[Heathkit] meter protection

[Mike McCarty]

Dennis Berry wrote:
> I have seen on several websites that advocate putting the back to
> back diodes on a meter, which I am not rejecting as wrong, but I do
> have a question.
> 
> As I recall most of the Heath meters are about 1200 ohms and 200 uA
> movements.  Wouldn't this indicate full meter movement is at 0.24V?

Yes.

> Do diodes that conduct up to say 0.5V or 0.6V still protect the
> meter?  I know it's better than a flashover voltage, but is 0.5V
> enough to still damage the meter?

Yes. The voltage across the diode won't get to much over 0.5 to 0.7V.
That is only 2x the meter movement limit. It might be enough to bend
the needle. It's not likely to be enough to burn out the winding on
this meter.

The issue I have with that is it affects meter accuracy.

Diodes don't wait until 0.5V and then start to conduct. The current
through them is related to the voltage across them exponentially.
So, they act as a non linear resistance shunt.

> Just trying to learn and understand, not pick on the method or Cal.

It's commonly done, and I've seen it in commercial equipment. It
affects meter accuracy. If the meter is a fairly "large" one for
high current and has very low internal resistance this isn't apparent.
If the meter has a relatively high voltage drop (like this one) it
is sometimes noticeable.

For an example of commercial equipment with such an arrangement,
see the Sprague TO-6.

In order to reduce the effect of the diode on accuracy, some suggest
putting two diodes in series, the idea being that conduction is not
significant until about 1.0 to 1.5V. That certainly would reduce
the affect on accuracy, but it also means that the voltage on the
meter may rise to 2V or so, which is likely enough to burn it out.

YMMV

Mike
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