PIC Step & H-Bridge

[eSilviu]

Originally Posted by dalescnc hi all Anyone have a link to circuit using L297 with 2 L6203 outputs with filter back to the ref voltage or gated back to the ref ? thanks

hey, what allegro stepper drivers?! they can microstep, 2A/35 or 50V, and can be drived with step/dir/en signals, right from parallel port

[pminmo]

Microstepping is a function of the translator, and chopper circuitry, not the driving bridge. L298's and L6203's can be employed in microsteping circuits, however since the have limited functionality on decay settings, they won't be as robust as a discrete bridge. The LM18245 has the dac and pwm functions in the part besides the bridge, the L298 and L6203 doesn't. My experimentation with the L6203 while not conclusive, leads me to believe it dissipates a large amount of power for the current it is driving.

[dalescnc]

Thanks for the reply only reason im asking is the LMD is very hard to get here. So looking for a good alternative tried l297 & l6203 but wasnt happy with the current hissing on the feed back to the ref. will try a comparator and exp with the feed back. They cheap so if i blow a couple its ok ;-)

[pminmo]

The current hissing is possibly a product of higher current ripple which gets back to the decay modes.

[dalescnc]

Hiya all eventually got a couple of LMD18245T for a good price i think ZAR 190.00 and plenty more to come lol Built 4 of the infamous picstep boards ;-) bows down to Alan the problem i have is im running the boards @ 50volts i have 3 85mm frame motors bi polar rated @ 5.6 amps my Z is belt driven with a T5 belt 15 tooth gear. The current setting i have @ 3 amps with all the boards fitted on a old sub woofer heat sink wich i had in my shop. The LMD's do not even get slighty warm due to the heat dissipation im getting. I wanna make sure im getting the full 3 amps to the motor or if i can maybe push it to 4 amps? From what i have read its just a guide line as to the current setting i have'nt seen much about the resistance of the coils taken into the calculation. Hence i dont think im getting the full 3 amps. Even with such a big heat sink 6" X 8" . Any sugestions would be appriciated. Thanks

[pminmo]

You maybe ok pushing the 3A setting as that is a continuous rating. You only run 100 percent of the rating at specific angles when micro stepping on one phase, so during rotation each phase varies between zero current and 100 percent. Since your ratio of supply voltage to motor coil is high the on time duty cycle is lower dropping the actual average current. Worst case would be stopping at a 100 percent point for long periods of time. I guess if your burn up the LMD's then you know for sure, yea or ney.....

[Boomslang]

I wanted to use AN822 as well, but the code MC supply does not provide a way to set the current (I know assembly, but I'm not really in the mood for coding... I want to get my cnc running). You would have to write your own "chopper" code or use inline resistors.
If you use the code change the step/direction input code to use IRQ's and remove all the RS232 parts/code.
One bonus is you can get the PIC's from MC as demo chips...

[dalescnc]

Thanks for the post! with a digital meter even if thats fast enough could i measure the current while running the any of the motors ? or should i wait till i load the machine with a cut out then while is cutting measure the amps? i just think that i might be able to get more out of what i have due to the size of the motors. I know if i over kill the current setting of the LMD's the proc should kick in. Maybe its worth killing 2 LMD's for the sake of education and perhaps others can learn by it lol Just every thing is running to cool for me and i think i could get another 20-30 % of what i have the Z is ok due to the lead screw but the X & Y belt drive im not really happy with. ? the most current the motors take is when in motion or in holding. As i have mention the motors i have are rated 5.6 Amp would that be per winding? They are monster's, apperciate your feedback Dale

[student1616]

hhhhmmmm...

man that was pretty costly.this Lmd18245 is far more costly than i imagined.
there has to be some replacement part which work same as LMD.

i already built the bi-polar driver using l297+l298.It worked well,but motor buzzes a lot.so i want to go for micro stepping.as bi-polar driving is better than unipolar,i want to go with picstep.
so has any one tried other chips than LMD18245

[Boomslang]

So I'm not the only one thinking the LMD is costly... maybe it will be worth my effort to change the circuit/driver and code, as others can benifit from it as well.

Quick question without me having to read all the datasheets: What would be the best (don't forget cheap) chip to replace the LMD (it does not need to be pin compatible)? The obvious answer for me is the 298, but the technology/features of that chip is a bit old nowadays...

[dalescnc]

Been a few months now and i have had absolutley no problems with the picstep baords. 2nd best to comercial controlers. If anyone is starting out with cnc dont waste your time with any other boards. I have 4 control boards mounted onto an old sub woofer heat sink no matter how hard i labour the machine the boards remain at cool temp remember heat KILLS. Just a note to say thanks to Alan for the great boards !!!!