Drive-by Q on Stepper Motor Drivers

Forgive me as I am just getting a lot of eye-tracks over the board and site and while interested I'm only sorta kinda maybe thinking about doing a 3d printer a couple projects down the road. But I have ordered half a dozen surplus steppers powerful and sized enough to use if I decide to ... they look like fascinating toys at any event.

Anyway, I've looked at how to control them, looked at what reprap does, and my eyebrow muscles strained at trying to find an A3983.... I see elsewhere in this subforum that there's a new version in the works with the A3977. But both these chips seem a bit expensive and difficult to procure (Digikey, "not in stock" but orderable). But I see the A4983 is almost only half the cost ... and Pololu has this breakout board for it which has most of the electronics on it.

Am I missing something or would using the breakout board and soldering the connectors reprap uses to the right pads on the breakout board give me pretty much transparent functionality with reprap's boards?

Of course the question is motivated by the idea that if, down the road a bit, I do decide to do a reprap, and I buy a couple of these breakouts in the meantime to play with, can I do the above and "get away" with it for a reprap compatible printer.

Additionally, just throwing the question out on the 4983's on the wild possibility that their capabilities (16th step) might have been missed.

-- TWZ

I found that the A4983 can only handle about half an amp without heatsinking, whereas the larger A39xx series can handle a bit more (don't know how much exactly).

My mendel is using 4x A4983 breakout boards from pololu, and I've just strapped heatsinks to them, which allows them to give me 1.5A or so.

this is plenty for the axes, still yet to see if it's enough for the extruder. If not, allegro makes a stepper driver chip which drives a pair of external mosfet bridges rather than having them internally, or I may just jury-rig some up to one of my A4983s.

I have posted about them on these forums

For another option, TI's cheaper DRV8811 is pin-compatible with the TSSOP A3977, but the official stepper boards use the PLCC A3977 so the boards would need a redesign to use it.

I cant see why you can't just drive a bigger mosfet bridge from the internal one.

or am I missing some thing here?

Maybe I need to take another look at the internal structue of these devices.

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Bodge It [reprap.org]
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BIQ Sanguinololu SD LCD board	BIQ Stepcon	BIQ Opto Endstop
BIQ Heater Block PCB	BIQ Extruder Peek clamp replacement	BIQ Huxley Seedling
BIQ Sanguinololu mounting	BIQ standalone Sanguinololu or Ramps mounting	Print It Stick It Cut it

My rep strap: [repstrapbertha.blogspot.com]

Buy the bits from B&Q pipestrap [diyrepstrap.blogspot.com]
How to Build a Darwin without any Rep Rap Parts [repstrapdarwin.blogspot.com]
Web Site [www.takeaway3dtech.com]

The easiest way to drive a bigger mosfet is to set up a n- and p-channel pair, however p-channel mosfets can't handle as much power as n-channel mosfets because the majority carriers are physically larger or something fundamental like that.

also, the chip has built-in dead time and synchronous rectification and stuff which works best when the top- and bottom-side mosfets are individually controllable. I'm really not sure how it would translate through an external buffer.

SO, it's possible to slap a high current inverting buffer on the output, but probably not ideal. I may try it at some point, as there are some stunning fets out there these days like ST's 0.0008 ohm one and thousands with perfectly sufficient intermediate capabilities

I'm a bit new to this stuff, but wouldn't you lose the current limiting feature which makes it possible to overdrive the stepper at overvoltage without burning it out?

-- TWZ

the sense resistor is external, so you can just hook the tail of your external bridge to it and retain current sensing.

I think I will be trying this method out also as I have some bigger steppers that can not be controlled with 2A controllers.

I do have 3 4.2A micro stepping controllers so its not yet an urgent requirement.

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Bodge It [reprap.org]
=======================================

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BIQ Sanguinololu SD LCD board	BIQ Stepcon	BIQ Opto Endstop
BIQ Heater Block PCB	BIQ Extruder Peek clamp replacement	BIQ Huxley Seedling
BIQ Sanguinololu mounting	BIQ standalone Sanguinololu or Ramps mounting	Print It Stick It Cut it

My rep strap: [repstrapbertha.blogspot.com]

Buy the bits from B&Q pipestrap [diyrepstrap.blogspot.com]
How to Build a Darwin without any Rep Rap Parts [repstrapdarwin.blogspot.com]
Web Site [www.takeaway3dtech.com]

Sorry to be such a pain, but I'm still not getting this.

The sense resistor can tell the controller that the current coming out is too high, yes, but as I see how I think the (second/slave) bridge would be connected, the controller has no control over the current or voltage coming out of the slave.

As the bridge inputs are effectively rendering the controller outputs as purely digital inputs to the slave-bridge, the only thing the controller could do would be to turn the slave-bridge off, then back on which I would think could lead to some unpleasant resonances given the motor coils involved.

If there is some kind of standard practice connection finesss to overcome the above could you point me to a link?

Thanks.

-- TWZ

Triffid_Hunter

How about a picture of your heat sinks?

TWZ - the driver chip has NO idea where the feedback signal is coming from - just that there is a voltage on a pin that it has to compare against its internal standard. So if you scale the sense resistor to give the right voltage just before the overload current/voltage is reached at the amplifier outputs, the driver chip will shut off current to protect the amplifier FETs as you want it to.

And yes, the driver chip will just basically turn everything off if it senses an overload. The fly back diodes in /around the driver FETs take care of the stepper coil's back EMF. Steppers are generally driven digitally - full power or nothing, so if you are getting heating in your output FETs it should not be happening. At a typical on resistance of 0.001 ohms, and say 30 volts, the FET should only be emitting .03watts, whch basically means the FET should be at ambient temp. If the FETS are getting hot it means ringing oscillations are occuring (quite common)and the FET is acting as a load dump. The fix is to put a high wattage, low value resistor across each coil of the stepper. Try a value of about 1.5 times the motor coils impedence as a start.
The other thing to cause FETs to get hot when operating is not enough drive voltage. I always aim for about half the value of the output voltage onto the gate or 2 volts less than Gvmax, whichever I get to first. so in the example above, I would aim for 15 volts if it didn't exceed Gvmax. If for example Gvmax was only 10 volts, I would aim for 8 volts.

freds, [wooden-mendel.blogspot.com]
I'm planning on getting a full 40mm chipset heatsink and arranging my 4 drivers under it swastika-style, with the adjustor pots hanging out the side and dimples cut for the Vbb capacitor. I think that'll work much better than what I've posted in my blog as a first attempt.

TWZ, yeah it's not ideal. As RonP says, you would need some low value resistors on the gates to turn off the external bridge when the current chopper kills the output. that's why I suggested the chip that's designed to drive an external bridge as the preferable option.

The internal fets in the A4983 have Rds(on) of about 0.45 ohms, so I^2.R heating is significant. Also, the chip's package is tiny - about 3mm per side, so it has only a tiny surface area with which to dissipate the heat- Tj-amb is about 115C/watt with no heatsink.
This is why we're discussing externalising the power switching in the first place- the 16x microstepping and current chopper are most useful, but the abysmal thermal dissipation isn't so great.