Re: E-Meter autoscan

[Lee Hart <leeahart@earthlink.net>]

David G Leatham wrote:
> If you are refering to the (was Cruising Equipment) Link 10...

Yes, that's it. The Cruising Equipment E-meter and the Heart Interface
Link 10 are one and the same.

> could add another E-Meter or just another meter so that the E-Meter
> would not have to autoscan.  I wonder if a simple resistor and a
> zener diode could be used to make an expanded scale analog meter
> suitable for those high volt EV's using a millamp meter and redoing
> the numbers by hand, using the E-Meter as reference to calibrate the
> handbuild.

Yes, that works exactly as described. An expanded-scale voltmeter is a
milliammeter, resistor, and zener diode, all in series.

All by itself, the milliammeter would be something like 0 to 0.1v full
scale. The series resistor scales this up. For example, a 0 to 1ma meter
is 1/1ma = 1/0.001amp = 1000 ohms per volt. So a 0 to 100 volt scale
takes a 100v x 1000 ohms/volt = 100,000 ohm (100k) series resistor. To
be strictly correct, this is the total resistance including the
resistance of the meter itself. Use a small potentiometer in series and
you can calibrate it.

A zener diode in series subtracts its zener voltage from all readings. A
50 volt zener in series with the above 0-100v meter makes it a 50-150v
meter. Note that zener diodes are far from ideal at low currents; the
actual zener voltage will deviate substantially from the "rated" zener
voltage. You will probably have to hand-calibrate the scale.

If the meter you use is very low current full scale, like 50 microamps,
then the resistor value gets very large, and you may have trouble with
leakage currents. Fingerprints, dirt, or moisture on the surface of the
zener or resistor will affect the readings. If the zener is in a glass
case, then light shining on it will affect the reading.

> a battery monitor that used an inductive pickup for amps.

Inductive pickups are used for measuring AC current, but don't work for
DC. However, there are Hall-effect sensors that measure the magnetic
field generated by the DC current in a wire. They are generally more
expensive, and have trouble maintaining accuracy over a large range of
current.
-- 
Lee A. Hart                Ring the bells that still can ring
814 8th Ave. N.            Forget your perfect offering
Sartell, MN 56377 USA      There is a crack in everything
leeahart_at_earthlink.net  That's how the light gets in - Leonard Cohen