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switching loads with transistors - I'm going insane — Parallax Forums

switching loads with transistors - I'm going insane

YanroyYanroy Posts: 96
edited 2005-09-07 20:25 in General Discussion
Hey,
It's been a little while since I been on this board, getting moved into my new apartment and all.· But now that I'm settled in, I've begun work on a new project.· I need to switch a 12V solenoid on and off (it draws 1A) with the output from a 74154 (1 to 16 decoder), which for some reason that is making me go insane is active-low, and I can't find any equivalent that is active-high.· So I've been trying to figure out which P-channel FET or PNP transistor would do this job for me.· I understand NPN transistors very well, and my knowledge of N-channel FETs is enough to get around, but these P-channel FETs are driving me crazy!· Everything I thought I knew is backwards... and of course, I can't find the good book I have on this subject, so I'm trying to go by this terrible, terrible textbook I had·a long time ago.· My logic circuitry is going to be running at 5V, I need to switch a 12V load at 1A, preferably without a relay in between... less complexity and expense.

If someone could recommend some specs for the device that would perform these functions (or even a part number, preferably one Jameco carries as I plan on placing a jameco order this afternoon), I'd be very, very grateful.· Thanks a lot.

Comments

  • Jon WilliamsJon Williams Posts: 6,491
    edited 2005-09-03 19:43
    This article has lots of useful information: http://www.parallax.com/dl/docs/cols/nv/vol1/col/nv6.pdf

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    Jon Williams
    Applications Engineer, Parallax
  • NewzedNewzed Posts: 2,503
    edited 2005-09-03 19:54
    Use a TIP120 Darlington transistor.· Very cheap and they will handle 5 amps.

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    Sid Weaver
    Do you have a Stamp Tester yet?
    http://hometown.aol.com/newzed/index.html

    ·
  • KenMKenM Posts: 657
    edited 2005-09-04 00:06
    An overkill mosfet application.

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    Ken
    IRL520 drive circuit.jpg
    869 x 555 - 53K
  • FlyingFishFingerFlyingFishFinger Posts: 461
    edited 2005-09-04 02:35
    Or use an inverter and your Npn transistors...
    Rafael
  • Don BuczynskiDon Buczynski Posts: 31
    edited 2005-09-06 03:44
    I'm using a Basic Stamp, 74154, and this circuit to drive a solenoid. ·

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    Don Buczynski

    http://www.buczynski.com
    TIP127.jpg
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  • KenMKenM Posts: 657
    edited 2005-09-06 14:53
    Don,

    I am curious, what voltage do you put on the bottom of the 4.7k resistor to turn off the load?

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    Ken
  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2005-09-06 15:55
    While the transistor might appear to be off with an OUTPUT;HIGH on the I/O pin, you still are going to have lots of heat generated.
    Looking at the datasheet...
    http://www.datasheetcatalog.com/datasheets_pdf/T/I/P/1/TIP127.shtml
    ... the Hfe (DC gain) for this transistor is 1000
    and the internal resistance across B-E in parallel with the 10K resistor in your schematic makes an equivalent resistor value of 4.48K ...· 1/[noparse][[/noparse](1/10K)+(1/8.12K)]=4.48K
    When the I/O pin is HIGH you have a 7V potential (12V-5V=7V) across the 4.48K and 4.7K resistor or about .76mA of current [noparse][[/noparse]7V / (4.48K+4.7K) ] = .000762527 Amps
    With an Hfe of 1000 the output or drive current across the C-E is going to be about 762mA (3/4 of an Amp)
    When the I/O pin is LOW you have a 12V potential across the 4.48K and 4.7K resistor or about 1.31mA of current [noparse][[/noparse]12V / (4.48K+4.7K) ] = .001307189 Amps
    With an Hfe of 1000 the output or drive current across the C-E is going to be about 1307mA (1.3 Amps)
    ·
    It may be that this circuit "works" because 3/4 of an Amp in the "off" state is not quite enough to drive your solenoid, where as 1.3 Amps is plenty in the "on" state
    to do so.· The current configuration might work, but much of the energy is wasted in the form of heat.· I would·go with a second transistor for the high-side
    driver or·ideally·I would use an NPN or NMOS rather than a PNP or PMOS, this way you would avoid the·need to deal with the high-side voltage management.


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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • Don BuczynskiDon Buczynski Posts: 31
    edited 2005-09-07 20:25
    The bottom of the 4.7k resistor is tied directly to an output pin of a 74154. Attached is a diagram from the 74154 spec sheet of the·circuit·for each of the 74154 outputs. Originally, I was going to drive the TIP127 with a NPN transistor until I realized that the 74154 output was already doing that.

    I arrived at the 4.7k and 10k resistor values through experimentation and my trusty resistor substitution box. I'm sure the circuit could be improved as my solenoid duty cycle needs are very low. I have about 40 such circuits powered by a single 12 volt 1 amp supply with no adverse heating or current·leakage issues.

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    Don Buczynski

    http://www.buczynski.com

    Post Edited (Don Buczynski) : 9/7/2005 8:31:38 PM GMT
    154output.gif
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