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Grounding a 317 regulator circuit.

Discussion in 'Electronic Basics' started by Nikolas Britton, May 22, 2005.

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  1. Hello all, I need some advice on the correct way to ground a standard
    LM317 circuit. Right now I have the base of the transformer and the
    negative side of the DC output connected to earth ground. To me it
    seems weird having to earth ground the DC negative side, but if I don't
    do that I'll get 12.5Vac / 20mA from the DC negative post to earth
    ground. I thought the DC section was suppose to be isolated because of
    the transformer?

    Parts list (so far):
    *The transformer is 60Vac with a center tap to give me 30Vac, pulled
    from a dead Sony amp.
    *KBPC10 Bridge rectifier, metal casing, I think it's rated at 10A /
    *Sprague powerlytic 2600uF 150V filter cap.
    *230 ohm 1/4 watt resistor.
    *10K potentiometer.
    *Big heat sink.

    BTW, IIRC, when I first tried grounding it I connect the earth ground
    to the heat sink that had the rectifier and the LM317, plus side of the
    DC output, mounted on it but then I got 12.5Vac / 7A on the negative
    side to earth ground....

  2. Ban

    Ban Guest

    Niko, with 30Vac you will get more than 42Vdc across the cap, which is above
    the absolute max. rating(input/output differential) of the LM317. This means
    when a short occurs on the output the regulator might blow. There is a
    LM317HV version available, which is good for 60V.
    Now to the connections:
    1. connect the center tap to the neg. side of the cap and leave the - tab of
    the rectifier open, so only 2 of the diodes are used. you do not need to
    connect a earth. View the diagram with fixed font.

    -. ,--------.
    )|( | ____
    )|( | | | +1.25...37V
    120Vac )|(---. | +-+-------o------|317 |-------o--o
    )|( | | A A | |____| |
    )|( | '-+ | |+ | ___ |
    -' '---)------(-+ === .----o----|___|-o
    | A A /-\ | | |
    | +-+-- | |+ .-. ---
    | - open | === | |<-. ---
    | | /-\ | | | |0.1u
    '----------------o | '-' | |
    | | | | |
    (created by AACircuit v1.28 beta 10/06/04
    Be aware that your rectifier is not rated for 60Vac, better to take some
    discrete diodes with 3A/200V(1N5402) rating.
  3. Thanks.

    I looked at the LM317HV and it's max output is 500mA, I need amps more
    then I need volts. I payed zero dollars for the parts I have now
    because I pulled them all from dead equipment than came my way. If I
    had to buy new parts then my selection would be limited to whats
    available at the local radio shack. What about a power resister is
    series to drop the voltage down to 40Vdc, I have lots of 5 and 10 watt

    Here are the rectifiers and regulators what I have in my parts bin, I
    found another box that has a lot of rectifiers, transistors, diodes,
    and ICs that I forgot about so I still need to sort and catalog those
    S30SC4M, 1, Schottky Rectifier
    KA317, 1, Voltage Regulator TO-220AB varitable 1.5A Positive
    LM7912CT, 1, Voltage Regulator TO-220AB 12V 1.5A Negative
    LM7905CT, 1, Voltage Regulator TO-220AB 5.0V 1.5A Negative
    LM7808CT, 1, Voltage Regulator TO-220AB 8.0V 1.0A Positive
    LM340T15, 1, Voltage Regulator TO-220AB 15V 1.0A Positive
    LM340T5, 1, Voltage Regulator TO-220AB 5.0V 1.0A Positive
    RS603M, 1, Bridge Rectifier RS-6M 6A 200V
    RS405L, 1, Bridge Rectifier 4A 600V
    RS603M, 1, Bridge Rectifier RS-6M 6A 200V
    KBPC804, 1, Bridge Rectifier BR-8 8A 200V
    KBPC10, 2, Bridge Rectifier - 10A MaxVrms=35 MaxVrrm=50 Vr=50 Ifsm=300A

    I decided on the 317 because I had the circuit diagram in a book and
    had all the parts needed to build it on the spot. Also at my desposal
    is a crap load of power transistors, big diodes, box full of
    transformers of all sizes, large caps, etc if someone has a better way
    to do what I want.
  4. If you look at the LM317 datasheet at, you will find that the
    mounting tab is connected to the output terminal - you DO NOT want to
    ground that tab, or the heat sink the part is mounted on (unless you
    take measures to insulate the tab from the heatsink.

    Peter Bennett, VE7CEI
    peterbb4 (at)
    new newsgroup users info :
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    Vancouver Power Squadron:
  5. tempus fugit

    tempus fugit Guest

    What DC output are you trying to get from the regulator?
  6. 1.5V ~ 24V.... I need a general purpose benchtop power supply that can
    handle at least 3 amps

    I just found these circuits that uses a 317 and power transistors to
    get 3 ~ 5 amps out:

    What is the purpose of the diode (D1) in the 2nd link? and would it be
    possible to put a switch, for safety, in series with the base of the
    power transistor so you can "turn on" the extra amps only when I need

    I have two NTE378 PNP Transistors that might work for this circuit but
    I'm not sure if they are overkill. The collector current (continuous)
    is rated at 10A and I think the transformer is rated only to 7A , too
  7. Yes!, I found that out when I hooked my oscilloscope ground lead to it.
    I disconnected the scope lead when it sparked, only a small spark, and
    got my meter out to check the volts/amps, the meter showed 12.5Vac /
    7Adc. I didn't believe the meter readings so I tested it with a 100ohm
    resister, burned up in milliseconds!

    I wanted to have everything mounted inside a metal chassis. The heat
    sink I have was designed for this chassis and is directly mounted to it
    but being that the 317 is in a TO-220 package it would be hard to
    isolate it, unless you had plastic screws.
  8. kell

    kell Guest

    Radio Shack has (had?) a TO-220 mounting hardware with a mica
    insulator, washers etc. I saw them as recently as last year. But only
    a large, fully-stocked RS would have them.
  9. The alternatives:

    Isolate the heat sink from the chassis, and leave a warning label
    inside saying the heat sing is not isolated from the circuit.

    Isolate the chassis against outer touch, without reducing its heat
    emissing properties too much. A thin layer of plastic, maybe?

    Choose another regulator circuit, where the collector of the power
    transistor remains at ground potential.

    I have designed several such power supplies, The main part is a npn
    power transistor sitting between ground (output ground) and the
    negative end of the rectifier bridge, which is NOT grounded anywhere.

    The positive end of the rectifier becomes the positive output from the

    Now we only need to control the base of the power transistor so the
    voltage over the transistor is what needs to be subtracted to get the
    output voltage we want between the ground (the collector of the
    transistor) and the positive supply.

    First I add a series resistor in the emitter of the power transistor,
    and a smaller transistor which controls the current by pulling down the
    base of the power transistor. This sets the maximum current this
    circuit will allow through the power transistor.
    The small transistor, e to minus, c to base of power transistor, b to
    the top of the current sensing resistor, the emitter resistor of the
    power transistor. 0.6V/1Amp=0.6Ohm, for example.

    rectifier plus Output plus

    Output ground
    rectifier minus

    Then I need to compare the output voltage with the reference voltage.

    The reference I use is often a zener, or led's used as zeners.

    The output voltage is floating compared to the regulator circuit and
    that poses a little problem.
    The sensor or output voltage needs to float with the output, but send
    control signals to the regulator part, which is completely under
    ground, so to speak :) because it is below the zero voltage of the

    This is not so difficult, we can use current or even light to control

    The "sensor" must take a sample of the output voltage difference, and
    use it to control a current in a wire to the regulator unit.

    There are many fine engineers here who can help you with the rest if
    you want to continue this lesson in power supply design.

    Well, even if we complete this design and build it the chassis will be
    grounded, and some care is needed for that chassis not to get in touch
    with anything which it could short. We prefer boxes which are not
    electrically active in any way.

    For example a toaster where the chassis is grounded, and you poke some
    bread crumbs out with metal wire. If you hold the other hand around the
    toaster's chassis you can die if the chassis is grounded metal. If it
    is isolated, and even better, made out of plastic, you have a better
    chance of survival.

    Sometimes we need direct contact between the metal chassis and the air
    so much that we can sacrifice the rule that a chassis should be

    I built a computer monitor once, designed a 95 Volt 1A power supply as
    described above, and bolted the output transistor directly to the sheet
    metal outer chassis I made for the tube.
    Its collector was the ground point in the regulator I built so there
    was no need for isolated mounting, maximizing the possible power and
    reducing the working temperature.
  10. Thanks everyone, today I built a wooden box for it and reassembled the
    circuit inside of it.

    I'm going to keep it, for the moment, a standard 317 circuit. The only
    thing I modified from the original circuit I posted was to add an extra
    sprague powerlytic 2700uF 150V cap after the regulator stage to help
    stabilize devices such as audio amps. I tested the circuit with a 200
    watt 12v car audio amp and I was impressed by the little LM317. Is it
    possible for the 317 to output 2.4 amps?, this is what my amp meter

    At first I had a 4"x4"x1/8" heat sink on it but with the audio amp it
    was getting too hot so I had to upgrade the sink to a 10"x1.5"x3/8"
    with about 20 1"x1/8" fins. With this sink I was able to keep it in the
    110 to 130 (F) range with the audio amp, also I put a dab of thermal
    grease on the metal tab.... 2.4 amps?!?!
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