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AT Power supply circuit breakers

Discussion in 'Electronic Design' started by Robotnik, Feb 2, 2004.

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  1. Robotnik

    Robotnik Guest

    I've been handed an interesting project at work (not related to my REAL

    Some 12 volt hallogen track lights managed to overheat their factory power
    supply. (three 20 watt bulbs, in paralell)

    I decided to use an AT PC power supply after having no luck with finding any
    12volt power supplys that would output a load of 5 amps... (well, I couldn't
    find any inexpensive ones).

    The power supply I donated is rated at 9.5 amps on the 12 volt side, I
    figure that gave plenty of room (I assume these ratings were maximum, not

    In my own testing at home, they seemed to work fine, running for several

    However, when I put them back in their own environment, the power supply had
    shut off sometime during the day.

    I'm trying to decide if it was an overheating issue, since it lasted a
    while, or if it's an electrical current overload issue.

    I'm thinking that heat build- is the problem, the power supply (separated
    quite a distance from the lights, they're not heating it any) is in a
    somewhat confined space, I think it's heating to a certain point. I ended
    up cutting up some cardboard and making a "snorkal" for the air to flow into
    the power supply from another location, rather than sucking hot air back

    Any thoughts on this?


  2. Bill Sloman

    Bill Sloman Guest

    Personal computer power supplies seemd to be designed on the basis
    that they are going to be fan-cooled. Did your set-up include a fan?

    Adding a vertical chimney to a power supply is better than nothing -
    the longer the chimney, the more you will encourage natural
    convection, driven by the density difference between the hot air in
    the chimney and the cold air outside the chimney - but it is nothing
    like as effective as a fan.

    Hve you tried measuring the temeprature of the air in the chimney,
    just above the power supply?
  3. Ken Taylor

    Ken Taylor Guest

    Does the supply require a nominal load of some value on the 5V line? This is
    common for AT supplies - you may need to derate the supply if you don't do
    this. Otherwise I'd be going for heat as Bill has already said....

  4. Robotnik

    Robotnik Guest

    This power supply does have a fan on it, but the power supply is oriented
    such that the air flows sideways, left to right. I have I guess a
    "backwards chimney" on it, the cool air is being taken in from a different
    location, where there shouldn't be as much heat.

    Are you suggesting that I put a chimney on the exaust too (or instead)? I
    hadn't thought of that. I could even put another fan at the top of the
    chimey, or somewhere in it.

    And I have not had a chance to check the temperature. However, This is the
    Second at power supplly that I have tried, the first one was only rated at 7
    amps, and it didn't last a day either. It has a temperature controlled fan
    though, and when I figured out it didin't work, it was very veryr warm to
    the touch. At first I didn't think twice about the temperature, and that's
    why I treid a higher-rated (on current) power supply.

    BUt the one I'm trying currently does not have a temp controlled fan, it
    stays on constantly (until the breaker trips).


  5. Robotnik

    Robotnik Guest

    Well I don't know tons about electronics, I'm not completely sure I
    understand your question.... But Both of these ran for a few hours at my
    house with plenty of ventaltion, and no problems. I think it operates
    without a load on 5v. Do you suggest that I put some sort of load on it?

  6. Syd Rumpo

    Syd Rumpo Guest

    The inrush current (the initial surge) for an incandescent lamp will
    be maybe ten times the normal running current - the filaments have a
    positive temperature coefficient and are much lower resistance when
    cold. You might get lucky, but I'd find a different solution.
  7. Robotnik

    Robotnik Guest

    So are you suggesting that the resistance will increase as the filaments
    heat up?
    I see no problem with that, that means they'll draw less current as their
    temperature increases.
    Turning them on seems to be no problem, the power supply shuts off after a
    long period of runing time, even after the lamps are heated to a fairly
    constant temperature themselves.

  8. Ken Taylor

    Ken Taylor Guest

    It wouldn't hurt - if the supply starts wandering from spec. under heat
    stress then without a properly regulated 5V supply (no load) it may end up
    shutting down. There's usually a minimum load and a hunt on the
    manufacturer's website should reveal this. Failing that, find a 6V light
    bulb. It's a cheap test.


  9. This is inherent with any resistive load that has a positive temperature
    coefficient. Most materials behave this way.
    Not really. They draw their _rated_ current when fully hot. The problem is
    that when cold, a lamp will typically draw perhaps 10* this current. The
    normal supplies avoid this being too much of a problem, by having a fairly
    high internal resistance, thereby limiting the maximum current during this
    This is because the supply is designed to handle the 'turn on' surge, but is
    switching off, perhaps because the thermal cutout is set slightly too low.
    Unfortunately a PC supply is not designed to handle this type of surge, and
    if it has enough current rating to deal with it, may well significantly
    shorten the life of the bulbs (slowing the rate of initial 'switch on' a
    little has a large effect on bulb life). One solution is to add a NTC
    resistor in the line feeding the lamps.

    Best Wishes
  10. Robotnik

    Robotnik Guest

    Okay, I think I see what you mean. Any suggestsions on the value of the
    resistor? (supply rated at 9.5 amps 12V, lights in total (eventually) 5 amps
    12 volts).


  11. Robotnik

    Robotnik Guest

    Alright, thank you!

  12. Turning them on seems to be no problem, the power supply shuts off
    Look in circuits for halogen projectors. You are looking for something like
    a 1R to 0.1R unit. This will change resistance from 1ohm when 'cold', to
    0.1ohm when 'hot'. The parts are quite common in older slide projectors, and
    overhead projectors for exactly this reason. Modern designs tend to use a
    switch mode supply, and have 'soft start' built into the supply. With the
    cold resistance of the bulbs (which should be about 0.25ohm), you will just
    about be 'in spec' for the supply as it starts. The parts are sold as
    'inrush current surge limiters', an SL121R008, would probably be close to
    what you need. This is a '12mm' diameter type, designed to limit the maximum
    current to 8A, with a nominal 1R resistance when cold.

    Best Wishes
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