I have a cheap bench power supply model 303D Arksen, 30V, 5A which hasn't been used much. It was fine until today I noticed I have no current when I select high current mode. In low current mode the current is restricted to 50mA, and that works like new. I suspected a blown fuse but it doesn't have fuses. The built-in voltmeter and ammeter work without issue. It will supply the voltage I set it to, but under any load there is no current unless I select the low current output mode. The voltage display does not go to zero under load, it stays the same. I verified it holding voltage with a multimeter and a scope. Seems like an open circuit. I wasn't able to find a schematic.

I have verified 25VDC on +V pin 4 of the LM324N quad op-amp IC. I've verified both TO3 power transistors are good and working. I have verified both current pots work with power off, and by checking for varying voltage drops with power on while adjusting them. I have yet to find a bad component or connection. Any help is appreciated.

 

Do you have a schematic of the supply you can post?

 

Do you have a schematic of the supply you can post?

I haven't been able to find one. Thanks.

 

What size load do you have connected to the supply? Do you have an appropriate resistor to use?

 

What size load do you have connected to the supply? Do you have an appropriate resistor to use?

I am using a 600Ω 10W resistor now. I don't have another 12V power supply to check it under load but it's possible my power supply was damaged from overcurrent earlier.

 

Okay I tried a different load of 3Ω and got 3A. It limits the current to 3A by limiting the voltage output. So this power supply is working now. Thanks everyone for the help.