[volt-nuts] J511 current regulator diode (FET) temperature coefficient

[Charles P. Steinmetz]

Dave wrote:

>Measuring the voltage drop across R718 which is the 100 ohm 
>collector resistor for TR703, I see a huge change just taking the 
>lid off the meter:
>
>Lid on: 0.45200V after 8 hours powered on
>      *     *     *
>The voltage across this resistor was almost 0.5V at power on from cold.
>
>Are these diodes subject to failing with poor temperature regulation?

They don't have very good temperature regulation to begin 
with.  Vishay specifies the J511 with a tempco of -0.34% per degree 
C.  (If I recall correctly, this is significantly less than some 
other manufacturers claim for the J511 -- but I long ago gave up on 
CRDs, so that's an aged recollection.)

If yours changed from 5 mA cold to 4.25 mA hot, and I've done the 
maths right, that implies a temperature change of 28C if the tempco 
is -0.34% per degree C -- which does not sound at all implausible 
between ambient temperature rise in the meter and self-heating of the 
J511.  Of course, if your J511 is from another manufacturer and has a 
higher tempco, it would take less than a 28C change to produce the 
same shift (I also did not try to guess how much less than 5 mA yours 
sourced when cold).

Note that (like zener diodes) the J5xx series goes through a zero 
tempco point between the J507 (nominal 1.8 mA, +0.08% per C) and J508 
(2.4 mA, -0.05% per C), so you might be able to do better with a 
carefully chosen parallel combination.  Or, if good temperature 
compensation is needed at this location, you may want to build a 
temperature-compensated current source with an LED reference and a 
transistor (that is probably what I'd do), cascoding it with a FET if 
necessary to lower the output conductance.

However, if this particular meter is performing differently than 
others of the same model with the same CRDs, the tempco of the 
current source may not be the source of the problem.  (Or does that 
meter have a fan that is not working properly, or obstructed internal 
ventilation paths?)

Best regards,

Charles