Circuit conondrum

Discussion in 'General Electronics Chat' started by rifme, Jan 14, 2013.

  1. rifme

    Thread Starter New Member

    Jan 14, 2013
    3
    0
    Greetings,
    I've built a charge controller for solar/wind generation based upon the 555 timer chip. The circuit is working great as far as I can determine except that the load relay coil is energized when the circuit is powered up, and does not shut off when it's supposed to. The problem seems to me to be the (Q1) 2N2222 transistor that is supposed to drive the gate of an IRF540 mosfet (Q2) that enables the relay coil. The voltage on the base of Q1 alternates between 0 and .73 or so volts depending on the circuit status (charge vs. dump). What is confounding me is that when the base voltage of Q1 goes from 0 volts to .73 volts, it only drops the Q2 gate voltage by .5 volt; i.e. from 5 volts to 4.5 volts. If I short the collector of Q1 to ground, the relay drops out as it's supposed to. The transistor Q1 should do this but does not. I've check my solder joints with an ohm meter to no avail. I'm thinking that it must be a high resistance connection somewhere between resistor R6 and ground. I've changed the transistor Q1 two times so far with no results. What the heck is going on?
    I'm attaching a couple of drawings and list of components. Any help is appreciated. Thanks.
    mike
     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    I am no expert, but I don't know why you need a BJT, a MOSFET, and a relay. Can't the 2N2222 drive the relay without the MOSFET?
     
  3. crutschow

    Expert

    Mar 14, 2008
    12,977
    3,221
    Check the pinout connections on Q1. You may have the emitter and collector reversed. If they are reversed the transistor will still sort of work, but with very low gain.
     
  4. rifme

    Thread Starter New Member

    Jan 14, 2013
    3
    0

    Thank you very much! You're spot on..... I had an "epiphany" of sorts, and discovered this about an hour after I sent my post. I had the Q1 transistor connected back-asswards. I seem to have a problem switching from "conventional" current theory to "electron flow". Screws me up every time! Thanks much for your response.
    mike

    I'll attempt to cancel this post but am not sure I know how just yet... :-0
     
  5. crutschow

    Expert

    Mar 14, 2008
    12,977
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    Why are you switching from "conventional" current theory to "electron flow"? The only time you need to really concern yourself with electron flow is if you are studying semiconductor theory or how electron tubes (valves) work. I like current flow since it goes in the same direction that the arrows do in bipolar transistors and diodes, and it goes from the typical positive power supply voltage towards ground.
     
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