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Negative Overvoltage Detect


Member level 5
Jul 22, 2014
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In the past I have used a 431 (shunt ref/reg) to detect over-voltage (or sometimes under-voltage) conditions on a HV rail. I would normally have a LV rail in addition that has circuitry to "action" the overvoltage condition.

In the absence of a negative equivalent of the 431 the circuit becomes more complex (I assume) if the HV is negative. This is because the reference voltage is referenced to the negative terminal so is the circuit is flipped the precision of the circuit is governed by the accuracy of the 12V rail. I have come up with a "fix" for that using two references but I have this nagging doubt about if I'm missing something and there is an easier way of doing it.

I have shown the 12V is positive here but that could easily be -12V (generated by an isolated PSU).

Of course I could use the positive circuit with the circuit floating at -HV potential but I don't want to do that.


  • +VE.PNG
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  • -VE.PNG
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One article tells how the TL431 is internally a complex IC even though it has just 3 pins which are sufficient for us to operate it. It contains components whose names we might recognize (voltage reference, comparator, high-gain switching) so it does a precision task.

Schematics commonly hook up TL431 in a circuit where the power supply is positive polarity. If current needs to be limited flowing through the device, a resistor is added at the positive terminal as with a zener diode. I haven't seen it explained whether the IC needs a low-ohm path to ground in order to function properly.

I haven't seen an explanation how its terminals should work in a circuit where the supply is negative polarity.

A P-type transistor is useful in circuits where the supply is negative. And transistors have a built-in 0.6V reference. Adjust bias so that a small change of supply amplitude causes a large change in the transistor. These two circuits have identical components except for supply voltages:

PNP with negative supply small change in voltage causes PNP to conduct a lot.png

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