resistor does not do that. \$\endgroup\$ – Andy aka May 30 '18 at 15:32
  • \$\begingroup\$ I actually meant the zener diode in the example circuit you posted. Some other designs will control the SCR thrystor with a dedicated integrated circuit to have a finer voltage control. I do not understand if a control IC, there are many from TexasInstruments, would be overkill in my case. I just need to short circuit supply if voltage is above 12.5-13.5V (acceptable range) \$\endgroup\$ – Francesco May 30 '18 at 15:38
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    \$\begingroup\$ Using a zener to pass current to the SCR that causes it to fire is a little prone to error in that it may work at 12.5 volts on one unit or it may not work until 13.5 volts on another unit. I've used your numbers. That implies a nominal value of 13 volts +/- 4% and most zeners are toleranced to +/- 5%. Hence, this is why controllers are used - they will use a precision voltage reference and a comparator. Once the level is exceeded it will drive the gate of the SCR hard and this guarantees a tighter control of the voltage that triggers the crowbar action. I think you should use an IC. \$\endgroup\$ – Andy aka May 30 '18 at 15:45

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