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reducing current output

Discussion in 'Electronic Basics' started by vjm, Aug 3, 2003.

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  1. vjm

    vjm Guest


    I have a power supply that will output 30 VDC @ 1000mA. Is there a
    way to limit the current output of this supply to the 25-30mA range?
    What components would I need to use? Will they generate heat?

    Any help would be appreciated.

  2. Terry

    Terry Guest

    At 30 volts a load designed for 30 volts will take only as much
    current as it requires; up to the maximum capability of the power
    supply, 1000 milliamps.
    For example if you had a 30 volt lamp that consumed 100 milliamps
    you could connect ten such lamps at the same time for a total of
    10 X 100 = 1000 ma.
    If you connected 11 such lamps you would be overloading the power
    supply by 100 ma.

    To go further you need to explain what you are trying to do and
    need to learn Ohms Law. It sounds as though the circuit you wish
    to operate from that supply may need less than 30 volts?

    An example.

    Today I adjusted the output voltage of our bench supply to 5
    volts in order to test some lamps of unknown voltage and also to
    see if they were 'good'. They each took about 40 milliamps. I
    then discovered that they were 12 volt lamps and adjusted the
    output to to around 10 or 12 volts. Each lamp then took a bit
    less than 100 milliamps.
    Each bulb/lamp therefore is consuming approx. 95 ma at a normal
    voltage and brightness of 12 volts.

    Using Ohm's Law the resistance in ohms of each lamp (when working
    normally) is Voltage divide by Current;
    That is 12/0.095 = 121 Ohms.

    If I had increased the voltage to 30 volts the lamps would have
    taken way too much current (almost three times as much) and would
    have 'blown' their filaments!

    Have fun. Terry.
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