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

[David C. Partridge]

Yes the TC of these devices is awful.

I need to check against other meters, but I'm fairly sure this will turn out to be a "feechur" of the circuit design.

So I'm thinking in terms of replacing this with something better, but what?  I'd rather not (being lazy) try to design a good zero TC bipolar 4.7mA current source for this.  

Do you have suggestions - ideally with a low part count - the LT3092 looks attractive, but the distributors don't seem to stock them, while the LM334 needs an extra diode and resistor.  

Thanks
Dave
-----Original Message-----
From: volt-nuts-bounces at febo.com [mailto:volt-nuts-bounces at febo.com] On Behalf Of Charles P. Steinmetz
Sent: 05 July 2012 03:34
To: Discussion of precise voltage measurement
Subject: Re: [volt-nuts] J511 current regulator diode (FET) temperature coefficient

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





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