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Matched Transistors, Exact Resistance, and 1 Pot

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  • Matched Transistors, Exact Resistance, and 1 Pot

    I would like to do the fine tuning described in the intermediate SG manual. It mentions after haivng matched transistors and testing for 7 100ohm resistors of exact values then only a single 1kohm 1w pot is needed to adjust for max rpm. What would the schematic be for this? Thanks!

  • #2
    If you would like to experiment with a pot all you need to do is put it in the trigger path.

    I do not know how your board is setup currently but you probably have an intermediate resistor before your base resistors, put the pot there.

    A word of caution,, you want to lower your intermediate resistor a bit from whatever value it is currently so that you can have the variance of the pot but do not remove it completely. you want some resistance there in case your pot were to short.

    The actual hookup of the pot is done by connecting one side to the middle post and the other to either the left or the right post depending on which way you want to turn it to raise or lower the resistance.

    So again what I am saying is leave some resistance in the path regardless of the pot, or say if you were to turn the pot all the way down to zero there would still be resistance there on the way to the bases.


    • #3
      Ok, so do I need 7 resistors plus 7 pots, or 7 resistors and 1 pot that can control the path to each transistor base?


      • #4
        What do you have on your base resistors currently?

        WHat you would be doing is putting the matched 100 Ohm resistors on your base instead of whats there now. Then working back from there you would have another resistor that would be your intermediate resistor, this is sort of a safety in case the pot messes up ( does serve other purpose but for this conversation think of it that way),, then the next thing is the pot which is connected to the trigger wire.

        Make sense?

        Actually just look at the original monopole diagram. This also shows the light bulb which you do not need for it too work. That has another purpose but we won't go into that, some use them some don't.

        Look at this diagram:
        Last edited by BobZilla; 08-18-2014, 07:43 PM.


        • #5
          Possible Wiring Diagram?

          The fail safe resistor makes sense.

          Since the pot goes in the trigger wire, is that why only one is needed or does there have to be one per transistor somehow?

          Check out the image to see if I would be wiring it correctly. Thanks for the help.

          Click image for larger version

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          • #6
            The picture is real small, hard to tell what I am seeing but it looks like you have the proposed pot at the trigger bottom going to the emitter? If so then that is wrong, you want it before the base on the top side.

            So from your base you would have the 100 Ohm resistors which bus together, then you would put that intermediate resistor which just assures some resistance when the pot is fully open or if it shorts, then the pot which has the trigger top connected.

            Looked at the picture again and it may be right, just can't see it well enough.

            Since the pot goes in the trigger wire, is that why only one is needed or does there have to be one per transistor somehow?
            Yes only one and it is in series with the trigger wire. See what it does is allows variable resistance on the trigger wire on it's way to the base, that's it really. That will affect how much your transistor is opening the C to E path.


            • #7
              Thanks for the help. I changed the pot so it is now going to the base. I had it on the wrong end of the tigger wire. The new pic is of my own set up.Click image for larger version

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