Wax Motors Add Motion to Your Projects

[electronicsNmore] has uploaded a great teardown and tutorial video (YouTube link) about wax motors. Electric wax motors aren’t common in hobby electronics, but they are common in the appliance industry, which means the motors can be often be obtained cheaply or for free from discarded appliances. Non-electric wax motors have been used as automotive coolant thermostats for years.  Who knows, this may be just what the doctor ordered for your next project.

As [electronicsNmore] explains, wax motors are rather simple devices. A small block of wax is sealed in a metal container with a movable piston. When heated, the wax expands and pushes the piston out. Once the wax cools, a spring helps to pull the piston back in.

The real trick is creating a motor which will heat up without cooking itself. This is done with a Positive Temperature Coefficient (PTC) thermistor. As the name implies, a PTC thermistor’s resistance increases as it heats up. This is the exact opposite of the Negative Temperature Coefficient (NTC) thermistors we often use as temperature sensors. PTC’s are often found in places like power supplies to limit in rush current, or small heating systems, as we have in our wax motor.

As the PTC heats up, its resistance increases until it stops heating. At the same time, the wax is being warmed, which drives out the piston. As you might expect, wax motors aren’t exactly efficient devices. The motor in  [electronicsNmore’s] video runs on 120 volts AC. They do have some advantages over solenoid, though. Wax motors provide smooth, slow operation. Since they are resistive devices, they also don’t require flyback diodes, or create the RF noise that a solenoid would.

19 thoughts on “Wax Motors Add Motion to Your Projects

    1. Ha. EEVblog. You mean the guy who reviews electronic CAD packages by launching the application then playing with icons for 20 or 30 minutes while wondering out loud what features he can find without reading any docs? Now THOSE are a genuine waste of time!

        1. Why would you want to watch someone mirror your learning curve with all your mistakes (assuming you check no docs or tutorials as in the vblog) as a “first impression”? First impression to me is a mini-review based on some experience – like how hard was it to make a simple schematic and PCB layout? Not how pretty it is and how you found what some of the shinny buttons do – sort of. Anyway, I watched one of those, I think when I was looking at DipTrace. I sat through the whole rambling thing waiting for some useful info and have not been back to the site. It was kind of hilarious in a sense – waiting, waiting, waiting, nothing! Between two ferns for EE’s.

          1. I gave him another shot. One on solar roadways and one on spark gaps on PCBs. I can see they are popular but the guy has a knack for squeezing 5 minutes of info into a 30 minute gab fest! Imagine those are classes and you are taking notes. What parts do you write down? I admit to skipping in 2 to 5 minute chunks through the solar road calculations and don’t think I missed anything (though he missed all the physics but made up for it by having done a full day’s data collection on a solar panel so his empirical data was good). If my classes were like that I would have been fired a long time ago. I would give it a try but I like to use a pedagogically sound plan and rehearse demonstrations which is a lot of work for video blogging. At least he doesn’t do those awful un-boxing videos – does he? There is certainly a hunger for info from a knowledgeable mentor.

  1. Interesting but waaaaaaaaaaaaay too long. A couple of short paragraphs, a technical drawing of the inside components and a 20 second demo video would have been more information and way less of a timesuck.

    1. Agreed. So many folks use videos as a shortcut, figuring they can wing it and get something good enough. It’s especially frustrating for tutorials.

      And there’s a special place in hell reserved for people who make video tutorials for software then upload a video that’s too compressed or low res to follow the cursor or read the UI.

  2. In-rush circuits use NTC thermistors (not PTC as described in summary). You want high resistance at cold to limit in-rush into bus capacitance and low resistance once active. BTW – since NTCs are always burning power (they’re inefficient) and also thanks to power-factor correction stages, NTCs are far less common in power circuits than they used to be.

    1. NTC thermistors are robust though, and the power dissipation can be fixed with a parallel MOSFET and some fancy drive electronics. Not exactly a cheap solution, but had to use one once in a 400W @ 18V-36V application. A 20A NTC is about 1.25″ dia x 0.25″ thick, massive chunk of material. Had a huge capacitive load to mitigate inrush current for and a >90% efficiency target. The parallel MOSFET easily met efficiency target, and also kept NTC cool once bypassed, as one HUGE problem with NTC inrush limiting is that they don’t work when rapidly cycling power and not given cool-down time. A lone NTC also changes the operating characteristics a lot when you have to work -55C to 100C ambient.

  3. I just opened one myself but the resistance when cold was 2.7k ohms and when heated it was about 1k ohms? Its a wax motor from the dishwasher detergent dispenser. Im confused now.

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