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PNP transistor saturation voltage

  1. Oct 13, 2009 #1
    Hi,

    Anyone here can tell me how to control the saturation voltage of a pnp transistor?

    I was actually doing a project and my design using a pnp transistor to energize a 5V relay.Since I am using 5V supply,I cant afford to drop too much voltage at VCE of the transistor.

    The design was tested and it worked fine for the past 2 weeks.I was able to get 3.5V at the relay which was enough to energize the 5v relay.

    Strangely,the circuit wasnt working yesterday and there was only 0.7V at the relay coil which the VCE of the transistor taking a large chunk of the voltages.

    Anyone can advice what could be the reason the VCE suddenly so high?I increase the Ib to 6mA but still couldnt get the VCE down.

    Or something wrong with the relay resulting the VCE so high?I tested the relay by directly applying power to it and it worked fine.

    Or it could be the transistor itself was damaged and acted strangely.

    Please advice.Thank you.
     
  2. jcsd
  3. Oct 13, 2009 #2
    Hi Stealthray-
    One thing to be careful about when driving relays with transistors is that the relay coil has a lot of inductance L. When the transistor (I presume open collector) suddenly cuts off the relay coil current, there is a large dI/dt. Thus there is often tens (or more) of volts across the transistor from collector to emitter (recall V = L dI/dt) of both polarities, and this can wipe out the transistor. Put a reverse biased diode fron the collector to the emitter.
    Bob S
     
    Last edited: Oct 13, 2009
  4. Oct 14, 2009 #3
    Hi Bob,

    TransistorCE.jpg

    I am not really sure if I got what you meant.Refer to the diagram,I put a diode at the CE junction of the transistor.Is this the right way of connecting the diode to the transistor CE junction?

    Thank you.
     
    Last edited: Oct 14, 2009
  5. Oct 15, 2009 #4
    Troubleshooting power supply and circuit

    Hi,

    I decided to make a new thread on this since the last thread did not solve my problem.

    https://www.physicsforums.com/showthread.php?p=2392499#post2392499

    TransistorCE.jpg

    Refer to the circuit diagram,where in my old thread,the CE junction of the transistor is taking all the voltages and there is less than 1V voltage on my 5v power relay.It seems that the transistor was not working.I ordered new transistor and retested the circuit with a diode added at the CE junction.Sadly,it was not working.The relay still did not have enough voltage to energize and most of the voltage was at the CE junction of the transistor.

    Now here is the problem which I found out:

    I have 4 DC variable power suply which I use to do the testing.I measured all the power supply with a multimeter and they looked good.

    During the testing,I found out that only 1 power supply which can power up my circuit and energize the relay.

    Another 2 power supply though provide the voltage correctly could not energize the relay as most voltage is applied to the CE junction.When I use the IR transmitter to trigger the base of the transistor,there was clicking sound from the relay but it could not energize.

    Another 1 power supply goes into fault (the same condition if I short circuit it) when I trigger the base of the transistor.

    As you can see,only 1 power supply can be used.Although,all the power supply looks fine and produce accurate voltages as measured by my voltmeter,only 1 can be used to work correctly on my circuit.

    I was wondering what is the real problem here?Why the same power supply (from same manufacturer) could produce different effect on my circuit board.Could it be something wrong with my circuit?

    Please advice.

    Thank you.
     
    Last edited: Oct 15, 2009
  6. Oct 15, 2009 #5

    uart

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    Re: Troubleshooting power supply and circuit

    - The probem is not in the part of the circuit that you posted, it's somewhere else.

    - You don't need the CE diode, it does nothing in your circuit. The other diode is needed however.

    - What is the part number of the transistor?

    - Show the cicuit of the base drive of the transisitor.

    - Do your power supplies have (adjustable) current limits?
     
  7. Oct 15, 2009 #6
    Re: Troubleshooting power supply and circuit

    Hi,

    Attached is the full circuit diagram which I use in the testing.

    scan-1.jpg

    I am using transistor pnp 2N2905A and the ir receiver is TSOP1238 Vishay.

    Regarding the power supply,yes it is adjustable.I think it has more than 1A rating.Out of the 4 power supply,only 1 of them can be used to on the relay when the IR receiver was triggered.

    Thank you.
     
  8. Oct 15, 2009 #7
    Re: Troubleshooting power supply and circuit

    I'm sure you know, in an PNP, you have to let the current out of the base in order to turn it on.

    you can do a quick test, and simply ground the base with a crocodile click and see if the relay will turn on.

    if it does, then the problem is somewhere else.
     
  9. Oct 15, 2009 #8
    Re: Troubleshooting power supply and circuit

    Ok,I will try it tomorrow.

    But the thing that puzzled me was why 1 of the power supply can power up the relay while the other 3 power supply cant. And I tested all the 4 power supplies on the IR transmitter and they all worked fine.

    For 2 of the power supplies,most of the voltages were at the ce junction and only 0.7V was dropped at the coil.The weird thing was when I point the transmitter to the receiver,there were non stop clicking sound from the relay as it was on and off non stop. but the measurement from voltmeter showed it did not energize and no connection at the NO/COM pin.

    Another power supplies showed short circuit fault when I point my transmitter to the receiver.

    The last power supply was able to energize the relay and work correctly.
     
  10. Oct 15, 2009 #9
    Re: Troubleshooting power supply and circuit

    then in that case, the power supplies have adjustable current limit. somebody was messing around with them earlier and lowered the current limit.

    if the power supply is analog, current limit is not hard to find, if it's digital you have to go the menu and find current limit.
     
  11. Oct 15, 2009 #10
    Re: Troubleshooting power supply and circuit

    Yes,there are 2 knobs at the power supply,one which is the voltage and the other current.

    In all my testing and lab work,the lecturer told us to turn the current knob just a little bit.And then turn the voltage knob to the voltage we want.

    Are you saying,that I need to turn more on the current knob to raise the current limit.

    If that is the case,if I was indeed set a low current limit,and my relay will probably need:

    5V/50ohms = 0.1A, then this will result in 4V++ at the CE junction and only 0.7V at the relay coil?

    Thank you.
     
  12. Oct 15, 2009 #11

    uart

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    Re: Troubleshooting power supply and circuit


    Also, do you know what an adjustable current limit is and how to use it?
     
  13. Oct 15, 2009 #12
    Re: Troubleshooting power supply and circuit

    I have no idea what is that for.But I think it limits the output current of the supply.

    I measured the relay coil when pnp transistor turns on and it showed 0.7V.

    So I = 0.7V/47ohms = 14.9 mA.

    Can I say this is the max current that the power supply produced based on the current knob setting?If I turn the knob more,Will I get more current and this increase the voltage at the relay coil and lower the voltage at the CE junction?

    Thank you.
     
  14. Oct 15, 2009 #13

    uart

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    Re: Troubleshooting power supply and circuit

    Yep, that's the trick. :) It sounds like some of the power supplies simply have the current limits set too low for your circuit.

    Basically the current limit is there (on a good lab power supply) to try and help prevent you damaging things if you make a short or other mistake with in your experiment or prototyping exercise. You should always set the current limit before you start working on a given circuit based on the approx maximum current that you expect the circuit will need. In this case I suggest you should set the current limit to approx 200mA.

    To set the current limit you should first adjust the voltage to just a few volts and the current limit to near zero. Then apply a short to the supply terminals* and slowly increase the current limit until you get the desired value. Then just remove the short, set your voltage and start your experiment.

    * Some power supplies have a "set current" switch (or button) which does this for you automatically so you don't need an actual external short.
     
  15. Oct 15, 2009 #14
    Re: Troubleshooting power supply and circuit

    Thank you so much for helping me to troubleshoot and find out the root cause of the problem.
     
  16. Oct 15, 2009 #15

    uart

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    Re: Troubleshooting power supply and circuit

    That would have been because the circuits you were working with only needed a little bit of current. Now this one needs a bit more, that's all.

    Edit. I hadn't seen the above reply when I posted this. I see you fully understand the current limit thing now. :)
     
  17. Oct 15, 2009 #16
    Re: Troubleshooting power supply and circuit

    By the way,there are 3 terminal at the power supply outlet, -ve ,ground ,+ve

    What is the difference connecting to -ve,+ve and +ve and ground?

    I measure the voltage and they are all the same.

    Thank you.
     
  18. Oct 15, 2009 #17

    uart

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    Re: Troubleshooting power supply and circuit

    Ground is connected to the power system earth. For simple circuits with just a single supply you'll usually have "-ive" as your zero volt reference and in such cases it's often a good idea to also have this connected to the system ground. Instrumentation (like scope or signal generator for example) will usually have a ground reference so once you connect them to your circuit it will be grounded anyway.

    Only in certain circumstances will you want to have your power supply isolated from ground and a good power supply should give you that option.
     
  19. Oct 15, 2009 #18
    Re: Troubleshooting power supply and circuit

    Thank you for all the info.
     
  20. Oct 15, 2009 #19

    berkeman

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    Re: Troubleshooting power supply and circuit

    Multiple posting the same question is not allowed. I've merged your two threads into one here.
     
  21. Oct 15, 2009 #20
    Re: Troubleshooting power supply and circuit

    My apology on that.I thought making a new thread for a new detected problems on the same issue might make the question more straightforward and simple.

    I will not repeat the same mistake again:wink:
     
    Last edited: Oct 15, 2009
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