How to choose a transistor

Discussion in 'General Electronics Chat' started by crease-guard, Apr 10, 2010.

  1. crease-guard

    Thread Starter New Member

    Nov 8, 2008
    3
    0
    I'm working on a circuit to control a small dc motor with momentary push button controls to control the volume on my audio system. I found the following circuit and laid it out in Eagle to interface into my motorized pots:

    [​IMG]

    Now, this circuit is designed for a 12v dc motor drawing 1 amp max at load, but is only drawing about 100mA in this setup. My motors are 3v drawing 100mA. Two questions:

    1. I know I don't need the TIP31 but will they work with my smaller voltage?
    2. If I were to look at my requirements for power and voltage, how does one go about selecting an appropriate NPN transistor?

    I understand the concept of the NPN but I just haven't quite wrapped my head around how to pick one.

    Thanks
    Jay
     
    Last edited: Apr 10, 2010
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Give some overhead with the voltage and current. If you can find a less expensive Darlington that will handle, say, 500 milliamps, then use that.

    Because this is not a PWM scheme, your design can get lots simpler. Use a DPDT switch for the polarity switching. If it's center off, you can just use the one switch to replace the entire circuit, otherwise you will need a pushbutton switch to run the motor.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Well, rather than dealing with transistors and wiring for an H-bridge, have you considered using an L272 dual power opamp? One IC has up to 1A from each output. They're cheap and small, too.

    Here's an example schematic:

    [​IMG]

    See the ST Microelectronics datasheet for an L272 or L2720.
     
  4. Bychon

    Member

    Mar 12, 2010
    469
    41
    This guy asked how to choose transistors, not design a motor controller.

    You go to a website of a company that makes transistors and use their sorting program to start choosing transistors that can handle the voltage you need, and some extra so they survive inductive spikes and other small interferences. Then you choose among those for transistors that can handle the current you need to use. Then you check for power ratings because you can calculate the power the transistor has to survive. You also have to sort for "special" abilities like, frequency limitations, leakage current when turned off, and current gain abilities.

    You can spend all day sorting transistors if you'r setting up an array of devices for a production run, an R&D department, or your own work bench, and you can find the right one in 15 minutes if you know exactly what you need for today.
     
  5. crease-guard

    Thread Starter New Member

    Nov 8, 2008
    3
    0
    Sgt Wookie, didn't know about that IC, I'll look into it as it looks like exactly what I need and infinitely more simple.

    Been there, my interface is a bit more complex that actual physical switches. I needed multiple switches from one set of switches. The original controls were designed using a 2 wire interface and resistors in series that would send back different voltages based on which button was pushed. I used a voltage comparator along with some logic gates to make discrete logic outputs for each switch. I then used a pair of Maxim 16054 to latch one of the push buttons to control a set of TS5L100 IC so that I could turn 8 switches into 32 switches. Each of the 32 "Switches" can be latched with a 16054 or momentary (using opto isolators) depending on my needs. So given my setup, I needed a circuit that would use the switch pulse to move the potentiometer motor as opposed to a true PWM type circuit.

    The circuit I made works great...much to my surprise as it was really the first circuit I designed from scratch without any help from those much more knowledgeable than me in these areas.

    I've already got the part for the above circuit, if the TIP31 NPN will work with the lower voltage then it's easier for me to just build that one as I've already laid out the design. However, I will look into the IC Sgt Wookie recommended as it seems to be a much more simple design.

    Thanks for the help.
    Jay
     
    Last edited: Apr 11, 2010
  6. crease-guard

    Thread Starter New Member

    Nov 8, 2008
    3
    0
    What I need is a transistor that will drive a DC motor rated at 3v and 100mA current draw. Now, I didn't want to assume that just because a transistor was rated for a higher voltage and current draw that it would do anything below that...because I'm not knowledgeable enough to know if that assumption is correct.

    Jay
     
  7. Bychon

    Member

    Mar 12, 2010
    469
    41
    We get a lot of people asking things like this. They wonder, if a car battery will provide 300 amps, why doesn't it shove 300 amps through the headlights? or, if a light bulb is connected to a 15 amp circuit breaker, will it use all 15 amps?

    I don't even think in those terms, so I miss and give the wrong answer.
     
  8. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,750
    759
    Hmmm..... retched ...I wonder when, or how long..want to bet..
     
    Last edited: Apr 11, 2010
Loading...