Parts Identification Help Needed

Discussion in 'General Electronics Chat' started by kbyrne, Feb 12, 2012.

  1. kbyrne

    Thread Starter Member

    Dec 10, 2011
    The enclosed Gif is part of a power supply taken out of a VCR machine. With the schematic can anyone assist me in:
    1. Identifying the Resistor shaped parts They surround the bridge rectifier on all sides. What is their purpose also.
    2. The inductors before the transformer are not marked and are enclosed in a round white object. How can I figure out their value? What is their duty in a circuit like this? I cannot figure out where else to go on the internet for assistance in figureing this out. Can this type of transformer work like a regular transformer with just the necessary parts or it a special type? Thanks from a newbie to electronics. I have two matching junk box transformers and desire to design a lM350 @ 3Amps power supply with each.
  2. jimkeith

    Active Member

    Oct 26, 2011
    The resistor shaped parts surrounding the bridge are probably capacitors that are added to help control voltage transients caused by diode reverse recovery. If replacing, the value is probably not very critical--perhaps 0.1uf.

    The inductors before the transformer are most likely a common mode coke with two coils wound on the same core--these are used to prevent RFI from getting out to the power mains, or in from the same--also not very critical in value unless you are actually doing the CE qualification--if replacing, just make sure that the current rating is adequate. To determine their inductance, connect in series with a light bulb and measure the voltage drop and current through the inductor--then calculate the reactance--then the inductance.

    The transformer appears to be a simple line frequency type--very suited for making your low voltage power supply. The common mode choke is unnecessary in your application.

    Woops, on second thought, this may be one of many distributed power supplies that run on perhaps 40kHZ that is generated via a much larger power module. If so, the bridge capacitors and common mode choke make more sense. To determine this, power the transformer primary via a powerstat and low voltage filament transformer for safety and low voltage--bring the voltage up slowly to determine how much voltage the primary will support before the current skyrockets--if very low (perhaps 1V or less), it will be unsuited for line frequency application.
    Last edited: Feb 12, 2012