# Need help with limiting resistors

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

1. ### allahjane Thread Starter Member

Sep 19, 2012
75
1
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
7,338
1,574
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
75
1
Sort of, but I want to limit the current to safe value and not to shut it down completely

4. ### crutschow Expert

Mar 14, 2008
12,580
3,083
I don't understand how your load can take (leech?) more than its rated current if the supply voltage doesn't change?

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.