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decreasing votage to regulators ?

Discussion in 'Hobby Electronics' started by mark krawczuk, Oct 10, 2004.

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  1. HI, i am in the process of building up a power supply using lm 7805 regs
    but the voltage before its had a capacitor across it , staright of the
    rectifier is about 37 volts dc, which is too much for the reg, plus with the
    cap across it would increase it abit.

    is there a simple way to reduce the voltage to the regualtor ?
    thanks, mark k

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  2. MC

    MC Guest

    A different transformer (if practicable).
    A high-power zener-diode (eg. 12V) in series with the regulator.

    How much current do you need to use at 5V ?
  3. hi, only a maximum of 1 amp.
    thanks, mark k
  4. **ONLY a maximum of 1 Amp! Yikes! Do the math. Far too much for a TO220 3
    term reg. Here is what I would do, if I were you:

    * Use a series pre-regulator, with a 2N3055 (with heatsinking) and a zener
    (say: 12Volts).
    * Use either a current booster with your 7815, or an LM338K (with

    The back of your Dick Smith catalogue should contain all you need. As does
    the National Semiconductor data sheets.

    Your requirement is a VERY inefficient use of materials. MUCH better to
    lower the input Voltage to a more reasonable level.
  5. no its not, LOOK at the data for the lm 78 XX regulators, they do handle 1
    MARK K


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  6. **They do handle 1 Amp, but they are limited in their ability to dissipate

  7. hi, whats top posting ?
    i accidently posted two posts exactly the same , cause my puter locked up
    and wasnt sure the firsty one went thru.
    mark k


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  8. **This is top posting.

    **And this is how humans post. AFTER the post made initially.
  9. Peter Howard

    Peter Howard Guest

    You didn't say what the VA rating of the transformer is. e.g. how much
    current it is able to give, though I do get from subsequent posts that you
    want 1amp from your 7805 reg.
    However, if the tranny is able to provide 1amp, you may find it is
    constructed so as you can remove turns from the secondary winding.
    Somertimes the secondary is wound over the top of the primary, other times
    they are side by side on the core. In either case, if the secondary is
    traditional construction of layers of wire separated by layers of lacquered
    paper you should be able to remove the top layer of paper and begin
    laboriously unwinding the secondary without disassembling the interleaved
    laminations of the iron core. It may be difficult until you get a layer or
    two off which gives you more to room pull the wire back through the
    "windows" of the core.
    Unwind 10 or 20 turns, scrape the free end of the lacquered wire bright,
    apply 240v to primary (safely insulated connections and earth wire connected
    please)and measure with an AC voltmeter how much the AC voltage from the
    secondary has dropped. This will give you a clue about the "turns per volt"
    of the secondary and how many more turns you need to remove to reach a
    target voltage of maybe 10 VAC. When you have got there, you can cover the
    exposed winding with some of the salvaged paper or even ordinary brown paper
    impregnated with varnish (not essential but looks nice and protects the
    winding) and re-terminate the end of the winding.
    Data sheet says that min input V for 7805 is 7.2VDC so you could even aim
    for a lower target than 10VAC. Remember that the higher the inputV, the more
    power the reg must dissipate to output 5V and the less current you will be
    able to draw before the thing goes into thermal shutdown. Go lower than the
    min 7.2 VDC input and it won't regulate. Diff between regulated output and
    min input is termed dropout voltage and some reg types are made to have
    small dropout voltages.
    BTW, one amp from a 7805 is pushing it. May I suggest an LM317 in a TO3 can?
    Data sheet available online from Nat Semi.
    If you're not comfortable with the idea of butchering the tranny or it seems
    like too much effort, a lower voltage transformer is the best answer.
    However, altering the tranny does work. I've done it most recently with a 30
    VAC transformer from an obsolete Telecom PABX power supply which is now part
    of a trickle charger for a workshop 12v battery.

  10. MC

    MC Guest

    Get a different transformer, something around 6.3VAC or 9VAC.
    With the transformer you seem to have now you will be wasting
    (and dissipating) about 40Watts to get 5V at 1A.
  11. Hello Mark,
    I realise you asked your question some time ago, but if
    you are still reading this group you may find this article
    and the links interesting.
    John Crighton
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