Increase the resistance going to base

Discussion in 'General Electronics Chat' started by sdowney717, Apr 22, 2015.

  1. sdowney717

    Thread Starter Well-Known Member

    Jul 18, 2012
    If I increase the resistance value AIR4 coming from the current coil AICR5, will that let the transistor AIQ1 turn off? Or maybe shunt some off from the coil to ground through a resistor?
    It looks like current flow from the coil is keeping the transistor NTE199 NPN base biased and that is keeping the transistor flowing current which is keeping the run relay K4 engaged.

    This circuit is very sensitive to detecting an AC load and keeping the 1965 boat generator running.
    The problem is I added 2 digital gauges to measure the AC voltage, and that is enough to keep the gen on after it autostarts when it detects a load. It will not autostart with the gauges on in the circuit, but the gauges keep the gen running and it should be off if there is no other loads than these 2 gauges. No point keeping the gen on to see what the gauges read.

    I measured the vac coming off coil, with no load and gen on, it reads 0.4vac
    With both gauges reads 1.9vac
    With a 10 amp load reads 4.1 vac
    Higher loads, it reads about the same 4 vac.

  2. sdowney717

    Thread Starter Well-Known Member

    Jul 18, 2012
    Ok, I think I figured it out, I also have the wiring schematic etc... so that helped.

    They call this an amplifier board.
    AIR4 and AIR1 form a voltage divider circuit, like a potentiometer.
    NTE199 turns on about 0.4 or 0.5 volts
    So depending on what the values of those two resistors, I can adjust the voltage the transistor base sees.


    I randomly calculated that with 2.0volts coming in you want 0.4 volts going out.
    So i picked R1 as 100 ohms, and R2 as 25 ohms, just arbitrarily to give the voltage drop.
    What I have to do is measure the resistors ohms they used and go from there.

    I dont suppose it matters much which resistor I change, but I think maybe R2 is better.
    This is part of an Onan Control-O-Matic from 1968.
    Years ago I had to work on this board, and had to guess at a transistor since corrosion had eaten the numbers off the part.

    I wonder how accurate this will be since I see they show this as a range from 0.5 – 0.9 V
    Base Emitter ON Voltage VBE(on) VCE = 10V, IC = 2mA 0.5 – 0.9 V

    I dont suppose there is much current here, so maybe I could substitute a potentiometer for R2 (AIR1) and dial it in.

    I suppose the electrolytic AIC1 keeps the transistor from pulsing on and off, pushes back like a battery, cause it looks like they rectify T1 sense coil and it is pulsing DC.
    Last edited: Apr 22, 2015