Chopper Help!!!

[ez-cnc.com]

I made my own board design using a pic and 4 irf540. Now im interested in adding a current chopper. Any ideas for a simple chopper circuit??? I was also thinking to add a simple pwm using a 555 with no feedback(just a setable pwm) is this ok???
I mean addind a pwmed signal that can be set, but no current feedback to comapre.
Thanks.

[H500]

An open loop pwm is not worth doing since the performance is so much lower than a "real" chopper. Unless the pwm freq is very high (hundreds of kilohertz?) it might not even work since the pwm will turn off the drive before the rotor have a chance to step into position.

Simplest way is to use a pair of comparators to sense the currents. If there's a PIC with 2 comparators, you won't need extra chips. Chopping is very demanding on the PIC. You only have 20-50uS to do everything. Adding an external monostable would make things much easier.

[ez-cnc.com]

I saw somewhere also the posiblity to add an external transistor current regulator, is it a good idea??? ( humm lots of power to dissipate )
Bu ¡t i was thinking of an external current chopper, any ideas????
May be a unique sense resistor and a mosfet chopper.Ç
Thanks.

[pminmo]

I see where you have a website, is this for a commercial application?

[smarbaga]

you could use a current monitoring chip and have its output going into your pic, which intern may decrease a digital pot driving a voltage amp, which drives power transistors.
with mosfets they are either on or off, making it hard to regulate the current but easy to chop it.
things can start out simple and get comples real quick.
but the rewards are usually worth it

[ez-cnc.com]

well its not a bad idea to regulate a transistor. But its the same situation as having an external not chopped regulator, lots of heat to disipate. There has to be an easy way to make an external chopper (with feedback) with few components. But my problem is that I have to put this on the high side. May be using some kind of linear optocoupler to sense on the high side??? Any ideas???
Thanks, Matias.

[BCwanderer]

Why do you have to chop the high side?

[ez-cnc.com]

Yep u r right, no need to do it in the high side, ill make it in the low side. But the sense resistor is gonne have to be really low value so that the voltage dropis really low and doesnt afect the other transistors.
It just came to my mind, what about sensing direclty in between source and drain of the mosfet??? am i too crazy???

[DaRy]

Try this link. There is a simple current limiting chopper in the "Practical Examples" section using only simple components. I've not tried it myself thou.

http://www.cs.uiowa.edu/~jones/step/current.html

[kreutz]

Try this thread: unipolar motor microstepping on that thread, We designed a fixed frequency chopper, and it is already tested two ways: first, by using one LM555 as pwm oscillator, used also to set the blanking time for the current compare circuit, and second, using a pwm channel on a microcontroller to do the same thing with adjustable blanking time.