B&K Precision 1655A - Manual - Manual (Page 31)

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Refer to the separately supplied schematic diagram while
reading the following circuit descriptions.
VARIABLE VOLTAGE CIRCUIT
Input power is applied through the POWER ON switch to
variable auto-transformer TI. The output of TI is set with the AC
VOLTS control from 0 to 150 volts. The selected amount of
voltage is applied across isolation transformer T2 to the
ISOLATED OUTPUT receptacle(s).
Model 1653A is protected by input fuse F1, while Model
1655A uses circuit breaker CB1. This limits the maximum input
current. Additional protection for the auto-transformer is offered
by fuse F2, which limits the maximum output current at low
voltages which would not open the protective device in the input
circuit. In Model 1655A, this is a 4-amp fuse on the rear panel.
In Model 1653A, a 3-amp internal fuse is used.
METERING CIRCUIT
Since the output of the unit is ac, the meter is connected across
a bridge rectifier to provide the dc necessary for meter operation.
The rectifier circuit senses the average level, while the meter is
calibrated to read the equivalent rms value of a sine wave voltage
the back-to-back diodes connected directly across the meter limit
maximum voltage and thus offer overrange protection.
When the VOLTS function is selected, the voltage across the
ISOLATED OUTPUT is applied to the meter circuit. The meter
is shunted by the VOLTAGE CAL trim pot which calibrates the
voltage scale.
A 1-ohm current sensing resistor (two series 0.5-ohm in Model
1655A) develops a voltage proportional to output current. In the
AMPS function, this voltage is applied across the meter circuit.
The meter is now shunted by the AMPS CAL trim pot which
calibrates the 2 amp current scale. In Model 1655A, the 4 amp
scale is selected by sensing the voltage developed across only one
of the 0.5-ohm resistors.
When the LEAKAGE function is selected (Model 1655A only)
the current from the leakage probe is applied to the meter circuit.
Diodes D4 and D5 are reverse biased below 500 µA for expanded
scale feature. At higher leakage current, the diodes become
forward biased and shunt part of the current to compress the
meter scale.
SOLDERING IRON TEMERATURE CONTROL CIRCUIT
(Model 1655A Only) Input power is applied through triac TR1
to the soldering iron outlet on the rear panel. Breakdown diode
D1 does not gate the triac into conduction until its 30 volt
threshold is reached. Thus, the triac does not conduct for the
entire input cycle. SOLDER TEMP control R2 forms part of an
RC phase shift network which varies the phase of the trigger
voltage in relation to the anode voltage of the triac. This varies
the non-conduction period, which adjusts the soldering iron
temperature. The circuit is automatically reset each time the ac
passes through zero.
CIRCUIT DESCRIPTION