12v to 9v using zener

Thread Starter

daryll

Joined Jan 22, 2021
6
just to run this past you people , I have a small 555 based clock generator which runs on 9v , my synth has the +-12 power supply , so would this work..

12v to 9v zener.jpg
current draw for 555 clock is < 15ma
 

Thread Starter

daryll

Joined Jan 22, 2021
6
the white rectangle is the 10 pin euro connector the +12,0v, and -12 wires are from the euro power supply
 

KeithWalker

Joined Jul 10, 2017
1,582
What is the zener and what is its power rating? What is the recommended current through it? With only a 10 ohm resistor in series with it, it will be drawing 330 mA and the zener will be dissipating almost 3 Watts. I think you need a much higher value for the resistor.
 

dendad

Joined Feb 20, 2016
3,781
What is the zener and what is its power rating? What is the recommended current through it? With only a 10 ohm resistor in series with it, it will be drawing 330 mA and the zener will be dissipating almost 3 Watts. I think you need a much higher value for the resistor.
Yes. Smoke ensues! Check that it is 100R, not 10R as marked on your diagram.
 
The load must be connected to the output end to prevent zener from getting damaged. Series resistor 10 ohm is a current limiting resistor and when high current is drawn it has to pass that current through it, thus 5-watt resistor is necessary.
 

dendad

Joined Feb 20, 2016
3,781
The load must be connected to the output end to prevent zener from getting damaged. Series resistor 10 ohm is a current limiting resistor and when high current is drawn it has to pass that current through it, thus 5-watt resistor is necessary.
It is good practice to design the Zener regulator to handle the full current.
In this case, the current specified is about 15mA.
So, allow some for the Zener, 5mA, let's say a total of 20mA.
The resistor needs to be 12V-9V = 3/.02A = 150R. P=3x.02 = 60mW.
I'd go to a bit higher current, and a 400mW 9V Zener can pass 36mA, so why not go to a total of 30mA?
R = E/I = 3/.03 = 100R
The 100R resistor will dissipate 90mW so just about any resistor will do.

The series resistor should not be 10R as that is very wasteful of power,
A total of 300mA, so the Zener will need to be a 5W version. 2.7W dissipated, heat sinks needed...
If that sort of current is needed, us a 7809 regulator.
 
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