# Simple Under Voltage Detect

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#### Suraj143

##### Active Member
Want to detect if the supply voltage goes below 10V, then the relay should turn ON. Is this ok?

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#### Nigel Goodwin

##### Super Moderator
No, the relay will be permanently ON.

#### ronsimpson

##### Well-Known Member
Maybe of R1 & D1 reverse places.

#### Suraj143

##### Active Member
Hi,

What adjustments do I have to do?

#### crutschow

##### Well-Known Member
Here's a circuit that uses a low cost TL431 adjustable zener as a comparator to accurately set the trigger voltage.
The circuit switches when the Ref voltage, as determined by R2 and R3, equals 2.5V.
You can make the trigger point adjustable by replacing R2 and R3 with a 10k ohm pot.

#### Suraj143

##### Active Member
Hi Crutshow

Thanks for the circuit.I have two questions.

1)
If I want a trip point of 10V what voltge do I have to set by trimpot? Is it 2.5V or 10V?

2)
Do I really need a base bias zenner diode?

#### ronsimpson

##### Well-Known Member
If I want a trip point of 10V what voltge do I have to set by trimpot? Is it 2.5V or 10V?
The TL431 has a 2.5 reference. So divide the 10V down to 2.5V. That will help you pick the two resistors. R2, R3
Do I really need a base bias zenner diode?
When the input to the TL431 is below 2.5 volts the output is almost Vb.
When the input to the TL431 is above 2.5 volts the output is in the 2 to 2.5 volt range.
So the Q1 circuit needs to know the difference between Vb and 2.5V. Q1 turns on at 0.6 to 0.7 volts. If the Zener is 3.3V then Q1 will turn on/off at about 4 volts. You could turn D2 & R7 into two resistors, voltage divider and get similar results.

#### Suraj143

##### Active Member
Ok thanks for the clear explanation.

So when I adjusting the trip point I have to supply by a 10V and measure across R2 and it should be 7.5V volts?

Is that correct?

#### ronsimpson

##### Well-Known Member
Yes.
You have 2.5V + 7.5V = 10V.
Because 7.5V is 3 times 2.5V then the resistor also need to be 3:1.

#### crutschow

##### Well-Known Member
Because 7.5V is 3 times 2.5V then the resistor also need to be 3:1.
An interesting side note is that those two values of resistors (10.2k and 3.4k) are the only combination of 1% resistors that give an exact 3:1 ratio.
Other values can give close to a 3:1 ratio but not exactly.

#### Nigel Goodwin

##### Super Moderator
An interesting side note is that those two values of resistors (10.2k and 3.4k) are the only combination of 1% resistors that give an exact 3:1 ratio.
Other values can give close to a 3:1 ratio but not exactly.
What about a 3K and a 1K ? - which was what I used.

However, not so clever - as I mis-remembered the FVR reference voltages, and I really needed 4 : 1

#### crutschow

##### Well-Known Member
What about a 3K and a 1K ?
If you can buy a 3.00k, 1% resistor, that's fine, but the standard 1% value is 3.01K.

#### crutschow

##### Well-Known Member
You could turn D2 & R7 into two resistors, voltage divider and get similar results.
As long as their values are low enough to give sufficient base drive to Q1.

#### crutschow

##### Well-Known Member
Okay.
I wasn't previously aware of stock 1% resistor values in other than the standard 1% resistor sequence.

#### schmitt trigger

##### Well-Known Member
What is disturbing about the circuit that the OP attached, is that it is a well drawn schematic, meaning that this non-functional circuit is published somewhere!

And people will start copying it.
Wondering how come it does not work.

#### Nigel Goodwin

##### Super Moderator
What is disturbing about the circuit that the OP attached, is that it is a well drawn schematic, meaning that this non-functional circuit is published somewhere!

And people will start copying it.
Wondering how come it does not work.
Fairly obviously as transistors require 0.7V to turn ON, and the 10V zener is between base and emitter, it will always have plenty more than 0.7V drive to the base holding it permanently ON. Even when the power is high enough for the zener to conduct, it will only limit the drive voltage to 10V, not remove it.

Obviously however drew the circuit hadn't the slightest idea how zeners work.

#### Suraj143

##### Active Member
Hi, I made the below circuit & it worked nicely.The only problem is when it is reaching the trigger point (around 10.6V) the LED is flickering.

How to overcome this?

Thanks

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