Temperature Switch - HP A Series Operating And Service Manual

NOTE
0.75V below the crowbar trip voltage setting.
To prevent transients from falsely tripping
the crowbar, the trip voltage should be set
higher than the power supply output by at
least 0.75Vdc.
To set the crowbar trip voltage, perform the following pro¬
cedures.
NOTE
Do not connect a load to the supply when
setting the crowbar trip voltage.
1. Turn the CROWBAR ADJUST (screwdriver con-
TOl) fully clockwise (trip voltage set at maximum).
2 Measure the power supply output voltage and set
the output voltage (with the VOLTAGE ADJUST screw¬
driver control) for the desired crowbar trip voltage. If the
desired trip voltage is beyond the maximum output voltage
that can be attained by adjusting VOLTAGE ADJUST, per¬
form the next step. If the supply can be set to the desired
crowbar trip voltage, go on to step 4.
3. If the output of the supply cannot be set to the
desired crowbar trip voltage;
Turn
VOLTAGE ADJUST to minimum
setting.
b. Turn off the supply and temporarily
shunt a 12k, 5%. %W resistor across
resistor R5 on the main power supply
board.
c. Turn on the supply and set power supply
output to desired crowbar trip voltage.
-CAUTION *-
Do not set output voltage more than 2V
above the maximum output voltage rat¬
ing of the supply.
Resetting Crowbar
If the crowbar trips during normal operation (supply
output goes to near zero), remove the input power to the
supply and then disconnect any load from the power sup¬
ply. Re-apply input power and determine if the crowbar
again trips. If it does, there is a problem in the power sup¬
ply Check the line fuse and the temperature switch, TS1
(if provided) for open. If either of these devices are open,
refer to the operating and service manual for troubleshoot¬
ing procedures that can be used to isolate the cause of the
overvoltage condition. If the supply does not crowbar when
the load is removed, check the load circuit or the trip point
setting.
Temperature Switch
A temperature switch, TS1, is included on some sup¬
plies (Figure A-2, Note 6) to shut down the unit in case of
internal failures where the current through the SCR crowbar
path could result in excessive dissipation. The power leads
of the device will open if the current flow throughjihe heat¬
er leads causes the temperature to reach 115 ± 1.5 C. t
takes between 4 to 40 seconds (depending mainly on the
rating of the supply in question) after the SCR fires for the
device to reach the activation temperature. Once a temper¬
ature switch has opened, it must be replaced as described m
the next paragraph.
The temperature switch is polarity sensitive and its
power leads must be connected as shown on Figure A-2,
Note 6 The side with the ceramic insulator must be con¬
nected to the positive side of the rectifier (marked with a
plated polarity dot on the P.C. board). The other power
lead must be connected to current sampling resistor.
The heater leads are not polarity dependent and can be
connected in any manner. When soldering in any of the
temperature switch leads, apply heat to the leads for as shor
a period as possible as it requires only 115°C for the switch
to open. The power leads are particularly susceptible to
hoat anH the caution note below must be observed.
CAUTION
4. Slowly turn the CROWBAR ADJUST counterclock¬
wise until the crowbar trips (output falls towards 0 volt).
5.
The crowbar remains activated and the output
shorted until the supply is turned off. To reset the crowbar,
turn the supply off. Before turning the supply back on, re¬
move the shunt resistor installed in step 3 (if necessary.) and
turn VOLTAGE ADJUST to minimum. Turn the supply
back on and set the power supply output voltage at least
When soldering in the power leads of a
temperature switch, connect an alligator
clip to each lead to help dissipate heat.
The switch will open at approximately •
115°C, so solder the leads as. quickly as
possible.
A-2