transfer characteristics

Discussion in 'The Projects Forum' started by ninjaman, Apr 19, 2014.

  1. ninjaman

    Thread Starter Member

    May 18, 2013

    I have a book called "Discovering Electronics" by, Roy Edwards. it is meant for students and has some interesting projects in. one is called "transfer characteristics".
    I have an excel spreadsheet that I cant upload. it has the results. it doesn't look like what is in the book.
    for the power supplies I use a battery soldered to a potentiometer, this is attached to a breadboard with the rest of the circuit. the other supply is from a dc dc step up booster I bought off ebay. it uses 2577 chip and is "stable"
    (im not to sure of the proper or full function of this device but it stays at the same voltage and doesn't shift, my take on "stable", some more educated views may differ? please explain if so!)

    I used two "cheap" multimeters(as in, I got one free when spending more then £5 on ebay purchase and I got two for the price of one at maplins. so two for £4) they are the bog standard yellow meters you find everywhere.

    I set the right side voltage to 5v which would remain that way throughout. then I started low and increased the base current taken readings from the collector as I went. as the base current got higher I found that when I touched the potentiometer the current would increase. is this capacitive coupling increasing the current. I found that getting a steady result from the collector meter was difficult. sometimes it would keep rising slowly, sometimes it would jump around. im guessing that using cheap stuff is the issue.
    I tried to simulate the circuit using multisim but a convergence error kept popping up. I wont use ltspice as I don't know enough about it or electronics measurements to use it properly.
    so, im stuck with,
    1. getting dodgy readings that rise and sometimes jump around.
    2. not sure what the problem is with the potentiometer
    3. keep getting convergence error when using the pictured circuit.

    it does say in the book about using short leads for test probes. though I don't think there could be interference from anything around. I think I will try the same project when I go back to college. but any suggestions would be great, thanks!

    all the best
  2. ninjaman

    Thread Starter Member

    May 18, 2013
    this is the project from the book with the results that I got.
  3. DickCappels


    Aug 21, 2008
    The dodgy readings cold be a noisy pot, poor or worn contacts in your breadboard, with a 220k resistor on the base, electrical noise (like 50 Hz or 60 Hz) from you hand being capacitively or otherwsie coupled from the shaft of the pot to the base of the transistor.

    The idea of it being mechanical -loose contacts or a noisy pot would be more likely if the meter jumped when the table to is tapped.

    The idea of it being electrical noise would be more likely if the current increases when you touch the pot.

    Can't see a reason for a convergence error, but you might try (in spice) putting 10 meg resistors from the base to ground and the collector to ground just in case it thinks one of the nodes is floating DC-wise.