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Correct series-or-parallel-or-off switch wiring?

  1. Jul 27, 2007 #1
    Is this a correct way to wire a DPDT toggle switch so that you can run current through two identical resistance heaters either in parallel or in series (or not at all, so that you have off/half/full power)?

    Code (Text):

       |                           |
       |                     +-----|----------------------+
       |                     |     |                      |
       |                  + -|- - -|- +                   |
       |                  |  |     |  |                   |
       |                     *     *                      |
       |                  |           |                   |
       +----/\/\/\/\/\/------*     *-------/\/\/\/\/\/----+
       |                  |           |                   |
       |                     *-----*                      |
       |                  |           |                   |
       |                  + - - - - - +                   |
       |                                                  |
       |                                                  |
       |                                                  |

      Hot                                              Neutral
    The idea is that when the switch is in the down position, current flows through the left heater, down the left half of the switch, across the jumper, up the right side, and through the other heater. (And the other way for the other half of the AC cycle.)

    When the switch is in the up position, current flows through the left heater, up the left half of the switch, to neutral AND (in parallel) down the right side of the switch and through the right heater to neutral.

    With the switch in the middle, nothing flows and that's the OFF position.

    I'm concerned about safety as well as correctness here, because it will be handling 120VAC power.

    For example, is there a rule about polarity for a toggle switch, such that the center connections should be neutral to avoid making the switch live, or anything like that?

    (It's not a homework question, BTW; it's more of a "do I understand toggle switch wiring?" and "will this kill somebody?" kind of question.)

    I realize that there's no need for the current to flow through both sides of the switch in the down position. (I suppose it might be just slightly better to use two diagonal jumpers from the center connections to the opposite-side bottom connections. That way current could flow through both sides in parallel, so that if one happened to fail, the other would still keep it working.)

    Thanks for any advice.
    Last edited: Jul 27, 2007
  2. jcsd
  3. Jul 27, 2007 #2


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    Staff: Mentor

    Looks right to me (nice ASCII drawing, BTW). As for safety, just be sure that the switch exposed metal parts are Earth grounded. How are the wire connections made to the switch? Are you using a light wall switch type with screw terminals on the back, or are you using a discrete-type switch where you have to solder the wires on the lugs on the back? If the latter, be sure to use heatshrink tubing to cover the soldered ends (remember to put on the heatshrink tubing onto the wires before soldering!).
  4. Nov 10, 2007 #3
    Sorry for the very belated response... I thought I had notification enabled but I didn't...

    I'm using a 20 amp DPDT toggle switch with quick connects, and using insulated crimp-on connectors crimped onto the wires and slid onto the connect tabs.

    Thanks for the help!

  5. Nov 10, 2007 #4


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    Science Advisor
    Homework Helper

    It probably wouldn't be allowed in practice though. The left heater is hot when the switch is turned off - a danger to anyone servicing the equipment.
  6. Nov 10, 2007 #5
    Good point. Which makes me wonder... is there an alternative series-or-parallel wiring scheme with a DPDT switch that would not have that problem?
  7. Nov 10, 2007 #6


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    Science Advisor
    Homework Helper

    Not with a single switch, that I can think of (without using an extra control circuit and relays).
    You would either need a special 3 position switch or 2 switches (on/off plus low/high).
  8. Nov 10, 2007 #7


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    Science Advisor
    Gold Member

    I recently had the back off of my oven and found that one leg of the 240 volt power cord is tied directly to the oven coil (lower). I see no reason why it would not be permitted.
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