How do I figure out these resistors?

Discussion in 'Homework Help' started by DarkMavis, Oct 19, 2015.

  1. DarkMavis

    Thread Starter New Member

    Aug 24, 2014
    [​IMG] [​IMG] [​IMG]

    I was looking at 555 timer circuits. I can see what this is supposed to turn the transistor on and off at about 10khz. But I haven't really looked into transistors yet. I'm kinda learning stuff either as I need it, or, like now, when I see something that raises a question. I guess the one closer to the IC is like the grid resistor on a vacuum tube? They haven't labelled them...what does the other one do? The one between the base and emitter? How would I calculate the values of both using this circuit as an example? Oh, and how critical is it? Surely there's a wide range of what works? I don't know how this will react as the battery voltage drops from the 9.8v or whatever of a fresh battery, but I'm just interested in the actual maths, not their specific circuit. I guess if they just use a value somewhere between a fully charged battery and a "dead" battery, everything will be within acceptable tolerances?
    I haven't looked into transistors much, but I thought to use them as a switch, you just needed a current flowing from base to emitter, and that as long as it was enough current for that tranmsistor, then collecter to emitter would simply function like an open switch. (or closed..can never remember which way round, because it's a bad choice of word) Did I miss some things? I guess I did, bceause I don't see a point in those resistors based on what I've read so far. Is there some relationship between the base to emitter current and the collector to emmitter current that must be kept within a certain range? Is that the point of the resistor on the left?
    Then, what's the one from base to emitter for? Is it some non-essential thing, or does it have to be a specific value?
    It's not easy learning electronics from the ground up, because all the knowledge out there is not arranged into a logical order. Almost everything you can learn, requires you to already have learned something else

    Oops. Should have checked how you post images on thius forum first

    GRrr. can't seem to get it to work.
    Last edited: Oct 19, 2015
  2. DarkMavis

    Thread Starter New Member

    Aug 24, 2014
    [​IMG]Nope. Can't seem to make images work. Maybe I'll try another hosting site
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
    Bipolar Transistors come in mainly two forms, npn, and pnp, the base, emitter voltage is usually 0.7V, so in your drawing the resistor from pin 3 to the base drops approx (9v-0.7v)=8.3V this would set the base current, needed because otherwise the transistor would burn up, the base to emmiter resistor is just to turn off the transistor, not needed really in this situation.

    Other transistors like Fets, have a Gate, Source, Drain, they behave more like valves

    Gate is the base,
    Source is the emitter
    Drain is the collector

    same construction Ntype npn, and Ptype pnp,no gate bias resistors are needed as they are very high impeadance, more capacative, so resistors are used to discharge the gate.
  4. WBahn


    Mar 31, 2012
    Or, better yet, try uploading your images to the AAC server where they will be archived along with your post.

    Which interface are you using (orange or blue)?
  5. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    You can upload them to this site. Use the upload a file button.
  6. GS3

    Senior Member

    Sep 21, 2007
    Sometimes that resistor is not really needed. (Probably that diagram is such a case.) It may serve to shunt some leakage so that the transistor only conducts when you really want it and not due to some stray current. You will learn as you go along. Be patient and persistent. It's a long road. Lots of fun too.
  7. dl324

    Distinguished Member

    Mar 30, 2015
    This is the homework forum; you need to post your best effort before asking for help.

    What would happen if the timer output was high and the base resistor was shorted?
    Analyze the circuit with and without the resistor in parallel with the base-emitter junction.