# 12v to 9v using zener

#### 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..

current draw for 555 clock is < 15ma

#### AlbertHall

Joined Jun 4, 2014
11,504
What is the white rectangle?
What are the blue link wires for - they don't seem to connect anything.

#### 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
1,917
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.

#### AlbertHall

Joined Jun 4, 2014
11,504
With only a 10 ohm resistor in series with it,
Looks like 100 ohms to me.

Joined Feb 20, 2016
3,909
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.

#### crutschow

Joined Mar 14, 2008
27,718
You could try a 3.3v Zener in series with the 12v supply.

#### AlbertHall

Joined Jun 4, 2014
11,504
Yes. Smoke ensues! Check that it is 100R, not 10R as marked on your diagram.
Oh, yes! I was seeing the Ω as a 0.

#### AnalogKid

Joined Aug 1, 2013
9,353
Why is running the 555 circuit on 12 V such a big problem that it requires this? what is the problem?

ak

#### Boutwell156

Joined Mar 12, 2021
1
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.

Joined Feb 20, 2016
3,909
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.

#### Yaakov

Joined Jan 27, 2019
3,492
Why is running the 555 circuit on 12 V such a big problem that it requires this? what is the problem?

ak
Yes, according to the datasheet, maximum supply is 16V which is plenty of headroom.