Capacitor vs Resistor for feeding transistor base

Discussion in 'General Electronics Chat' started by doug08, Jul 20, 2011.

  1. doug08

    Thread Starter Member

    Jan 30, 2011
    153
    2
    In a circuit that has a .1uf capacitor feeding the base of a transistor, what is the difference from using a resistor to feed the base of the transistor. Is the capacitor used for lowering the current/voltage and filtering the input? What would be the equivalent resistor to use? 30K?

    Thanks
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You really need to post a schematic of what you're talking about.

    Caps can be used in parallel to a base current limiting resistor. The cap helps improve transistor turn-on and turn-off times.
    They could also be used from the base to a power rail; to filter the signal or perhaps to delay the transistor from turning on during power up, or to cause it to turn on during power down.
     
  3. doug08

    Thread Starter Member

    Jan 30, 2011
    153
    2
    Radiation detection circuit. I built it EXACTLY to specs, with the exact components, but the led is supposed to flash no less than 1 time every ten seconds w/no radioactivity. Mine flashes once every 1.5 seconds. I left the detection circuit alone, and played around with the capacitor marked in red. Raising the value slows the flashing, but also slows the reaction time. Circuit is completely shielded in copper/grounded as the plans called for. The guy who designed it was more than nice to help me out with it, but I hate to keep bugging him. He says the detection portion of the circuit is picking up a signal from somewhere and it is being amplified by the amplifier circuit. Thanks.

    [​IMG]
     
  4. PeeSeeBee

    Member

    Jun 17, 2011
    43
    7
    You cannot simply replace a capacitor with a resistor when it is used for coupling (or wherever it is used for that matter). The biasing on the base of the 2N4401 would be messed up by the voltage on the collector of the MPSA18 if you used a resistor. The coupling capacitor blocks any dc, but allows ac or pulses through to the next stage.
     
  5. doug08

    Thread Starter Member

    Jan 30, 2011
    153
    2
    you were right. Have to use a capacitor. How about adding a resistor in parallel? I notice that if I hold my finger across the capacitor, the led works the way it should, as does the detection. Add a few million ohms across the cap? Also noticed that sunlight, or any bright light directed at the LED slows the flashing down.....weird. I know that LED's can produce voltage when exposed to light, but how does this cause the circuit to slow the flashing? Does the voltage created by the sunlight on the LED travel through the 220ohm resistor, through the 104 cap to the emitter of the 2n4401 where it has an effect on the flashing? Only thing I can think of.

    Thanks.
     
    Last edited: Jul 20, 2011
  6. doug08

    Thread Starter Member

    Jan 30, 2011
    153
    2
    When I build something, I do it right. I personally feel that this particular circuit is a 9.5 out of 10 for difficulty in getting it to work properly, even with proper precautions taken. Love to see someone else make one that works right off the bat.

    [​IMG][​IMG][​IMG]
     
  7. PeeSeeBee

    Member

    Jun 17, 2011
    43
    7
    As you suggest, the resistance of your skin is changing the bias on the following transistor. Can I ask a few questions?

    1) What are you using as a radiation source to determine that it is operating correctly when you put your finger across the capacitor?
    2) Is it possible that light is causing the monitor to react? Have you tried covering the sensor to see if the monitor still reacts?
    3) Have you tried putting the unit into a microwave oven (without turning it on!) to see if the flashing decreases?

    It might be worth disconnecting the PIN diode to see if the flashing rate remains the same.

    Also removing the 0.01uF capacitor (the one you are bridging with your finger) will determine whether the amplifier is picking up random noise, or whether the pulse stretcher part of the circuit is self-triggering.
     
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