PWM. When the voltage rises above 13V, pin 7 of U4 latches low, and thus Q5 will turn off
if permitted by diodes CR12, CR14, CR16 and CR17, controlled respectively by the Remote
Turn-on, Thermostat protection, Overvoltage protection and Turn-on delay circuits.
(3) Overvoltage Protection
The overvoltage trip setpoint is set by potentiometer R8. The output voltage is sensed by
R26 and R27. The amplifier has positive feedback latching, which requires removal of AC
power for resetting after the OVP has been activated. C23 prevents noise tripping of the
latch. Once Pin 8 of U4 latches high, Q5 is turned on and thus the control pin is pulled low,
causing shutdown of the modulator. Also, Q8 is fired and the output of the power supply is
crowbarred. The OVP LED is lighted through J3/8 for the 600W/1KW and through J3/12
for the 2KW/2.5KW and 5KW.
(4) Mode Detect
R38 samples the voltage at the output of U6. In the 600W/1KW and the 2/2.5KW, U4
compares the output of the voltage channel to GND (zero) whereas in the 5KW the voltage
channel is compared to the current channel through R39. When the voltage channel is in
control the output of U4 at pin 14 is high and that lights the VOLTAGE MODE LED
through R41. If the Current Channel is in control the output of U4 at pin 14 is low and the
CURRENT MODE LED lights through R40.
(5) Remote Turn-on
This feature allows the user to control the power supply from a remote location with either
12-24 Vdc or 24-115 Vac or a dry contact closure. U3 (U2 for the 5KW) provides isolation
from the power supply ground. With the internal LED of U3 (U2) not activated, Q4 is
turned on, which shuts down the modulator through CR12 and Q5. Any event that activates
the internal LED, allows the modulator to function and thus let power pass to the power
supply output. The internal LED of U3 (U2) can be activated in two ways: (1) Application
of external voltage (AC or DC) across TB1/J1-15 and TB1/J1-16; (2) Dry contact closure
across TB1/J1-16 and TB1/J1-17.
With dry contact closure, the power is supplied by the relaxation oscillator consisting of T1,
Q3 and associated components. Each time that Q3 conducts, it discharges C18 through the
primary of T1, inducing a voltage in its secondary winding. This voltage is rectified by CR8
and filtered by C15. The Remote Turn On may be disabled by connecting a jumper from
cathode of CR11 to ground.
4.5.2 OVER CURRENT PROTECTION
A measure of transformer primary current and switch current is monitored by the current
transformer(s) on the inverter Bd. The signal is rectified and filtered producing an
unregulated dc voltage that is used for peak current limiting.
The rectified voltage from the inverter is fed into the inverting terminal of a comparator and
is compared to a reference that is attenuated down from the 5Vdc of the PWM. The
reference is set by a potentiometer. This circuit is activated when the switch current in the
inverter exceeds 22 Amps as in fault conditions like saturation of the power transformer or
short circuit on the inverter. Pins 5 and 8 (Pin 9 for 1KW) of the PWM are pulled low
shutting down the PWM. Peak Current Limiting is not available on the 20-337-XXX A100
Bd. of the 600W/1KW units.
83-473-000 Revision J
4 - 3
Theory of Operation
