Wednesday, February 7, 2018

Electrical Circuit Question

If a switch is on, will the resistance be less than off?  Logic tells me resistance in off should be ∞, and something less in on.


  1. The resistance across the switch? Off it should read infinity. With the switch on resistance should be very close to zero.

    If you have a three-way switch, it will have one common point and two switched terminals. In the (we'll call it) Up position the switch is closed between the common terminal and (we'll call it) A terminal. It will be open between the common and the third (we'll call it) B terminal. Flip the switch and it will be closed from common to B, open from common to A. Between A and B should always be open.

  2. The Off resistance is not quite infinite. There will be leakage currents, down in the nano-pico-tiny range, but at sufficient voltage, it will be there. And that doesn't take in to account inductive and capacitive effects for AC.

    And so, yes, the On resistance will be less than almost-infinity, although not quite zero. Especially if the contacts have experienced arcing.

    That said, I feel like I'm being set up. What am I missing?

    [Here's the question that's bugged me for most of life: if a bridge is open, you can cross it; if it's closed, you can't. If a switch open, then, it must be closed.

  3. The resistance when the switch is on will entirely depend upon what is in the circuit.

    Before trying to measure resistance, PLEASE make sure it is completely de-energized.

    With the switch on, the resistance will be based on whatever is downstream of the switch. A 60W incandescent light bulb will be around 240 ohms (probably a bit less; the filament resistance rises with increased temperature).

    A switched receptacle with nothing plugged into it will also have a (theoretically) infinite resistance.

  4. A mechanical switch will be an open when off, a short when on.

  5. Exactly, Clayton. When off, the switch opens the circuit by presenting a very high resistance. When on, it presents a low resistance to allow the current to flow. Typical values are gigaohms or more when off and milliohms or less when on for mechanical switches.

  6. More or less, yes.

    When the switch is Off, the circuit is usually "Open". Sometimes this is described as a "Broken" circuit.

    This is equivalent to a circuit that has infinite resistance. At whatever voltage is supplied to the circuit, the current-flow is zero.

    When the switch is On, the circuit is closed. The amount of current flowing is proportional to voltage-divided-by-resistance for the entire circuit.

    Most of that resistance is in the light (or motor, or battery charger, or whatever) at the far end of the wires.

    Most wiring systems are very low-resistance. (From the perspective of the person designing the wiring system: if the wires have any appreciable resistance, voltage at the far end of the wires, where they are attached to the light/motor/battery-charter/etc., will be lower than the voltage at the power source. Most users want full voltage at the far end of the wires, so the user prefers wires with negligible resistance.)

  7. If a tree falls in the forest, and there is nobody there to hear it, does it make a sound?

  8. If the switch is in a circuit, there may be other paths for the current to take, so I'm not sure that testing it in situ is reliable.

  9. To a good engineering approximation a mechanical switch will have infinite resistance when open or off and nearly zero when closed or on. If the resistance in the closed/on state is appreciable it will heat up. The more current drawn by the load the more the voltage drop across the switch.

  10. So the consensus is, the answer is "obvious to the meanest intelligence", but Clayton's intelligence is far from mean, or even THE mean, and he's well educated in technical matters.

    Why then is this not obvious to Clayton? What is he seeing that the rest of us are missing?

  11. Switch on, switch off. Switch on, switch off.

    In this way you will achieve nirvana, Grasshopper.

  12. DJMoore: I thought I knew the answer, but I am increasingly humbled watching YouTube videos. I thought natural rubber was no longer used for tires.