Power supplies

Discussion in 'General Technical Questions and Answers' started by KE0CPH, Jan 20, 2017.

ad: L-HROutlet
ad: l-rl
ad: abrind-2
ad: Left-2
ad: L-MFJ
ad: Left-3
ad: Subscribe
  1. KE0CPH

    KE0CPH Ham Member QRZ Page

    I'm wanting to make my own linear PSU for my radio needs.

    I already have a metal case to store the components.

    My main issue is putting out enough amperage at 13.8 volts.


    Just for example, the above uses a 12v regulator but I'd imagine it'd work for 13.8 as well. Since those individual regulators can only handle 1.5 A, could I connect the regulators in parallel to allow more amperage through?

    Any thoughts?
  2. W1VT

    W1VT Ham Member QRZ Page

    This site has a lots of practical information on linear power supplies.

    Typically the hardest part to obtain is the power transformer of the right voltage and sufficient current for your needs. At high power levels, you need to be pretty close to the right voltage, or you are going to be wasting a lot of power in the regulator circuitry.
  3. KE0CPH

    KE0CPH Ham Member QRZ Page

    I've got a few microwave transformers that I could probably re-wind to get close to 14.5 -15 volts.
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    How much current do you actually need??
  5. VK2TIL

    VK2TIL Ham Member QRZ Page

    Older Handbooks (it's in my 1986 edition) described a project for "An RF-Proof 30 Amp Supply"; it's a very good "tutorial" on the design of high-current power supplies.

    The article, with some notes, is available here;

    http://www.dd1us.de/Downloads/an rf proof 30 amps power supply 1_0.pdf

    I built one almost 30 years ago; it's still my main power supply.

    The most difficult item will probably be the transformer; I re-wound one from a valve (tube) TV set using 3 x 1.5mm wires.

    I used a different heatsink, one that I bought cheaply from a surplus shop.

    Most of the other parts should be available.

    I drew the PCB pattern by hand before etching; it's not a complicated pattern.

    But I see that FAR Circuits have it for only $10.50.
    WA7PRC likes this.
  6. K8JD

    K8JD Ham Member QRZ Page

    Regulators connected to 2 or 4 "Pass Transistors" that can handle a lot of current. U also need a big transformer with a 20 V secondary.
    K7XRL likes this.
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Microwave ovens lately use switchmode power supplies and the transformers don't work at 60 Hz.

    You might have some old ones, though.

    How much current are you looking to achieve?
  8. AA3EE

    AA3EE Ham Member QRZ Page

    There's an article in the older ARRL handbooks for a 28V, 10A power supply. Far Circuits sells the board. It's based on the LM723 and is very easily adaptable to 13.8V. 2 Pass transistors (2N3771) are OK for 20 amps intermittent, 4 would be better. Probably <$35 in the regulator circuit.

    Dave AA3EE
  9. KA9UCN

    KA9UCN Ham Member QRZ Page

    Yard sails, junk stores, auctions often have automotive12v battery chargers. They have a 24v CT transformers and 2 rectifiers in a case with a meter. Often for just a couple dollars.

    Joe KA9UCN
  10. AG6QR

    AG6QR Premium Subscriber QRZ Page

    Don't go connecting regulators in parallel. Minor inconsistencies between the regulators, maybe due to something as simple as a small temperature difference, will have them fighting with each other to set the output voltage, and at best, there's a good chance the load won't be shared equally between them. Better to use a single regulator, along with one or more pass transistos. If you use multiple pass transistors, take care to balance the load between them.

    Also, it's not only the regulator and pass transistors that must be able to handle the current. The transformer and rectifier diodes have to be up to the task, as well.

    The repeater-builder website, linked above, has good information on how Astron uses multiple pass transistors to add lots of current handling capability to a modest regulator. Astron is well-known for making straightforward and reasonably rugged power supplies that are well-suited to the requirements of amateur radio. In fact, it'll be hard to build your own for less money than you'd spend on a decent used Astron, but the building exercise may be educational, so I would encourage you to pursue it if that appeals to you. Just don't expect to save a lot of money unless you already have most of the parts on hand.

    Another example of a linear supply that uses pass transistors is the Elenco XP-720K. It's not well-suited to amateur radio use, because its current capability is too low, and the highest current rail of it is at 5V, not 13.8V. Nevertheless, its manual has a pretty good explanation of how it's put together, and how the pass transistor works in conjunction with the regulator to allow a 7805 regulator plus the pass transistor to deliver much higher current than the regulator alone would be capable of. You may want to browse the manual just for the explanation of the (fairly straightforward) circuit. http://www.testequipmentdepot.com/elenco/pdfs/xp720k.pdf

Share This Page