For those of you that design circuits...

Discussion in 'Darkroom Equipment' started by Greg Davis, Jul 16, 2013.

  1. Greg Davis

    Greg Davis Member

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    I have two identical magnetic stirrer/hotplates that do not work. One will run for a couple minutes then shut off completely until I remove the fuse and put it right back in, the other does absolutely nothing at all. Looking inside, the power comes in, goes to a circuit board then the motor and hotplate. The two dial switches are on a separate circuit board. Nothing looks damaged, in fact, they look brand new on the inside.

    I do not need the hotplate function, so I was thinking that I may be able to bypass the PCB's altogether and wire the dial potentiometer directly to the motor and power. Testing if the motor was any good, I disconnected it from it's PCB and connected it straight to the power cord inlet and it worked just fine, though at full speed only. I know only the basic concepts of circuits, so I do not know if I need to have any resistors or things so I don't fry the motor or switch. My question to you all is, what do I need to read with my multimeter to find out the limits of the switch and motor, then calculate what parts I may need in between. The device does say it uses a 10A fuse and is 125V. If any of you have an iPhone or iPad, I can "FaceTime" with you and check things in real time.
     
  2. spijker

    spijker Subscriber

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    If you don't need the hotplate, just disconnect it from the PCB and leave the motor connected to the PCB. It seems that the motor speed is electronically controlled. The shut down after a few minutes could be a self protection circuit kicking in. Maybe the hotplate draws too much current due to a defect. Taking the fuse out and put it back in (or just disconnecting it from the 120V) probably resets the protection circuit.
     
  3. Peter Simpson

    Peter Simpson Member

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    Yeah, if the motor is electronically controlled, you DO NOT want to just put the pot between the motor and the line. The pot is rated for very low power and will turn into smoke if you try that. The problem is with something on the electronic control board. If you can send a pic of that I might be able to tell you what to replace.

    Are these Chinese made or from a major manufacturer? (I'm looking here for two things: quality of design and possibility of IDing the parts)

    There's also the junkyard challenge alternative of replacing the whole motor control circuit with an off-the-shelf lamp dimmer...
     
    Last edited by a moderator: Jul 16, 2013
  4. Greg Davis

    Greg Davis Member

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    This is the one day I didn't bring my digital camera to take photos of this with. I will see what I can do in the meantime. The one that works intermittently does this whether the hotplate is on or off. Unplugging it from the wall and back in does not make it work again. The PCB with the switches has only three small resistors connected to led bulbs, otherwise it doesn't have anything on it. It is connected to the main PCB by a ribbon cable.

    The units are Fisher Scientific brand, but they are a discontinued model of their Isotemp series.
     
  5. jp498

    jp498 Member

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    Make as few changes as possible to achieve your goal, especially as you don't know what you're doing and it's 120v. I'd suggest just removing the heating element.

    The heating and cooling and motion could have also caused circuit board pads to lift (where wires or components are soldered on); in which case you'd have to repair those with resoldering or bypassing torn traces with wire jumpers)

    You could also find some retired appliance repairman in your community if it's that simple. It used to be a common profession before appliance were made disposable.
     
  6. Greg Davis

    Greg Davis Member

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    Well, I have done some troubleshooting with it. The heating element is disconnected, but the problem persists. Since they are identical, I thought I would see if I could eliminate some possibilities. The motors of both work, and I swapped the switch PCB's with no change. The first unit still turns on for a while, the second doesn't come on at all. The main PCB has a part that touches the machine's frame with a thermal paste. If it is not in contact with the frame, then the motor only spins at it's fastest speed regardless of the speed setting. I replaced the thermal paste with fresh paste, but no change. I used an infrared laser thermometer to see if any part may be overheating. One small part circled here gets hot. The thing seems to turn off when this piece reaches 140 F, which only happens if I use a medium to slow speed on the motor. It gets hot at higher speed, but not hot enough that the motor shuts off.
     

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  7. Peter Simpson

    Peter Simpson Member

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    OK, Fisher Scientific is probably worth fixing. One more thing that I should have said at the start: be VERY VERY CAREFUL if this thing is open and you apply power. IT CAN KILL YOU.

    The circled part is a low voltage DC capacitor and it shouldn't be getting to 140F. But that's probably a result, not the cause of your problem.

    The heating coil might be independent of everything but I wouldn't assume that disconnecting it won't affect other stuff on that circuit board.
    You can try Googling for a schematic with the exact model number, it's a slim hope, but you might get lucky. I would pull the thing apart and try to reverse engineer the AC control side of the board and figure out whether the DC supply is still working. Those are the two most likely problem areas. My guess is it might have gotten zapped -- this is a risk for anything that stays plugged in all the time. A second possibility is that something overheated.

    The white things by your thumb look to be opto couplers, used to isolate logic circuits from AC circuits. The bent over part with heatsink goop on it just to the right is probably the Triac that controls motor speed or hotplate temp...there should be a second one somewhere...again, can't tell how the hotplate is controlled -- is it a separate, independent control, or does it connect to that board as well?

    If you can get a good clean shot of both sides of the circuit board, and then annotate what connects to each of those wires, I might be able to do this by remote control. You can PM me, no need to clutter up this thread until we figure out what's wrong.
     
  8. Greg Davis

    Greg Davis Member

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    In the photo, the black cloth covered wires go to the motor, the white ones connect to the heating element. It is controlled by a separate dial, but it connects to the same PCB as the motor.
     
  9. Steve Smith

    Steve Smith Member

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    Unfortunately this is one of those things which many of us could sort out in minutes if we had it in front of us but is nearly impossible to sort out remotely. You need to find someone local who knows about these things.


    Steve.
     
  10. Peter Simpson

    Peter Simpson Member

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    OK, looks like there's another triac just above your circled part -- it's a surface mount part, lower power capacity, which makes sense. The big one with the thermal paste on it must be for the heater control. Either or both of those could be blown, or it could be that the micro (I'm guessing it's just to the left of the transformer, but that photo isn't quite high enough res for me to read the part numbers) is shutting things down.

    First thing is to see if the DC supply is working OK -- is the micro getting 5V or 3.3V? There's a bridge rectifier marked BR1 just above the top left corner of the transformer. That probably feeds the fat cap to its right and the chip above it is marked VR1, for Voltage Regulator. There should be a regulated DC output from that chip, which will power the control logic. I'll bet your thermometer measured VR1 instead of the cap above it. What's the big silver and black thing on the edge of the PCB, just to the right of your circled cap?
     
  11. Greg Davis

    Greg Davis Member

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    Well, I am not going to pay to have them repaired since they are university property, but the university would rather throw them out than have them repaired. I got them from university surplus since they won't buy more. I will spend a couple dollars on parts, though, since I can get that from the department's petty cash. I don't understand why I can't remove the PCBs completely and just wire the motor to a variable speed switch. Is this possible? I see homemade mixers doing this same thing with computer fans. I understand I have to convert the AC to DC, but surely there must be a simple solution rather than tracking down all the broken pieces.
     
  12. polyglot

    polyglot Member

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    That PCB? It's a variable speed motor control. Sure you can replace it, but the replacement is far more complex than a potentiometer due to the power levels involved. The replacement is going to work similarly to the PCB already in there except it won't be designed to fit in the case. The quickest/dirtiest thing you can try is an AC lightbulb dimmer between the mains and the motor, it will probably give you coarse speed control. You might also blow up the dimmer if it's poorly designed (they don't expect inductive loads).

    Computer fans are 12V BLDC motors, this thing is an AC squirrel-cage motor. They have totally different supply and control requirements.

    See the two surface-mount black things above and below the capacitor that's overheating? My guess is that the top one (3 pins) is a triac which is doing phase control on the motor. Replacing that on the stirrer that doesn't work at all might get you somewhere, or it might not.
     
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  13. Hexavalent

    Hexavalent Subscriber

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    If you choose to use a simple lamp dimmer for speed control, it may have a tendency to wander with varying loads. This is not much of an problem for quick-mixing chems, but for slower, longer-term mixing, wandering speed can become an issue.
     
  14. Greg Davis

    Greg Davis Member

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    I will try to get some better images tomorrow. I may have a guy that an look at it in person.
     
  15. Peter Simpson

    Peter Simpson Member

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    Yup. That's the ghetto way out and it might work fine. Certainly worth the $9.99 to give it a try. If it doesn't work on the bench you can go back and reverse engineer the original controller.
     
  16. jp498

    jp498 Member

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    If the part has thermal paste, it's not a capacitor, what's the label for that part on the board? like c??, r??, q??. If the part that gets hot has become disconnected from the heat sink it could have been destroyed by the heat.
     
  17. Greg Davis

    Greg Davis Member

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    The part with thermal paste is not the part that gets hot. In the photo I attached earlier, the part I circled is the one that gets hot, it is labelled "C2". The part with thermal paste is at the top of the board, and the paste can be seen as a grey goop on its surface.
     
  18. Steve Smith

    Steve Smith Member

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    When I said someone local, I meant someone like many of us here who tinker with things and have the knowledge to repair it rather than a professional repairer.

    Is there an electronic engineering department in the university? (look for a lab technician rather than an engineering student - they won't have a clue!).


    Steve.
     
  19. Greg Davis

    Greg Davis Member

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    There is, but virtually everyone is absent right now since it is the summer. Most people aren't back until late August.
     
  20. jp498

    jp498 Member

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    C2 is a capacitor. If it's getting hot, something else is causing it to get hot. You'd need a tech to figure that out.
     
  21. John Koehrer

    John Koehrer Subscriber

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    Would something like a Beseler Resistrol or speed control for a router carry the current rather than A light dimmer?
     
  22. Peter Simpson

    Peter Simpson Member

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    If all you want is motor control, buy a light dimmer and give it a try. Seriouly...it might work fine. They're rated for 600 watts, and that's a pretty small motor. Home Depot has the wall mount knob type for under $10.