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4QD-TEC: Electronics Circuits Reference Archive
Analogue Opto
Isolator

This is a circuit to use a standard, low quality opto isolator to transfer an
analogue signal with reasonable linearity and without complicated feedback loops
to monitor and linearise it.

The circuit was designed to interface a mains driven thryristor motor
controller to low voltage analogue circuitry. In this application you could
isolate the thyristor controller with a transformer and earth one side of the
motor, but you still have a potentially high voltage situation if there is a
wiring fault or component failure. Also motors and thyristor controllers are
potentially good generators of noise: in a system which used several motor
controllers it was considered essential to isolate the input signal!

Thyristor motor controllers, without lots of setting up and without
tachogenerator feedback can give around 15:1 speed range. This isolator was
certainly linear enough for this sort of application and performed very well in
a production situation.

Production boards

This is another circuit which has been produced and could be produced again
if quantities are sufficient: 30-50 boards would be a suitable initial quantity.

Transmitter

It may be instructive to compare the first part of this circuit with the voltage controlled clock generator.
You may notice a distinct resemblance in overall form.

Current Mirror

Tr1, Tr2 and Tr3 form a precision current mirror (see also Current sources and mirrors - a
voltage applied to Vin causes current to flow through R1 and R2 giving rise to a
'mirror' current which is sinked into Tr3's collector.

Oscillator

Consider C1 (4n7) at first uncharged. The current sinked by the mirror
changes it and the base of Tr5 falls. When the base voltage of Tr5 is near the
positive line, Tr4 is conducting but when it falls to about 9.15v below the +ve
line (voltage determined by D2), Tr5 starts to cut off and Tr6 starts to
conduct. This will turn on Tr9, whose collector voltage is coupled back to Tr6's
base via C3 to cause positive feedback so Tr9 turns hard on, drawing a pulse
through C4 and the opto-coupled diode, D3.

During this pulse, R10 and R11 cause Tr4 to conduct, shorting out C1 and
re-setting the relaxation oscillator.

Receiver

The pulse in the opto coupler's transmitter causes its transistor to conduct.
The original circuit worked very reliably on a cheap coupler such as 4N38. When
the opto conducts, Tr11 turns off, D5 goes reversed biased and C6 charges up
thorough R37, holding the opto coupler on for a defined pulse time. As soon as
C6's current falls too low to maintain conduction, Tr11 turns on again,
discharging C6 via D4,and D5. This discharge path is low impedance so discharges
C6 very quickly. The circuit can therefore work with a very short time between
pulses. Clearly, is the pulse is 90% of the duty cycle, the average voltage on
Van is 95% of the supply voltage.

At low input voltages (into Vin) the oscillator charging current will be low
and the opto pulses widely spaced. Tr11 will be conducting most of the time
therefore and the average output voltage, Van, will be low. As the oscillator's
frequency increases with increasing input voltage, pulses will be more frequent
and the average output voltage will rise.

The voltage on Van is always a fixed pulse width at variable frequency: the
motor controller it was used with had a ramp (as do pretty well all motor
controllers) which averaged out this pulse train so the controller responded
only to the average voltage.

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Page's Author: Richard Torrens

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