Electronics > Beginners
What is the purpose of this 3rd transistor ?
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Beamin:
What does the transistor do on the top of this schematic? Why does it have so many parts around it, and is C750B an polarized cap? I have never seen a cap/ground schematic diagram like that. Its a back light dimming circuit to a yeasu radio where the input is off a software controlled board.
I figured out the middle transistor is the actual amplifier for the LEDs voltage and bottom left trans acts as a switch/dimmer controlling the power trans. I labeled the wires that are off the diagram but also there is a link to it in ze… er...ah... TRXBench er...ah...video (yes its slow to watch him explain zeez things).













https://youtu.be/epGTkALH5as?t=1340
Nerull:
That's a zener controlled transistor series voltage regulator. The zener maintains the base at a specific voltage, and emitter follows at a diode drop less.

Nusa:

--- Quote from: Beamin on April 27, 2019, 11:40:01 pm --- and is C750B an polarized cap? I have never seen a cap/ground schematic diagram like that.

--- End quote ---

That's C7508, not C750B (notice C7507 label on adjacent cap). And yes, it's an electrolytic cap. There are many different capacitor symbols internationally. That one originates from Japan, I believe.

The *** presumably refers to a footnote somewhere about what value should be used.
Beamin:

--- Quote from: Nerull on April 27, 2019, 11:52:31 pm ---That's a zener controlled transistor series voltage regulator.

--- End quote ---


I figured it adjusted the voltage but how does that work? Does it make sort of a PWM circuit using the caps for timing the trans on and off or work off the 0.6 volt drop?
Nerull:

--- Quote from: Beamin on April 27, 2019, 11:57:28 pm ---
--- Quote from: Nerull on April 27, 2019, 11:52:31 pm ---That's a zener controlled transistor series voltage regulator.

--- End quote ---


I figured it adjusted the voltage but how does that work? Does it make sort of a PWM circuit using the caps for timing the trans on and off or work off the 0.6 volt drop?

--- End quote ---

The transistor acts as a variable resistor that maintains a desired voltage drop for any given output current. I edited a diagram into my earlier post which might help.
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