# Thread: Role of current limit on driver IC

1. ## Role of current limit on driver IC

Hello,
I was examining several chips like L298 and saw that they all have a resistor shunt tying the H bridge to ground. I was wondering about why it is useful to limit the current. Does it play a role when the motors are moving, or just to limit the current at rest?

Frank

2. Hi,
Step motors are current driven devices. In order to get any sort of performance out of them they need to be driven with a fairly high voltage, usually 5-20 times the voltage that would 'normally' result in the rated current thru the windings.

Ie. if the motor is rated 2.4V, 3A you can't use a 2.4V power supply. You should use something like 24V but applying 24V without any way of limiting the current would result in 30A thru the winding and a destroyed motor.

3. Thanks, it makes sense that the stepper current rating is the only important parameter for getting strong manetic fields.

While I am still browsing the posts, is there on the forum a good reference where I can see the combination of a good motor and driver properly tested together so I can get a good idea of the values ?
Frank

4. I'm sorry, I don't really understand what you mean...what values do you mean?

/Henrik.

• Maybe I am going into too much details. I get the general concept that the voltage on the driver board can be 10x bigger than what is on the motor.

I mean something like Mr. X designed a system called Y with a Nema A motor rated for B current and C voltage giving D torque being driven by a controller board of E volts current limited to F amps.
Frank

• Hi Frank,

Finding detailed references like that you mention won't be easy, there's not many people having the lab equipment like dynomometers necessary for those tests.

The performance of the motor depends a lot on the drive being used some are good, some are even better. Geckodrive makes some very good step motor drives at reasonable prices. The motor manufacturer should have speed/torque curves for the motors where you can see how much torque they deliver at a certain speed and supply voltage. Generally speaking, the available torque drops with increased speed. If you double the voltage you get twice the speed with the same torque.

Have a look at the Step motor basics document from Geckodrive, although tailored towards their drives it's a good read when it comes to understanding step motors.

/Henrik.

• The supply voltage depends more on the driver than the motor. The best systems will use a 50v or higher supply. Within limits, the higher the voltage, the better the performance.

• Hi Frank

The stepper motor winding is very inductive. To drive a pulse of current into the inductance it helps if the drive source is a high voltage. Make the voltage too high and the motor insulation will breakdown. Make the voltage too low and the required ampere turns will not be achieved until the pulse is nearly ended resulting in reduced power or even lost steps. With a stepper motor the required current has to be achieved early in each step to give torque to the spindle.

The resistor is only there to give the integrated circuit something (voltage drop proportional to current) to measure so that it can shut off the drive when the desired current has been achieved. Put more current into the motor winding and it will fuse.

For the above reasons when picking your motor consider the inductance of the coils as presented to the driver. This is why parallel connection is better for speed than serial connection of the windings because the lower the inductance the greater the speed with which the current can be made to rise. (Power is voltage x current less losses so the faster the current pulse rises to the limited value the more power is achieved with each pulse.)

The theoretical motor sizing starts with a knowledge of the power required for the machine axis. This is a complex calculation and involves many unknowns which can only be estimated. A good guide is to look at the motor sizes used on similar machines to get the right feel. You can make the motor too big and cause problems if your machine is not very ridgid so try an get this estimate adequate but not by a very large margin. That said the current can be reduced by the electronics in the drive.

If you decide to purchase ready made drivers there are lots to chose from including those from Zapp and Gecko. No matter if you buy or make drivers the lower the inductance the easier it is to achieve the rated power. Also the higher the inductance the greater the switching spikes!

Hope this helps

Regards

Pat