Control a TIP121 used as a relay switch from 5V and 12V??

Discussion in 'General Technical Questions and Answers' started by 2E0ILY, Dec 6, 2018.

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  1. 2E0ILY

    2E0ILY Ham Member QRZ Page

    I have a TIP121 Darlington used to control a 12V relay from an output pin of an ATMEGA328. I now also want to switch the TIP from 12V. The base has a 1K resistor in series, can I choose a value to accept 5V from the ATMEGA and 12v from the 20mA limited12V output of my transceiver PTT safely and saturate it? Thanks.

    Needless to say I am new to this.... ;)
  2. AI3V

    AI3V Ham Member QRZ Page

    You should be fine with the 1k, it's not a critical value at all.

    What I do is divide the load I by the transistor gain and make sure I have at least maybe twice the needed base I.
    More base I won't hurt :)

  3. WA7ARK

    WA7ARK Ham Member QRZ Page


    Is not the PTT circuit in the radio such that it has 12V on the PTT line when the radio is in receive mode, and 0V on the PTT line when the radio is keyed?

    If so, then you would have to use inverted logic with respect to the base of the Darlington; the Darlington will be on during receive (base current), and will be off during transmit (no base current).

    It would be trivial to invert the ATMEGA output pin logic in order to create the required OR function: the Darlington is turned off either when the PTT line is low or when the ATMEGA pin is low.
    Last edited: Dec 6, 2018
    K6CLS likes this.
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    Snipped from the TIP121 data sheet:

    Since the gain of a Darlington is ~1000, if it was switching 0.5A, its base current to saturate it would need to be 0.5mA. The base to emitter voltage is ~2.5V, so if being driven from a 12V source, the base resistor needs to be R=E/I = (12-2.5)V/0.5mA = 19K Ohms or lower.

    The ATMEGA pin will likely be able to source about 4.9V (if Vcc=5.0V). The base resistor between the ATMEGA pin and the Darlington base would need to be (4.9-2.5)V/0.5mA = 4.8K Ohms or less.

    I would use a 4.7K or 5.6K resistor... 1K just wastes power.
    K7JEM and AI3V like this.
  5. K6CLS

    K6CLS Ham Member QRZ Page

    (Did you mean NOT function?)

    I've used 2n3906 / bc557 to do this. PNPs are off when the base is high, they turn on when the base is near 0v and can sink some current. So drive the base of the TIP121 on the emitter of the NPN.

    But as pointed out above, it depends on how the code logic should be. Also, when the Atmel is powered off, it won't hold the PNP base high, so there's a chance of false triggering the TX relay... Early morning thoughts.
  6. 2E0ILY

    2E0ILY Ham Member QRZ Page

    Thanks, no it's a Kenwood TS-590 and one pin goes high (to 12V) when in TX mode with a selectable delay. So both my U3S exciter and the 590 should be able to do this now I know re the base resistor :) Thanks again appreciated.
  7. 2E0ILY

    2E0ILY Ham Member QRZ Page

    OK, I'll increase it then, thank you!
  8. 2E0ILY

    2E0ILY Ham Member QRZ Page

    OK, I may just raise it a bit, to be "green" :) Cheers Rege.
  9. K7JEM

    K7JEM Ham Member QRZ Page

    A 10K resistor to the controller, and a 22K resistor to the TS-590 from the base of the transistor should work OK. If either of them go "high", the transistor will turn on. If both are "low", the transistor will be turned off.
  10. K7JEM

    K7JEM Ham Member QRZ Page

    Looking at the data sheet, there are some internal resistors across the base-emitter junction of that transistor, so maybe lower value resistors are in order, just to be safe. Maybe 4.7K to the ATMEGA, and 10K to the TS-590. Also, remember to put a reverse diode across the relay coil to keep inductive spikes from damaging the transistor.

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