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Looking for a sanity check on a stepper driver circuit

I've got a radio-controlled step-sequencer/driver built, tested, and functional for driving a single steper motor.

As it sits, it controls one stepper motor - Call that one "Stepper A." The four separate outputs of the sequencer are each as shown in the "Driver" drawing below. Each connects to a "Phase #" line in the "Motor side" drawing below that. The line of "========" in the "Motor side" drawing is the demarcation between the existing circuit (above the line) and proposed additions (below the line.)

My goal is to control a second stepper - Call this one "Stepper B" - separately from the first. At any given moment, Stepper A might be active, OR Stepper B might be active, or both might be INACTIVE, but there will (or at least, according to plans, there *SHOULD*) never be a time when both A and B are active simultaneously.

I've been doing some thinking, and it seems as though I should be able to do what I'm looking for WITHOUT building a second sequencer and the associated circuitry that goes with it. Existing circuitry on the R/C board I'm already controlling "Stepper A" with can easily provide me with an A and a B signal, to drive points A and B in the "Proposed additions" section of the drawing.

What I'm looking for is someone to confirm or deny the sanity involved in my thinking.

Anybody see anything blatantly wrong with my "brainstorm" as drawn below? (ignore "cosmetics" such as lack of any back-EMF diodes in the drawing - it's much simpler for me to draw it without them, with the understanding that they are needed, and actually present, in the circuit)

I *AM* wondering if I ought to toss some "steering" diodes in between Stepper A and Stepper B, say at the locations marked "D?" though... Good idea, or unneccesary "overkill"?

Driver (four separate circuits, all identical)

,------O Phase "X" |E (To "Motor Side" phases, below) B | Control "X" O-----------K NPN (2N6387) (from sequencer logic |C via optocoupler) | | --- -

Motor side Phase 1 O--+-------D?---UUU---, ---, | | | Phase 2 O--(-+-----D?---UUU---+ | | | +------, |--- Stepper A Phase 3 O--(-(-+---D?---UUU---+ | | | | | | | | Phase 4 O--(-(-(-+-D?---UUU---' | ---' | | | | | | | | | +------------O +12VDC | | | | | Present stepper power connection | | | | | (would be deleted for the | | | | | second motor/selector additions) | | | | | ======================================================================== | | | | | | | | | | | | | `------UUU---, | ---, | | | | | | | | `--------UUU---+ | | | | +---, | |--- Stepper B | `----------UUU---+ | | | | | | | | `------------UUU---' | | ---' | | | | Proposed motor "selector" ,-------------------' | and second stepper motor | | additions B |C | B O----------K NPN (2N6387) | |E | | |

+12VDC O------+ | |E | B | | A O----------K NPN (2N6387) | |C | | | `----------------------' 
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Don Bruder - dakidd@sonic.net - If your "From:" address isn't on my whitelist,
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Reply to
Don Bruder
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Well, it certainly isn't going to work with NPN transistors wired as shown above.

Reply to
Chuck Harris

you'll need them on both motors.

without them curent will flow forwards through half of the "unselected" motor and backwards out the other half of it... then though the "undriven" wuindings of the "powered" motor.

so your selector won't work, and torque will be reduced.

even with the diodes

you'll find that the deselected motor will have less torque than when it was powered.

switching between motors when the sequencer is in a different configuration than before the last switch could prove interesting too.

--

Bye.
   Jasen
Reply to
Jasen Betts

Steering diodes are necessary on both motors - all windings, and those should be PNP transistors if you planning on switching +12 connected to the emitters.

Other than that I don't see why it wouldn't work

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I figured that would probably be the case. Diodes it is, then, and on both motors.

I'm assuming you mean holding torque (Which is pretty much irrelevant to my project)? Obviously, I'm going to lose worth of voltage when a given coil is powered, but I can't see any other reason for loss of power - assuming the coil *IS* powered.

On that topic, I already expect that I'll see a "twitch" as the newly selected motor powers up and the mag-fields "line up" to match whatever state the sequencer is in. This is non-critical to the project - In fact, due to the combination of small step-size (both motors are 1.8 deg/step units) and the fairly high-ratio reduction gearheads on them (According to "count the teeth and do the math", one is a 51.5:1 reduction, and the other is a 62.3:1 reduction - assuming I didn't mis-count gear teeth) that "twitch" might not even be measurable without resorting to much more precise tools than I have, or have any interest in using - Worst case I can come up with is the "new" motor powering up

180 degrees out of phase from the current sequencer state, causing a 2 step jump as it synchs up - Barely noticable, and completely irrelevant for the project. 
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Don Bruder - dakidd@sonic.net - If your "From:" address isn't on my whitelist,
or the subject of the message doesn't contain the exact text "PopperAndShadow"
somewhere, any message sent to this address will go in the garbage without my
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Reply to
Don Bruder

AUGH! That's what I get for making the drawing from memory of assembling the circuit... OK, I've got my transistors wired backwards in the drawing - Good catch. Other than that little brain-fart, any other obvious problems?

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Don Bruder - dakidd@sonic.net - If your "From:" address isn't on my whitelist,
or the subject of the message doesn't contain the exact text "PopperAndShadow"
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Reply to
Don Bruder

Which confirms what I already suspected - A second time, even :)

Yah... Somebody else caught that little brain-fart, too... Swap E and C on all the transistors, and you've got what I actually meant.

Great - I'm *NOT* totally crazy! :)

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Reply to
Don Bruder

Or use a PNP it will drop a lot less voltage as a high side switch. NPN as a high side switch drops four volts and PNP drops point six.

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yeah that's what I meant.

that's ok then.

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Bye.
   Jasen
Reply to
Jasen Betts

If holding torque is not an issue, then just diode it up, and have fun! This can be expanded to as many motors as you want, as long as you're sure that you understand that you can only drive one at a time. ;-) I once modified some other guy's Z80 stepper driver's timing loops, so instead of going "CLACKETY-CLACKETY-CLACKETY etc," the actuators went, "bvvvvvvvvvvvp." And the client and all of his minions went, "Ooh! Aah!" It was very gratifying. :-)

Cheers! Rich

Reply to
Rich Grise

I agree - I also wouldn't use an emitter follower in this app, because it exposes the poor little E-B junction to the inductive kickback, which with a stepper, is not a bad thing (the kickback, that is - the high E-B reverse voltage is a bad thing in any transistor circuit), if you want any speed out of it. ;-) (the higher you can let the spike go, the faster the magnetic field at that pole will decay.) So, you protect your transistors by giving them a high BVcbx and shunting them with zeners. (or tranzorbs, if you're switching insanely high amounts of power. ;-) )

Good Luck! Rich

Reply to
Rich Grise

NPN high side switch drops point six volts, PNP drops point two or less. ;-)

Cheers! Rich

Reply to
Rich Grise

I think you have it backwards. the common type of self biased collector input npn power supply requires a full four volts higher across it to stay in regulation - a switch should do the same.

But just the base needs a greater voltage on it so one way to keep the output or efficiency up is to return the base bias to a higher voltage power supply than the collector input.

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