Simple way to adjust the zener voltage

Thread Starter

iONic

Joined Nov 16, 2007
1,650
I thought I had asked this question before but can't seem to find it...was probably in someone elses thread.

Something like this?

 

BrainFog

Joined Jan 24, 2011
122
I am still inexperienced. Logically I would say it looks like it would work. I am however unsure if the rectifier diode with have an effect on the operation of the zener.

One idea I am interested in having a go at, which would allow you to use them at higher currents, is using a zener to control a transistor. If done correctly you could adjust any zener to any voltage.

I am curious to hear what others think.
 

Thread Starter

iONic

Joined Nov 16, 2007
1,650
I just tried a 5.6V Zener in series with a 1N4002 and got 6.3V. So it does work and is simple... one extra part. Not saying it's the best solution for any application but it is functional.

Sgt. your "Rubber Zener" sounds more "flexible" than just adding a diode, but also might require more parts. I'm interested in seeing a circuit!
 

Audioguru

Joined Dec 20, 2007
11,249
A "rubber diode" circuit changes its voltage when the temperature changes which is good in an audio amplifier when the circuit's transistor is bolted to the heatsink of the driver or output transistors so it keeps their current constant.

The diode in series with the zener diode also changes its voltage when the temperature or current changes.
 

#12

Joined Nov 30, 2010
18,210
Just wanted to drop in and say thanks. I met this circuit about 35 years ago and never thought of using it as an adjustable zener diode.

I learn something here almost every day.
 

t_n_k

Joined Mar 6, 2009
5,455
With a bit of luck you might even have the situation where the temperature coefficient of zener voltage just cancels the silicon diode forward drop temperature coefficient of -2.5mV per °C.

Interestingly a 5.6V silicon zener has almost zero temp coefficient of zener voltage. Apparently the avalanche effect plays some part in this. According to Wikipedia for zener operation above 5.6V the zener voltage has a positive TC.

This checks out with the attached data ...
 

Attachments

Audioguru

Joined Dec 20, 2007
11,249
Did you notice that the temperature coefficient list from Fairchild varies for the 13V and 15V zener diodes? Because Fairchild wrongly copied the list from Philips who invented the BZX series of zener diodes. I don't have Philips (or NXP Semi their new name) datasheet anymore but the Motorola datasheet copied Philips correctly.
 
Top