Can I check Linear Power Supply Unit FETS in situ?

Discussion in 'Power Electronics' started by nmmnmm, Jul 7, 2017.

  1. nmmnmm

    Thread Starter New Member

    Mar 7, 2014
    Trying to fix a 5v Farnell 16RB Linear PSU. Done lots - usually just replace the electrolytics..... I'd been in to fix an unrelated fault and checked the PSU which was working fine until the VERY INSTANT the Tool-Changer jammed on a nearby CNC Milling Machine causing the Milling Machine to briefly pull lots of current at which point the Farnell bombed! I fitted a spare PSU and returned to base with the defective on where I noticed a diode (N4003) that had been overheating (pcb discoloured). It broke as I was trying to removing it to examine & test. I replaced the electrolytics, diode and a resistor that sat next to it but l can't get a decent output. I also replaced the LM723CN but no joy. In situ the Bridge Rectifier reads 0.625 both ways. The PSU should output 5v on 2 cables that join together further down the line, but one reads 5.3 the other 6.4. The pcb has 3 trimmer pots, two tamper protected (current limiters presumably) and an unprotected one I assume for voltage adjustment. I marked the slot position but adjusting it fully each way makes no difference to the output. There are other parts on the board that I would like to test in situ if possible. MBR1560, MJE3055T and TIC126B. I have an Isotech IDM73 multimeter and a twin trace DC coupled 'scope. Thanks for taking the time to read this.. Any suggestions please anyone?
  2. dl324

    AAC Fanatic!

    Mar 30, 2015
    Firstly, use paragraphs to organize your thoughts.

    By your usage of testing in-circuit - no.

    You can, however, verify correct operation by taking measurements in the circuit.

    Post a schematic if you want help.
  3. ian field

    AAC Fanatic!

    Oct 27, 2012
    MOSFETS are best checked out of circuit because random charges on the gate can cause unexpected results.

    In most cases the existing gate drive circuit will douse the gate capacitance, if you lift the drain terminal there shouldn't be any leakage at all - but a fault in the gate circuit can make this test unreliable.

    Usually I take them out and do several tests. You can charge the gate with a continuity tester so it conducts - the charge should hold for a while or the oxide layer is suspect. You can charge the gate reverse, there should most definitely be no leakage at all.