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Calculating Size of Resistor needed?

  1. Dec 19, 2009 #1
    Hey again. I have a circuit that I've made and I was told I should put resistors in so that I don't blow out my transistors and buttons. Ok, I can do that... but I have no idea what size of resistor I need! Picture:


    I'm using TIP41C Transistors:

    Information Sheet

    I think what I needed from this sheet was the "Continuous base current" of 3A?

    12V = 3A * R

    R = 4ohm?

    Does that sound right? Am I oversimplifying it?

    Attached Files:

  2. jcsd
  3. Dec 19, 2009 #2
    What current do these loads draw?

    You want the transistors to saturate, which means Vbe > Vce.

    Generally, the current gain at saturation is 10 or less.
  4. Dec 19, 2009 #3
    .5 amp each, I think. Either that or 1.5A each
  5. Dec 19, 2009 #4
    What current must flow in the solenoids? If you have a spec for these components please post. I don't think I_B = 3A is the relevant parameter, but I need to understand the load to consider how the transistors will work.
  6. Dec 19, 2009 #5
    Sorry, I'm not sure exactly which solenoids we're using.

    Assume the current draw for each solenoid is .5A each

    Edit: Just got an email back from one of the guys. He says they're 1.5A each
    Last edited: Dec 19, 2009
  7. Dec 19, 2009 #6
    So you need Ib > 150 mA.
    That t'sistor on the right cannot be saturated as it is drawn.
  8. Dec 19, 2009 #7
    ?? why not? What is Ib? The current in b? and why does it have to be > 150mA?
  9. Dec 20, 2009 #8
    1 because Vbe can never go higher than Vce in this circuit
    2 Ib is the current into or out of the base
    3 because you want Ic to be at least 1.5A and the current gain at saturation is presumably 10x or less.
  10. Dec 20, 2009 #9
    You've asked a simple question, but the design may call for a bit more complication. For example google the term "solenoid driver" and look for some images. The driver depends on the solenoid properties and application requirements.

    Are the solenoids to be operated momentarily or latched? If momentarily you must ensure that a person holding their finger on the button does not overide the proper solenoid operation.

    If latching the solenoids it is common to reduce the current in the latched state. Also, you need to understand how to bias the transistors. To do that, draw each transistor driving its load with 12V at the top and ground at the bottom, as if the other transistors and loads do not exist, if you wish.
  11. Dec 20, 2009 #10
    Argh. *googles current gain* Though, I think I understand what you mean. I just now have no clue what to do to fix my circuit :(

    As for the solenoids, the actual shifting of the car takes milliseconds, but somebody holding the button down shouldn't affect anything, just vent CO2 to the atmosphere.

    *googles biasing of transistors*

    Found a good youtube lecture... maybe it will give me some ideas...

    Ooooh... wait a second. Can I use a darlington pair to further amplify the current output? Like this: ?


    I'm thinking that's what was meant by the fact I didn't have enough current gain? After all, I thought about what I needed (higher current in the base), and thought I could use two transistors... then remembered that's exactly what a Darlington pair does :P

    I get Ib > .625 mA in this configuration assuming " A typical integrated power device is the 2N6282, which includes a switch-off resistor and has a current gain of 2400 at IC=10A." <--- Wikipedia.

    Note: I've never had an electronics class, so keep it simple, eh? Or at least link me to some literature that so I can understand what you mean if that's easier :P

    Attached Files:

    Last edited: Dec 20, 2009
  12. Dec 20, 2009 #11
    The attached gif (left side) is taken from this page on robot solenoid:


    You want to use the NPN transitor as a "low side switch" as shown on the right. The transistor should be fully on (saturated) when the pushbutton closes. Notice the Vce(sat) in the data sheet? That would be the transistor voltage Vce when biased to turn on all the way, so the solenoid would see 12V - Vce.

    You may need a flywheel diode to protect the transistor from excess current surge when it turns off (the coil needs to discharge energy and the transistor could get zapped). It would help if you get a spec for the solenoid. I need to consider the bias conditions ...

    Attached Files:

  13. Dec 22, 2009 #12
    Got the spec sheets.

    See attached.

    Qty1 L01SS6594000060 (12VDC)
    Qty1 NF1BAN524N00060 (12VDC)
    Qty3 230-365 Electrical Din Plug Connector

    Attached Files:

  14. Dec 22, 2009 #13
    3.5w and 6w @ 12v comes to 300 mA and 500 mA for coil currents.
  15. Dec 22, 2009 #14
    The second pdf offers an option to purchase with the surge suppression diode. I suggest purchasing with this option unless you plan to choose your own suppression diode:

    17N = DC Solenoid W/Surge
    Supression Diode

    See if such an option is available for the other solenoid model, and try to find a sketch of the solenoid driver design in the numatics documentation.
  16. Dec 22, 2009 #15
    How does this help me? I don't have any clue what to do with these values.

    Couldn't find a sketch of it, but I did come across this simple design in Google Images:


    I suppose I could modify this for my purposes, but I don't know exactly what's going on. Why is there a ground AND both connections to the 12VDC?
    Last edited by a moderator: Apr 24, 2017
  17. Dec 22, 2009 #16
    This is an obsolete part but you'll get an idea of how you would use it. Datasheet for a high current high side switch:

    http://cds.linear.com/docs/Datasheet/1089fa.pdf [Broken]

    My sketch is a low side bipolar transistor switch and yours is a highside FET switch. It would be good if you can find a replacement similar to that LT 1089 part.
    Last edited by a moderator: May 4, 2017
  18. Dec 22, 2009 #17
    Thanks for the ideas. Unfortunately, I believe the solenoids have already been ordered (or sponsored). As for the circuit, I am in the midst of getting packed to take a flight home for xmas, but I'll be able to take a look at it in more detail.

    I've given it a quick look, however, and I'm a little confused about what's happening. Does that transistor on the right only get saturated when the solenoid is on/open? Is that a function of the internal makeup of the solenoid itself?
  19. Dec 23, 2009 #18
    Have a good trip home. This page gives the bias circuited needed to use the TIP41 transistor as a low side switch (but the size of the flywheel diode is still uncertain):


    We can go through the design, build, and test procedure when you're ready.
  20. Dec 25, 2009 #19
    So, this system de-clutches for down-shifting only, and interrupts ignition for up-shifting???

    No doubt you will laugh at my ignorance, but I dread to imagine what would happen if I drove my manual (stick-shift) car in this way. I would love to know what this is for - a motorbike?
  21. Dec 26, 2009 #20
    Thanks, SystemTheory. I looked at your link, and suddenly it makes a lot more sense. Not sure why, but it suddenly came to me :) I can try to find values for the solenoids we're using.

    I got confirmation from the pneumatics guys that they've already ordered the solenoids, sans suppression diode :(

    I would prefer to use NPN transistors, as I know I can get them really easily from the school (the TIP41Cs that I think I mentioned earlier?). Unless it makes life THAT much harder, as I'm sure they carry PNPs as well...

    Does it make any difference that I need to power 2 solenoids at once? Can they simply be put in series?

    Adjuster: Hehe. Yeah, it might not work so well for your car at home, but for an FSAE car, however, it works just fine. Our car is run by a Honda F4i 600cc motorcycle engine and associated (but customed) transmission. So yes, it's basically a motorcycle drivetrain.
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