Zener diode switched N MOSFET low voltage cut off switching

Thread Starter

CrustyS

Joined May 20, 2019
27
This idea is to cut power to a dashcam when a specific battery voltage is reached. The method is to select an appropriate Zener of about 12.2-12.4V and use it to switch a MOSFET supplying power to a 5V buck converter thence to the dashcam. I am pretty sure I could squeeze this into a cigarette lighter 5V adapter. I would use a logic level MOSFET so an appropriate resistor would be needed between the Zener and the gate.

I do have an alternative plan of using an Attiny 85 to switch the MOSFET but I was hoping to be as simple as possible.

Any comments most welcome, even "that's a dumb idea" if it is !
 

crutschow

Joined Mar 14, 2008
34,468
To get a more accurate voltage than using a Zener, which have a wide voltage tolerance, you could use an inexpensive programmable reference, such as the TV431, as an accurate voltage comparator.

LTspice sim of example circuit below:

Pot U2 allows adjustment of the trip point.

R4 provides a small hysteresis (positive feedback) so it doesn't dither around the set point (as the battery voltage may rise a little when the load is removed).

The P-MOSFET can be just about any with a sufficient voltage rating, and an ON-resistance low enough to not require a heat-sink (<1W MOSFET I²R dissipation) with a max load.

1680908113189.png
 
Last edited:

flecom

Joined Apr 9, 2023
2
the TLV431 HV variant seems to have an absolute max rating of 18v, so guessing this would not work with a 5s power tool battery (max >20v when charged)?
 

Thread Starter

CrustyS

Joined May 20, 2019
27
To get a more accurate voltage than using a Zener, which have a wide voltage tolerance, you could use an inexpensive programmable reference, such as the TV431, as an accurate voltage comparator.

LTspice sim of example circuit below:

Pot U2 allows adjustment of the trip point.

R4 provides a small hysteresis (positive feedback) so it doesn't dither around the set point (as the battery voltage may rise a little when the load is removed).

The P-MOSFET can be just about any with a sufficient voltage rating, and an ON-resistance low enough to not require a heat-sink (<1W MOSFET I²R dissipation) with a max load.

View attachment 291680
Thank you for taking the time to reply I will give this option serious consideration.
 

Thread Starter

CrustyS

Joined May 20, 2019
27
Thank you for taking the time to reply I will give this option serious consideration.
To get a more accurate voltage than using a Zener, which have a wide voltage tolerance, you could use an inexpensive programmable reference, such as the TV431, as an accurate voltage comparator.

LTspice sim of example circuit below:

Pot U2 allows adjustment of the trip point.

R4 provides a small hysteresis (positive feedback) so it doesn't dither around the set point (as the battery voltage may rise a little when the load is removed).

The P-MOSFET can be just about any with a sufficient voltage rating, and an ON-resistance low enough to not require a heat-sink (<1W MOSFET I²R dissipation) with a max load.

View attachment 291680
I think this is the best solution for what I have in mind. I will use FQP27P06 for the MOSFET, just waiting for my TL431's to arrive.

Thanks again
 

Ken Comeau

Joined Oct 26, 2018
1
Maybe I need more coffee this am but WHY
- use a 5V buck converter
- use a uController

A straightforward solution is use a [low-power] comparator. - with say a 8V2 Zener voltage reference Then no need for an exact voltage reference.
 
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