Why wont this work?????

[epineh]

I was thinking about a simple bipolar stepper driver, what if you got a simple microstepping driver chip that drives a small stepper directly, say the Allegro A3967, which has nice features, but limited to 750 mA of motor current, and added a little circuit on the output to drive a push-pull type stage.

Lets say 4 of IRF 540's.

I didn't get a price on the chip but lets say it costs $10.00 and the FET's cost about $15.00-$20.00, thats a $30.00 bipolar stepper driver, with 100V 20 amp capability...

There would have to be some kind of connection for the current sensing, but little else. The resistors are just to give some kind of balancing load, and would be the same value, and rated to suit the additional FET gate currents

I'm sure there is some reason it cannot be done, but I can't see it, so who is going to burst this bubble ??? I have put a pic of the schematic to extract the positive and negative signals to fire the push-pull stage, don't laugh I did it in a bit of a hurry.

I bet it is Al or Kreutz.

[epineh]

No answer, does this mean it will work ?

I realize that IRF 540's are probably not the correct transistor, but apart from that...

[Madclicker]

There are chips that are made to drive mosfets.

[epineh]

Yes but the Allegro chip will accept step and direction, take care of the microstepping, current control, current decay control scheme. All in a chip for about 5 bucks, the only problem for CNC apps is the low current rating, which is why I thought about "tinkering" with the output stage a little.

This would result in a very cheap microstepping driver with the capability of running up to 100V supply and 10-20 amps...

Russell.

[NC Cams]

You want the fets to get voltage fast to turn them on and likewise to turn them off.

Depending on the fet, lets say an IRLZ44 logig level, you'll see a voltage drop going thru the diode. This drop, even on a logic level fet, can affect the saturation potential of the fet - this causes speed and heat problems.

We played around a LOT with fets when we did the R/C car thing. 5 volt logic level fets ran warm at 5 volt enhancement, they were stone cold and FAST at 10volts.

Same for IRFZ44. These would run OK at 10-12 volts, there was noticeable heat in them. Yet when we hit them with 18vdc Vgs with a 20 vdc Vgs limit, they too ran stone cold and switched FAST and hard.

The advice - use fet drivers and as much voltage as the gates will safely handle. The circuits will switch fast and hard and the fets will run cool. Lame drivers aren't worth the failed parts that they ineveitably cause.

If you persist on using diodes, use Schottky's, they switch faster and with less voltage drop than 1N4000 through 4004 series.

[epineh]

The 3967 will accept 30Volts as a motor supply voltage so I should be able to fire a pretty high gate voltage.

Cheers NC Cams.

[ViperTX]

Well you should put together the whole schematic and build it. Let us know how it goes.

[epineh]

Originally Posted by ViperTX Well you should put together the whole schematic and build it. Let us know how it goes.

Roger that...just have to finish my little A3986 project first.

I especially like the DIP package, whole lot easier to play with than the 3986's TSSOP