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TL431: Overvoltage Crowbar

Danny Turner
Prodigy 20 points
Part Number: TL431

I am designing an overvoltage crowbar circuit and I would like to use the TL431 as an adjustable comparator like in the circuit below (taken from Axotron's website): I will use a pot for R1 to adjust the "trigger" voltage.

I need to protect a 36V supply, so my circuit needs to trigger around 40V. Since the TL431 can output 36V max, I need to make some adjustments to the circuit. My first idea was adding a Zener diode in series with the TL431, connecting R2 to the anode of the TL431 (and cathode of zener) and anode of zener to ground. This makes the anode of the TL431 referenced to the zener voltage (for this example, let's use 10V). I will then set the TL431 to "trigger" at 30V. Will this cause any problems for the anode to be referenced to a voltage other than ground?

  • 0
    TI__Mastermind 19370 points

    Hi Danny,

    Can you draw your circuit so I can better understand your description?

    From my understanding you are using a Zener to elevate the anode voltage of the TL431. This will limit the voltage difference between the anode and cathode of the TL431. The issue I see is that if you are using a Zener then the accuracy of the TL431 is compromised and that is the benefit of using a TL431 over other crowbar configurations.

    I would find a way to limit the upper end so the TL431 can still function.

    -Marcoo

  • 0 in reply to Marcoo Z
    Prodigy 20 points

    Hi Marcoo,

    Yes, you are correct in your understanding. If I use a 10V zener, this will elevate the anode of the TL431 to be 10V, and if I pick the resistors to "trigger" the TL431 at 30V then the crowbar will trip around 30+10 = 40V. It doesn't need to be very precise because I was planning on using a potentiometer for R1 so I am able to fine tune the overvoltage trip point to be exactly what I want. Also the pot will give me a little bit of wiggle room to adjust the trip point higher or lower, to account for the voltage drop in the cables. For more clarification, this OVP circuit will be at the output of the power supply, and there will be a long length of cable (tens of feet) to the UUT. There will be sense lines connected down at the UUT, so depending what the voltage drop of the cables might be the power supply could be set at a higher voltage and I don't want this to trigger the OVP.

    I am pretty much just using the TL431 for its ability to be adjusted (unlike a normal zener) and not so much for its accuracy, although that is an added bonus.

    -Danny

  • 0 in reply to Danny Turner
    TI__Mastermind 19370 points

    Hi Danny,

    In that case I do not see an issue with the design during normal operation. Just make sure that T1 will trigger when the voltage on the base of T2 is diode voltage + 2V from the TL431.

    -Marcoo