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Author Topic: L297 stepper motor controller  (Read 16598 times)
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Hi, I have a few questions about the L298 chip. Ive wired one up to control the direction of electroplating. It has to deliver about 1.4A. Ive noticed theres a 2.4V drop across the L298 and also it gets roasting hot. Is this normal?

My power supply reads 6.5V  1.4A and the voltage across the plates is 3.6V. Im also dropping voltage across the wire going to the plates. The L298 datasheet says that Vsupply must be 2.5V higher than logic Hi (5v in my case). Is this critical?

Also I dont have current sense resistors. I just wired the sense pin to ground as I set the voltage to give me the correct current using my power supply.

Its all working fine Im just a little worried about the heat and whether my lack of current sense resistors matters.

Thanks
Nicky

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Daniel
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hi

yes that is normal that it would get roasting smiley You need a heatsink at that current or the chip will die soon.  2.5V X 1.4A means the chip has to dissipate 3.5 Watts of heat...

The better way to design for your task would be to use a MOSFET H-Bridge, as MOSFETs have a very low on resistance, on the order of  milliohms. With them, you would drop perhaps .2V across the bridge. The downside is that they are static sensitive, harder to drive as they need more current, and they don't like to switch in "high-side" applications.  But if you can find someone who makes a prepackaged driver IC with logic-level inputs, you will have solved most of that. There is an article here that describes a 30A MOSFEt H-bridge that could be built for under $20. Alternately, you might want to check the local hobby store, as the RC motor drivers that they sell ready-to-go will switch up to 100A. Add some servo code and you are set.

Lack of sense resistors  is OK if you have are controlling the current. The Motor supply does have to be minimum 2.5 higher than logic, as it has to overcome the drops on the internal transistor junctions to function properly.

D
« Last Edit: July 26, 2007, 04:08:13 am by Daniel » Logged

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Hi Daniel,
Thanks for that. I just had a quick look for a mosfet hbridge IC but they seem to have higher minimum voltages (around 10V). I also am out of time with this project so will have to stick with the L298s for now. So Im sitting here looking at an old AMD cpu cooler, some artic silver heat paste and a drill. Im sure I can get this thing cooled with with this combo. Time to get creative.

Logic Hi plus 2.5 would be 7.5 which gives me a bit too much current. I have a load of diodes and 3.3V regulators. Do you think it would be worth running the L298 at 3.3V instead of 5V? Then I could drop the supply voltage to 3.3+2.5 = 5.8V which would probably be fine current wise.
Thanks and all the best
Nicky
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Daniel
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Quote
So Im sitting here looking at an old AMD cpu cooler, some artic silver heat paste and a drill. Im sure I can get this thing cooled with with this combo. Time to get creative.

Who needs a drill? I always find that a random piece of aluminum, say a frying pan, along with a mini vise-grip for adhesion, makes an excellent ad hoc heatsink. smiley

Quote
Logic Hi plus 2.5 would be 7.5 which gives me a bit too much current. I have a load of diodes and 3.3V regulators. Do you think it would be worth running the L298 at 3.3V instead of 5V? Then I could drop the supply voltage to 3.3+2.5 = 5.8V which would probably be fine current wise.
Thanks and all the best
Nicky

Your best bet is to put in those current sense resistors ( .5 ohms, at least a watt or so each), as they will do what they're designed to do, i.e. set a current limit. Otherwise you could feed the enable pin of the HBRidge with a PWM signal from the Arudino, to modulate the current. As the datasheet says in haiku-like english:

"Between this pin and
ground is connected
the sense resistor
to control the current
of the load. "

( The datasheet also says the package will dissipate 25W, but I imagine that is with a heatsink.)

D
« Last Edit: July 26, 2007, 04:44:00 am by Daniel » Logged

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aha. click. Now I see the point in those resistors. SO i can run VSS at As +2.5 but set my current correctly. doh!
thanks again
N

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hi

nope.. Vss= VIH + 2.5 = 4.8....
Try 9V power with 5V logic, and the resistors.

D
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oops. typo meant Vs not As.
Just got some very fat 2W 0.5ohm and 2.2ohm resistors. Back to work,
Thanks
N
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