Problem- Low Current interface, High Current Motor?

[Clackvalve]

Hi,

I'm thinking of buying a small (compact) stepper interface, rated less than 2 amps per phase, but I want to run it on a 2.5 amp per phase stepper motor. Can anyone tell me if I can get away with just putting an FET on each phase?

I am CNC'ing a large'ish' Mill, so I need fairly powerful motors to run it. I know it is easier and simpler to buy a >2.5a rated controller, but this controller has button controlls so I don't have to boot the very slow, very old laptop to move the leadscrews (I also have mobility problems) The mill is a far eastern model purchased in the UK from WARCO, model ZX-15 and I am on a non existant budget.

I'm no expert when it comes to electronics so be gentle with me. Any information would be great, thanks

Ian

[serriadh]

It isn't necessarily a good idea to run steppers at their current limit; they'll get pretty hot. Running them slightly under that limit would be fine.

At a guess, if it were an easy thing to uprate a motor driver, it would be done more often and geckodrives wouldn't do such good business ;-)

[Clackvalve]

Hi serriadh,

Thanks for the comments. I think I may not have made myself clear. I don't want to run the low current driver at max, I wondered if it was possible to add a power FET on each phase to take the load, so the driver wouldn't strain and burn out.

Thanks anyway...... Clackvalve

[ger21]

A drive won't try to deliver more than it's rating. If it's a 2 amp drive, it'll deliver up to 2 amps, all day long. You won't damage it by using higher current motors. The drive dictates the current, not the motor.

[Clackvalve]

Hi, Thanks for the info ger21, I didn't know that. I guess there must be a current limmiting circuit of some sort built into the controller?

I'm trying to find out if I can add a fet to each phase, so I can use the motors at there full potential....

Clackvalve.

[JonLed]

Quote: "I'm trying to find out if I can add a fet to each phase, so I can use the motors at there full potential....

Clackvalve. "


I'm not sure of all the details, but it should be possible to add a secondary amplifier to the output of your drive. This would mean another power supply with a larger transformer and smoothing caps (unless the supply you have is sufficient for the load). It would also mean having the correctly designed switching circuitry for the FETS in the 2nd stage, considering the 1st stage output characteristics.

There maybe some sacrifice in performance, but maybe not if properly designed.

Jon

[Clackvalve]

Hi, JonLed,

The switching circuitry is the bit that is beyond me, I know that when these steppers are moving there is a portion of back emf to consider but I think I could just bung a fast diode in front of the fet to stop that doing any damage.

Power supply is not a problem, I have a variable voltage 30A PSU to hand so no probs there. The driver I am considering is in the UK If your interwested its here>>>> http://www.virtualvillage.co.uk/step...06408-001.html

cheap but as I said in a previous message the buttons are useful.

Clackvalve.

[JonLed]

That uses a L298 driver, if you're good you could parallel another to it, see data sheet. There are tons of DIY drives on this site you could go with instead you messing around with mods. How largish is this machine anyway?

Jon

[Clackvalve]

Jon,

I have seen a few L298 circuits dotted around but they all seem very simple circuits I am all for KISS philosophy but I would like to be able to add a simple remote so I can use the mill without having to boot up my rediculosly slow very old laptop every time I want to use the mill. Hand cranking is very difficult, I have early onslaught Arthritis.

That is why I am so desperate to use the controllers I described. Not only are they cheap (£8.99) but they have forward, reverse, fast & slow buttons I can use. I have to admit I haven't had a look in the files lately so I'll have a butchers to see if there is any inspiration.

Thanks Jon....... Ian

[JonLed]

A simple micro-controller could be interfaced to just about any stepper drive to give you those functions you want. You didn't mention how largish this machine is. Are you sure 2.5 amps is going to give you enough torque to move it?

Jon

[JonLed]

Ok, I found the mill here http://www.warco.co.uk/ZX-15-Milling...B040AEC96.aspx

Do you have any experience making PCB's?

Jon

[Clackvalve]

Yes, I have made many pcb's. As a radio ham I have built many commercial kits. I have made a few "spider web" PCB's, and chemically etched them. I dont have the UV stuff now though I have still got the heated etching tank. Of course mostly RF/audio.

Yep, that is my mill, I should say it is of oriental origin and has no ball race type bearings "YET" and there needs to be some polising of gibs and ways etc all making it more difficult to crank so I thought I would combine both projects into one. That way I won't have to take it apart twice.

The micro controller sounds a little involved, I was hoping to keep this strait forward, hoping that the down time is kept to a minimum. I have no commercial interests but I do have an estimated 5 year project I am about half way through. There will of course be a time that I will not be able to use the machinery so time is limmited for me. Again that is why I was hoping that the controllers I found would do the job.

Ian