Help with Transistor Project

Discussion in 'The Projects Forum' started by repdav, Feb 25, 2011.

  1. repdav

    Thread Starter New Member

    Feb 25, 2011
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    I have a IC timer that I want to turn on/off a small electric DC motor.
    Here are the specs:

    Voltage to both Motor and IC Timer is 1.5 volts (D cell battery)
    DC Motor Load = 175 ma
    IC Timer Max Output = 1.35 ma

    What transistor and what resistor to base should I use?

    Thanks

    <SNIP>
     
    Last edited by a moderator: Feb 26, 2011
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    A single transistor does not have high enough beta. You could use a low-threshold MOSFET. Fairchild has them, specified for Vgs=1.5V, but they are all surface mount devices, which you may not want to deal with.
    Your next choice, in my mind, is a two-stage BJT driver. See below.

    EDIT: OK, I see the error of my ways. See post #6.
     
    Last edited: Feb 26, 2011
  3. SgtWookie

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    Jul 17, 2007
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    I'm not trying to be picky here, but 330 Ohms is too small for the timer output; it'll be sinking almost twice what our OP claims it's rated for - except they said it's rated for 1.35 current output, which makes me think they'll need the NPN first, and a PNP to source current to the motor. A reverse-EMF diode would probably be a good idea, too.
     
  4. Ron H

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    I used the LMC555 datasheet for the timer, and observed that it can sink more current than it can source. I think you will find this to be true of all CMOS 555s. Hence the choice of a PNP for the first stage.
    VOL is about 0.2V typical for 1.35mA sink current. Vbe for 2N3906 at Ic=16mA and Ib=1.6mA (close to our operating point) is about 0.8V. Ib1=(1.5-0.3-0.8)/330=1.5mA. Since sink current is not a dissipation issue, but an output drive issue, I figured this is close enough. If you assume VOL is 0.3V, then Ib=1.2mA. If it would make everyone feel better, the circuit would still work if Ic1/Ib1=20, so let Ib=0.8mA. Then R1=620Ω.

    I did simulate this, and Ib1 was 2mA, but for the CMOS 555 model I used, the VOL was 12mV. This means the output NMOS has Rds(on)=6Ω. This is with Vgs=1.5V. I chose to believe National's datasheet rather than the model.

    I was going to put a diode across the motor, but I forgot. My bad.:(

    EDIT: OK, I see the error of my ways. See post #6.
     
    Last edited: Feb 26, 2011
  5. CDRIVE

    Senior Member

    Jul 1, 2008
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    This circuit is based on your original information stating that the timer output voltage will be high @ 1.35mA max. It's basically what Sgt.Wookie described. Though I used a 2N6726 you could try using a 2N2904 for Q2.
     
  6. Ron H

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    OK, now I get it. Wookie and Cdrive are trying to pound into my wooden head the fact that the timer's output is high when it is active. Duh.
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    The datasheet for Intersil's Cmos 555, their ICM7555 shows detailed graphs of typical output current at various supply voltages with various voltage drops at different temperatures.
    I think National's LMC555 and texas Instruments' TLC555 are the same.
     
  8. CDRIVE

    Senior Member

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    Ron, I just assumed it was sourcing because he didn't say 'sink' current. The Sarge and I could be wrong though.

    It's Saturday and getting near my time to head for my watering hole and place of social interaction. When I get home I'll be much smarter!:D
    Chris
     
  9. CDRIVE

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    Jul 1, 2008
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    I forgot to ask... Do y'all think that one 2N7000 logic level MOSFET would get enough gate voltage to do the job? I wasn't sure and that's why I didn't go that rout. The max sustained Id is only 200mA, so its limits are being approached. The gate threshold Vgs(th) = .8V min, 2.1V typ, and 3V max. If you have a bunch of 2N7000-s to test you can probably select the most sensitive one but I'm sure the OP is not in that position.
     
  10. Audioguru

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    A 2N7000 Mosfet has a max on-resistance of 5.3 ohms when its gate-source voltage is 4.5V and is slightly less with more voltage. Then the voltage loss with the 175mA motor current is almost 1V. The 3V motor might not even start running when powered from only 2V. But there is no 4.5V available.

    Why are you looking at the "threshold" voltage rating? It is when the Mosfet is almost turned off not when it is turned on.
     
  11. CDRIVE

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    Well, you answered my question. I'm glad I didn't go in that direction because it would not have worked.
     
  12. Ron H

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    I mentioned some 1.5V MOSFETs that Fairchild has, but they are all SMD, and most of them are leadless, which makes them difficult to work with for hobbyists.
     
    Last edited: Feb 26, 2011
  13. SgtWookie

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    I don't really know if our OP's timer has an active high or active low output. Since they said 1.35mA output max, I had to assume that it was sourcing current, thus active high.

    It would help a lot if our OP would tell us more about their timer; is it a CMOS 555 timer? And if so, what is the exact part number? After all, they are not all identical.

    Don't think I'd try a MOSFET solution, because it would get pretty flaky at best as the battery discharged - there's just not enough margin with a 1.5v battery.
     
  14. repdav

    Thread Starter New Member

    Feb 25, 2011
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    The timer is not a CMOS 555. It is a simple Brinks model I picked up at Walmart. I am using just the timer portion and hav removed the 110 volt parts, ie the 24 volt relay by which the timer actuates.
     
  15. SgtWookie

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    That's it? :confused:

    Well, then I guess I'm done with this thread.

    Good luck.
     
  16. Ron H

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    How is it that you get 1.5V@1.35mA from it? You said 1.5V comes from a battery, but what limits the current?
    And you said you had an IC timer. What's that all about?
     
  17. CDRIVE

    Senior Member

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    Where did you get the specs for the Brinks timer? BTW, I seriously doubt that Brinks manufactures the timer chip, though they may put their name on it.
     
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