Doubling down on motor drivers

Motor driver chip too weak for your needs? Just use two of them. That’s the advice which [Starlino] gives. He stacks motor driver chips to product move powerful controllers.

When stacked as shown, the driver combos should be able to drive at 4A. This is partly because he ganged together the outputs in pairs, and also because of the stacking. That’s a lot of juice, but [Starlino] documented his testing stage which shows that they’re up to it. It’s a bit hard to see from this angle, but he is using a serpentine heat sink. It snakes its way between the stack of chips, then over the top chip before folding back and spreading its wings. The motors he’s using have a stall current of 3.7A, and he included resettable fuses graded at a 2A hold current. He’ll be glad to have that extra protection is something goes wrong with the drivers.

[Thanks Roger]

Comments

  1. Mike says:

    I don’t know, seems a bit dodgy. Unless they are both from the same batch, I would expect the two to have slightly different response time, and over time, I think you could fry them.

    • Barry says:

      The SN754410 is just a Quad H Bridge, merely a few PNP and NPN transistors in an IC shell. I doubt that the tolerances on the chips would be great enough to cause any problems.

      My Robotics Professor did this when the current from some motors was literally setting single chips on fire (, the problem was a weird bug in the code that caused the signal to alternate rapidly, but we tried this method before we found the source of the problem, as it is hard to debug when your robot is on fire).

      • SD says:

        I did this last year for a class project (Kinect + Toy Gun + Turret combo driven by a really big DC motor for moving the gun platform around …). We had a few instances of blowing up the SN754410 chips, but they were mostly attributed to shoot-through condition. Apparently the guy who programmed the PIC controlling this concoction wasn’t sensitive to the timing constraints for switching directions. Polarity was switched too soon and the back EMF popped chips like popcorn.

  2. tim says:

    nice, I love the heatsink design.
    The stacking thing is not new, many people do this as a cheap way to drive high amperage motors.

  3. Sven says:

    Ruining the chips because of timing differences or oscillations in this type of configuration is not unlikely. Also i wouldn’t even consider running 4A in a low-cost DIL socket. Maybe in a high quality socket.

  4. ggm says:

    Aren’t BJT transistors like the ones in this IC bad candidates for parallel operation because of the thermal runaway situation?

  5. NewCommentor1283 says:

    1) they’ll all have the same response time… they are analog and identical shadow mask

    2) correct that the socket might not be rated for 4A, but for short term its okay as long as you inspect for blackened contacts hourly/daily/ect

    3) its a very good idea but i would have added equilization resistors… if theres a difference in chips, it’ll be the voltage drop from power-in to motor-out, so put each output pin in series with a 0.1 or 0.01 ohm resistor so they share the currect aprox 50%-50% each, even when they have vastly different voltage drops (EG: 0.69v vs 0.72)

    • NewCommentor1283 says:

      they teach you to do that in (ancient) linear powersupply designs made from discreet transistors… if using 10 transistors in parallel to get 10x output current, then add 0.1 or 0.01 ohm in series to EACH output transistor THEN tie all in parallel

    • jordan says:

      #3 is a cool trick that I hadn’t heard before. Thanks!

    • Chris C. says:

      +1 to #3, that’s some sage advice right there.

      • Le Samourai says:

        I’m not understanding exactly what he’s saying there in #3. The drive pins have the series resistor, and the other end of the resistors are ganged to form the output? Or do you need another resistor to tie the outputs from the 0.1 ohms drive-side?

      • Mikey says:

        Yeah, this seems a bit unintuitive to me as well..

        I mean, even with low voltages (like say 3 volts), running 3v through a .1 ohm resistor, that resistor would be trying to draw 30 amps (and probably glow red-hot)…

        Let alone, if this were being used to drive a 12 volt bi-polar stepper with a .01 ohm resistor — that resistor would be trying to draw 1200 amps!

        Can someone please clarify WTH you’re all talking about?

        • Le Samourai says:

          @Mikey: You’re not sending the current through the 0.1 ohm resistor to ground (“parallel to ground”) you’re putting it inline with the motor drive power lines to do this “balancing” they are talking about. Motor coils have a finite resistance, between a few ohms to a few hundred ohms, depending on the motor.

          What I don’t understand is how these series resistors do any “current balancing”…

  6. Otte says:

    Ah… a Gillette Fusion ProGlide Power approach to stepper motors. I like!

  7. Alistair says:

    An L298n dual full bridge driver isn’t very expensive and will do 4A when you tie the two drivers together (That’s one of the examples in the datasheet btw)
    They come in a multiwatt-15 package so you can bolt them to a huge heatsink too.

  8. Przemek Klosowski says:

    You can’t double up everything. The problem is that they will never be exactly the same: even if the output stage semiconductor devices are from the same batch, they can be influenced by temperature drifts, connection lengths, etc. If their temperature coefficient is positive, it’ll run away from equilibrium: more power through one part means higher temperature means more power.

    Bipolar (traditional, pnp/npn) transistors have this problem; MOSFETS don’t. The problem can be remediated by ballast resistors, which however are tricky (they are in the main power path).

    Bottom line/TL;DR: sometimes it can be done but it actually requires some thought.

    • GS says:

      And exactly why is it that mosfets don’t share this same problem? They have unique parameters from chip to chip too as far as I remember when doing the same thing for a low side PWM power control circuit.

      http://www.solorb.com/elect/solarcirc/pwm1/

      • William says:

        MOSFETs have a positive thermal coefficient of Rdson, which means that the hotter chip in a parallel set will carry less of the current.

        Side-effect: if you don’t heatsink your MOSFETS properly and keep them cool, the performance gets really bad. Particularly problematic is when there’s a feedback loop around the whole system trying to drive constant power into the load; the FETs get warm, dissipate more power, get warmer, etc until something burns.

        BJTs on the other hand do not. Common case is that one in a parallel set will take the vast majority of the current and then blow and then the next one goes, etc.

    • Otte says:

      TL;DR indeed – look, Starlino’s done it, and it works for him. So what’s the point of these ‘doesn’t work’ comment?

      • Chris C. says:

        The point is that there are caveats to doing this, which warrant a disclaimer so that people who don’t know better (including you) won’t blindly copy it.

        BJTs do not lend themselves to being paralleled. If you do it, you should add balancing resistors. And even then (but especially without), two in parallel will not run at double their individual ratings without eventually failing.

        • Otte says:

          Chris… you dont know me and you dont know what I know about semiconductor devices. Stating that someone doesnt know better, without ever seeing them, talking to them, seeing their work, or otherwise having any knowledge about them, is just being presumptuous.

      • Chris C. says:

        Chances are none of us here know you personally. Our sole impression of you is based on your comments. So far you’ve complained, failed to grasp the point, and chosen to get indignant rather than intelligently discussing any of the facts myself or others have laid out. Therefore that is how you will be known, by your own actions, regardless of presumption or your opinion of yourself.

  9. Hackerspacer says:

    He stacks motor driver chips to product move powerful controllers.

  10. Roger Wilco says:

    That’s why i don’t like tablets, Mike tried to type “He stacks motor driver chips to produce more powerful controllers” and instead got “He stacks motor driver chips to product move powerful controllers”.

  11. echodelta says:

    Tablets or capsules, spell-check ain’t grammar check.

  12. andres says:

    you can find cheap as dirt igbt ipm modules on ebay,just ridiculous overkill for small motors and they’ll last just about forever, no need to worry about killing them. $20 is the normal range for one, unless this is all you had, i would go the igbt route. but in a pinch, if it gets the job done, what’s the difference. good work!

    • fhunter says:

      Local electronics shop – turnaround about 2 hours.
      Ebay – minimum a 20 days from buying to receiving, and dirt cheap turns into really expensive with shipping.

      • andres says:

        i live in south texas, and there’s not an electronics shop for about 150 miles that carries a decent assortment of components. on ebay, i found 2 nice 30 amp, 600 volt modules for $29.80 shipped, took less than a week to get here. FSBB30CH60 modules if you’re interested. besides op amps, comparators, dinky mosfets, bjt’s and lots of passives, you won’t find a lot of selection nearby.

  13. MikeW says:

    Otte,
    It’s the difference between prototyping and production. Starlino did a great job and got his problem solved. The point being brought up is it won’t always work without a little extra design effort. This is why things seem to work in a lab and you get to production and have huge fallout. He is not making a lot, but if everyone does this some will have fallout. The comments are meant to help improve the design not denigrate what he has accomplished. My 2 cents worth.

  14. Barefoot says:

    Huh, I just read about this technique a few days ago on the adafruit.com forums — Motor Sheild FAQ. Scroll down about half-way to “I need more power!”

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