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I was curious to see how the foot pedal of a sewing machine drives the universal (?) motor inside the appliance. I had expected a simple variable resistor like the ones in vintage machines but that's not what I found. My experience is limited to digital circuits, so I would appreciate any explanations you can offer.

From a mechanical point of view, the R4 variable resistor is the main control input, and the switch S1 is linked so that it opens as R4 is pushed to one of its endpoints thereby breaking the circuit and resetting the triac.

There is one unknown component on the PCB- it looks like the other 0.5W resistors, but only has a single black band going down the middle.

My (naive) assumption would be that C3 or C1-R1 provide a low impedance path for the AC, so how can the rest of the circuit affect the motor speed? Also, is L1 part of an LC circuit or does it serve a different purpose? The appliance is rated at 0.9A @ 110V, how can they get away with only 0.5W resistors in that case?

Schematic

PCB overlay

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    \$\begingroup\$ Google triac dimmer. It's what this is and a universal motor is not unlike a light bulb as the load in this regard. The current though R4 set the fireing angle of the triac and thus the average output voltage. R1-C1 can be seen as a snubber across the circuit. \$\endgroup\$ – winny Apr 1 '17 at 7:40
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    \$\begingroup\$ A resistor with a black band might just be a jumper. "0". But it's probably some kind of UL fusible type resistor. \$\endgroup\$ – Daniel Apr 1 '17 at 8:39
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    \$\begingroup\$ @Cross It is just a light dimmer . Mains powered Electric drills use a similiar circuit .I would check your drawing because it wont work at all as shown .Double check the variable resister connections . \$\endgroup\$ – Autistic Apr 1 '17 at 8:47
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    \$\begingroup\$ @utu2012 That's a stretch. L1 is a must to limit the dI/dt to a value the triac can withstand without latch-up/destruction. Reduced EMI is a bonus and triac dimmers are extempt from many EMI requirements. \$\endgroup\$ – winny Apr 1 '17 at 12:26
  • \$\begingroup\$ @Autistic Thanks. I double checked the VRs but as far as I can see it's exactly like the schematic. \$\endgroup\$ – Cross_ Apr 1 '17 at 18:28
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It's a dimmer circuit, electronics-tutorials has a really great article on it: http://www.electronics-tutorials.ws/power/diac.html

simple dimming circuit

"If we wish to control the mean value of the lamp current, rather than just switch it “ON” or “OFF”, we could apply a short pulse of gate current at a pre-set trigger point to allow conduction of the SCR to occur over part of the half-cycle only. Then the mean value of the lamp current would be varied by changing the delay time, T between the start of the cycle and the trigger point. This method is known commonly as “phase control”."

Looks like L1 is just there as a low pass filter, it's hard to tell without values, but probably the switching on and off of the Diac creates a sharp rising edge that would create unwanted audible noise in the circuit.

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