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odd(?) transistor switch behaviour

Discussion in 'Electronic Basics' started by tempus fugit, Feb 26, 2007.

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  1. tempus fugit

    tempus fugit Guest

    Hey all;

    Still plugging away at getting my pedalboard pop free. I use a transistor to
    switch amp functions, as shown below:


    control voltage ___ |/
    o--------o-|___|- -|
    3k |>

    (created by AACircuit v1.28.6 beta 04/19/05

    The 15v is from the amp, and switching it to ground turns the amp function
    on or off (e.g. channel switching, volume boost). It switches dead quiet in
    one direction, but there is a pop when I switch back. I put the circuit on
    the scope, and found that it switches cleanly to ground, but when I switch
    back, there is a little spike in the waveform just before the beam goes to a
    higher position on the screen. I assume that this is my audible pop.

    Any ideas as to why it's there and how to get rid of it?

  2. James Beck

    James Beck Guest

    My best guess is that the entire circuit jumps to +15V when you kill the
    ground to it. It is still connected to the amp's ground and when the
    coupling caps charge to +15 the thing pops. Try using a high side
    switch and see if that helps (a PNP switching the +15V).

  3. Eeyore

    Eeyore Guest

    Try a much higher value base resistor. Just a hunch.

  4. John Fields

    John Fields Guest


    | |
    | [100nF]
    control voltage ___ |/ |
    o--------o-|___|- -| GND
    3k |>
  5. tempus fugit

    tempus fugit Guest

    I'm gonna check and see if I have any PNPs sitting around here and give that
    a try. I wish I'd have thought of that before I built the circuit into my
    pedalboard - I wouldn't have needed to use an inverter to get the control
    voltage right.

  6. tempus fugit

    tempus fugit Guest

    Hi Graham;

    Thanks again for the insight. I'll try a higher resistor while it's hooked
    up to the scope, but I can't actually use a higher resistor on the
    pedalboard. The 15v comes from the amp through a 100k resistor, and the Ron
    of the transistor is too high to actually fully switch functions if I use
    more than 3k (actually it still doesn't quite fully switch with 3K, but I
    cant go any lower because it's being driven by a 4049.).
  7. tempus fugit

    tempus fugit Guest

    Thanks for the tip John. I'll scrounge up a 100nF cap and see if that fixes
    things. Also, is the value critical? I'm assuming not, but I'm just
    wondering how you decided on 100nF.

    Thanks again
  8. John Fields

    John Fields Guest

    WAG, basically. ;) I figured the 15V was coming from a pullup, and I
    thought it likely to be somewhere between 10k and 100k, so 100nF
    would give a time constant of 1ms at 10k and 10ms at 100k. What it
    does is increase the time it takes for the transistor end of the
    pullup to get back to 15V, so that should also "soften up" the
    notch. The value isn't critical except that it might be too small.
    It occurs to me that the notch might also be in the switching signal
    on the base, so you might want to look at that line and filter it if
    you find the notch there. Which transistor are you using?
  9. tempus fugit

    tempus fugit Guest

    Thanks again John. The transistor is a generic 2222.
  10. Eeyore

    Eeyore Guest

    So that's 150 uA.

    With a typical small signal NPN device you'll need no more than 3uA of base
    current to switch that !

    Eh ?

    That makes no sense at all..

  11. Eeyore

    Eeyore Guest

    He sucked his finger.

    It's amazing how often 0.1 uF comes up as a result of that.

  12. John Fields

    John Fields Guest

  13. tempus fugit

    tempus fugit Guest

    OK I'm glad this came up because it's been puzzling me for a while. The amp
    has an LED indicator for each channel, so whichever one is on has its LED
    lit. If I ground the connector (i.e., 15v to ground) the channels switch and
    1 LED is on. However, if I connect the 15v to ground using a 15ohm resistor,
    one LED will light fully, and the other will still be partially (or fully)
    on. The same goes when using a transistor as a switch. I originally used a
    10k resistor for the base, but 1 LED remained partially on. Using a lower
    value resistor (like 3k) turns the LED almost completely off. I figured from
    this that driving the base with more current reduced the Ron to a low enough
    value that it switched completely. It does seem odd, though, with a 100k
    resistor in there that an extra few ohms would make that much of a
    difference. You may remember from my previous posts on this topic that I was
    going to try a low Ron N-channel MOSFET to get away from this problem. Also,
    I've tried using an optoisolator to do the switching, but it also leaves one
    LED partially or fully lit.

    Can you shed some light on this (no pun intended).


    Oh, one other quick question - I'm using a 4049 inverter set up to be a
    latching flipflop and it seems to have died on me. When the input of any of
    the inverters is grounded, its output should be a logic high shouldn't it?
    I've been testing the various inverters in the package to see if there is
    only one damaged or if the whole thing is shot, and none of them give a
    logic high with the input grounded, although they do show 0VDC when a high
    is applied to the input.
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