OHVM II - Status

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "David Sharpe by way of Terry Fritz <teslalist-at-qwest-dot-net>" <sccr4us-at-erols-dot-com>

OHVM - Opto-isolated voltmeter

"No Power" OHVM circuit tested successfully today.
Worse case error was <5% without 1% tolerance
resistors, trimming, nulling, or span (amplifier
gain) adjustment.  Only complication is LED input
must see a minimum of 0.6mA (LED must be in "ON"
state) for circuit to work properly.  A solution
to this problem is a wide bandwidth, low offset
instrumentation amplifier with offset/gain
tweekability (as Rick W.'s excellent circuit
recently posted); program in a LED current offset
of 0.6mA with '0' input, then null out VLED offset
on instrumentation side.  Linear span of circuit
(see raw data below) is:

from 0.6  --> 10mA (with +/- 12VDC powering);
possibly 0.6 --> 13mA (with +/- 15VDC powering)

Op-amp I'm using is a National LF412 low offset,
wide bandwidth, high performance FET input, dual
op-amp.

Will probably use a commerical GDT / PWM control
chip driving a +/- 15V regulators to derive
isolated power to run amplifier on HV side,
circuit is relatively simple and biplar power
requirements are less then 20mA, versus using
batteries (and still needing local voltage
regulation for high accuracy...  :^C  )

Raw Data presented as Vin (across 1k resistor)
to Vout.  All measurements made with Fluke 87 III
DVM's within annual calibration cycle.  Note
clearly that linear region is with LED Ifwd > 0.6mA
(article suggested Ifwd should be designed for 1mA
in area of interest), but linear region is clearly
significantly wider then that, without much circuit
heroics.)

Vin            Vout
0.00           11.59
0.10           11.48
0.20           11.54
0.30           11.51
0.40           11.49
0.50            2.17
0.55            1.40
0.60            1.47
0.70            1.53
0.80            1.61
0.90            1.71
1.00            1.84
1.50            2.54
2.00            3.06
2.50            3.50
3.00            3.92
3.50            4.52
4.00            4.92
4.00            5.59
5.00            6.25
5.50            6.85
6.00            7.40
6.50            7.92
7.00            8.42
7.50            8.92
8.00            9.40
8.50            9.89
9.00           10.36
9.50           10.81
10.00          11.24

Regards
Dave Sharpe, TCBOR
Chesterfield, VA. USA