zener BJT regulator in cold temperature?

[treez]

Hello,
Is the Zener BJT linear regulator in figure 3 more likely to drop out in cold weather or hot weather?
https://www.bristolwatch.com/ele/zener_power_supply.htm
I believe in cold ambient it would be more likely to drop out but do you agree?

 

[Audioguru]

Ambient temperature changes the base-emitter voltage of the transistor a little and changes the voltage of the zener diode a little.
They work well between -55 degrees C or less to 150 degrees C or more. Of course if they have power dissipation then they heat themselves higher than the ambient temperature.
Look at their datasheets to see.

 

[treez]

the datasheet just says the vbe decreases by 2mV/degC.
That I assume, would mean more current at higher temperature?

 

[Audioguru]

the datasheet just says the vbe decreases by 2mV/degC.
That I assume, would mean more current at higher temperature?

No.
When the transistor temperature increases then its Vbe decreases 2mV/degC so the output voltage also decreases 2mV/degC if the temperature of the zener diode does not change. Then Ohm's Law says that since the voltage across the load decreases a little then the load current also decreases a little.

You might be able to select a zener diode voltage that has the opposite temperature coefficient to the Vbe of the transistor so that they cancel and the output voltage does not change when the temperature changes.

 

[treez]

Though if VB stays the same with increasing temperature, and vbe reduces with increasing temperature, then vout (Vemitter) actually increases?
there are no statements about what happens to Hfe when temperature increases:
https://www.onsemi.com/pub_link/Collateral/2N5550-D.PDF

graph fig 3 on page 3 shows that I(c) increases at high temperatures for a given vbe........thus ic/ib increases at higher temperature, thus hfe increases at high temperature, therefore, the zener/bjt regulator must feature a base resistor low enough in ohmic value that enough base current can flow, at low temperatures, to allow enough collector current to flow to supply the load, at low temperatures.

Thus I conclude that the "danger temperature" for the zener /BJT regulator is low temperature, because at low temperatures, if your base resistor is too high in ohmic value, your output voltage may drop out?....is this correct?

 

[dick_freebird]

Zener and forward diode tempcos can be made to cancel
(first order) but you have to beware things like high temp
zener leakage and low temp beta degradation as actors
that won't be noticed in a commercial temp range design,
but could make accuracy degradation accelerate further
from known territory.

I would not expect a dropout unless the BJT was operated
at a pretty low current density (then current starved times
degraded beta, might be on the wrong side of load-point).

Many commercial devices do not test for extreme-temp
attributes at all, and you might not be told just how much
scatter there is in the population, or how far out, in the
parameters that matter.

 

[Audioguru]

The simple circuit should be designed so that the hFE of the transistor and its resulting increased base current does not starve the zener diode from having enough current. Then the change in hFE with temperature changes makes little difference in the output voltage.

 

[BradtheRad]

Wikipedia discusses zener temperature coefficients in its article:

https://en.wikipedia.org/wiki/Zener_diode

As a rule...
Below 5V, zeners operate on the zener effect, and exhibit negative TC.
Above 5V, they operate on avalanche effect and exhibit positive TC.

"Thus the 5.6 V diode is useful in temperature-critical applications."