On Mon, 01 Dec 2008 16:01:58 -0500, Bryce
put finger to keyboard and composed:
Franc Zabkar wrote:
On Sat, 29 Nov 2008 11:12:45 -0500, Bryce
put finger to keyboard and composed:
My v7a vtvm (built by a guy who looks like me but a lot younger)
uses a 1.5v battery for resistance measurements. I'm tempted
to replace it with a zener reference. Looks easy: the battery
is grounded on neg side, so is one side of the 6.3vac.
I guess we didn't have zeners fifty years ago. Dunno why Heath
didn't use several 1n34's in series (I think we DID have those!)
- thermal stability I suppose.
Is there some reason that a zener powered from rectified filament
voltage won't work here?
I was curious, so I had a look at the manual:
http://bama.edebris.com/manuals/heath/v7a
Here is the circuit:
http://bama.edebris.com/download/heath/v7a/v7aschem.gif
It seems that you first set the FSD by selecting your particular ohms
range (with no resistor connected) and tweaking the OHMS ADJUST pot.
The meter should then reflect the OC voltage of the 1.5V battery. If
you then measure a resistor whose value is equal to the internal range
setting resistance, the meter reading should be exactly mid scale.
However, the lowest range setting resistor is 9.1 ohms (instead of
10R) which suggests to me that the designer may have allowed 0.9 ohms
for the ESR of a typical battery of that time. If you were to now use
a precision 1.5V supply with an ESR of zero ohms, then your scale
would read 10 ohms for a 9.1 ohm resistor.
- Franc Zabkar
Half-scale is 10 ohms and I agree with your analysis. I have always
thought of an aging battery as having a larger than usual internal
resistance; i.e., open circuit voltage may still be about nominal
but the output voltages collapses under load. Doesn't that mean
this sort of ohmmeter is inherently inaccurate without a "nominal"
battery regardless of the ohms adjust setting? Not a big error
for an analog readout though.
Bryce
I've never thought about multimeters much until your post, but it does
seem to me that anything that relies on an unregulated battery supply
cannot possibly lay claims to any sort of precision.
In your case I would try a precision voltage reference for the 1.5V
supply and a 10R 1% resistor in the lowest ohms range.
- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.