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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

    Only one of the devices, either the U3Swith its ATMEGA or the TS-590 will be connected, it's impossible to connect both simultaneously, so that's one less worry! Thanks for the reply Joseph.

    I have a diode across the relay, I have been using the box (antenna change over) with the U3S for a while, but now want to use it with the 590 and an external amp.
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    Here is how I would do it as shown in an LTSpice simulation:


    I plot the relevant node voltages and currents. I had assumed (before I saw your more recent posting) that the two drive signals could overlap. Of interest is the simulated realistic delay in the relay coil current, both on pick-up and drop-out...

    By examining V(C), you can see the snubbing action of D3. Note that the collector voltage V(C) is clamped to one diode drop above 14V.
    Last edited: Dec 6, 2018
  3. AI3V

    AI3V Ham Member QRZ Page

    I would add a couple steering diodes, one each on each of the 2 output lines.

  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    I put steering diodes into the first (un-posted simulation). However, since the base-to-emitter voltage is clamped by the TIP121, and only goes to ~1.6V, there is no way that the 12V signal can back-feed into the ATMEGA output pin. Anyway, the OP says that both sources will never be connected at the same time....
    K7JEM likes this.

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