Plugging up an Emitter with a 339?

Discussion in 'General Electronics Chat' started by pntrbl, May 21, 2008.

  1. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
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    Can I use the output of a 339 on the emitter of a transistor to control it? When the output is down the 339 will sink the current of a fwd biased transistor and when up, .... plug it up?

    SP
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    What is a 339? Sounds like you left out some of the part number.

    Do you have a link to a datasheet?

    [eta] Oh, you mean an LM339 comparator. dOH! :rolleyes:

    An LM339's output cannot source current, but it can sink up to about 20mA.

    So if the load on the NPN transistor won't exceed 20mA, then yes - it can sink the current from the emitter.

    Whatcha up to?
     
    Last edited: May 21, 2008
  3. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
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    My apologies. It's an LM339. I do have a datasheet ... but most of it goes right over my head!

    SP
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    A transistor is either turned on, or it is turned off. It is not plugged up.
    What are you talking about?

    The output of a comparator is the open collector of an NPN transistor. It does not go positive, it just turns off. Its minimum sink current is only 6mA and its max saturation voltage is 1.5V at 6mA.
     
  5. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
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    The output of the LM339 will have a pull up resistor on it. So ... in an attempt to express myself more clearly, can I use a turned off LM339 output to stop the current flow thru a transistor by connecting it to the emitter?

    I'm thinking if the LM339 is sinking the current from the emitter of a turned on transistor, I can shut it off by removing, plugging if you will, the sink.

    Perhaps not. It's why I ask.

    I tend to express myself in ordinary everyday terms because there's no EE on me. The phrase a little knowledge is a dangerous thing just about sums up what I remember about electronics. But I'm learnin'. Besides that, it gives me something to think about on the freeways .....

    SP
     
  6. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
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    I'm going round and round on an ignition kill circuit for the Briggs we've been manipulating Sarge. I'll post something that will probably light up and fry here soon! LOL!

    SP
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    Don't let the emitter voltage go higher than about 5V higher than the base of the transistoir to prevent avalanche breakdown of the base-emitter junction.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    I think you're trying to have the cart push the horse. ;)

    An LM339 is liable to die a quick and painful death if you try to use it to sink the current from the kill switch tap. The magneto is generating brief bursts of current that will turn the output transistor into a blazing cinder.

    We're tapping off a bit of current from that circuit through a 10k resistor and a diode to charge up the tachometer filter network - and you're getting nearly 8v when it's running at top speed. The magneto is also driving the primary side of the ignition coil, which takes a pretty good bit of current. That means the magneto is likely to be generating spikes once per revolution that pack a pretty good punch, current-wise.

    You're going to need something pretty heavy duty to make sure that current is sunk; a TIP120 might do it, but the emitter is going to have to have a good ground to the engine block. Long, small-gauge wires may have too much inductance. It might require the use of a MOSFET or relay to turn the thing off. MOSFET might be dicey, because I don't know what the peak voltage is on the kill tap. The tach filter has a 330k bleeder resistor that keeps draining off the charge; but that was the plan. If you removed it (R12) you'd find out what the peak voltage was by measuring across C1 with a digital voltmeter with the engine at full throttle, but you'd have to pull R13 also or risk frying the TL082. An old-fashioned volt-ohmmeter with a needle wouldn't work well for this, as it's resistance is too low.

    So why not try that little experiment? I wouldn't be surprised if you saw 25 or 30 volts.

    After you're done measuring the voltage, make sure that you re-connect R12 before you put R13 back in, or you'll fry the TL082.

    Did you figure out how to determine if the batteries were charged yet?
     
    Last edited: May 21, 2008
  9. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
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    15.6v across C1 when wide open Sarge. 9.2v at idle.

    SP
     
  10. pntrbl

    Thread Starter Active Member

    Apr 21, 2008
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