Help repairing a Tenma 72-7660 power supply

[mmorningstar17]

Hello everyone. I have the above listed power supply and it recently stopped putting out more than 1A. (I've been using it to power a 12v 2-3A radio for the past few months. This is a 30V, 10A benchtop variable power supply. Everything else works fine, but it just will not put out more than about 1 amp. This is a nice supply and I'd love to salvage it but i'm not sure where to start. I'm assuming some power component burned out but without schematics or a service manual, I'm at a loss. (I've searched for them) I would sure appreciate some help! Thanks in advance! -Mike

 

[betwixt]

I do not have the information you need but if the voltage is still adjustable and only the current is limited, the most likely culprit is the current sensing resistor. If it's value increases, as it might do after prolonged use, it will make the current limit kick in too soon.

Brian.

 

[mmorningstar17]

I do not have the information you need but if the voltage is still adjustable and only the current is limited, the most likely culprit is the current sensing resistor. If it's value increases, as it might do after prolonged use, it will make the current limit kick in too soon.

Brian.

Thank you. Any idea how I might identify that resistor? There are a bunch of high wattage resistors glued to a heat sync, but without a schematic, it's hard to tell.

 

[betwixt]

I don't have any technical information but the resistor would typically be a high stability one but not necessarily have a high power rating or be fixed to a heat sink. Typical values are less than 1 Ohm. It could be in several places in the circuit and on either the negative or positive output lines. If you follow them both back, look for a low value resistor in series with one of them. Typically, PSUs like that use a low value series resistor to drop a small voltage proportional to the current being drawn. The voltage is measured to sense the current and then compared to the current limiting 'reference' voltage. As the sensed voltage exceeds the reference threshold it pulls the regulator output voltage down so the current limit can't be exceeded. If the resistor increases in value, as most do after a prolonged 'cooking', the voltage dropped across it increases so the supply thinks more current is being drawn than really is and it goes into limiting mode prematurely.

Brian.