VITALAB MICRO & MICROLAB 200 THEORY OF OPERATION
VITAL SCIENTIFIC SERVICE MANUAL
2.13
switching a resistor parallel to R20 and P2. Now the reference
voltage (+/- 6V) is selected which corresponds to 30 C. This
voltage can be adjusted by means of P3.
If the 'T-SET1' signal is active, FET switch IC3B is closed,
switching a resistor parallel to R20 and P2. Now the reference
voltage (+/- 4V) is selected which corresponds to 25 C. This
voltage can be adjusted by means of P4.
If both the 'T-SET0' and 'T-SET1' signals are active the
'TEMP-OFF' (temperature off) signal is active. IC5 will amplify
the difference between the output voltage of IC6 (actual
temperature) and the selected reference voltage (required
temperature) in such a way, that 10V at its output corresponds
to 0.1 C temperature difference between the actual and the
selected temperature.
Dual followers.
The output signal from the differential amplifier, which can be
negative as well positive, depending whether heating or cooling
is required, is fed directly to the inverting input of one of
the followers and to the non-inverting input of the other
follower. The two outputs are connected together via diode's
and connected to the switched power supply.
This signal which is connected to the switched power supply
(built-up around a L296) varies between 0V and -15 Volts. When
the input signal is 0 Volts (measured and selected temperature
are exactly the same) the output voltage of the power supply is
also 0 Volts. When the difference in the measured and selected
temperature is more than 0.1 C. the input voltage will be the
supply voltage (-13 to -15 Volts) and the output voltage of the
switched power supply will be maximum 1.0 Volts. This maximum
voltage is chosen for current limiting through the Peltier
elements.
If the output voltage from the two followers is less than -10
Volts the output of comparator IC4D will go high, and via
opto-coupler OC1A the signal 'temperature ok' is high, meaning
that the measured temperature is within 0.1 C. The
microprocessor can monitor this signal.
The two outputs of the followers, before they are connected
together by means of two diodes are switched trough by means of
Summary of Contents for Vitalab Micro
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