# Simple current Limiting Circuit

Discussion in 'Electronic Design' started by Aung Ko Ko Thet, Nov 7, 2012.

1. ### Aung Ko Ko ThetGuest

Hi All,

There is a need to limit motor current in forward/reverse control of 230V DC motors.
I am thinking of using this circuit:http://en.wikipedia.org/wiki/File:Current_Limiter_NPN.PNG.

But as it is difficult to adjust R1 to pull up Q1 to turn it on at a high voltage (Actually we used full wave pulsating DC to drive the motor at a current of 2A in each direction), I am thinking of using a FET for Q1. I will use a 1W zener at the gate of Q1 which is pulled up to 230V and limited to approximately 10V.

One of my senior designers said "it is not a good idea to use such circuit for high voltage DC".

Any comment and suggestion?

Thanks

2. ### Guest

not likely it'll work (for long) Q1 will potentially be dissipating
many 100s of watt

pulsed DC suggest you are driving it with some type of pwm controller

-Lasse

3. ### JamieGuest

Sounds like a bad idea to me. You need to shop around for a Mosfet that
can handle the heat, because you would be putting it into linear state.

There is 2 ways to look at this, you said you're using a full wave
source? This implies that you are using a simple on/off circuit with no
speed control? If that being the case then using a zero crossing
detector to Reset a simple base timer like a 555 and a current monitor
node to set the time on delay to trigger the gate could allow you to
fire an SCR at the start of each cycle. As current increases the time
constant will reach the peak of the wave, when it starts triggering on
the down slope due to excessive current causing longer delays from the
timer, you'll get less voltage out to the motor. Phase firing the
supply is a basic and long time method to control output.

Second option you have is to use a switching power mosfet that is
biased on at all times until the load current reaches a set point and
thus can then start to PWM modulate the Mosfet switch.

Normally the duty cycle will be 100%, keeping the switch on until
output current reaches max or over at which point you use the overage to
set a reduce duty cycle in the switch..

This switch of course is going to be a high speed carrier so you
should have the output of this switch passing into an L (inductor) where
R heating will be minimized and the pulse can looks like DC on the
output. Using the induction of the motor to me is kind of a hack way but
it may also work.

THe driving PWM gate ref can be made using a dual comparator so you
can generate a triangle wave for the carrier into an input of the other
comparator and the remaining input on that one will be used as the sense
node. This will give you a 0..100% duty cycle abilities.

Jamie

4. ### Phil AllisonGuest

"Aung Ko Ko Thet"

There is a need to limit motor current in forward/reverse control of 230V DC
motors.
I am thinking of using this
circuit:http://en.wikipedia.org/wiki/File:Current_Limiter_NPN.PNG.

** ROTFL

One of my senior designers said "it is not a good idea to use such circuit
for high voltage DC".

Any comment and suggestion?

** A 230VDC motor that pulls 2A is a powerful machine with a LOT of spinning
inertia.

If you need to reverse the direction of the output quickly, then go for
MECHANICAL solution.

A reversing gear box.

.... Phil