Power Supply Question

Discussion in 'General Electronics Chat' started by PRS, Jan 2, 2009.

  1. PRS

    Thread Starter Well-Known Member

    Aug 24, 2008
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    35
    I have an aluminum heat sink with 8 TO3 transistors mounted on it. Four are PNP, four are NPN. I want to use it to make a 12 volt supply. I could use the PNPs to make the negative supply and the NPNs to make the possitive supply, or I could buy 2N3055s and have a single-ended supply.

    But what I'm wondering is this: What is the best strategy -- series or switching? I want to use it for experiments in general, but mostly for radio transmission. Switchers are said to produce many harmonics and I want as clean a supply as possible.

    But mostly I want to know this: How do I arrange all these xistors in such a manner that they all work together, combining their amperage. My thought is to combine via parallel connections -- base to base, emitter to emitter, collector to collector -- all the xistors into the role of one series pass xistor and use feedback to control the output.

    Any thoughts? I'd sure appreciate it. :)
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Transistors are all different unless you buy thousands and test them and sort them into groups. Since they are different then they don't share the current when in parallel.
    One hogs most of the current then burns out followed by others burning out.

    Their current gain and their base to emitter voltage can be forced to be almost the same if low value series emitter resistors are used.
     
  3. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    You can balance them to a degree by using low value emitter resistors to merge the transistors.

    The main thing is what you're going to use them for, if it is for high power analog (RF transmitters, high power audio amplifiers, etc) you probably don't want digital, since it is noisy by design. Switching units can handle MUCH higher power levels though, since you are operating the transistors in their two most efficient modes, cutoff and saturation, which dissipates very little wattage. Wattage is generated during switching, while the transistor is in its analog region.

    There is a reason switching power supplies for computers are so small for the power they generate. My opinion, if you need high power digital switchers you can't go wrong using a computer power supply, it has everything including safety features for circuit protection. You would need some other circuitry to keep it happy, to keep current flowing through the ±5V.

    For RF think analog.
     
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