Revised August, 2004
10 TEGAM WAY • GENEVA, OHIO 44041 • 440-466-6100 • FAX 440-466-6110
•
4-1
PRINCIPLES OF OPERATION
SECTION 4
PRINCIPLES OF OPERATION
The power supply uses a transformer with two secondary
windings. The low-voltage winding is a center-tapped
winding. Diodes D9 and D12 provide rectified + power
filtered by C26 and regulated at +6 V by D7. Diodes D10 and
D11 provide rectified - power, filtered by C25 and regulated
at -6 V by D8. The high-voltage winding on T-1 charges C23
through D7 and C24 through D8. Since these two capacitors
are connected in series, this circuit functions as a voltage
doubler to provide a high voltage nearly equal to twice the
peak voltage of the high voltage winding. This high voltage is
at least 650 V at low line in order to provide 500 V test
voltage with 1 IDA of I x R drop across R37.
R37, a series resistor, is followed by Q1, a high-voltage
shunt regulator, to regulate the high voltage supply at the
selected value. R38 provides protection for Q1 from excess
current caused by external voltages. R6-R9 operate as a
four-section voltage divider, dividing the voltage from U2, a
2.5 V precision voltage reference, to 0.25, 0.5; 1.25, and 2.5
V. The voltage selected by S1 A, the ‘TEST VOLTAGE’
selection switch, is fed into pin 9 of U1, a quad operational
amplifier. The high voltage output is divided by R3 and R4 to
1/200 of the high voltage output. This divided voltage is fed
to pin 10 of U1, where it is compared with the selected
reference voltage. If the divided output is higher than the
reference, the output (pin 8) of U1 goes more positive,
increasing the base current drive to Q1. Since this is a shunt
regulator, the output high voltage decreases, until an
equilibrium point is reached. Thus, the high voltage is
regulated at the selected value.
When a resistor is not being tested, S3 (the ‘TEST’ switch) is
closed, shorting out the high voltage. This also provides
added safety in the event that the external circuit under test
contains capacitors. If so, they will be charged during ‘TEST’,
but discharged automatically as soon as S3 is released.
