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DC motor control

Discussion in 'Sensors and Actuators' started by clove333, May 21, 2012.

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  1. clove333


    May 21, 2012
    MinnKota mystery

    I picked up this beatup looking trolling motor at an estate sale. I tested it and it didn't work
    so we negotiated a deal and I've finally gotten around to disassembling it to see what's
    It's a MinnKota 55A and the armature was in amazingly good shape considering it's age.
    In fact not too much was wrong but I've come to repairing the variable motor speed
    control and I'm stumped.
    The circuit is not complex, it's dirt simple.
    Basically it's a TO-3 npn transistor with the collector connected to the battery + thru the
    motor. You could say it''s 'sinking' the motor LOL.
    The emitter is at the ground or negative battery potential.
    In series with the base is a 1N4148 or similar with the band towards the xistor.
    Next is an 11 ohm 10 watt which I guess is current limiting.
    After that is a wire wound pot of 10w - 120 ohm design that connects to the battery +.

    I thought, Ah ha it's obviously the wire wound pot since it is up top and those kid of pots
    always go intermittent anyhow. I took it apart and cleaned it since it actually was pretty
    spotty as I turned the shaft. Interesting design. Since it's emulating the throttle control on
    an outboard motor, when you turn it all the way CCW it disconnects, or opens,
    the base circuit terminating any drain on the battery (while you're casting away, I guess).

    Okay, here's the problem. When I switch to the variable mode (which connects the
    emitter to ground) the motor goes to some in between speed and wont vary no matter
    where I set the pot (except when I turn it to off). I've tested all the parts three times in
    frustration and everything looks good. I have a VOM and a Heath IT-27 so I'm fairly sure
    of my measurements. Unfortunately I dont know enough to really do a proper SPICE simulation of how it's supposed to work and I cant find a cross ref for the power xistor.

    It measures as a low beta device, but not as low as the type found in a TV horizontal
    oscillator, more like a typical 2N3055 or 2N3771. This pass transistor has a 'house' marking on it of S39316. ECG, SK, NTE show nada. I looked at the Fairchild database
    and some exclusive silicon valley service I managed to con a trial membership on.
    But still no joy.

    The whole machine was made in 1973 according to the date stamped on the motor
    armature and the datecode on the xistor. It's a typical Fairchild device of the period,
    using the same low profile aluminum package that Motorola was using at the time.
    Was it a darlington that drifted? I dont know so I'm hoping for some enlightenment.
    Everything tests as working yet it doesn't perform.

    Perhaps the manner of implementation caused a problem?
    I dont think it's the motor. It was almost pristine, showing hardly any carbon buildup
    from the brushes.
    The way the speed control is set up, the select switch selects between Hi or Var with
    a center position of 'off'. So it's definitely 'break before make'.
    In the circuit that switch connects the battery - (minus) either directly to the motor
    or engages the xistor emitter (which is left floating otherwise).

    Suggestions, detections or selections. I need some help.

    Oh yeah. I have this neat old German meter my dad brought home from the war that
    has various shunts and it has a high amperage one that does 12A AC/DC max.
    According to that meter the motor draws approximately 4A running with a surge of
    perhaps 8 to 11 amps when first started. Difficult to tell surge exactly on an analog meter.
  2. clove333


    May 21, 2012
    I've got this simple motor speed control that ought to be easy to sort out, yet I'm having a
    devil of a time making it work right. I made a schematic of this trolling motor when I found
    it just comes on at an intermediate speed and wont vary. Maybe someone can give me some
    insight that'll help me sort it out.
    What it is is a TO-3 pkg npn power transistor wired so it sinks the motor as a load in series
    with the collector and the emitter is connected to ground. The base is also connected to the
    12v power source thru a 120 ohm wire wound pot (speed set), then a 10w 11 ohm resistor
    and finally a 1N4004 diode with the band (anode?) towards the base.
    That's it!
    The pot was intermittent but I fixed that by cleaning it. The motor runs fine connected direct
    across the battery and the diode reads ok. The power transistor has a house marking on it.
    It was made by fairchild in 1973 and shows a beta similar to 2N3055 or 2N5303. What it
    really is is open to speculation since I cant find a cross ref for the number: S39316.

    The only thing I can think of is maybe changing the transistor for a darlington or making one
    by adding an MPS-U05 or similar.

    Any ideas, anyone?
    Last edited: May 24, 2012
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    What current does the motor take when connected directly across the motor?

    Have you confirmed (with the power off) that the resistance between the +ve supply and the base of the transistor varies as you turn the pot?

    If you remove the pot from the circuit, does the motor run? (i.e. with the base open)

    I would not recommend a darlington because the pot and series resistor will allow enough base current to turn it hard on even at maximum resistance. A 2N3055 *may* be a reasonable replacement

    CDRIVE Hauling 10' pipe on a Trek Shift3

    May 8, 2012
    This is odd. Trolling motors aren't exactly light battery loads. Yet, what the you describe sounds like a wasteful, (heat generating) analog control instead of PWM. Is this very old?
  5. CocaCola


    Apr 7, 2012
    Last edited: May 25, 2012

    CDRIVE Hauling 10' pipe on a Trek Shift3

    May 8, 2012
    Ha, unless a DMM has a super fast sampling rate and also has a 'Peak Detect & Hold' feature, an analog meter beats a DMM in this application every time. Admittedly, not by much though. ;)

    CDRIVE Hauling 10' pipe on a Trek Shift3

    May 8, 2012
    It bugs me when people don't read all the pertinent information. :rolleyes: :eek:

    Anyway, it's time to replace the control with power efficient PWM. ;)

  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Not all the pertinent information was in this thread until CocaCola noted the other thread that I have merged with this one.
  9. CocaCola


    Apr 7, 2012
    The transistor date code was there, but the additional merged post also verified that information...

    Anyway CDRIVE don't feel to bad, I have a tendency to read the 'readers' input and commentary on news sites and I can state without a doubt half the population have no reading comprehension at all and completely miss everything... Missing a small detail here and there is just normal...
  10. clove333


    May 21, 2012
    Sorry I've been silent. Had to go chase some income in the hinterlands.
    I originally posted the verbose version but found I wasn't getting any replys so I streamlined my description and posted in another likely area. I'm glad that has been
    more responded to as both together are my first try at garnering help. I thank the big boss
    super for combining them now that there is a dialogue of some sort.

    Anyhoo. The measurement of the motor 'I' draw was done without water and the weight
    of a boat to load it so 'I' draw in real world is more. The owners manual I managed to cadge from M-K's archives recommends a 60 amphour battery minimum. Regarding
    the design it is from the 'Fender/Dynaco' school of design as opposed to what Saul Marantz might've done.
    That's not to denigrate Fender or Dynaco as their product was designed for a
    market and held up quite well all things said. Remember the teething problems of the
    Dyna Stereo 120?

    In the M-K 55A, the parts list shows two items for the speed control (excluding the rocker
    switches).. The wire wound pot up top and the other three parts (located in the nose of the motor housing for heat sinking) and proudly described as 'solid state unit' for replacement purposes.

    What my crude analysis of the circuit shows is the designer needed to have the least
    possible resistance in series with the motor so that when the pot was simulating full
    throttle, motor speed approached the revs available in the HI setting. Taking into account
    variations in beta of the power xistor the 11 ohm resistor sets a base current limit to
    protect the xistor from destruction if the prop became fouled and stalled the motor.
    I believe it was done this way as an emitter resistor would be in the load path and lower
    max revs.
    In my empirical experimenting I thought it might be a good idea to have a power diode in
    series with the battery in case someone connected backwards. A significant loss of revs occurred, even in HI mode.
    Back to the circuit. The diode sets a starting bias point so the xistor is just turned on at minimum throttle. It also might prevent failure of the xistor from battery polarity reversal.

    Obviously the circuit worked at one time and something changed as time passed.
    I'm sure there were reliability problems with this design as product ten years later used a more sophisticated design that was more robust and less dependent on part tolerance.
    It was the same basic concept at heart though.

    Perhaps I should try a signal diode instead of the 1 amp 400x type now installed?
    That would change the bias point.

    What would change in the circuit that would cause the type of aberration it exhibits?

    I'd like to make this unit work like it did originally. Installing different parts and circuit will
    get messy as there isn't much room and the whole upper unit is made of nylon plastic
    and bakelite which is getting brittle with age. Also 'stock' looks ever so much better
    than a kludged together uniqueness, even if unique works mo' better. It gets terribly
    complicated and I just want to use the damn thing. With the new propeller I had to get
    and odds and ends I've got about thirty-five dollars invested and there you have it.

    Part of my purpose in starting this thread was hoping for a bit of 'edemicashun' from those with greater knowledge and experience than I. If it sounds like I'm blatantly seeking
    help here, I am. It's not a deep circuit. I could parts substitute until it worked but I was
    hoping for a more elegant solution (especially if subbing still yielded no joy).
    One more point about the way the original circuit was built. I dont think a 10 watt
    resistor was needed or a ten watt potentiometer.
    However, fishing gear tends to get treated pretty poorly on the whole so I think that this overkill was an attempt to buy reliability at the pennies extra the higher rating parts cost.

    Barring any suggestions or input that gives me a 'lightbulb' moment my next task will be
    to put the unit in variable mode and measure voltage drops in the major current path and
    sidechains. That's including switches. soldering and wires. Ten days to two weeks to do
    that if I dont alter priorities on projects in the 'on deck' circle.

    By the way. I like the suggestion to go to a PWM style. In fact I found a circuit in one of
    those Sams/Tab 1001 circuit ideas books that looked interesting and made a prototype.
    I wont detail any more on that yet as I am trying to shake the tree of knowledge and have
    a few ripe apples hit me on the head with this fine example of early 1970's .consumer
    Cheers y'all.

    PS regarding Steve's question about removing the pot. It is internally configured such that
    turning all the way CCW disconnects, just like the throttle control on an outboard would
    kill the motor. That function works correctly. However, when the pot goes 'in circuit' any
    spot on the rotation gives the single speed (4 to 120 ohms).
  11. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    May 8, 2012
    Wow, the mention of Dynaco brought back memories. I believe it was the mid to late 50's when my Dad built his first Dynakit. The exact dates and time period are now foggy but after testing his work I do remember exactly what he said to me. "Son, this is off limits to you and your sisters", "If you touch it I'll cut your b*lls off and nail them on the wall!" I miss you Dad. You had a way with words.

    As, far as your project is concerned, PWM is easy and definitely the way to go. Nearly full torque will be delivered at all speeds. Very handy when Spatterdock and such gets wraped around your prop.

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