Mysterious voltage drop when motor connected

Discussion in 'General Electronics Chat' started by Hisopio, Jun 27, 2016.

  1. Hisopio

    Thread Starter New Member

    Jun 27, 2016
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    Hello, I'm working in a very simple project where I use a PIR motion sensor to run a little DC motor.
    I'm using 4 AAA batteries so 6v, and I got the motor from an old walkman that used 3v to run.
    Whenever I test the motor directly with as low as 1.5v it works perfectly.
    When placing the Mosfet and the PIR sensor in the circuit I'm measuring around 5.3v at the end. That should be more than enough.
    The problem is whenever I connect the motor the voltage drops to 1.5v. It runs, but too low to make it do anything.
    The Mosfet is a 88L02S
    And the PIR
    Here's the circuit :
    [​IMG]

    Thanks for your help.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Have you tried it without the LED?
    Also, do your batteries power the motor nicely if directly connected?
    What is the gate voltage when things are not going well?
     
    Last edited: Jun 27, 2016
  3. Hisopio

    Thread Starter New Member

    Jun 27, 2016
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    Tried without the LED yes, no change.
    Motor runs perfect when connected directly.
    Gate voltage reads 3.3v without the motor, and starts at 3.2v but drops fast to 2.5v with...
     
  4. wayneh

    Expert

    Sep 9, 2010
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    Hmmm, that's interesting. You've got a form of positive feedback there, where the voltage drop caused by the load of the motor causes a drop in the gate voltage at the MOSFET. What happens to the voltage at the batteries? Maybe that PIR device is using more power than you might think, to power the IR lights.
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    The PIR sensor puts out a nominal 3V when activated. The FET Vgs(thr) can be as high as 3V according to the datasheet. If it is (nearly) that high then the FET would pass only a small current, insufficient to drive the motor properly. You might do better using a bjt as a switch, instead of the FET. Alternatively, use 2 x bjt to provide a clean 6V gate signal for the FET.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    You have the FET drain and source shown reversed on the diagram.
    You need a logic-level type MOSFET [Vgs=3V or less for the rated Rds(on) value in the data sheet] or go to a BJT.
     
  7. BobaMosfet

    Active Member

    Jul 1, 2009
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    I would also put a snubber diode in parallel with the motor, since it's inductive. It will create massive voltage spikes in the circuit, which can damage your mosfet. I agree with another poster-- use a BJT. Most mosfets generally require at least 10V before they full activate. Vgs(thr) just means it starts transconducting at 3V-- not saturation.
     
  8. Hisopio

    Thread Starter New Member

    Jun 27, 2016
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    Batteries voltage stable without it, but drops when the motor is connected to 1v then 2.75v for half a second and then back to 5.5v.

    It's weird, I tried with a BJT in the first place and the motor hardly moved at all.
    I'm going to try that again, maybe I missed something...
     
  9. crutschow

    Expert

    Mar 14, 2008
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    You need a base resistor, of course, if you use a BJT.
     
  10. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    If the battery voltage drops too much then the PIR will malfunction, so its output will be erratic, trying to switch the FET on and off repeatedly.
     
  11. Hisopio

    Thread Starter New Member

    Jun 27, 2016
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    Okey, you were all right, it works much better with a BJT. The motor is moving quite fine with 1.7v-1.5v.

    The only doubt I have is that the base is getting barely 0.76v, I found that very low, so I'm not using a resistor.
    Do you think that I should put one anyway?
    This is the BJT I'm using.
     
  12. crutschow

    Expert

    Mar 14, 2008
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    0.76V is the normal Vbe for a BJT as the base-emitter junction looks like a forward biased diode when ON. Why did you think it should be higher?

    Yes, a BJT needs a resistor in series with the base to limit the current.
    Without it, the base-emitter current is basically limited by the short-circuit output current from the PIR sensor.
    If nothing else, that's probably not doing the sensor any good.

    (Short Rant: I'm always amazed at those who try to use transistors without even a basic understanding of how they work, since a short Google search will tell you all you need to know about them.:rolleyes:)
     
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  13. Hisopio

    Thread Starter New Member

    Jun 27, 2016
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    Thanks a lot everyone, I'll put one then.
    I'll come back (hopefully soon) with news of the finished project.

    (Short reply to short rant: Me too I'm amazed of those who think that we normal people can become engineers with just a quick look at the thousand of millions of Google pages out there. o_O But thanks. I appreciate a lot your help!).
     
  14. BobaMosfet

    Active Member

    Jul 1, 2009
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    @Hosopio-- We're glad you asked. If the majority of information about electronics on the web was correct, fewer people would be asking for help. It's one of the reaons I became an advanced designer. I can tell if it's wrong, or how it's supposed to work if I find something interesting and fix it. It isn't always the author's fault either-- huge scores of published schematics have minute errors due to the publisher personnel typesetting things without undestanding what they were doing, or creating nifty illustrations or BOMs without understanding that decimal point means something where it is, or any of a number of possibilities that occur.
     
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  15. wayneh

    Expert

    Sep 9, 2010
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    It's not unreasonable to expect someone to learn to use a tool properly before they attempt to use it. Suppose you just bought a table saw (like I recently did). Would you rush to cut some wood with it without first reading about safe operation? I hope you answered "of course not". I'm scared to death of my table saw and take every precaution I can think of, which was expanded by extensive online research.

    I know that using a transistor is not nearly so scary, but it's good form to understand a component you intend to use in a design.
     
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  16. Hisopio

    Thread Starter New Member

    Jun 27, 2016
    19
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    I agree with you, but a table saw, while more deadly indeed, it's way much simpler to use.
    Learning to use a transistor properly is not at all something easy, specially because you have to start learning lots of other electrical basic concepts before even attempting to understand anything. Like what's a volt, and a watt and an amp and then ohms and ground and hertz and resistors and diodes and leds, and I'm not even speaking about soldering that it's an art by itself. And then when you think that you learnt enough (after hours, and days, and weeks of reading) and you try to put some very simple circuit together and that, of course, it doesn't work properly, then you spend more nights (because of course, you work on something totally different during the days) googleing tons of different answers (everyone has his own solution) and then when you finally find the one that you think it's going to solve everything, it looks like this:

    [​IMG]
    so yes, I don't pretend to understand how to properly use a transistor, and probably never will. But hopefully I would continue to find cool generous people like you all to help me with my simple questions, 'cos is either that or my projects will never exist.

    Sorry if I bother much, and thanks a lot again!
     
  17. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Why use a large power mosfet with a gate to source voltage rated at 20v?
    Also if running off 3 AAA cells the motor cannot be that high a load?
    Sure you cannot get away with a 2n7000?
    What are the details on the sensor and the motor?
    Your LED is virtually across the supply.
    Max.
     
  18. Hisopio

    Thread Starter New Member

    Jun 27, 2016
    19
    3
    Yes thanks for your answer. The mosfet was a mistake. I'm using this one now HJ882. It's just that I like the idea of recycling and I'm getting the parts off an old PC mother board.
    It's working better now.
    I'm using the LED instead of a protection diode for the motor back spikes. Couldn't find one yet.
     
  19. BobaMosfet

    Active Member

    Jul 1, 2009
    109
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    If you got a tablesaw, I'd strongly recommend you purchase one of those stop mechanisms (it may already have one) that if your flesh comes in contact with the blade it stops it _right now_. It's essentially testing for capacitance through the blade all the time. It will ruin the saw if activated, but it will save a finger.
     
  20. wayneh

    Expert

    Sep 9, 2010
    12,120
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    So far I've built a crosscut sled for it, and that alone makes it far more accurate and safer too. I've made plans to build a riving knife for it, for ripping, but so far I haven't made that happen. I need 13 gauge metal and a way to cut it to my pattern. So far I don't know how I'll do either.

    An outfeed support is probably my next priority.

    I have to disagree with the TS: A tablesaw is far more challenging than a transistor. To use a transistor, once you have one rated for your load, you just need to limit current through the base-emitter to about 1/10th of the collector-emitter circuit, and you need the base at ~0.7V above the emitter voltage to turn it on. That's not so tough.
     
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