Building a 12VDC powered toy... kinda stuck

Discussion in 'The Projects Forum' started by kalikid1210, Jul 16, 2008.

  1. kalikid1210

    Thread Starter Member

    Jul 15, 2008
    21
    0
    I'll keep this clean, honest!

    I've built a few toys using 24VDC, 350 Watt, Currie brand Scooter motors. Using the Scooter's speed controller module I've got some very impressive results. Variable speed and fully portable using (2) two 12-VOLT batteries...only problem is they're so heavy!

    So I happened upon a 12VDC GEAR motor, the type used for an automobiles' power windows from www.surpluscenter.com:
    RPM 150 at 12 VDC no load
    Voltage 12 DC
    Amps 3-1/2 no load
    Reversible
    Duty continuous



    I am looking to 'make' this motor variable speed but have a couple of questions:
    1. Can this be done using a POTENTIOMETER?
    2. IF SO, what size?
    Any help you Circuit Critics can yield would be great!
     
    Last edited by a moderator: Jul 16, 2008
  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,694
    904
    First, you are going from a 350W motor to a ? wattage motor. Are you sure it will work even at full speed? Do you know its full load current draw?

    A potentiometer will control a brushed DC motor, but you will lose torque and generate a lot of heat, particularly if it turns out that the full load current is 29 A (i.e., 350W).

    The best way to control speed electronically is probably like the speed controller you have does it. I suspect it uses PWM, but before getting there, the big question is whether the 12V motor is enough.

    John
     
  3. kalikid1210

    Thread Starter Member

    Jul 15, 2008
    21
    0
    [​IMG]

    THIS is the reading on the motor. What can be known besides the voltage?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Found these specs:
    Specs:
    3 Volts: 35 RPM, 1 Amp (No Load)
    6 Volts: 77 RPM, 2.4 Amps (No Load)
    9 Volts: 120 RPM, 2.9 Amps (No Load)
    12 Volts: 163 RPM, 3.3 Amps (No Load)

    Shaft Dia.: 11/32" (9mm)
    Shaft Length: 1-1/4"
    Motor Dia: 1-3/4"
    Length overal: 7-3/4"
    Weighs approx. 1.5 lbs.

    Denso R730556-7030 730556-7030 162000-2231 12V 12U Made in Japan
    on this page: http://www.skycraftsurplus.com/index.asp?PageAction=VIEWPROD&ProdID=814
     
  5. tomocar

    New Member

    Jul 16, 2008
    5
    0
  6. kalikid1210

    Thread Starter Member

    Jul 15, 2008
    21
    0
    SGTWOOKIE-I appreciate the detailed info! The more i"m findin out about this motor and it's requirements the easier this becomes. And man that website has EVERYTHING!

    TOMOCAR-Thank you for the link to the kit, I will purchase this ASAP! Man, just what I was looking for.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    That PWM controller might need an upgrade for the MOSFET. Consider changing to an IRF540, or since you're operating at such a low voltage, an IRFZ34 or IRFZ44. These higher-rated MOSFETs have less resistance when they're conducting, thus generate less heat.

    Your no-load current is considerably less than the current will be when operating under load.

    Skycraft Surplus is a techno-geek's dream come true. :) I have spent many a happy hour in that place.
     
  8. kalikid1210

    Thread Starter Member

    Jul 15, 2008
    21
    0
    Uh OH! I just bought that PWM controller....hmmmmm. Should I stop the presses and cancel my order?

    Is the heat generated considerable and/or detrimental to the components?

    Also, I have to reserach what this MOSFET is...such a great forum to train the brain.

    Thanks again Sarge
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Well, the MOSFET is just a single component in that PWM controller.

    The main components of that kit are a 556 timer IC, a MOSFET, a pcb (printed circuit board), the heatsink for the MOSFET, a pot, power terminals, and then some miscellaneous resistors and capacitors - all necessary for it to work properly.

    Power MOSFETs are pretty inexpensive nowadays, and can control a remarkable amount of current for their size. The IRF530 that the PWM kit comes with is good for a constant 7A load, with peaks up to 15A. The IRF540 has a higher current rating, and a lower ON-resistance.

    You should probably test the motor under load before you build the PWM controller.

    Note that window motors generally have a weak section in the drive gear part; that's so if a window gets "stuck" or something is jammed in the window, that the torque isn't enough to cause damage to the window or the door itself. The drive gear shears instead, protecting all the other components.

    If you want to protect the original IRF530 from overcurrent, you could use a 6A fuse or circuit breaker in the motor's supply line. If you upgraded your MOSFET, you could increase the size of the fuse correspondingly.

    Basically, you don't want to load the motor so much that it gets near stalling. Better to blow a fuse or breaker than to cause damage.
     
  10. kalikid1210

    Thread Starter Member

    Jul 15, 2008
    21
    0
    Sarge-I was actually hoping to melt some stuff down and post it to my YouTube account.

    Well since my wife is keeping the purse strings ultra tight nix that previous statement. I will use a fuse in series as you recommended to prevent some costly damage....

    And to test the motor under load I would use my DC amp clamp on one of the wires while the motor is under load? Or am I floating in the sea of ignorance?

    Thanks,
    Pete
     
  11. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    MOSFETs make a glorious purple flash if you overload them just right. And you should see the lemon-yellow smoke I've had come out of DC motor windings - beautiful stuff!

    That's an excellent choice.

    Looks like your motor may have a stall current of 16A at 12V: http://animaxter.org/motors.html

    6 Amp circuit breakers are available for four or five bucks. Depending on how quickly the machine would be wanted back in service after a near-stall condition, and how frequently near-stall conditions occur, the breaker might be a better choice than the fuse.

    As the Wook noted, good MOSFETs don't cost much - might be simpler to just upgrade them.
     
  12. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Well, the DC amp clamp may work, but it's a brushed motor, so the readings might jump around a lot if it's a digital meter. If it has averaging, you'll need to use it.

    But, you can measure current by measuring the voltage drop across a known resistance. The issue here is that you'll have to filter the voltage across the drop to get an average reading. A few capacitors and resistors, and 10 feet of 10 gauge wire would work pretty well. For every Ampere of current through a 10' length of copper wire, you will see a difference of 10.2mV from one end to the other. With your motor running on 12v no load, you should measure about 33.7mV from one end of that wire to the other, which works out to 3.3 Amperes - which is what the specs say it should be.

    But, the brushes in the motor will make and break contact with the commutator as the motor spins, which will cause a great deal of noise on the wire. That's where the filters come in; it's purpose is to average out these voltage fluctuations so that you can get a steady reading.

    Have a look at the attached schematic. The signal generator is putting out a 1kHz 12v square wave that's on for 9.5mS and off for 0.5mS. The simulated O-scope trace below shows how the filters have smoothed out those fluctuations. The meter's reading 11.1mV, which is about what I expected it to read with the motor model I had set up.

    You should have a fuse or breaker in the line anyway, just to make sure you don't overstress the motor while you're testing. 10A would probably be good for the moment.
     
  13. theamber

    Active Member

    Jun 13, 2008
    318
    0
    Make sure the fuse you use in the motor circuit is slow blow fuse or a delayed action fuse.
    Inrush currents of up to ten times normal operating current are commonly experienced every time the motor is started from a dead stop and will blow a fast acting fuse inmediately. The dealy fuse will have more mass but no more ampacity than a fast acting one.
    However semiconductor devices such as Mosfets tend to be intolerant of overcurrent conditions, and as such require fast-acting protection against overcurrents in high-power applications.
    You could use an optocoupler to isolate the two circuits.
     
  14. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    No, he could not use an optocoupler as the power MOSFET is what's supplying the current to the motor.

    However, he may need to use a high-current Shottky diode across the motor to guard against spikes. I don't know if the PWM kit he ordered has that or not.
     
  15. kalikid1210

    Thread Starter Member

    Jul 15, 2008
    21
    0
    I'll ask the distributor if it includes a HIGH-CURRENT SCHOTTKY DIODE.

    And I have good skills in sweating copper fittings from my A/C work BUT soldering will be a brand new endeavor. A quick link to a soldering site for tips would be appreciated.

    Thanks again to SGTWOOKIE,thingmaker3, and theamber...
     
  16. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Soldering electronic stuff is a good bit different than sweating copper fittings.
    The best solder for electronics is Sn63/Pb37 - that's 63% tin to 37% lead. It can be hard to find. 63/37 is "eutectic" solder; it does not have a "plastic" state; when cooling it goes directly from a liquid to a solid.

    60/40 is the next best. It does have a plastic state, but it is far better than 50/50 solder.

    Do NOT use acid core solder or plumbing flux. That stuff is corrosive, and will eat your project up.

    Rosin flux is typically used for electronics. Rosin core electronic solder can be found, too.

    A pencil-type soldering iron about 30W will work. Temperature controlled soldering irons are better. MPJA.com has some decent prices on such things:
    http://www.mpja.com/solder_equipment.asp
    Harbor Frieght has a decent one here:
    http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=96375
    but getting replacement tips from Harbor Freight may be a problem.
    You are better off to start with a decent iron, than to buy a cheap iron and wish that you'd bought a better one. Don't use a soldering gun, you'll get poor results.

    So, a basic list of things:
    1) Soldering iron (preferably a temperature-controlled soldering iron, at least 30W) and extra tips. This one looks decent: http://www.mpja.com/productsdirect....&item3=15140+TL&item4=15141+TL&item5=17204+TL
    2) Flux (rosin or other non-corrosive flux)
    3) Solder (preferably 63/37, small diameter) A couple of these: http://www.mpja.com/prodinfo.asp?number=17462+TL
    4) 91% Isopropyl alcohol (find this in your grocery or pharmacy, for cleaning components and removing flux residue)
    5) Acid brushes (metal handled cheap small bristle brushes; available at auto parts stores, for scrubbing with the alcohol)
    6) Solder wick, 2mm and 4mm widths (for removing unwanted solder) Here: http://www.mpja.com/products.asp?dept=445
    7) Copper scrubbing pad (kitchen/cleaning supplies at your grocery; works great for cleaning the tip of the soldering iron, doesn't cool the tip like a wet sponge does)
    8) Round-nose pliers for bending component leads. Here:http://www.mpja.com/prodinfo.asp?number=15400+TL
    9) Scotch-brite scrub pads (cleaning supplies @ grocery store, works great for cleaning component leads)
    10) A wrist strap and anti-static mat. Radio Shack has wrist straps. You're in TX, and static electricity will be a real problem. One small zap and a MOSFET will be toast; it happens quicker than a blink. Keep MOSFETs and CMOS in ESD-protective enclosures (pink plastic or metalized mylar bags) until you're ready to install them.

    Something else that you will find really handy is an item called "Helping Hands" - a couple of alligator clips and a magnifier on a cast iron stand. Harbor Freight Tools sells them: http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=319

    Get some practice before you try assembling your kit. Go to Radio Shack and pick up one of their general purpose pre-drilled PC-boards, and an assortment of resistors and/or diodes. Grip the component lead next to the body of the component with round-nose pliers (best), needle-nosed pliers or hemostats, and bend the lead on the side away from the component. This avoids stressing the component body, which can crack it. Round-nose pliers make a nice neat and smooth radius, placing minimal stress on the wire and component.

    Your solder joints should appear clean and shiny, with the outline of the wire lead still visible. More solder is not a good thing; it will take too long to cool down, possibly overheating the component.

    Here are some soldering tips:
    http://www.aaroncake.net/electronics/solder.htm
    http://www.epemag.wimborne.co.uk/solderfaq.htm
     
    Last edited: Jul 18, 2008
Loading...