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Scroll Saw Motor Driver — Parallax Forums

Scroll Saw Motor Driver

ctwardellctwardell Posts: 1,716
edited 2014-03-10 11:48 in General Discussion
The motor driver on my Craftsman Scroll Saw bought the farm.

The replacement driver board from Sears is $67.00 plus shipping, I can't bring my self to pay that for a board that will likely eventually die as well.

The factory was nice enough to grind part numbers off of critical components...

I replaced the triac for giggles, didn't help.

So it seems it's time for a project, I realize I'll spend way more in time and money, but I'll have a controller that's more robust and easier to maintain.

The saw has a PM DC motor with an AC tach attached to the shaft for speed feedback.

The controller is using a triac to feed a fullwave bridge with a freewheeling diode across the motor.

I'll likely use a propeller and use the tach feedback to do PID control of the motor speed.

I'd like to get some suggestions for the motor driver circuit.

I could use a triac feeding a rectifier like it does now, or I could rectify first and then use an SCR or mosfet.

Snubberless Triac or add a external snubber?

Any suggestions?

Thanks,

Chris Wardell

Comments

  • ctwardellctwardell Posts: 1,716
    edited 2013-09-23 12:14
    Found some useful info here:

    http://www.littelfuse.com/~/media/Electronics_Technical/Application_Notes/Switching_Thyristor/Littelfuse_Phase_Control_Using_Thyristors_Application_Note.pdf

    C.W.
  • ElectricAyeElectricAye Posts: 4,561
    edited 2013-09-23 20:26
    I'm not knowledgeable enough to offer any technical help on this, but I commend your attitude and your efforts to fix your tools "just because." Sometimes it seems downright criminal to throw something away when you know you can fix it, even if doing so doesn't save any money.

    Good luck and keep us posted on how it goes.
  • ctwardellctwardell Posts: 1,716
    edited 2013-09-24 03:02
    ElectricAye wrote: »
    I'm not knowledgeable enough to offer any technical help on this, but I commend your attitude and your efforts to fix your tools "just because." Sometimes it seems downright criminal to throw something away when you know you can fix it, even if doing so doesn't save any money.

    Good luck and keep us posted on how it goes.

    Thanks ElectricAye.

    I know I should just be pragmatic and buy the darn replacement, but who ever said I was pragmatic...

    C.W.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2013-09-24 05:53
    You have a PM DC motor? Brushed or brushless?

    You will have to convert your AC to DC to powere the motor. And you will have to figure out what voltage it wants to operate at. Is there an Amp rating on the unit? That would ballpark a maxium and probably is a bit over the actual motor current.

    Nothing wrong with hacking what might become yet another junker without your inspiration.

    No guts, no glory. If it is low power, you might just try a room light dimmer switch as a speed controller.

    Here is a very general introduction... http://control2.net/p/picmicro-dc-motor-control-tips-n-tricks-w2788.html
  • prof_brainoprof_braino Posts: 4,313
    edited 2013-09-24 05:58
    ctwardell wrote: »
    Found some useful info here: Switching_Thyristor/Littelfuse_Phase_Control_Using_Thyristors_Application_Note.pdf

    That is an interesting read. I'm always pretty nervous when it comes to mains current, but this looks complete enough to maybe give it a try.

    How much of the original circuit do you plan to reuse? Do you know which part(s) failed?
  • ctwardellctwardell Posts: 1,716
    edited 2013-09-24 06:12
    Loopy,

    Thanks for the document link.

    It is a brushed DC motor, no markings, no details on the Sears parts site other than that it's a motor...

    The original controller has a Triac (BTA16-600B) driving a FW bridge made from FR307 (1000V 3A Fast recovery) rectifiers. It also has a FR307 across the output of the bridge as a flyback diode.

    The nameplate rating on the saw is 120V, 1.6A, so we have some ballpark figures. The manual states 400 to 1600 SPM (strokes per minute), so we have operational limits.

    I jumpered the across the triac A1 and A2 terminals and the motor runs, but at excessive speed, so I know the motor is good.

    C.W.
  • ctwardellctwardell Posts: 1,716
    edited 2013-09-24 06:20
    prof_braino wrote: »
    That is an interesting read. I'm always pretty nervous when it comes to mains current, but this looks complete enough to maybe give it a try.

    How much of the original circuit do you plan to reuse? Do you know which part(s) failed?

    I replaced the triac based on the most likely to be bad theory of shotgun repair techniques...no joy.

    The diac the drives the gate has the part number ground off, replaced it with a likely candidate...no joy.

    There are four small signal transistors, they all check OK.

    There are two LM324's, really the only other actives that could be bad.

    The power supply pins on the LM324's are at 15.7 volts, that seems reasonable.

    I suppose I could replace the LM324's but I have this gut felling that won't help.

    The remote potentiometer checks out, not much else left.

    C.W.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2013-09-24 06:25
    You might search image for "LM324 motor control" to get some hints of what is going on. I often find schematics this way for reverse engineering.

    I presume this is ONE direction of rotation, so the LM324 may have many outputs in parallel to provide more power. Is the motor running at 15-16 volts or at 115V?

    http://images.search.yahoo.com/yhs/search?_adv_prop=image&fr=yhs-ddc-linuxmint&va=lm324+motor+control&hspart=ddc&hsimp=yhs-linuxmint
  • ctwardellctwardell Posts: 1,716
    edited 2013-09-24 06:37
    Loopy Byteloose wrote: »
    You might search image for "LM324 motor control" to get some hints of what is going on. I often find schematics this way for reverse engineering.

    I presume this is ONE direction of rotation, so the LM324 may have many outputs in parallel to provide more power. Is the motor running at 15-16 volts or at 115V?

    http://images.search.yahoo.com/yhs/search?_adv_prop=image&fr=yhs-ddc-linuxmint&va=lm324+motor+control&hspart=ddc&hsimp=yhs-linuxmint

    The motor is running from rectified 115V, based on the triac and diac it looks like it's using phase angle control to vary the speed.

    The motor also has an AC tach output, the LM324's are likely using that to try to regulate the speed.

    C.W.
  • Mike CookMike Cook Posts: 829
    edited 2013-09-24 07:17
    Any capacitors?


    Had a pool pump controller go out, thought it was the out transistor that controlled the relay went bad. Digital display still worked, but would reset when It activated the load. Replaced two electrolytic caps and all’s good now. I’m assuming the heat cooked the caps.
  • ctwardellctwardell Posts: 1,716
    edited 2013-09-24 07:29
    Mike Cook wrote: »
    Any capacitors?


    Had a pool pump controller go out, thought it was the out transistor that controlled the relay went bad. Digital display still worked, but would reset when It activated the load. Replaced two electrolytic caps and all’s good now. I’m assuming the heat cooked the caps.

    There are a few electrolytics, none of them are bulged or look bad in any way, there are also a few what look to be mylar or some type of film capacitors.

    I haven't done any probing with the scope because this thing isn't isolated and I don't have an isolation transformer.

    C.W.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2013-09-24 07:53
    As a rule of thumb, replace all the electrolytics regardless of good appearance. They may be defective or out-of-spec and are impossible to test in-circuit. Once you remove them for testing, why not install fresh ones?

    Generally, the higher power devices are subject to the greatest stresses, so they fail first... unless there are other obvious issues (such as corrosion or heat stresses). Last year I sucessfully repaired my A/C controller board via reverse-engineering. But it became obvious that the cooling coils were leaking badly due to rust, so I replaced it this year.

    I use a TV to VGA box for TV. My old one failed and replacement of the capacitors revived it for another two years. About a month ago, the digital chips went haywire and while I could get audio on all channels, the digital output would hang on one still image. So it too was finally replaced.

    My wifi router failed. All that was required was a new wallwart. This seems a common lightning strike problem that causes many to buy a whole new router.

    Switches and pots are other sources of failure, sometimes easy to resolve, sometimes impossible.

    The main point is that I am getting an extra year or two of use via these DIY repairs. And I improve my ability to reverse engineer as I try more.
  • ctwardellctwardell Posts: 1,716
    edited 2013-09-24 08:06
    Loopy,

    I'm going to try a few more part replacements this weekend.

    C.W.
  • Mike CookMike Cook Posts: 829
    edited 2013-09-24 08:11
    Neither did the ones I replaced, they looked new. It was a last ditch effort before I threw it out!

    I did not have the exact value but had some that were slightly over rated in value and voltage. Might be worth a shot if you have some on hand.
    ctwardell wrote: »
    There are a few electrolytics, none of them are bulged or look bad in any way, there are also a few what look to be mylar or some type of film capacitors.

    I haven't done any probing with the scope because this thing isn't isolated and I don't have an isolation transformer.

    C.W.
  • LawsonLawson Posts: 870
    edited 2013-09-24 10:21
    I assume this circuit is built on a single or double sided PCB? I'd really recommend tracing the PCB and making a complete schematic. Even with out the part numbers of all the pieces, laying out how they are connected can tell you a lot about the circuit. Especially if you can compare with similar circuits. It'll also help with understanding what the circuit is supposed to do. And when what the circuit is supposed to do is compared to what the circuit IS doing, the point of failure often becomes obvious.

    Marty
  • ctwardellctwardell Posts: 1,716
    edited 2013-09-24 10:59
    Marty,

    It's double sided with a lot going on under components which makes it hard to trace.

    I may slog through it an make a schematic but I'm going to try replacing the two electrolytic caps first.

    C.W.
  • CircuitsoftCircuitsoft Posts: 1,166
    edited 2013-09-24 10:59
    Care to throw the board on a flatbed scanner and post the image?
  • ctwardellctwardell Posts: 1,716
    edited 2013-09-24 11:46
    Circuitsoft wrote: »
    Care to throw the board on a flatbed scanner and post the image?

    Here are some pictures.

    C.W.

    SSCBottom.jpg
    SSCTop.jpg
    SSCBottom.jpg
    1024 x 1802 - 236K
    SSCTop.jpg
    1024 x 1964 - 282K
  • LawsonLawson Posts: 870
    edited 2013-09-24 20:43
    Yea, the traces under the big DIPs are going to be a pain to follow, though you can usually see under the DIPs enough to puzzle it out. Since you have pictures you can overlay them to make tracing easier. Reduce each side to grey-scale in Gimp, flip the back side photo, and overlay them with the top photo in red and the bottom photo in green. (it'd help if the pictures were more "square") Another good trick is to set the board over a bright light so you can see traces through the board.

    Marty
  • kwinnkwinn Posts: 8,697
    edited 2013-09-24 21:39
    If replacing the capacitors does not fix the problem you can use Marty's suggestions to trace out the visible connections, and once they are added to the schematic the rest can be determined with an ohm meter or a led and resistor continuity probe..... Of course you could avoid all this by using a propeller to control the triac.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2013-09-25 02:45
    It
    fthe electrolytics are obviously filtering a/c ripple, it would not hurt to increase the capacitance if you find you have bigger capacitors with the same foot print. It doesn't hurt to go to a higher voltage rating either. These upgrades will add life to the repair.

    I strongly suspect that manufacturers provide caps that are intended to live short lives as many of the other parts just never fail.
  • Dave HeinDave Hein Posts: 6,347
    edited 2013-09-25 05:21
    Does the controller work at all, or is it completely dead? I read somewhere that the controller will shut down if the tach reading is too low to prevent driving the motor when it gets stalled. Could there be a problem with the tach feedback?
  • ctwardellctwardell Posts: 1,716
    edited 2013-09-25 05:38
    I changed the electolytics, no joy. I used identical values, but of higher quality.

    Dave, it's dead as far as driving the motor, the power supply seems fine, 15.7V present on the LM324's.

    If I jumper the A1 & A2 terminals of the triac the motor runs, so the rectifiers are OK.

    The tach leads show a resistance of around 50 ohms, I measured 4.15 volts AC when running the motor with the triac jumpered.

    I'm going to go ahead and trace everything out this weekend.

    It is now a matter of being stubborn; way past the point of being pragmatic or saving any money...

    Thanks for the help guys.

    C.W.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2013-09-25 09:32
    Once an interesting puzzle takes hold, it is hard to set aside. I suspect you will be successful.
  • ctwardellctwardell Posts: 1,716
    edited 2013-09-28 19:14
    No joy so far.

    Pretty much ready to punt and build a new controller.

    C.W.
  • VinceCVinceC Posts: 1
    edited 2014-03-09 14:39
    So, did you get anywhere with a new controller. I have pretty much the same problem with my Sears Craftsman Scroll Saw. Thinking just to get a motor speed controller off of e-bay, unless someone came up with a dependable design?
    ctwardell wrote: »
    No joy so far.

    Pretty much ready to punt and build a new controller.

    C.W.
  • ctwardellctwardell Posts: 1,716
    edited 2014-03-09 14:55
    VinceC wrote: »
    So, did you get anywhere with a new controller. I have pretty much the same problem with my Sears Craftsman Scroll Saw. Thinking just to get a motor speed controller off of e-bay, unless someone came up with a dependable design?

    I decided it wasn't worth the time and set it aside.

    The motor is Permanent Magnet DC, so the AC/DC speed controls made for universal motors will not work without adding a fullwave rectifier.

    C.W.
  • kwinnkwinn Posts: 8,697
    edited 2014-03-10 11:48
    Try gating the scr with a DC voltage (battery) to see if the motor runs. If it does the problem is in the triggering circuit.
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