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In the datasheet for TL431, page 26, figure 32, is an example schematic of a crowbar circuit. I've been toying around with this one, since this is the first time I use a TL431. Instead of a TRIAC I use an opamp to catch the trigger (Vref to opamp = ~2.5V).

For Vcc = 5V, setting R1 = 2k4 and R2 = 2k, the "trip" point for TL431 will be at 5.5V. If Vcc goes >5.5V, the opamp will trip and switch its output. Works nice on my breadboard.

But, I just found something that I cannot find an answer/explanation for in the datasheet; if the trip-point is set at 5.5V and the total Vcc load is 0.5A, the actual trip will not occur until Vcc >= 6.1V. Even worse; if the load is 1A, I managed to turn Vcc up to 8V before the current limiter in my lab supply started to choke, and the opamp did not even react.

Why does TL431 not "trip" when there's a fair amount of load? The load current isn't even going through TL431, so what's going on here?

Where in the datasheet can I read about this?

EDIT The original post was very vague. Rewrote it for clarification.

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    \$\begingroup\$ "The load current isn't even going through TL431" That's what's going on. Put more current through the gate. \$\endgroup\$ – Ignacio Vazquez-Abrams Apr 23 '17 at 19:33
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    \$\begingroup\$ It does seem rather odd. There shouldn't be a dependency on the load current. I would suggest you confirm the voltage using an external volt meter. Maybe the supply output is drooping due to internal resistance or resistance of wire in your test setup, or whatever. Also, what resistor value are you using for the unlabeled resistor in the schematic of figure 32? \$\endgroup\$ – mkeith Apr 23 '17 at 19:44
  • \$\begingroup\$ @mkeith: My Fluke measures Vcc (immediately from the lab supply), and that's what I'm looking at when the "trip" happens. For all unloaded cases, the trip goes at ~5.5V (shown by the Fluke). The unlabeled resistor was 8k2, but after reading up I lowered it first to 470R and then 82R, but the error remains. \$\endgroup\$ – bos Apr 23 '17 at 19:47
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    \$\begingroup\$ OK, so at first you said your circuit was the same as the diagram. Now you say you have an extra op-amp in it. Please use the schematic editor to add a circuit diagram to your question. Edit the question, then, in the edit window, click on the icon that looks like schematic symbols. In future, please add an accurate schematic to all questions. It is very frustrating to have the question change. Also, some people who may be able to answer it, may have already read it and moved on. So put your best effort into the question right at the beginning. \$\endgroup\$ – mkeith Apr 23 '17 at 20:31
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    \$\begingroup\$ I think it's some unspecified series impedance that is getting you. That's what your description sounds like. You seem to be measuring from a point prior to that impedance. The higher load current causes a larger voltage drop across this impedance. That schematic (and your points of measurement) are probably important, at this time. \$\endgroup\$ – jonk Apr 24 '17 at 5:33

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