# Under voltage protection Design

#### kaao

Joined Jul 31, 2023
10
Can u guys help me the design of undervoltage protection circuits.

#### Jerry-Hat-Trick

Joined Aug 31, 2022
449
You will have to be a bit more specific than that. Are you talking about AC mains voltage or DC and what is the expected voltage and what value of undervoltage is acceptable and what do you want to happen when the voltage falls below that limit, and will that action be reversed when the voltage recovers and how much hysteresis (so it doesn't hunt when the voltage changes when current is drawn)

In simple terms, what are you trying to do? Specifying a requirement is the first vital step in any project.

#### kaao

Joined Jul 31, 2023
10
You will have to be a bit more specific than that. Are you talking about AC mains voltage or DC and what is the expected voltage and what value of undervoltage is acceptable and what do you want to happen when the voltage falls below that limit, and will that action be reversed when the voltage recovers and how much hysteresis (so it doesn't hunt when the voltage changes when current is drawn)

In simple terms, what are you trying to do? Specifying a requirement is the first vital step in any project.
A system that works in 30v dc conditions, cuts the power when it drops below 8v, then gives power again when it goes over 15v.

#### crutschow

Joined Mar 14, 2008
33,352
cuts the power
How much current?
Can the switch be a high-side MOSFET or do you need a relay?

#### Jerry-Hat-Trick

Joined Aug 31, 2022
449
Sounds like you need to use a comparator (or Op Amp) with hysteresis. Power the comparator with your nominal 8V to 30V single supply and use a 5.1 volt Zener (in series with a resistor) connected to the +ve supply voltage to have a fixed 5.1V into one input of the op amp whilst the other input is supplied by a two resistor voltage divider. A feedback resistor will give you the ability for the ouput to swing from rail to rail at your 8V and 15V thresholds which in turn can drive a relay or a transistor. I need a pencil and paper to do the calculations - someone may beat me to it, or come up with a better solution!

#### sghioto

Joined Dec 31, 2017
5,099
A system that works in 30v dc conditions, cuts the power when it drops below 8v, then gives power again when it goes over 15v.

#### kaao

Joined Jul 31, 2023
10
How much current?
Can the switch be a high-side MOSFET or do you need a relay?
switch will be Mosfet not relay,thanks

#### kaao

Joined Jul 31, 2023
10
Sounds like you need to use a comparator (or Op Amp) with hysteresis. Power the comparator with your nominal 8V to 30V single supply and use a 5.1 volt Zener (in series with a resistor) connected to the +ve supply voltage to have a fixed 5.1V into one input of the op amp whilst the other input is supplied by a two resistor voltage divider. A feedback resistor will give you the ability for the ouput to swing from rail to rail at your 8V and 15V thresholds which in turn can drive a relay or a transistor. I need a pencil and paper to do the calculations - someone may beat me to it, or come up with a better solution!
thanks a lot, i'll try

#### kaao

Joined Jul 31, 2023
10
in this circuit why do we need 2 opamps

#### sghioto

Joined Dec 31, 2017
5,099
Not op amps. The LM393 is a comparator with two individual circuits.
One provides the shut off at 8 volts the other turns power back on at 15volts.

#### kaao

Joined Jul 31, 2023
10
Not op amps. The LM393 is a comparator with two individual circuits.
One provides the shut off at 8 volts the other turns power back on at 15volts.
thank u, kind regards.

#### Jerry-Hat-Trick

Joined Aug 31, 2022
449
why do we need 2 opamps
Not sure you do. This is the arrangement I mentioned yesterday with calculated values

You can think of it as R4 being in parallel with R2 when the output goes high and R4 in parallel with R3 when the output goes low. In practice, the calculation is an over simplification because the output never goes to the rails so if you want to tweak the values replace R4 with a 10K resistor in series with a 50K trimpot and insert a 10K trimpot between R2 and R3 with the centre tap into the comparator/op amp. Lowering the value of R4 increases the voltage window and vice versa

#### kaao

Joined Jul 31, 2023
10
Not sure you do. This is the arrangement I mentioned yesterday with calculated values

View attachment 299595

You can think of it as R4 being in parallel with R2 when the output goes high and R4 in parallel with R3 when the output goes low. In practice, the calculation is an over simplification because the output never goes to the rails so if you want to tweak the values replace R4 with a 10K resistor in series with a 50K trimpot and insert a 10K trimpot between R2 and R3 with the centre tap into the comparator/op amp. Lowering the value of R4 increases the voltage window and vice versa
How do you provide opamp supply

#### Jerry-Hat-Trick

Joined Aug 31, 2022
449
How do you provide opamp supply
Needs to be an op amp which works with a single supply up to 30V which should still function at 8V. Rail to rail would be nice but not essential. I have a box full of LM358 op amps which are actually dual, but you can ignore one of them. Not rail to rail but really handy for a host of applications. They are specified to work from 3V to 32V single supply.

#### sghioto

Joined Dec 31, 2017
5,099
thank u, kind regards.
How much current will this 30 vdc system require?

#### kaao

Joined Jul 31, 2023
10
How much current will this 30 vdc system require?
i dont know but not high current

#### sghioto

Joined Dec 31, 2017
5,099
Less than 1 amp?

#### sghioto

Joined Dec 31, 2017
5,099
Because the supply will be as high as 30 volts the Vgs voltage on the mosfet must be clamped below 20 volts.
Modification using a 5 volt zener diode to protect Q1.

EDIT: Moved pin6 to pin1

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