On Mon, 25 Sep 2017 15:35:28 -0500, "Dave M"
wrote:

T i m wrote:
On Fri, 22 Sep 2017 10:53:54 -0500, "Dave M"
wrote:

snip

I forgot to mention that for the zener solution, you really need to
select as low power a zener that you can find, such as 400mW or
lower.
The 1N4104 is a 10V, 250mW zener, with a test current of 1uA, and a
max current of 25ma, well within your requirements. In stock at
Mouser.com.

Cheers, Dave, I'll have a look.

As has been mentioned elsewhere, if we used a lower voltage zener (and
as high a tolerance as can be found in the right spec and the right
price *because*) to set the voltage on the -Ve leg of the meter and
then tweak the voltage up via resistors (variable initially) to give a
FSD at 15V then that should be sufficient?

I can't help feeling I've missed something with that solution though,
like the linearity ... ? ;-(


I gave the wrong zener current for the 1N4104 . I just looked at Mouser's
basic info for the diode, but neglected to follow up and look at the actual
datasheet. According to the datasheet, the zener test current is actually
250uA, whereas Mouser states it as 1uA. Quite a difference, but after
looking at the datasheets for some other 10V/200mW zeners, (datasheets that
had actual graphs for Iz vs. Vz) I see that they actually start conducting
well below 1uA, so I feel confident that the 1N4104 will do the job for you.

Thanks for the follow up Dave.

All the other zeners that I found are SMD devices, so I doubt that you'll be
interested in any of those.

Well, I have worked with SMD's but a wired device and could use on at
a pinch (soldered to the side of the meter contact etc) bit a wired
device is probably easier.

With that Zener in the circuit, I doubt that you'll have any noticible
nonlinearity; certainly not with the meter scale only having a 1-volt
resolution. Forget about the accuracy on the low end as well (except at the
volt marks), because you can't resolve to a basic meter accuracy
specification of +-3% of FSD.

Understood. I was thinking more on the idea of when using a lower
voltage zener to be well clear of the lower voltage levels being
considered and then the potential divider to set the FSD when the
input is 15V?

And also, wouldn't the impact of the battery voltage changing from say
10.5 to 14.4V be lessened (re the series current limiting / biasing
resistor) if the voltage difference between the reference and the
supply is greater?

That seemed like the best of all worlds (for the KISS solution)?

Cheers, T i m