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Diode Help — Parallax Forums

Diode Help

Not sure if this is the right category to post this in but I have a basic electronics question.
I am trying to set up a Light array where I have have 3 lights that need to be powered from 2 separate inputs When Input A is Hot, Lights 1 and 3 need to turn on. When Input B is hot I need all 3 lights to turn on. Currently It would seem I am back feeding into my 2nd light when Input A is hot, I believe in Need a diode in between light 2 and Input B to stop it from lighting up when Just input A is hot, but I am not sure what Diode to go with. this is a 12 Volt Circuit pulling 4.5 Amps.


  • @Zetsu
    We are good at guessing here and, frankly, I am fairly confident I know how to answer your question but a schematic could be of huge advantage and we could spare ourselves some of said guessing. Even a basic, simplified hand written draft would do.

    So the question is this: can you help us (and yourself) a bit and post it ?

  • No worries

  • B = Break

    TL = Turn Left

    TR = Turn Right

    L = Light.

    Currently when it turns Left or Right it lights up its center break light as well. I am trying to prevent that. The green Leads are what I think I need.

  • But I can see three inputs here, not two as described in the first post. Why's that ?

  • MaciekMaciek Posts: 670
    edited 2021-10-15 17:13

    How about this ? It assumes you're applying power from A and B and the 1 means the power is ON and the 0 means the power is OFF and these are symbolic control lines and not the power lines (black diodes). If You need to connect actual power from A or B to the lights then add the additional two diodes from B to lights 1 and 3 (red ones).
    It also assumes A nad B are mutually exclusive because (can they be 1 in the same time ? ). If both are 1 all lights will be lit (the same as only B is one).

  • @Maciek said:
    But I can see three inputs here, not two as described in the first post. Why's that ?

    Sorry when I was originally describing this post I had lumped Left and Right Turn into the same input.

  • MaciekMaciek Posts: 670
    edited 2021-10-18 16:52

    Ok, so it's going to be three inputs: B, TL, TR and you want to:
    TL - only left light lit
    TR - only right light lit
    B - all three lights lit (regardless of the state of the TL and TR)
    And the TL, TR can't be ON at the same time, right ?

    Does that describe your need accurately ?
    If so then the solution is this:

    It assumes the B, TL, TR are power sources (can be the same source if the B, TL, TR are ON/OFF switches like below).

  • Yes this describes the circuit correctly. I am wiring up this way now.

    Thanks for the help!

  • So, how did it go ? Did it work as expected ?

    One suggestion I'd consider implementing would be to add an, otherwise completely unnecessary, diode from the B to the L directly below in the same direction as the other ones. This would, more or less, introduce the same forward voltage drop making the voltages and currents and luminosities fairly equal on all lights (assuming all three lights are rated exactly the same, say X Watts each). The result may not be even noticeable if the diodes offered low ON resistance at the currents you need for the lights.

  • Perhaps my old brown eyes ever got some damage, due to all those shiny lights, but...
    Under the "current light" (sic) of component shortage, aren't there two unnecessary diodes at the last schematic?

  • Yes. If a single power source is used then absolutely yes. Just the two (the ones you left on the above picture) are fine.

  • Yeah, any other than a single power source setup would involve knowing the exact wiring configuration, in order to estimate every possible outcome.

    OP's mentions to the terms "brake", "turn left" and "turn right" definitelly suggests some kind of vehicle (not excluding any robotic "flavors"), thus a fuse/wiring diagram would "enlighten" (sic) the whole thread meanning even more. :smile:

  • Given what we have started with in the original post and that being "...3 lights that need to be powered from 2 separate inputs..." I'd say it's not bad as it is.

    I deal with, often only vague, descriptions of various "problems" every day and I always ask for a picture or a drawing to illustrate the problem. As it often turns out, the "problem" is not exactly as described initially and sometimes even a simplest graphical representation, a hand drawing, helps the one who comes with the said problem to understand it better and, at times, even solve it.
    But of course, having a proposed schematic, as detailed as it only could be in the first place, would surely help.
    I just hope it works out for the OP.

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