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 volts d. c. 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
ÒR3Ó 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
amperes; for the RS-35, it is 37 amperes. The short circuit limit is set by R1
in parallel with R1x at 2.5 amperes in the RS-35. Over-voltage in all models is
limited to 16.5 volts 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 volt d. c.
1 Voltmeter,
0-20 volts
1 Ammeter,
0-30-50 ampere
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-volt d. c.
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 the toggle switch in the low current position, turn on your power supply.
The meters should show 12+ volts and tiny current draw. Advance the
potentiometer slowly while observing the ammeter. Maximum current in low range
will be about 10 amperes. If all is well, back off the potentiometer and switch
to high range. Load to rating of your power supply. If all is well, back off
the potentiometer to the continuous duty rating of the supply (16 amperes for
an Astron RS-20; 25 amperes for an RS-35). Let it cook for a while. When the
power supply heat sink is hot, shut off the A. C. 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.
Select ÒLoad_Bank_2Ó from the Technical Corner sidebar for a schematic