AC Foot Switch for Small Power Tools

I wanted a foot switch to use with my Dremel tool. My current Dremel tool is a Proxxon IBS/E—the word is pretty much genericized by now, don't you think? I also have the MB-200 drill stand (which I modified for improved depth setting) and KT-70 compound table, a USB microscope for accurate drilling of PCBs, and other stuff. See this page for the whole story. These are all pretty much toys compared to the milling machines I've once had access to, but these are cheap, they will fit on my desk, and they are convenient to have immediately on hand for the occasional small job. And despite being small hobby tools, these are all actually pretty well made. When used for jobs appropriate for their size, I'm quite happy with them. But I digress. I wanted a foot switch for this thing. Proxxon does make a suitable foot switch, their No. 28 700 Footswitch FS, at a very reasonable price, but I saw one major drawback in it. It has a long inlet power cord, but a very short outlet cord. I think the idea is that the drill's power cord is long enough to reach down to the floor where the foot switch is—and it is long enough for that. But in order to bypass the foot switch for continuous operation, I would have to crawl under my desk to unplug the drill from the foot switch. I wanted a dedicated switch right there on my desk to bypass the foot switch! Therefore... DIY.

The PFS-135A foot switch and electrical safety

	First, I needed the mechanical foot switch / pedal itself. There's plenty of cheap plasticky foot switches / pedals available, but I soon found the PFS-135A, which is totally hardcore. It has an all-metal construction, and it looks pretty serious as well! (And it was immediately available locally, which also helped choosing.) It's pretty generic and no-name, available from many electronics suppliers, and quite cheap. Most reviews I saw on the Net rated it very highly. It is specified for 250 VAC and 10 A, and it has an SPDT microswitch and a 1.8 m cord with three wires, connected to the common, normally open and normally closed terminals of the switch. There is no safety ground wire to this metal-chassis device, which is why some reviews on the Net claim it is a death machine, and only suitable for switching low voltages, e.g. for controlling a relay.
	So of course I wanted to see what's inside the device. To open it, you first must loosen the single screw on the bottom of the chassis a couple of complete turns (or you can just remove it completely). Next, you slide out the bar that forms the pedal's hinge at the end away from the cord. Pressing down on the pedal slightly, right at the hinge, first push one end of the bar inwards with a small screw driver, then grab the other end with pliers and pull it out. The bar has a groove in its center position, which may get stuck on the edge of the pedal—just wiggle it onwards while varying how much you press down on the pedal. Once the bar is out, the pedal will come apart cleanly, with no tiny parts flying out uncontrollably (only one pretty large spring, which is easy enough to find if it does jump out).
	Here's the internal structure of the device. The bar you pulled out (far left in the photo) has a groove in its center, which is where the single screw goes to keep it in place. There's a hefty spring, not really attached to anything, but kept in place by protrusions in the bottom and top halves of the pedal when they're assembled. It's the only loose object you'll find inside on disassembly. The cord enters the pedal through a feedthrough, which grips the cord and also has a flex part protruding outward. In addition, the cord goes around a strain relief structure inside the pedal, so it's doubly certain not to get yanked out easily. Very good design in my opinion, and so far, quite safe!
	The cord is rated at 300 V and is 20 AWG each wire, which means 0.5 mm2 in people language. The connections to the switch are appropriately insulated with shrink tubing, and there's an extra plastic flap between them and the metal chassis. However, the rather thin conductors are soldered to the spade type connectors of the switch. This is not bad in itself, but the wires are soldered simply to one surface of the spade, so if the solder were to melt in some overcurrent condition, the live wire would immediately become loose inside the metal chassis, and could very well electrocute someone! A soldered wire carrying household AC should always go through a hole, and then turn back and wrap around itself, so even if the solder were to melt, the wire would still be held in place. I think this does constitute something of a risk in an ungrounded, metal chassis! There is a single screw terminal for a safety ground connection, but it is not connected to any wire.
	So while the device's construction is mechanically very good, I did want to improve its electrical safety a bit. Since I was going to use this device as a simple, momentary OFF/(ON) SPST switch, I would only need two terminals of the SPDT switch, and could thus use one wire in a 3-wire cord for safety ground (4-wire AC cables do exist, but aren't too readily available). I replaced the original cord with a length of thicker 1.5 mm2 3-conductor cable, which came from a grounded extension cord I dismantled for other parts (see below). The safety ground conductor of this cable is the standard yellow/green, as it should be. This thicker cable would just not fit through the original flex strain relief, so I cut away all its useful bits and used it simply as a plastic grommet (to line the threaded hole in the pedal's metal chassis). This provides no strain relief whatsoever, but especially with this new, thick cable, I do absolutely trust the strain relief inside the pedal to hold it in place! I crimped insulated connectors to all three conductors, and connected the yellow/green to the safety ground point, and the blue and the brown to the switch. I used a fresh, new microswitch, as I happened to have those immediately available in shelf in the correct form factor. It's a 250 V 16 A Honeywell-branded switch, whereas the original was unbranded and likely "cheap Chinese". It did look and feel quite decent, though. I insulated the unused terminal ("normally closed") with heat shrink tubing for good measure. Then I simply re-assembled the pedal.

Note, by the way, the switches are typically marked with two maximum loads, e.g. "16 (3) A". The first number 16 A is for purely resistive loads only! For a motor load, the second, much smaller, value (3) A applies! A motor load differs from an inductive load in its large inrush current at start-up. Finally, there's a lamp load whose current spec is even lower than the motor load's! That's for incandescent lamps, which can have an absurdly tremendous inrush current before their filaments heat up! Look up the datasheet. They're often quite interesting, and they'll tell you all you want to know. You should find resistive, lamp, inductive and motor loads all specified separately, and a bunch of other useful information.

Of course, a motor load refers to an actual motor being directly fed by the household AC, not to a switch-mode power supply feeding a motor, as is the case in your typical Dremel tool, regardless of brand. You might do well to go with the lamp load specifications instead!

The switch box for foot switch bypassing etc.

	With the foot switch on the floor, I wanted a separate switch box on my desk within easy reach, for two purposes: For bypassing the foot switch, when I wanted to run the drill continuously. There are times when I don't want to keep my foot on the switch all the time. And I'd rather avoid the hassle of unplugging the drill from its foot switch thing (especially if that meant having to crawl under my desk and hurting my back) and plugging it directly into an outlet instead. For isolating the foot switch, so the drill (or whatever else is hooked up) doesn't get powered on unintentionally if I step on the switch by accident! Sure, every hand-held drill has its own power switch, which will do just that. But I'm trying to provide for other applications that I may not have thought of yet. The circuit is simplicity itself, using a Schuko plug and socket (scavenged from a grounded extension cord) as AC input and output, and two illuminated DPST rocker switches. See the full schematic to get the complete picture.
	I constructed the thing in a small electrical junction box (not even a project box) which I fitted with three feedthroughs including strain relief. The rocker switches had the same kind of spade connectors as the microswitch, so I crimped a bunch of wire pigtails with mating connectors. To tie things together, I used 3-contact lever-type splicing connectors by Wago. These are so much more convenient than screw-type terminal blocks a.k.a. "sugar cubes"! And I especially love Wago's transparent connectors, as I can be certain that the stripped wire is where it should be, and that the wire insulation continues inside the connector by a safe margin. It's an absolute mess with the lid open... Before closing the lid, I tested all parts of the circuit for continuity and shorts with every position combination of every switch. Then I hazarded a live test with my Dremel tool, and observed that everything works as intended.
	With the lid closed, it's no worse than any of my other homebrew stuff. The cord on the left plugs into a multiple outlet extension cord on my desk. The cord on the right provides a socket into which I plug my Dremel tool, or whatever other tool. (Note that this cord is short, just like in the Proxxon foot switch. The difference is, this short cord sits right there on my desk, rather than on the floor underneath my desk.) The foot switch at the end of the cord towards the top goes around the back of my desk, and down to the floor. With the box sitting on my desk, the two switches for powering up the drill or for enabling the foot switch are always within easy reach. Perfect! I chose switches with built-in indicator lights, so either switch, when activated, lights up green. Also, when the "Foot enable" switch is on, pressing the foot switch causes the "Always on" switch to light up, which kind of makes sense, and is almost like a design choice. However, when the "Always on" switch is on, pressing the foot switch (which does not actually do anything) causes the "Foot enable" switch to light up, which is somewhat nonsensical. Oh well. Slightly more annoying is that when one switch is on, the other one is also dimly lit, just noticeable under ordinary room lighting. I think this must be due to capacitive coupling between the wires in the footswitch lead! Again, oh well.

This foot switch can be used with grounded devices, whereas the Porxxon Footswitch FS is ungrounded and has those flat "Europlug" connectors. However, Proxxon's choice of "Europlugs" automatically limits the power rating of the device that can physically be connected to it. My choice of grounded Schuko plug and socket does not! I'm almost tempted to substitute an ungrounded "Europlug" for the output just to be sure nobody else can do anything stupid with it either. So keep in mind the switch ratings, and obey them! A 3 A max motor load on the microswitch means about 700 W max (on European 230 V household AC), which is plenty for a small Dremel tool, or even the average Black and Decker hobby drill, but not necessarily for a serious hand drill, like the bigger rotary hammers by Hilti! (But with a 16 A max resistive load, you could control a 3500 W waffle iron with it just fine!) Any bigger motors you'll want to control using an appropriately rated relay or contactor. But feel free to control that contactor with this foot switch thing—or to include the contactor inside the same box.

A serious word of warning!

And, once more, do obey the current ratings of your switches!!! Depending on the switch, a 1 kW waffle iron may be ok, but a 1 kW hand drill may not be!!! Unsure why that's so? Stop right now!!! Do you need to google "A and V to kW"? Stop right now!!! Don't know the difference between resistive and inductive? Stop right now!!! Need to ponder, whether you can replace the broken 12V 800mA power supply for your WiFi router with the 12V 1500mA one you have on hand? Stop right now!!! (The correct answer is yes, you probably can. But how many examples can you think of, where you can't? Unless you come up with at least two, stop right now!!!)