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Transistor Help

AQucsaiJrAQucsaiJr Posts: 17
edited 2007-12-22 19:52 in Robotics
I need a BS170 Transistor to use as a switch to power a DC motor from My BS2 however the only place I can find it is on the Parallax web site.· would buy it from there however with shipping it makes it a little too pricey. My question is: Is there an equivelent to this transistor that may be more easily found?

Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2007-12-21 00:56
    What are the voltage and current requirements of the motor?

    -Phil
  • AQucsaiJrAQucsaiJr Posts: 17
    edited 2007-12-21 01:30
    Im not real sure cus it is not printed on it anywhere. It runs fine off the VDD and VIN power outputs on the homwork board though.
  • Mike GreenMike Green Posts: 23,101
    edited 2007-12-21 01:49
    A switching transistor like the 2N2222A would probably work. It can switch voltages up to about 60V and currents up to about 600mA which should be enough for most things that can run off a Homework board's regulator.
  • r2d2_botr2d2_bot Posts: 23
    edited 2007-12-21 03:38
    Have you looked on Jameco's web site (www.jameco.com)?· They have alot of good stuff!wink.gif

    [noparse][[/noparse]Tim]
  • AQucsaiJrAQucsaiJr Posts: 17
    edited 2007-12-21 13:06
    Sounds good mike I'll try it and let you know.
  • AQucsaiJrAQucsaiJr Posts: 17
    edited 2007-12-21 16:24
    I havnt asked this yet but I guess I should to avoid confusion on my part. What specs should I be looking for in the switching transistor? I know how they work however, when I was taught, they only used one type of switching transistor and didnt really teach me how to figure out what properties I needed to look for.
  • Mike GreenMike Green Posts: 23,101
    edited 2007-12-21 16:39
    The two most important specs are:

    1) The maximum collector-emitter voltage. This should be significantly higher than the maximum voltage to be switched.

    2) The maximum collector current. This should be higher than the maximum current to be carried when turned on.

    Other specs may be important in certain circumstances, but, for switching LEDs or relays or motors, these are the important ones.
    If you're going to be switching more than a couple of hundred milliAmps, you will probably need to pay attention to the kind of package
    the transistor has and whether it's capable of dissipating enough heat. You may want to use a power transistor that is physically designed to handle more heat (usually a TO220 package ... with the big tab).
  • AQucsaiJrAQucsaiJr Posts: 17
    edited 2007-12-21 17:49
    Thanks everyone.... I picked up a few transistors to try so ill let you know how it goes.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2007-12-21 17:55
    To Mike's list I would add:

    3) Collector-emitter saturation voltage at your working current.

    By multiplying the saturation voltage by the working current, you arrive at the power dissipation. This can be used to determine whether a heatsink is necessary or if the transistor is even usable in your app.

    Also:

    4) Minimum current gain (beta) at your working current.

    By dividing your working current by the minimum current gain, you can determine the minimum base current required to switch the transistor "on". This will help in figuring out the required series base resistor value [noparse][[/noparse] R = (input voltage - 0.6) / minimum base current ] or whether you need to add a gain stage just to drive the base adequately.

    -Phil
  • AQucsaiJrAQucsaiJr Posts: 17
    edited 2007-12-21 22:57
    Ok here is a better discription of what I have, I am using a basic stamp homework board to power the circuit. I am trying to activate two brushless DC motors without using a motor controller. I have no need for speed control right now just the abuility to turn them on foward and reverse. I want to power them through the 9VDC VIN pin slots on the HW Board using a couple of relays. The relays I have are the SPDT Micromini 5VDC Relay I got from Radio Shack. I can not power the relay coil off the PIN OUT of the BS2 on the HW Board because of the current requirements of the relay, so I purchased, what I thought were the 2N2222A transistors but turned out to be 2N3904 NPN switching transistors, to use the PIN OUT to switch the transistor which would then switch the relay to give my DC motors the power needed to make them go. Needless to say I have not been able to get this to work. Can somone possibly explain to me what I may be missing or need to change?
  • Mike GreenMike Green Posts: 23,101
    edited 2007-12-21 23:10
    The 2N3904 transistors should work. They have a lower collector current limit. You need to look at the examples in some of the Parallax tutorials (like the Industrial Control tutorial on page 85-86 ... www.parallax.com/Portals/0/Downloads/docs/books/edu/ic.pdf). You'll need a base resistor and a reverse connected diode as described in the tutorial. A 1N4001 diode from RadioShack will work fine. A 330 ohm or 470 ohm base resistor will work. If you didn't use the resistor, you may have damaged the transistor and/or the Stamp I/O pin.

    The examples on page 86 use a 2N3904 and a base resistor of 1K.

    Post Edited (Mike Green) : 12/21/2007 11:16:20 PM GMT
  • AQucsaiJrAQucsaiJr Posts: 17
    edited 2007-12-22 19:11
    Tahnks for the help guys I was able to get te motors too work. I used a 470ohm resistor between PIN 0 and the base of the 2N222A transistor. Grounded the Emitter and placed the VDD 5vdc output from the HW board inline with the relay coil to the emitter. There is plenty of power to switch the relay on and power the dc motors.

    I just have one more question. In the configuration I mentiond, would I need to worry about damaging the BS2 I/O PIN's?
  • Mike GreenMike Green Posts: 23,101
    edited 2007-12-22 19:52
    1) I assume you meant that you connected the relay between the 5V supply and the collector of the transistor. That would be correct.

    2) You do need the reverse connected diode across the relay coil as shown in the Industrial Control tutorial. The transistor may eventually be damaged if you leave that out.

    3) The 470 ohm resistor protects the I/O pins by limiting the current to a maximum of about 10ma (approx: E (5V) = I (10ma) x R (470ohm)).
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