[vcf-midatlantic] Apple IIe Voltmeter

[Herb Johnson]

> [I had no way] to precisely measure current. My other bench meters are analog,
 > lowish impedance, and not precise enough to measure voltage drop
 > across a small-value resistor. Additionally, my Simpson 260 and
 > Simpson 160 meters don't agree on the 100 mA scales --
 > I believe the Simpson 160 is correct, but needed a way to figure out.

My experience is that digital voltmeters put too much current load on a 
circuit, when measuring DC current at tens of mA. Put another way: they 
have significant series resistance and change the circuit, and so change 
the current. Your A/D technology also puts a load on the circuit, you 
can read off the schematic or parts to determine that.

But I found my Simpson 260 and other analog meters, put LESS load 
because they generally use 50uA movements (hence 20,000 ohms per volt, 
do the math). As to "figuring out the accuracy".....create a 10mA 
circuit from an accurately-measured battery and an accurately-measured 
resistor. Say 5V and 500 ohms, or whatever - the DVM can measure those 
fine, other resistors give other current sources, and math gives you the 
exact current. Many batteries can generally supply 10mA stable for 
minutes and hours or more, and your DVM can monitor their voltage anyway.

Then make those circuits with the Simpsons and recalibrate the scales, 
and create conversion factors (or fitness for use choice) for each meter 
AND for each scale. YOu can use the same apparatus to calibrate your 
Apple II "meter".

I prefer analog meters when making "runs" of comparative measurements. 
It's easy to see if one reading is "more" or "less" than another, the 
needle is literally in different positions.

I've found variations in current measurements among several Simpson 260 
meters (and among their scales) I've acquired. My guess is that some of 
the shunts were "cooked" by excessive current, or corroded by time and 
tide. I have not yet gone to the trouble of diagnosing them, I simply 
use the meters that have the best readings through the procedures noted. 
They are cheap today.

Why? Given the "digital is better" bias these days, only a few people 
call out the Simpson 260 as a valued instrument - Dave McGuire for 
instance, in that email list we used to use.

Jonathan, if you determine a cause for your analog meter's apparent 
errors, let me know.

Herb Johnson
old-school

-- 
Herbert R. Johnson,  New Jersey USA
http://www.retrotechnology.com