Just when you think you understand circuits, there's always a curve ball to derail you and leave you scratching your head.

The diagram below is a Triac/Diac speed controller for a reciprocating saw. At the top of the diagram, you'll notice there is a black wire from the right brush of the motor, a yellow wire to a resistor switch and a white wire connected to neutral. They are pigtailed together.

Although the diagram appears to be correct and works, I am confused as to why it works. If electricity chooses the path of least resistance, shouldn't the current leaving the black wire go directly through the white wire and back to neutral? Shouldn't it bypass the speed and trigger switches?

What adds insult to injury is that current bypasses the resistor in the speed switch when it is closed. Why does it bypass in. this case and not the other?

Reciprocating Saw Wiring Diagram


What you're missing is you have two circuits. Yes, the current flows from live, through the triac, through the motor, then down the white wire to neutral.

However, current also flows from live, through the capacitor (remember, it's AC we're dealing with here), through the resistors, the switch+resistor pair, and down the white wire to neutral. Some also heads off down to the triac to control it.

To make it more pictorial, you can split the circuit into two separate parts so it makes more sense:

First the main motor power circuit: enter image description here

Then the triac control circuit: enter image description here

Both (apart from the actual triac gate wire of course) are in parallel with each other using the same power supply.

  • \$\begingroup\$ Thanks for the clarification. The thought entered my head that it is a two circuit diagram, but the triac made it hard to understand. I wish I had a methodology for breaking down and simplifying circuits to avoid such confusion. Do you know any online resources for circuit simplification and analysis techniques? I desperatley heed it. \$\endgroup\$ – user148298 Aug 13 '16 at 15:31
  • \$\begingroup\$ The triac is just the point at which the two sub-circuits interact. Apart from that they are completely separate. You could replace it (conceptually in your mind) with a relay to make picturing it simpler. \$\endgroup\$ – Majenko Aug 13 '16 at 15:39
  • \$\begingroup\$ Thanks. I wasn't referring to this specific problem. I am looking more for something of a general approach, like a methodology for simplification. Sort of like an algebra to reduce complexity when you get stumped. Surely, EE guys must have some techniques they use as they work on very complex circuitry. \$\endgroup\$ – user148298 Aug 13 '16 at 15:44

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