Zener diode temperature coefficient versus diode operating current

[IconicPCB]

Is there a " rule of thumb " relationship between zener diode temperature coefficient versus zener diode  current.

I am specifically thinking of bare zener with no temperature compensation.

[amspire]

It is more a matter of testing for optimal zener current rather then using a formula.

There are two different voltage regulating effects that occur in a "zener" diode. Below about 4.7V, the zener effect dominates and it has a temp coefficient of about -3mV/C. About 8V, avalanche breakdown dominates and this has a positive temp coefficient - the size of the coefficient increases with breakdown voltage.

Somewhere in the 5.2V - 6.8V range, you have devices that can be set to near 0mv/C coefficient by finding the optimal current. So for a particular family of zeners, get 5.2V, 5.6V, 6.2V, 6.8V values and attach to a variable current source (ie power supply + resistor + multimeter). Vary the current, and using your finger to raise the temperature initially, look for a current with a zero coefficient. You can even try different base-emitter transistor junctions if you want to instead of the zener diode. The compensation is not perfect. If you pick a zero TC point of say 25C, you can get fairly good regulation over maybe +/-5 degC.

Now zero TC does not mean stability. Ordinary zeners will drift over time due to issues like surface defects and contamination. Zeners are also noise sources, and different zeners will have different amounts of noise. Some of this noise is very low frequency - such as 0.1Hz or 0.01Hz, so you cannot just eliminate it with a big capacitor.

Richard.

[IconicPCB]

Richard,

Thanks for taking time to respond.

I am aware of the two different processes commonly known as zener breakdowns and their intrinsic temperature coefficient profiles.

I am presently looking at the common garden variety 1N4735A diode with and without a series compensating diode ( 1N4148 ).
Over a narrow temperature range uncompensated diode shows up a positive fluctuation of a millivolt perhaps while at the same time same part number compensated with 1N4148 shows a negative fluctuation of perhaps a few millivolts over the same temperature change.

My description is a bit vague since i am using a 4.5digit instrument , have no way of measuring temperature other than a turkey stick electronic thermometer.
Thermometer is extremely bulky compared to the electronic components on the PCB and certainly introduces errors and delays in measurement.

The choice of temperature controlling thermistor was dictated by the content of the "glory box" so no joy on including an on-board thermistor of know properties.
The oven warms up to 40.1C and fluctuates +/-0.1C if the turkey thermometer is to be believed,
I come back to the question of current....The zener is running at about 5mA   and would like to get an idea which way to move the current in order to achieve improved compensation,
Hence the question about the "rule of thumb" or "gut feel".

[amspire]

4.5 digit meter is fine. You use two zeners, let one stabilise for half an hour and measure the difference to the other one on the most sensitive range of the meter. Make sure the first zener's circuit is well insulated from air currents. If your meter has a 10uV resolution, that is better than  2ppm.

Initially, you are looking for adjusting current for zero TC, so just use your finger to raise the zener temp quickly. Adding a separate diode will probably really slow down the settling time as the zener and diode will be at different temperatures.

When you have found the approximate current for zero TC, that is when you can worry about tightening up the temp control a bit. Amazing what you can do with household items and ingenuity.

[IconicPCB]

Thanks Richard.

I shall attempt to draw some general observations leading to a "gut feel".

[amspire]

By the way, a diode makes a perfectly good temperature sensor. The coefficient is -2.2 to -2.3 mV/C. So measure the voltage on a diode range of a multimeter at a known temperature. If it reduces by 22mV in an oven, you know it is about 10 deg above ambient.

[kalel]

By the way, a diode makes a perfectly good temperature sensor. The coefficient is -2.2 to -2.3 mV/C. So measure the voltage on a diode range of a multimeter at a known temperature. If it reduces by 22mV in an oven, you know it is about 10 deg above ambient.

Interesting!

[Kleinstein]

By the way, a diode makes a perfectly good temperature sensor. The coefficient is -2.2 to -2.3 mV/C. So measure the voltage on a diode range of a multimeter at a known temperature. If it reduces by 22mV in an oven, you know it is about 10 deg above ambient.

The TC of a Si PN junction depends on the forward voltage and can be different from the -2.2 to -2.3 mV / K.

It is more like having a linear slope that extrapolates to about 1.21 V at 0 K. So a diode / transistor running at low current and thus low voltage will have a high TC (could reach about -3 mV/K if the room temperature voltage is in the 300 mV range - e.g. an 1N5403 in the µA range). With a rather high forward voltage (e.g. relatively high current for a small diode) and thus maybe 750 mV it would be around -1.5 mV. The more typical case / rule of thumb is 610 mV at 300 K and thus TC =  (610 mV - 1210 mV )/ 300 K = -2 mV/K