I salvaged an old power supply from a piece of abandoned equipment. It is a linear power supply that is rated for 24VDC at 4.8A (http://www.mouser.com/ds/2/373/sl%20power_linears_ds-329280.pdf). I wanted to use it at work at my bench for occasionally testing various items. After adding a switch, indicator light (power "on"), alligator clip leads, and a fuse holder on the secondary of the 120VAC-24VDC transformer I tried powering on a 24VDC servo motor. Did not work. I disconnected the motor and measured the output voltage - 7VDC. Then I began the process of trying to figure out went wrong. This power supply uses a LM723 voltage regulator. I replaced the regulator, powered it "on", and then measured the output voltage - 24VDC. With the proper voltage I tried a load using a 20 ohm resistor. With a voltage meter attached I powered it "on" and the voltage immediately dropped from 24 to nearly 0 volts. Checked the 2A fuse and it had blown. Tried this again with a 150 Ohm resistor and it worked, fuse held. I checked the circuit board and I noticed a variable resistor connected to terminal #2 (current limit) on the LM723. Reading the specs it states:

"The LM723/LM723C is a voltage regulator designed primarily for series regulator applications. By itself, it will supply output currents up to 150 mA but external transistors can be added to provide any desired load current."

There are three 2N3055 power transistors in this circuit which I assume gives it the capability to output the 4.8A it is rated for. So is it possible that this adjustable current limit resistor is set to limit output current well below that? For some reason that doesn't make sense but everything else I have done to troubleshoot this issue has led nowhere. Any help is greatly appreciated. Thanks.


A malfunction at the junction
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Dwayne