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Need help in stablizing a power supply

Discussion in 'Electronic Basics' started by Everett X. Wang, Dec 6, 2004.

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  1. Hi All,

    I am working on a home project that needs to supply a 24V power to a
    brushless motor controller. The motor draws 24V with current less then 3.0A
    and my 12 to 24V DC to DC power supply is rated 700W. But when I connect the
    things together, the power supply will shut it down. My guess is the motor
    controller draws a high current at a very short time that overwhelmed the
    power supply, since my current meter indicated only 3 A from the 12V side.

    Can anyone give me a suggestion how to make my power supply work? Can I add
    a large capacitor at the power supply output? I also tried an other DC 2 DC
    (rated 4.5A output) power supply. The result is even worse. (It worked for a
    few seconds vs. minutes). My application is a mobile one and I can't use a
    desktop power supply. The power source is from 12V lead acid battery. Any
    solution to my problem?

    Thanks in advance.

  2. Guessing that the converter's oscillator is being affected by feedback
    from the motor. I'd try
    1) Adequately rated diode in series with motor
    2) The high value capacitor you suggested, plus a 10nF ceramic
    3) Fresh 12V battery (existing may have highish internal resistance)
    4) Choke-based filters on input and/or output of converter
    5) Combinations of the above

    This assumes it really is impossible to get a second 12V battery to
    give you the requisite 24V, or a 12V motor!
  3. CFoley1064

    CFoley1064 Guest

    Subject: Need help in stablizing a power supply
    Hi, Everett. I'm assuming you're using one of those automotive 12V-to-24V
    DC-to-DC converters here.

    First, you should look carefully at the difference between peak power rating
    and steady-state power rating. Your second DC-DC converter limited out, and
    that sounds like something one of those converters would do. But even if your
    first supply is only rated for 700 watts peak, it would probably be able to
    handle 3A.

    You might want to put an 0.22 ohm power resistor in series with the +, and then
    put your "mongo cap" across the output (I'd go with at least 4700uF, 35V for
    the cap rating). That will help with charge storage and help to even things
    out. Automotive doublers are made to handle a capacitive surge at turn-on, but
    the 0.22 ohm resistor should help. Then put a voltmeter across the 0.22 ohm
    resistor and measure voltage drop when your motor is on. By Ohm's Law, you
    should get 0.22V per amp of current. You can then measure the steady state
    current and see if it's really 3A (V = I * R = 3A * 0.22 ohm, somewhere around
    0.66V across the resistor). If it's too high, there may be something wrong
    with the motor or the driver circuit.

    .---------------. 0.22 ohm
    | 12V-to-24V | ___
    .------o+ +o----|___|--o-----
    | | | |
    +|Batt | | +|
    --- |12V 24V| 4700uF ---
    - | | ---
    | | | |
    '------o- -o-----------o-----
    | |
    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    Good luck
  4. Thanks for the suggestion. I got a 38,000 uF cap. But voltage is rated 25V .
    Should it be OK for 24V application? I don't think I will go over 25V.


  5. CFoley1064

    CFoley1064 Guest

    Subject: Re: Need help in stablizing a power supply
    I wouldn't do it, but it's up to you. I would guess your peak voltage will
    probably be above 25V, which will probably smoke the cap, and might smite your
    converter in rage on its way to electronics heaven.

    You can get a 4700µF 35V 20% Axial-Lead Electrolytic Capacitors for $5.29 USD
    at RadioShack (Catalog #: 272-1022).

    Also, you can get a couple of 0.47 Ohm/5W 5% Wirewound Resistors for $1.59 USD
    each at RadioShack (Catalog #: 271-130 ). Put them in parallel to get about
    0.23 ohms.

    For a less money/more time/probably equal or better quality tradeoff, try

    Good luck
  6. peterken

    peterken Guest

    Might consider using an inrush current limiter
    it limits the current at startup to a value depending on it's startup
    resistance, and (after it heats) acts *almost* as a wire
    I'd consider using a cold value of say 12Ohm limiting startup to 2amps, and
    with an operational value of say 0.5Ohms
    Just search the net, lots of them out there
    Only thing to keep in mind is it's operational current so the operational
    resistance stays low enough
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