Basic Transistor Circuit

Discussion in 'The Projects Forum' started by Thirtyniner, Oct 15, 2014.

  1. Thirtyniner

    Thread Starter New Member

    Oct 15, 2014
    Good afternoon!

    I have spent many hours reading about transistor circuit theory but I am still confused so I have come here for some much-needed help!

    I want to use a 12v, 20ma (max) source to switch on an array of 12v devices with a 12v supply and ground.

    I figure I can achieve this with (let's say) a 100ohm relay, requiring 120ma to energise. Not wanting to load my source too much, would it be a safe bet to find a transistor that will saturate at 5ma, thus needing Hfe > 24?

    Furthermore, will a high-side switching circuit be the best format for what I want?

    And what value of resistor do I place between my source/input and the transistor base:
    One that will limit it to 5ma?
    One that will limit it to 20ma?
    Or somewhere in between?

    I would appreciate any and all input as this has been racking my brain for days and the more I read the more confused I become.


  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    A basic 12V relay should have a coil resistance of say ~500 ohms, 100 ohms is too low.
    So assume a 500R coil, from 12V thats 12/500 = 24mA.

    Assume that the relay drive transistor will a have gain of 10 in saturation, so thats Ibase current = 24mA/10 = 2.4mA.

    So from 12V subtract the Vbe of the transistor say 0.7V, so thats 11.3V/0.0024 = 4700 ohms base resistor value.

  3. Thirtyniner

    Thread Starter New Member

    Oct 15, 2014
    Thank you for the response. I was under the impression that Vbe=0 upon saturation, is this not correct?

    You say a transistor will have a gain of 10 in saturation. What determines this saturation? I *want* it to output 24mA, say. But why does this mean that it's saturated if I give it 1/10 of that. If I change my mind and *want* it to output 240mA, will the same transistor now require 24mA to reach saturation (providing it is capable of outputting 240mA)?

    One more thing, I'm finding it difficult to find relays with coil resistances' as high as you mention, especially for panel mount types. For what it's worth, it needs to be capable of driving around 30A, so if you have any ideas where I might find such a relay that would also be much appreciated.

    Thanks for your help.
  4. wayneh


    Sep 9, 2010
    No, the drop will be roughly one diode drop, ~0.65V. Vce may drop towards zero for a big power transistor.

    It may be higher and is unlikely to be exactly 10. The value of 10 is a design rule-of-thumb that ensures saturation 100% of the time. Less base current than 10% of the emitter current risks having the transistor not fully saturated. If you want to save power and reduce the base current to, say 5%, it may work just fine but would warrant a test to be sure.

    Yes, exactly. Otherwise it will be starved by the low base current.