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Zener kinda turns on

Discussion in 'Electronic Design' started by panfilero, Apr 17, 2013.

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  1. panfilero

    panfilero Guest

    I was making a circuit that when you power it up with 5V a 5V LED indicatorturns on and if you power it up with 12V the 5V LED goes off and the 12V indicator goes on.

    But... it's not coming on with 5V...

    I spiced it, and it doesn't work in simulation either...

    if you look at the image on the link

    I put a 5.6V zener D1 in the circuit thinking that this Zener would be OFF until I provide it a voltage drop of 5.6V, and since it's in series with some other stuff (resistors and an LED) I thought it would definitely be off at 5V, but it's on just enough to turn on my FET M1 and not let my D4 LED light up... I'm basically measuring a little over 1V at the gate.

    I think I can remedy this problem by changing R2 to a lower value... like 1k maybe, but why is D1 turning on at all? Is it leakage current that's causing this circuit not to work at 5V? D3 the 12V LED is definitely off at a5V supply (as it should be) and the circuit works at 12V... but not at 5V....

    link is below

    much thanks!
  2. That 12V zener is not doing very much...

    But look at an I-V curve for the 5.6V zener, I'd guess there's still
    some current flow at 5.0V. If the FET gate is at 1 V then you have
    ~1uA of current flowing through the zener.

    George H.
  3. Robert Baer

    Robert Baer Guest takes about 4V to turn on a "typical" LED, so use of a 5V
    (or even 3V) zener will guarantee no light from the LED.
  4. Phil Allison

    Phil Allison Guest

    "Robert Baer"

    ** D4 is the 5V LED, D3 is the 12V LED.

    The dopey OP has failed to allow for leakage current in the 5.6 V zener and
    is trolling us as usual.

    .... Phil
  5. mike

    mike Guest

    Here' in the trailer park, we'd have just painted the 5V power plug red
    and the 12V one green.

    Over in town, they'd have used a dual-color LED, put a zener in series
    with the red one and called it green and orange.

    Or use rectangular LEDS and call it a bar graph. Probably works better
    for the color-blind.

    I've hosted a bunch of design reviews.
    Seen a lot of "solutions" that were WAY out of proportion to the root
    problem being addressed. Often comes under the heading
    "dimininishing returns". ;-)

    KISS 'R' US...
  6. OK I'm most likely just slow this morning, but how does that turn off
    the one LED when the transistor turns on? And doesn't it turn on the
    first LED at ~ 2volts or so. How about if you put a second LED in the
    leg without the transistor? Then it would turn on at ~4 volts and
    turn off with current through the transistor.

    I like Jan's circuit.

    George H.
  7. Guest

  8. Guest

  9. Jeroen

    Jeroen Guest

    Show us how you can do better.

    Jeroen -simplicity is a virtue- Belleman
  10. Guest

    First tell me what practical use this fake circuit requirement has.
  11. Guest

    If it were a BJT, the +12V LED wouldn't ever turn off.
  12. It might not turn off anyway, given Vgs(abs max) is 8V.

    I kinda like Jan's idea, here is an "over easy" mod of it.
  13. Jamie

    Jamie Guest

    Different color LEDs have different voltages. So the one with the
    transistor in line will operate a lower voltage led, this will then
    remove forward current from the higher voltage LED..

    THat is the way I see it, could be doing something different, who

    Also, if you put a diode in series with one LED, and then T-bone
    that node to another LED and then to a switch (transistor for example),
    that switch should cause a voltage drop to where the in series diode LED
    node will lose the required voltage..

    Just a thought..
  14. Ahh thanks,
    There's no-way I'm playing crossword puzzle component design with
    He'll kick my butt every time. I'm a 'class A' brute force kinda

    George H.
  15. Nice, reminds me of a more spendy LED indicator circuit that I use
    for control loops, I drive two leds to show when the 'control
    parameter' is above or below the set point. Both turn off at 'zero'

    George H.
  16. Fred Abse

    Fred Abse Guest

    Only downside is deadband between 8V and 9V, where neither LED lights.

    This simulation shows (Monte-Carlo'd for 5% tolerance zeners and 1%

    Version 4
    SHEET 1 880 704
    WIRE -384 48 -672 48
    WIRE -96 48 -384 48
    WIRE 112 48 -16 48
    WIRE 304 48 112 48
    WIRE -384 128 -384 48
    WIRE 112 208 112 48
    WIRE 304 256 304 48
    WIRE -672 288 -672 48
    WIRE -384 288 -384 192
    WIRE -64 288 -384 288
    WIRE 64 288 -64 288
    WIRE -384 352 -384 288
    WIRE -64 400 -64 288
    WIRE -384 464 -384 432
    WIRE -672 624 -672 368
    WIRE -384 624 -384 528
    WIRE -384 624 -672 624
    WIRE -192 624 -384 624
    WIRE -64 624 -64 480
    WIRE -64 624 -192 624
    WIRE 112 624 112 304
    WIRE 112 624 -64 624
    WIRE 304 624 304 320
    WIRE 304 624 112 624
    WIRE -192 688 -192 624
    FLAG -192 688 0
    SYMBOL LED 288 256 R0
    SYMATTR InstName D1
    SYMBOL nmos 64 208 R0
    SYMATTR InstName M1
    SYMATTR Value AO6408
    SYMBOL res 0 32 R90
    WINDOW 0 0 56 VBottom 2
    WINDOW 3 32 56 VTop 2
    SYMATTR InstName R1
    SYMATTR Value {mc,2.2k,tolR}
    SYMBOL res -80 384 R0
    SYMATTR InstName R2
    SYMATTR Value {mc,10k,tolR}
    SYMBOL LED -400 464 R0
    SYMATTR InstName D3
    SYMBOL res -400 336 R0
    SYMATTR InstName R4
    SYMATTR Value {mc,1.2k,tolR}
    SYMBOL zener -368 192 R180
    WINDOW 0 24 64 Left 2
    WINDOW 3 24 0 Left 2
    SYMATTR InstName D4
    SYMATTR Value BZX84C7V5
    SYMBOL voltage -672 272 R0
    WINDOW 123 0 0 Left 2
    WINDOW 39 0 0 Left 2
    SYMATTR InstName V1
    SYMATTR Value 5
    TEXT -440 656 Left 2 !.dc V1 0 12 10m
    TEXT 352 392 Left 2 !.MODEL BZX84C12V D\n+ IS=2.6665E-18\n+ N=.82284\n+ RS=.51617\n+ IKF=11.760E-3\n+ CJO=48.079E-12\n+ M=.32805\n+ VJ=.62172\n+ ISR=47.692E-12\n+ BV={mc,12.119,tolZ}\n+ IBV=.54544\n+ TT=825.22E-9
    TEXT 352 40 Left 2 !.MODEL BZX84C7V5 D\n+ IS=2.6665E-18\n+ N=.82284\n+ RS=.51617\n+ IKF=11.760E-3\n+ CJO=63.513E-12\n+ M=.33559\n+ VJ=.66795\n+ ISR=1.1222E-9\n+ BV={mc,7.6329,tolZ}\n+ IBV=.94329\n+ TT=2.7411E-6
    TEXT -24 656 Left 2 !.param tolZ=.05 ; +/- 5% zener tolerance
    TEXT -24 688 Left 2 !.step param X 0 20 1 ;dummy
    TEXT -24 672 Left 2 !.param tolR=.01 ;resistor tolerance
    TEXT -256 8 Left 2 ;plot I(D1), I(D3)
  17. Fred Abse

    Fred Abse Guest

    Not my solution, just an MC analysis of Fred Bartoli's solution.

    Not as elegant, or precise as yours, but cheap.
  18. My 2 cents: _ PAGE1.pdf


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