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I have a small space heater that each has a pair of quartz radiant heating elements. Each element is 400 Watts with individual latching button switches. This allows the heater to operate at 400W or 800W (either of one button pressed or both buttons pressed).

I find that the heater is often too hot and I'd like to add a 3rd option of 200W. This is accomplished by placing the two heating elements in series across the incoming line.

The ideal situation would still use only two button switches with the heater power selected in binary fashion. Two buttons gives 4 distinct states: Off, Low (200W), Medium (400W), High (800W).

The existing button switches are SPST and appear to be rated at 10A or so.

I've been doodling and the best arrangement that I can come up with requires replacing one of the button switches with a DPDT version. I'm having a real problem finding a suitable switch.

I'm hoping that someone can suggest a better alternative to what I have so far:

schematic

simulate this circuit – Schematic created using CircuitLab

The 4 combinations of SW1 & SW2 allow for heat selection of: Off, Low, Med, High.

What I'd like is suggestions of how to simplify this to, perhaps, two SPDT switches instead.

Worst case scenario is that I add a beefy DPDT relay controlled by the existing SPST SW2.

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  • \$\begingroup\$ +1 for the interesting question. I think it's impossible with only 2 SPDT, but only because I've failed repeatedly. I wonder if there's a Bridges of Königsberg type proof for its impossibility? You could try this question on the mathematics stack. If it helps, I can switch a heater between 4 heat settings with only 2 SPST, but none of those settings are 'off'. \$\endgroup\$
    – Neil_UK
    Nov 26, 2017 at 4:29

2 Answers 2

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schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ This is cool but what I don't like about it is that the full-heat mode is one switch up and one switch down. I'd prefer a binary-type solution, where both switches down (in) is full heat. But good food for thought. \$\endgroup\$
    – Dwayne Reid
    Dec 2, 2017 at 5:51
  • \$\begingroup\$ You mean up+off=open and down+on=full? \$\endgroup\$
    – Janka
    Dec 2, 2017 at 10:46
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This at least gets you down to a spst and a spdt:the relay and x10 can be replaced with switches

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  • \$\begingroup\$ I'm not quite sure how to read the schematic. Specifically, the white wire going to the right side of the heating elements is completely obscure to me. \$\endgroup\$
    – Dwayne Reid
    Dec 2, 2017 at 5:49
  • \$\begingroup\$ In your application it would be connected to a spst switch to hot. \$\endgroup\$
    – mtnCbn
    Dec 3, 2017 at 13:24

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