A very simple motor controller using a transistor

Discussion in 'General Electronics Chat' started by charliedurrant, Jan 21, 2012.

  1. charliedurrant

    Thread Starter New Member

    Jan 6, 2012
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    Hi Again,

    I now have my first circuit on my bread board with a motor's on/off controlled by an NPN transistor.

    The motor is of unknown make / model. It is small and was in a box full of odd bits.

    I bought the following transistor:

    http://www.datasheetcatalog.org/datasheet/motorola/BD237.pdf

    I made it a medium size one so it would fit in my bread board and bought it from maplin as they are down the road. The thought process in buying it was:

    a) make it largeish to withstand any abuse?
    b) small enough so it would fit in my bread board.

    Not a lot of thought but the range they had was small! I'm driving the emitter to base with 1.2v (rechargable AA) and the motor supply is via a 4.5v 1200ma uniross power adapter

    I now want to put in a potentiometer to allow me to vary the speed of the motor. I'm sure there are better ways.

    1) My primary question is how do I go about selecting a variable resister for the transister

    http://www.datasheetcatalog.org/datasheet/motorola/BD237.pdf

    I need to learn how to 'read' the data sheets - is there a reference I can refer to?

    2) My second qurstion is generally about choosing resisters. I understand that ~0.7V is required between emitter and base to open the gate. So... bare with me.....if that is the case, for the emiiter / base circuit we have a resistance. Therefore when combined with the protection resister we have a voltage divider? I measured the emitter to base resistance of my transister and there is some resistance ~30 MOhms (about as much as through my hands).

    So here are my assumptions, if 0.7V is required to allow current to flow through a PN junction then there is internal resistance. I assume when the 0.7v is reached (and before) the resistance decreases and current increases. So.....it's getting long this.... since resistance goes down but I still wish to maintain ~0.7v how do I choose the protection resister :eek:?

    reference:

    http://www.allaboutcircuits.com/vol_3/chpt_2/6.html

    noteably
    [​IMG]

    Once again, thanks in advance for any replies.

    Charlie
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Can you supply a sketch of your proposed motor driver circuit? Be sure to note on the sketch the power supply voltages you are intending to use.

    hgmjr
     
  3. charliedurrant

    Thread Starter New Member

    Jan 6, 2012
    18
    0
    No problem - give me a few minutes
     
  4. charliedurrant

    Thread Starter New Member

    Jan 6, 2012
    18
    0
    Hi,

    Here's the diagram.

    [​IMG]

    Charlie
     
    Last edited: Jan 21, 2012
  5. hgmjr

    Moderator

    Jan 28, 2005
    9,030
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    You do realize that you do not need a dedicated base power source. You can use your 4.5V power supply to deliver the required base current.


    hgmjr
     
  6. charliedurrant

    Thread Starter New Member

    Jan 6, 2012
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    0
    hgmjr,

    Yup I realise that I could use the 4.5v supply. Just experimenting...
     
  7. Markd77

    Senior Member

    Sep 7, 2009
    2,803
    594
    The way that circuit would work is, because V2 is always over 0.7V, the current through the resistor X1 and the base emitter junction of the transistor will be about (1.2-0.7)/R(X1)
    The current through the motor (if it has low enough resistance) will be the gain of the transistor times the base current.
    Unfortunately gains are a bit unpredictable.

    This circuit is more predictable:
    The voltage across the 2.2 ohm resistor is proportional to the current flowing through it and because the base emitter junction is about 0.7V, the voltage from the variable resistor controls the current through the 2.2 ohm resistor (and therefore the motor).
    The top 100 ohm resistor is to stop too much base current if you turn the knob too far, and the bottom one is to increase the voltage output at the other end of the scale (anything under about 0.6V the motor is off anyway).


    [​IMG]
     
  8. charliedurrant

    Thread Starter New Member

    Jan 6, 2012
    18
    0
    Mark,

    Thanks for the diagram which has real world values, I can purchase the parts and test it out. I knew there would be a better way of doing things!

    However, do you have any comment on my long ramblings about how to choose resisters for the base connection with respect to the varying resistance of the base - emitter junction with voltage applied.

    Also you refer to a 0.6-0.7v range from 0 emitter to collector to full. From the datasheet I can see 0.6v but it refers to

    Collector–Emitter Saturation Voltage

    Is that where you determined the 0.6v from?

    Charlie
     
  9. Markd77

    Senior Member

    Sep 7, 2009
    2,803
    594
    I hinted at how to work it out, the top section of my reply is about your circuit.
    The minimum possible motor current of your circuit depends on the value of the variable resistor. If it was 1K then the base current would be about (1.2-0.7)/1000 = 0.5mA
    Assume for the moment that gain is about 100:
    The minimum motor current would then be 0.5mA X 100 = 50mA
    The potential maximum current is very high (the variable resistor could be about 1 ohm at the lowest setting which would give a base current of about 0.5A, and the transistor won't last very long (without a hefty heatsink) if that happens.
    Unfortunately gain is hard to predict, the datasheet only gives a minimum, at a specified current, the maximum gain at that current could be four times higher.
    Then have a look at figure 3, the gain varies with temperature and current over a large range. It's best to avoid circuits that rely on the gain.
    The circuit I posted should give motor currents from 0 to about 300mA, you can increase the maximum current by using a lower value for the upper 100 ohm resistor.
    As for the 0.6 / 0.7V figure for VBE (sat) (not Collector–Emitter Saturation Voltage), that varies too but over a smaller range (see figure 4), it's the voltage from emitter to base that causes maximum current to flow through the collector emitter junction. A slightly lower voltage and the current is reduced, a little lower and no current flows at all, much like the graph on your first post.

    As a side note it's more efficient to control motors using PWM, but you seem to be studying transistors.
     
    charliedurrant likes this.
  10. charliedurrant

    Thread Starter New Member

    Jan 6, 2012
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    Mark,

    Thanks for the detail. It's funny how a few words can help so much. In time I will get more comfortable with all this. This is my first ever 'real' circuit and the most I knew from childhood was a bridge rectifier diagram and V=IR, although V=IR is very useful!

    Yes, it is simply a study. I have an ultimate goal that I want to achieve with respect to a programmable ECU on my vintage mini. I wont divulge what it is as I'm sure there are many people on here who will solve it easily! I've got to learn the fundamentals hence I'm just reading thtough the eBook.

    It's a challenge I've set myself and at the same time I'm sick of throwing out broken electrical goods and if I can repair one I'll be happy.

    I'll ignore the PWM comment as I know I'll get sidetracked and start reading about that for hours.

    Thanks again

    Charlie
     
  11. hgmjr

    Moderator

    Jan 28, 2005
    9,030
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    I should go ahead and alert you in advance that AAC does not permit discussions on the topic of automotive modification.

    hgmjr
     
  12. charliedurrant

    Thread Starter New Member

    Jan 6, 2012
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    Point noted.

    Out of curiosity, what's the reason? I can understand it if related to modern cars noteably breaking and related safety systems. In this case it's a MegaJolt ECU which is on a 1968 Mini Cooper which back when it rolled out of the factory came with electrics consisting of about 20 wires, two of which actually allow the engine to function with the rest controlling lights.

    Charlie
     
  13. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Concerns over safety issues have lead AAC to this policy.

    You are free to make any changes to your vehicle that you feel comfortable making. It is just that the forum prefers not to foster such activity. The skill level of our members and guests vary widely so it is impossible to determine each one's expertise in this area.

    That said, I wish you good luck with your project.

    hgmjr
     
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