# TL431 low voltage cut out

Discussion in 'The Projects Forum' started by RodneyB, Nov 11, 2015.

1. ### RodneyB Thread Starter Active Member

Apr 28, 2012
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I am trying to understand the TL431.

I have a 100k 10K trim pot and 30K resistor in series.

The supply voltage is 13.8 volts.

On the centre pin pf the trimmer I get a voltage range of 2.81 and 3.6 Volts

I want to know what the voltage that is on reference pin 1 of the tL431 switches on the TL431.

I have built this exact circuit with help in previous threads and now I cant get it to work.

2. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Apr 28, 2012
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4. ### Jony130 AAC Fanatic!

Feb 17, 2009
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The voltage higher than 2.5V at reference pin will turn-ON the TL431 and the PMOS will also be ON.
Try lower the supply voltage to 10V and now turn the pot and see if the circuit react.

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5. ### Dodgydave Distinguished Member

Jun 22, 2012
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Tl431 is a variable zener from 2.5v minimum, 36v maximum.

Last edited: Nov 11, 2015
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6. ### MikeML AAC Fanatic!

Oct 2, 2009
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Here is a sim of your values of R4, RV1, and R3, except I dont have the exact PFET. Note the cutIn and CutOut voltages. Is that what you expected? In case you still haven't got the pinouts right on the TL431 and/or the PFET, I show those too.

Note the voltage at V(ref). The trip point for the TL431 occurs when the voltage at Ref is 2.495V. R4, RV1, and R3 make a voltage divider. With the pot centered, the approximate trip point with the 10K pot centered will be:

V(in)*(R3+5K)/(R4 + 10K +R3) = 2.495

Pick a value for R4, say 100K. Now, knowing what you want for V(in) as the trip point, solve the equation for R3...

Note V(ref) in the plot. Note the step that occurs as the circuit switches state. You can see the hysteresis created by the positive feedback through R1.

If you want the trip point to be near 20V, write back because this circuit requires a small mod for the higher voltage...

Last edited: Nov 11, 2015
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7. ### RodneyB Thread Starter Active Member

Apr 28, 2012
633
13
Thanks Very much for the help, the circuit is once again working, I am embarrassed to admit a dry joint and I had put a 100K in R3 instead of 30K.

8. ### ian field Distinguished Member

Oct 27, 2012
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The TL431 is basically a comparator with a built in 2.5V Vref. There may be a 1.25V version, probably from Zetex.

Some of the various datasheets have an application example for an under/over voltage alarm using 2x TL431.

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9. ### Jony130 AAC Fanatic!

Feb 17, 2009
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TLV431 is a 1.24V version.

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