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electrolytic capacitor polarity help

Discussion in 'General Electronics Discussion' started by Pietro, Aug 10, 2015.

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  1. Pietro


    Aug 10, 2015

    I'm new to electronics and I just came across a circuit that i'm trying to make sense of. Here's a simle circuit with an electrolytic capacitor but it is connected with the polarity inverted. Can anyone explain please?

    Last edited by a moderator: Aug 11, 2015
  2. davenn

    davenn Moderator

    Sep 5, 2009

    dunno what's going on, I will talk to forum owner .... I cant even upload one of my own files
    will try to get sorted asap :)

    OK all sorted out and I have uploaded it to your original post
    and done a tidy up of the thread

    Last edited: Aug 11, 2015
  3. duke37


    Jan 9, 2011
    This looks to me to be a mutivibrator (oscillator). Collector to base, collector to base.
    The emitter/base junction of the PNP transistor acts as a rectifier to get the polarity right.
  4. TedA


    Sep 26, 2011
    You are correct, the electrolytic capacitor does see reverse voltage for part of the time during each cycle. The maximum reverse voltage across the capacitor will be limited by the 2N3906 Vbe as this transistor begins to turn on. Under 0.7V at room temperature.

    During the time the NPN transistor is on, the capacitor will see a normal polarity voltage across it, at least as soon as it has discharged back through zero volts.

    The reverse voltage on the capacitor is not the only thing about this circuit that's not so good. Yet it might work well enough, depending on what you want it to do.

    The saving grace is that the power supply voltage is so low.

    Have you built one up? Does it work?

    What color is your LED?

    The circuit's operation is very dependent on the characteristics of the transistors, the LED, the capacitor, and even the battery. The characteristics of these devices all vary from unit to unit and over temperature.

    If the circuit needs to work reliably, or many of these are to be built, perhaps some redesign is in order.

    For sure, use an aluminum, and not a tantalum electrolytic. A film cap might be a better choice, but we may be trying to make a silk purse...

  5. Colin Mitchell

    Colin Mitchell

    Aug 31, 2014
    Where do you get the reverse polarity nonsense from ???

    The electrolytic charges to about 1v in the forward direction and when the circuit turns OFF, the base of the PNP transistor rises about 1v above the supply voltage.
    The base-emitter junction of a transistor can go up to about 5v in the reverse direction before the junction "zeners."
    The electro does not actually get discharged via the transistor but via the 470k resistor.

    You should all look on my website to see how these circuits works because they work completely differently to anything you are even suggesting.

    I have not had one person with the slightest idea of how these circuits work.
    Last edited by a moderator: Aug 14, 2015
  6. TedA


    Sep 26, 2011

    Think again. If you cannot figure out how this circuit works, you might look at the voltage across the electrolytic cap with a scope. If you lack test equipment, there are free simulation packages to be found online.

    The circuit in the posted schematic will definitely apply reverse bias to the electrolytic cap, barring defective components, and dead batteries. If the circuit fails to oscillate, the reverse voltage will be DC.

    The 470k resistor discharges the cap down to zero volts, then charges it up enough in reverse to bias on the PNP transistor. If the cap never sees a reverse voltage, the PNP remains off and nothing happens.

    Hope this helps.

    Last edited by a moderator: Aug 14, 2015
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