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Need Transistor to relay help ASAP

Discussion in 'General Electronics Discussion' started by Slobin3d, Jun 13, 2012.

  1. Slobin3d

    Slobin3d

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    Jun 13, 2012
    i'm working on a defunct Mobility scooter speed controller, the relay that powers the forward is no longer functioning. I have determined that the relay is functional.

    I am trying to use a transistor to activate the heavier load relay. I found the signal from the throttle control. It rests at 5vdc and as forward throttle is increased the Vdc increase to a max of 10Vdc. For the reverse the opposite happens and the Vdc decreases to 0Vdc. The solenoid that activates the reverse engages around 4vdc.

    I want the Transistor to activate at 6Vdc and above and have a through put on the High power side of 24-40Vdc. I found a NPN 2N4401 transistor from Radioshack, from what I read it should be appropriate to my task.

    I'm having trouble making it work though. Did I miss my understanding of how this transistor works or how it activates? It's my first time dealing with transistors and have been reading for several days and my brains mush.

    Thanks
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    9,229
    1,861
    Nov 17, 2011
    How does your circuit look like? How did you connect the transistor into the existing circuit, and why do you need an additional transistor if the relay is functional?

    Harald
     
  3. Slobin3d

    Slobin3d

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    Jun 13, 2012
    from what I understand the circuit is set up so that E=24vdc B=6vdc and C= ground

    The relay Transfer the power from the battery direct to the motor at 70 amps. I need the transistor to activate the relay at 6Vdc+ once throttle is applied. there is an array of relays that control the power on/off and forward and reverse. I can manually activate the relay and the trottle functions as designed, I just cant get the Circuit board to provide the activation to the relay. Hence building an external circuit to do it.
     
  4. BobK

    BobK

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    1,648
    Jan 5, 2010
    That is not correct.

    Aside from the fact that the relay coil is not in the circuit, the transistor is backwards, and is shorting supply to ground if it turns on.

    The correct way to wire a an NPN transitor to turn on a rely is:

    C to one relay coil lead
    other relay coid lead to V+
    E to ground
    B to control voltage through a resistor.

    And this would work in the case the the control voltage was 0V when off, but yours is 5V when off if I understood correctly. In this case you are going to need to distinguish betwen 5V and 6V to turn the transistor on. The best way to do that is to use a comparator. So it is not such a simple circuit after all.

    Bob
     
  5. Slobin3d

    Slobin3d

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    Jun 13, 2012
    Ok so I have the C and E reversed. Now I'm lost again. What do I need to do to get this to activate and my 6Vdc?

    the transistor I have is


    Power dissipation: 350mW
    Collector emitter voltage: 40V
    Collector base voltage: 60V
    Emitter base voltage: 6.0V
    Collector current: 600mA
    Case type: T0-92
    Electrical Characteristics
    h(fe): 100-300
    f(t): 250MHz
    Case type: TO-92
    V(CE): 0.4
    V(BE): 0.95





    Or what would be the correct Transistor for my application?
     
    Last edited: Jun 13, 2012
  6. CocaCola

    CocaCola

    3,635
    4
    Apr 7, 2012
    He told you...

    You need to detect the transition from 5v to 6v and then you use that rise in voltage detection to turn 'on' the transistor...

    What you listed are the MAX values for the transistor, not when it will turn 'on' it will turn 'on' at a MUCH lower voltage below one volt actually...
     
  7. Slobin3d

    Slobin3d

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    Jun 13, 2012
    Ok, that a clearer explanation. What am I looking for in my values? I'm sorry for the Newb questions, this is my first time dealing with a transistor, and I need a little hand holding apparently.

    I read that using an inline resistor on base to reach the Saturation (on) is recommended, i'm guessing that would be easier then trying to find a transistor that is already set to that values I need.

    So using the transistor I have what is my base On voltage need to be? so that I I can get the appropriate resistor? This project is quickly surpassing my knowledge level:(
     
  8. BobK

    BobK

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    Jan 5, 2010
    A comparator is an IC that will compare one voltage to another and output a signal depending on which is larger. So you would feed 6.0V into the - input and the throttle output into the + input. This would turn the comparator on when the throttle voltage exceeds 6V. The "standard" compare is the LM339. But it outputs only a low, it is high impedance when it is logically on, and grounded when it is off. In this case, you put a resistor from V+ to the base and the comparator output. Sorry, but I am at work right now and don't have access to a program to draw a schematic.

    Bob
     
  9. CocaCola

    CocaCola

    3,635
    4
    Apr 7, 2012
    That is not how it's done, the inline resistor on the base is limit the current so you don't waste excess energy and to prevent damage to the transistors base...

    Around 1 volt according to the datasheet (it will vary slightly from manufacture to manufacture) you should always review the datasheet of any component, you are unfamiliar with... http://www.nxp.com/documents/data_sheet/2N4401.pdf

    As I said it doesn't work that way...

    It's never to late to learn, you just need to devote the time...
     
  10. Slobin3d

    Slobin3d

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    Jun 13, 2012
    let me see if I understand, or if I'm way off. I use the comparator to only pas voltage through it at my desired Base input? then use the output of the comparator to the transistor base to activate the Switch function of the transistor? I hope maybe? could I do the same thing with a resistor if it drops the 6vdc to the voltage the Base needs to activate, or will having voltage passing through the base, even if it's below the activation point damage the transistor or relay?

    Sorry for all the long questions but I really have to have this running tomorrow morning
     
  11. BobK

    BobK

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    Jan 5, 2010
    No, as CC keeps telling you, you cannot simply use a resitor. Transistors do not turn on or off instantly at a particular voltage or current, they are linear devices that will partially turn on at a voltage less than the threshold and this is dependent on the particular tranistor (even of the same kind) and temparature for example.

    That is why you need a comparator, to set an exact threshold at which the on / off state is discriminated. A single transistor cannot do this. We could probably come up with a two transistor circuit that would work if that would make you happier.

    Bob
     
  12. BobK

    BobK

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    Jan 5, 2010
    Here is a discussion of the schmitt trigger, which is a 2 transistor circuit that can function as a comparator.

    Bob
     
  13. Slobin3d

    Slobin3d

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    Jun 13, 2012
    thanks for bearing with me, it's a lot of new info in a short time, I'm learning lots of info, and I appreciate all the help
     
  14. Slobin3d

    Slobin3d

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    Jun 13, 2012
    OK, lets see if i understand now.......


    Voltage comparator to control output of signal voltage (effectively removing all voltage below 6v from signal) then use a resistor from the output of the comparator to achieve the desired input to the transistor's base to allow sufficient voltage to activate the hi voltage/amperage relay?

    I wouldn't need transistor at all except that the relay needs more then 6v to activate, correct?

    I think I'm getting the picture

    If i'm correct above, how do I setup the read the specs of a comparator to have it output the proper voltage, how do I read the transistor so that I can modify the output of the comparator to meet the transistors needs?

    i'm drawing a diagram in paint to hopefully explain what I'm thinking
     
  15. CocaCola

    CocaCola

    3,635
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    Apr 7, 2012
    Slobin3d you appear to pretty much have a grasp of the basics now...

    As for setting up the comparator and transistor, a Google search on both topics will yield a plethora of information on both... Yes, it's A LOT to take in but you have to start somewhere if you want to understand... Search out the newbie pages first, they should be obvious...

    Bob has hinted he will work up a schematic when he has free time, with the knowledge you gain between now and then doing a few Google searches when he does post of that schematic you might very well be able to understand a majority of it and how it works... It's not overly complicated but you do need to understand how the component function before it will make sense...
     
  16. Slobin3d

    Slobin3d

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    Jun 13, 2012
    Thanks, it's a crash course no doubt. I'm drawing up a basic schematic to see if I have it down. I will need some help to make sure its correct and fill the values so I can hopefully get to radioshack and get the appropriate parts to get this thing back together.
     
  17. Slobin3d

    Slobin3d

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    Jun 13, 2012
    ok here goes, What I need to figure out is what comparator to get, what resistors I need to make the the input voltage match the comparator's input specs, what resistor I need to match the transistors Base input and hopefully I solder everything up and the relay activates when Positive throttle is applied ;)
     

    Attached Files:

  18. Slobin3d

    Slobin3d

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    Jun 13, 2012
    doing more reading........

    [​IMG]

    It looks like the comparator uses V- as a reference voltage, and V+ as the variable voltage Making my thought of needing resistors on V+ and V- not needed?


    So if I have a constant Voltage +5Vdc on V-, what ever voltage beyond 5Vdc the signal wire sends will be sent through the comparator? Will the comparators output follow the increase in input voltage too? IE input is 5Vdc-10Vdc output will be 5Vdc-10Vdc as well.

    If so, will this change in voltage affect my transistor base input? Do I need some sort of voltage regulator between the Comarators output and the Transistors base to hold the voltage at a constant? then use a resistor to get that voltage to the level the transistor's base needs to function?
     
    Last edited: Jun 14, 2012
  19. BobK

    BobK

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    Jan 5, 2010
    Here is a schematic of a transistor switch controlling a relay using a comparator to set a voltage threshold. The pot can be adjusted to set the voltage between 0 and 12V, so 6V should be very near to the center. Or you could use fixed resistor in the ratio of 3:1 (top:bottom) to get a 6V threshold.

    When the throttle signal voltage is less than the threshold, the comparator output is 0V (ground), which grounds the base of the transistor and turns it off. When the throttle voltage goes above, the comparator is not grounded and the 3.3K resistor turns the transistor on. The diode is to protect from the voltage spike that occurs when the relay is turned off. This might very well (should) already be in the circuit, but if it is not, use a 1N4001 (or any of the 1N400X ones).

    Bob
     

    Attached Files:

  20. BobK

    BobK

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    Jan 5, 2010
    The - can be considered the reference and the + as the input.

    The comparator does not follow the voltage, it is a descriminator. Think of it as a switch, it connects the output to ground whenever the - input is > the + input, and it disconnects the output from anything when the + input is >. That is why, in my circuit on the last post, there is a resistor from the output to +24V.

    Bob
     
    Last edited: Jun 14, 2012
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