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.




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 Astron 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 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.




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.




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 Load_Bank_2 from the Technical Corner sidebar for a schematic


1 Service Manual for RM-35A, RM-35M, RS-35A, RS -35M.  1991 Astron Corporation, 8 Autry, Irvine, CA 92718

2 Telcon, Tech Service, Astron Corp, 9/09