Monday, November 17, 2008

Magic Smoke and Mirrors

The task is very simple. I have a solar tracker. I have a microcontroller that knows what direction the tracker should be pointing. I have a cordless drill that I've taken apart to do the actual work. So why is it taking me WEEKS AND WEEKS to get this working? I can't even turn the drill on yet, at least not without threat of releasing the magic smoke.

It should be extremely simple. Here is how a drill works.

OK, so it should be easy to just replace the human-operated switch with a microcontroller-operated one.

The microcontroller replaces the human and relay replaces the switch. It's a high-amp relay because on the very first magic smoke release test I found that the drill draws 20 amps (or more!). It's also a 15V battery, so I managed to remember the resistance of the motor as being .7 ohms which makes the V=IR math work out.

However, when I almost melted my entire apparatus on the second test I realized that the motor is only about .1 ohms. How on earth can I be feeding 15V to .1 ohms and only drawing 20 amps? Including a current-limiting resistor doesn't improve things much, since it just wastes a bunch of power getting really, really hot. After a little thought, I decided to actually look at the drill switch. I can't get it open, but I was able to confirm this much:

I don't know how, but that magic box is somehow limiting the current. One thing I can see in there is a transistor, which could definitely do the job....except it would have to be awfully high power to handle 20 amps. Or maybe it isn't? Maybe the 20 was transient and it normally feeds more like 2-3 amps? No idea. Which leads me to think maybe I need to give up and do something like this:

Which is stupid. If I knew how it worked, I could probably hook the microcontroller directly to the magic box and forego all my rubegoldbergian relay stuff (which should be a post on it's own...). It's also annoying that I know this can't be an unsolved problem. But I must not be thinking of the right keywords to find the solution.


  1. Presumably, you're using a drill motor because you actually need all that power (300 W) and torque? Because if you're using the drill motor because it was free, it sounds like you aren't doing yourself any favors. I know my drill goes at a different speed based on the depth of the trigger I pull. Since I don't expect you want it spinning as fast as possible, unless you have some sort of gear box, I presume you know how the switch handles going at different speeds?

    Have you tried looking up drill motor data sheets?

  2. Reasons I'm using this motor:

    1) I tried using some tiny-to-medium motors and they weren't nearly enough. This was the next biggest thing I had.

    2) I was already using the battery from the drill to power the project.

    3) I later realized that this whole thing will be outside in the cold. I probably need to overdesign the power so that it will work in much-reduced circumstances.

    But you've definitely nailed one of my nagging worries with the speed control. My drill works the same.

    I think the thing in the magic box that looks like a transistor IS a transistor. I looked up the data sheet on that but couldn't find it. However, I did find a comment in Russian that looked like it was saying the transistor was up to 8A. But it's a 20-25A motor. Maybe that's a transient number that the transistor can handle while 8A is continuous?

    Maybe I should ditch my whole rubegoldbergian relay setup and get power transistor that can handle good amount of current? Then I could control the speed directly and simply.

  3. My brother used TIP-120 NPN-D Darlington transistors for the motors he used in his autonomous submarine. However, his motor only drew 2.5 amps.

    A quick 'drill motor' search on google shows me some drill motor specs, and a lot of other motor vendors.

  4. Whoa whoa whoa.

    I've been looking for medium sized (i.e. bigger than aquarium, smaller than huge) pumps for a long time now. Do you know if they have a regular garden hose attachment?

    And does your brother's autonomous submarine have a webpage?

  5. submarine webpage

    I don't think the ones he used had garden hose attachements. However, from my limited understanding of what they're intended for, there has to be some sort of hose attachment.

  6. Yes, I see now that the pumps are submersible, which doesn't work for me. Well...I could probably disassemble and make it work somehow. But meh.

    This SSV is pretty cool, thanks.