Need help with limiting resistors

Discussion in 'General Electronics Chat' started by allahjane, Feb 20, 2013.

  1. allahjane

    Thread Starter Member

    Sep 19, 2012
    Hello all,
    I'm using a motor driver for my application that has been rated as 5 Amp max but the load can leech more than 5 Amps if that much current is available.
    I'm using a 12 v power source
    However my power source may vary over time and may exceed 5 Amp but voltage will always remain at 12 v

    So if I connect a 2.4 Ohm resistor in series then as per ohm's law the current will never exceed 5 Amps.

    Is that correct?

    also if when I use a power source with less supply than 5Amp will the limiting resistor lower the current further?
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    A 2.4 ohm resistor on a 12V supply will indeed limit current to 5amps, but only when there is no load (or it shorts directly to ground without any other parts). If you add a load then the current goes even lower.

    If this is for the L298 in your other post then that only has (each channel) a 2A average current, and 3A single peak.

    Rather then limit current you could sense current and trip a turn off circuit. I say "trip" as it needs a flip flop or delay to keep the H bridge off: a simple comparator will allow it to turn right back on.
  3. allahjane

    Thread Starter Member

    Sep 19, 2012
    Sort of, but I want to limit the current to safe value and not to shut it down completely
  4. crutschow


    Mar 14, 2008
    I don't understand how your load can take (leech?) more than its rated current if the supply voltage doesn't change? :confused:

    But if you really need a limiter, below is a two-transistor circuit that will limit the current to 5A. The circuit limits when the voltage across R3 reaches about 0.65V.

    Note there will be a small voltage drop at the output due to the load current through the 0.13Ω resistor.

    The P-MOSFET can be any 10A or larger device.

    If the limiter operates under a continuous limit condition then the MOSFET must be mounted on an appropriate heat-sink.

    Current Limit.gif