# Zener Diode Calculations

#### RodneyB

Joined Apr 28, 2012
697
Hi All

I am trying to make a simple battery monitor showing bad fair food and over charging using zener diodes and resistors. I know how to work out the value of the Resistor but how do I work out the value of the Zener diode, if I want the LED for arguments sake to illuminate at 12 Volts, do I put a 12 Volt zener in this is the calculation I amtrying to find.

Many thanks

Rodney

#### Wendy

Joined Mar 24, 2008
22,170
This is more of an electronics question, where we have a forum for such. I have moved to a better location.

#### WBahn

Joined Mar 31, 2012
26,265
Hi All

I am trying to make a simple battery monitor showing bad fair food and over charging using zener diodes and resistors. I know how to work out the value of the Resistor but how do I work out the value of the Zener diode, if I want the LED for arguments sake to illuminate at 12 Volts, do I put a 12 Volt zener in this is the calculation I amtrying to find.

Many thanks

Rodney
Are you powering your battery monitor from the battery being monitored?

If so, then you need it to work at voltages well below the lowest voltage level you want to detect.

Your Zener circuit needs to produce reference voltages and then you use a voltage divider to produce a sensed voltage that is proportional to the terminal voltage that you compare to the reference voltages.

#### crutschow

Joined Mar 14, 2008
25,631
For best stability with temperature you should use a voltage reference diode, not a plain zener. You could use a comparator, such as the LM339 to compare the reference voltage to the voltage from the battery and light the LED at the appropriate voltage. To reduce the battery voltage to equal the reference voltage use a two resistor voltage divider. The resistor divider voltage is Vb * R2 / (R1+R2) where Vb is the battery voltage, R1 goes to the battery and R2 goes to ground. Use appropriate resistor values typically between 10kΩ to 100kΩ or so.

#### RodneyB

Joined Apr 28, 2012
697
I have attached my Zener diode circuit Monitor, The one I built if I wanted it to light the LED at 12 Volts I had to put a 10 Volt zener. Also all the LED's were lit and as the voltage dropped they went out, I deally I would like only one LED on,the relevant one.

#### Attachments

• 25.6 KB Views: 52

Joined Jul 31, 2012
8
crutshow's idea of the LM339 would be the right way to go about this. If you are set on the zener diode and only want one LED on at a time with your current circuit, you could add a normally closed relay for each 'leg' of the circuit so when the next 'leg' of the circuit gets power, it opens the previous 'legs'.

The reason for the need of a 10V zener to power an LED at 12V is because the LEDs forward voltage. The one you tested must have a Vf close to 2V.