A Dummy
Load Bank for Your DC Supply
Jim
Hargrave, W5IFP and Larry Higgins, W5EX
Tired of using automotive headlights to load your 13.8 vdc
power supply? Not a good idea, because the initial load is near zero ohms,
which may trip your over-current protector. There is a better way, even
if you are now using large power resistors for the job.
This circuit uses transistors to gently increase the load
from nothing to the full dissipation capability of the transistors - in this
case 2N3055's. In its present form, the circuit is capable of cranking up to
30+ amperes load. Add another 2N3055 and 1 ohm resistor and you can test a
50+-amp supply. Junk box parts will work just fine.
Application
One of the most important power supply tests is that of the
over-current shutdown circuitry. With this device you can set the exact current
threshold. In AstronŠ supplies,
the resistors labeled
R3 and "Rx" set this threshold. As these supplies age,
they tend to shut down prematurely. When you are certain that this condition is
not due to a weak filter capacitor, weak pass and pass driver transistors or
defective base resistors, you may be able to adjust "Rx "
such that the current fold-back occurs properly. The shutdown value for an
Astron RS-20 is 22-23 A; for the RS-35, it is 37 amperes. The short circuit
limit is set by R1 in parallel with R1x - at 2.5 A in the RS-35. Over-voltage
in all models is limited to 16.5 v by the crowbar circuit. An SCR provides the
crowbar. Momentarily short R7 to check the voltage crowbar circuitry. This will
drive the output voltage to about 17. 1,
2
Parts List
1 Heat
sink capable of mounting a pair of 2N3055's.
1 Chassis,
large enough to mount parts
1 Fan,
12 vdc
1 Voltmeter,
0-20 volts
1 Ammeter,
0-30-50 amp
1 Resistor
1K 1 watt
1 Resistor,
27 ohm, 1 watt
2 Resistor,
1 ohm, 100 watt
1 Potentiometer
1K, 1 watt, panel mount
1 Diode,
1N4000
1 Switch,
3 amp, SPDT
2 Transistor,
2N3055
2 Mounting
kits for 2N3055
1 Barrier
strip with #10 screws
Assemble the unit with lots of space for airflow Use #10
wire for the 12-vdc lines #18 for the emitter switching circuitry. . Set up the
fan to blow on the heat sink and the 1-ohm resistors..
Operation
Use # 10 wire (We use speaker wire) to hook to your power
supply. Observe polarity. With toggle switch in the low current position, turn
on your power supply. The meters should show 12+ volts and tiny current draw.
Advance the pot slowly while observing the ammeter. Max current in low range
will be about 10 amp. If all is well, back off the pot and switch to high
range. Load to rating of your power supply. If all's well, back off the pot to
the continuous duty rating of the supply (16 amp for an Astron RS-20; 25 amp
for an RS-35). Let it cook for a while. When the power supply heat sink is hot,
shut off the AC and immediately turn it back on. The power supply must not
stumble, but must immediately restore the rated continuous duty current and
voltage. Repeat this test several times. Remove power, take a break while
things cool, return and turn the supply on. Again, there must be no stumble and
no shutdown.
Conclusion
With this simple tool, you will be
able to check your power supplies such that failure at a remote site can be
much less likely. Parts aging in an old supply can be detected such that early
replacement may prevent inconvenient failure.