# Simple current Limiting Circuit

• posted

Hi All,

There is a need to limit motor current in forward/reverse control of 230V D C motors. I am thinking of using this circuit:

nt_Limiter_NPN.PNG.

But as it is difficult to adjust R1 to pull up Q1 to turn it on at a high v oltage (Actually we used full wave pulsating DC to drive the motor at a cur rent 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

• posted

On a sunny day (Wed, 7 Nov 2012 03:10:24 -0800 (PST)) it happened Aung Ko Ko Thet wrote in :

I have tried something like that with a HV MOSFET and with a zener at the gate it would not stop oscillating. Without zener it would work, but the dissipation in the FET will be enormous in your case. I only used it for a few mA at 230V, only as short circuit protection, with the FET on a heatsink.

I think some sort of switcher in your case would work better?

• posted

motors.

circuit:

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.

high voltage DC".

The fet might have to dissipate hundreds of watts. Is the 2 amps average, or peak current?

There are better current-limiting circuits, but the basic concept may not work.

```--
John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    ```
• posted

DC motors.

rent_Limiter_NPN.PNG.

voltage (Actually we used full wave pulsating DC to drive the motor at a c urrent of 2A in each direction), I am thinking of using a FET for Q1. I wil l use a 1W zener at the gate of Q1 which is pulled up to 230V and limited t o approximately 10V.

t for high voltage DC".

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 think about integrating the current limit into that instead

-Lasse

• posted

motors.

circuit:

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.

high voltage DC".

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

• posted

"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:

** 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

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