Introduction
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I was part of the Resistance
About twice a year I need a load cell. Usually it is something like testing out the spec on a regulator circuit. Does it really pump out the juice like the datasheet says it will? The answer is usually not. But I need to know how much it can do. I also need to know how long it will do it. You might hit your target voltage right off but 10 minutes into a test and it gives up trying. That’s just not cool. So, limited life testing as well.
The other scenario I bump into quit often is needing to create a battery discharge curve. There are many types of data you can collect on battery charging and discharging. The two most common are constant current and constant load. Constant current is just a little harder than a load cell. I have done both and they both have their uses. The load cell allows for constant load or constant resistance.
Now you can go out and buy a fancy load cell that will give you any resistance you want with the tap of a keypad, but it will cost you a pretty penny and honestly there is just no fun in doing it that way. I decided I was tired of taping big resistors to typing paper, see figure 1. It looks tacky and it is awkward to work with, but it does work.
Figure 1 The Old Load Cell
Time to play
I decided that it was time to make a tool. So, I assembled the components to build my own load cell with fixed resistors. The first decision was the kind of base should I set them on. I settled on a piece of square aluminum tube, 3” by 3” by 21” long with 1/8” wall thickness.
I ordered little rubber feet on an 8-32 stud so it would not slide all over my desk, or work bench, when I used it. Then it was time to select the resistances I wanted. I chose 10 different resistances between 1 and 1000 ohms, leaving 4 empty spaces for future needs. I wanted to have the ability to pump a significant amount of power into them so I choose the 25W aluminum housing style with screw down tabs. I thought is would look silly to have a bunch of different form factors, so I got identical styles for all of them, even though it is unlikely I will get the 1K to 25W. I would have to put more than 150Vdc or 2-phase AC to get that much power out of it. And I ordered nice solid brass 4-40 slotted screws to hold them down, because, damn they look good.
Caution Hot
Now the tube has a lot of thermal mass and a significant surface area, so I probably did not have to worry about heat build-up, but I did not want to find myself in a situation where I had to worry about how hot the device got, so I designed a fan into the inside. The fan neatly fits inside the tube with a small gap that I filled with silicon to prevent air leakage. The fan is powered by 5Vdc which I provide using a wall-wort into a barrel connector. There is also a switch to turn in on and off, because having to unplug it to turn off the fan would just be unprofessional.
Appearances are important
I had the tube anodized so that it would look nice. It won’t make it work any better, but that is not always the point. Anodizing is an amazing process. Because of the way I intended to use it, we decided that Hard Anodization was not necessary. However, I think the shinier surface of the hard would have been a better way to go, next time. The guy at the shop recommended I sand the surface using 120grit sandpaper. Unfortunately, the tube is not flat, being an extruded part. So, I opted for bead blasting. It has a nice matt finish. Oh, and I chose to have it done in OD-Green, because, well because Green!, that’s why.
Company Label
You will notice that it has our company label on it. That is not really necessary because we are not going to offer them for sale. Still we have pride in what we do and putting the company label on a custom-made tool seemed right.
I have said this before and I will say it again. All of us are tool makers, kind of what sets us apart from the other animals. So, go out there and make yourself the tools that will make your lives easier.
Final thoughts
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