Chopper for AVR from complete newbie, comments pls

[genericuser]

Hello zoners,

I've been playing with the idea of connecting up my AVR 8535 to some steppers for awhile now (especially after seeing H500s "Avr Unipolar Chopper Drive"), and here's what I've come up with. Being a relative newbie at electronic circuitry I would appreciate some tips/pointers to anything I have missed. There's probably a lot, as I didn't really expect this circuit to work as it seemed just a little too simple, but I have it powering a 1.34 Amp 4V stepper with a 12V supply with great success. No overheating anywhere.

It get's it pulses from the 8535 AVR, which is just putting out half-step pulses with no connection to the PC, although this will be done shortly.

All this is currently sitting on my breadboard and I'd like to move it to a proto-board but would like some feedback beforehand.

FYI R6 trimpot is around 3k

Do you think that this design would work for larger stepper?

All comments appreciated

Cheers
generic

[H500]

Good work Gen! I originally tried to do the chopping by hardware using a single 324 quad op amp as the comparators, but it was too unstable. Glad to see your's work so well. Software chopping chews up too much cpu time.

One thing to note is that at high stepping rates, the inductance of the motors become the limiting factor and the current never reaches the point where the chopper would kick in. So the acid test is to use a low step rate and a higher voltage, 20 volts or more. Make sure you monitor the current in case the limiter don't work. Keep in mind that the current flow in the sensing resistor is bi-directional, so a simple voltage measurement might not give a true reading. A scope is best.

Also keep in mind that the voltage across the mosfets is TWICE the supply voltage.

To avoid resonance problems, micro-stepping is extremely desirable. In theory, it would be easy to use 6 MCU pins to make two 3 bit resistor ladder D/A converters to feed varying references to each of the two comparators.

Keep us posted on your progress!

[pminmo]

Great work generic,
Looks like you captured it in eagle, why not do a toner transfer pcb?

Phil

[genericuser]

Hey H500 & phil,

Thanks for the encouragement, I've spent the easter break fishing, not much on the bite which gave me some quite time to think about this project

Originally Posted by H500 One thing to note is that at high stepping rates, the inductance of the motors become the limiting factor and the current never reaches the point where the chopper would kick in. So the acid test is to use a low step rate and a higher voltage, 20 volts or more. Make sure you monitor the current in case the limiter don't work. Keep in mind that the current flow in the sensing resistor is bi-directional, so a simple voltage measurement might not give a true reading. A scope is best.

I working on getting a 24volt supply going to test it at a slow stepping rate as you suggested, but have no scope to monitor.

Originally Posted by H500 Also keep in mind that the voltage across the mosfets is TWICE the supply voltage.

Here's where my newbieness come into play, I'm not sure why the voltage across the mosfets is 2X supply. Can you explain briefly so I can further investigate? I assume your making sure that if I increase supply then I must also ensure mosfet max volts is in safe limit.

Originally Posted by H500 In theory, it would be easy to use 6 MCU pins to make two 3 bit resistor ladder D/A converters to feed varying references to each of the two comparators.

This makes sense to me and would be a good use of an avr, it's now on my next to do list.

Originally Posted by pminmo Looks like you captured it in eagle, why not do a toner transfer pcb?

my first attempt at eagle, I've never done a toner transfer board before. I'm going to attempt to implement some of the suggestions of H500 and if that works out I'll give it a go.

Cheers
generic

[pminmo]

RE - Twice the supply voltage. Same concept as a relay coil, when you open the switch the coil has a voltage potential stored that is roughly the same value as the supply voltage across the coil when it was on. That coil voltage is additive to the power supply, thus roughly twice the power supply voltage.

Phil

[H500]

Originally Posted by genericuser Hey H500 & phil, Here's where my newbieness come into play, I'm not sure why the voltage across the mosfets is 2X supply. Can you explain briefly so I can further investigate? I assume your making sure that if I increase supply then I must also ensure mosfet max volts is in safe limit. generic

When the chopper fet turns off, the current in the coil ceases instantly. Those who are familiar with inductors might ask how this can happen since it violates the fundamental characteristics. The answer is that the motor acts like a transformer rather than an inductor.

The + and - phase coils on a unipolar motor is wound bifilar, ie: two strands of wire are wound on the same core.

So, when the fet turns off, the current is actually transferred to the other coil, flowing back into the power supply. This transformer action is what doubles the voltage on the fet

If you are interested in the mathematical details, just read up on transformer theory. Otherwise, just remember to use fets with at least twice the voltage rating as the supply.

[genericuser]

Thanks Phil & H500,

I've done some reading on transformers and have a clearer understanding. The IRFZ44 fet has a 55V Drain-Source max, so I'll go with a 24v supply. I'm toying with the idea of building one, as trying to source a surplus 3A 24v supply for testing is proving difficult.

Will hopefully have phase 2 of design with micro stepping shortly.

Cheers
generic

[pminmo]

Originally Posted by H500 When the chopper fet turns off, the current in the coil ceases instantly. Those who are familiar with inductors might ask how this can happen since it violates the fundamental characteristics.

What fundamental chararestic does the inductor have that current flow can continue in an open circuit?

Phil

[Dr.H]

current flow will continue in the air for a split second. Thats what create a spark in your switch

[pminmo]

I thought the spark was because of air ionization creating a conductive path? The moment current is interrupted in an inductor, the inductor opposes current change, and voltage rises to maintain current flow.

Phil

[Dr.H]

Thats right

voltage raise to maintain current flow. It raise so much that it will create a spark witch is air ionization

[H500]

Try measuring the voltage across an inductor after the current is interrupted. You will find that it rises to several times the supply voltage (ie: more than just twice)