Stepmotor driver mc3479

Thread Starter

FatMooseHenry

Joined May 16, 2007
3
Hello :)

Im building a project, that involves a stepmotor driver(http://www.onsemi.com/pub/Collateral/MC3479-D.PDF)

I have assamblied it like fig. 12 in the datasheet. My application requires the motor to be held at a position, and therefire there must be current on the coils at all the time.

My problem is that the driver becomes really hot (but works fine). I always cuts the power when it reaches 75degree C. I have tried to reduce the current in the coils from 50mA to 25mA, but its still the same.

I have tried to put 47Ohm resistors between the chip and the coils, however, both the chip and the resistors gets hot. Again up to 75degree C.

Have anyone got any ideas? Am I doing it wrong?
 

n9352527

Joined Oct 14, 2005
1,198
Did you verify or measure that the coil current was really limited 50 or 25mA? Don't just rely on what you set with bias resistor. With 25mA, it's really hard to dissipate enough power to heat up the IC to 75degC. With typical Rja of 45degC/W, you'd need a bit more than a watt at room temperature of 25degC, which translate to around 40V voltage drop. What's your supply voltage?
 

Thread Starter

FatMooseHenry

Joined May 16, 2007
3
Hello
Sorry for the lack of information, I can see its hard to say anything about the system with the information applied :)
First of all, this is my first project with a stepping motor, so be gentle...

The motor, has been taken from a printer, and are of the type:
http://www.mitsumi-components.com/Catalog/compo/motor/m42sp/6nk/text01e.pdf

The Driver are driven with 12V, so the voltage across the motor are 11.3V. When the motor are in use, it will stand still, or it will move at approx. 2-3 times pr second.
One thing that has come to my attention is, that the datasheet for the motor states Rated Current/State : 400mA (peak)
And the driver states : 350mA/Coil drive capability. Is this a problem? So far I have believed that it is ok as long as I dont go over 350mA?

I have done some reading, and it seem like the most used method, is to minimize the voltage when the motor stands still. I guess I could do this, but still, it shoulnt get that hot?

When setting the bias for 25mA in the coils, the powersupply provides 19mA, then increases to about 26-27mA, so I guess the current in the coils should be ok?

Thank you for your help
Kind regards
Brian
 

Papabravo

Joined Feb 24, 2006
13,470
I don't know what you think this chip is trying to do for you, but if you try to put 11.3 volts across the coil you will get 400 mA. This implies a DC resistance of 28.25 Ohms. If the current is limited to 25 mA then only 700 mV will appear across the coils and 12-.7 or 11.3 volts will appear across the two output pins. So you pays your money and you takes your chances. Either the motor gets hot or the chip gets hot, take your pick. the motor probably has more thermal mass so you might as well let the motor get hot.

Limiting the current in this fashion does absolutely nothing for you in terms of holding torque or movement speed or anything. You need to let the motor be a motor.
 

Thread Starter

FatMooseHenry

Joined May 16, 2007
3
Thank you for the response.
So...what you are saying, are that I should not try to limit the power via the Bias pin, just...let it get hot, and if I do not limit it via the bias pin, the motor will get hot instead of the chip?

Kind regards
Brian
 

chakssam

Joined Oct 11, 2007
1
I am using the MC3479 to drive a bipolar motor. I am using C for programming, my question is how do i generate the CLK required for the chip...do i create a loop in a PORT that would go to this CLK and then toggle it accordingly, the max CLK is 50khz...
:(
 
Top