Zener diode Question

ian field

Joined Oct 27, 2012
6,539
I've always thought that zeners were terrible as voltage references due to their temperature drift sensitivity. How come a zener and an ordinary 1N400x in series tend to cancel their temp drift?
Sometimes it doesn't - the zeners can be grouped as either PTC or NTC, the break even point is somewhere around 5.6V.

The Vf of a regular diode becomes less as temperature rises - if that's the same sense as the zener you put it in series with, it doesn't improve anything.

The old NS application notes sometimes have examples of diode chains for temperature compensation, but I don't remember seeing much in the way of technical explanation.

Years ago there was a "favourite circuit" doing the rounds for a SLA battery charger based on the ST L200 linear regulator - most had a chain of diodes to set a particular temperature compensated charge current characteristic.
 

ian field

Joined Oct 27, 2012
6,539
If i want a 5V regulated output from a Zener diode from a VS of 12V , how do i calculate it ?

I assume i have to drop 7V on the series resistor to have 5V on the zener. But if the breakdown voltage of my zener is 5.6 then this will not be sufficient, correct ?

- Do i have to put a Zener diode of 4,7 breakdown voltage then ?
- Is the zener breakdown region available in any voltage range (untill it reaches it's limit) For example if i put 8 V on a Zener of 5,6 breakdown will it also have the self regulating function but then on 8 Volts ?

Thx Mike
The old standard was a 5.6V zener providing the reference for an emitter follower series pass transistor - you lose about 0.6 - 0.7V ish on the B/E junction, so it comes out at pretty close to 5.0V.

The lowest tempco is somewhere around 5.1 - 5.6V - I can't remember which.
 
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