I'm building a temperature-controlled voltage reference for my DMMs. I'm going to heat the cast aluminum box with a couple silicone flexible heater strips. Temperature will be about 40C, about 5W worst case dissipation. Heater is about 60x100mm. What's a good adhesive to use to glue the heaters to the enclosure? I seem to remember reading or hearing that ordinary RTV is good for these heaters, but RTV needs moisture to cure. How does RTV cure without air/moisture? How best to apply - use a little, a lot, clamp tightly, or OK to just spread and press the heater onto the goop?
Huh, I'm not sure. Pictures always help. For strap on heaters I think, no sharp edges, and some mechanical clamp, maybe rope/string and knot. Kapton tape is also good for holding heater wire down. I've had heater wire on glass (or Al) covered by Kapton ~50 ohms, ~10Watt max. for more than ten years in the field and no complaints. (knock on wood.)
Why heat a box? Just glue the reference into a small metal container, insulate it, and apply heat at the midpoint of the small-container's mount post inside the 'cast aluminum box'. Breezes and sunshine can change the cast box's temperature locally, but won't penetrate the insulation around the small container.
The old LM399 metal-can-with-internal-thermostat was a nifty part for its day, and about the size of a pea.
Seems the wrong way to go about it, heating an alu box. Once the sun shines on it it will be too hot, too strong coupling to the environment. Put your reference in a small thermally isolated box and for example heat it with a TO220 transistor on an small alu plate, like I do here:
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Are not there any stable references relative to temperature? Even a LM317 is stable, have some Microchip references too, what sort of accuracy are you talking bout?
Actually, this project is an experimental attempt to build a "poor man's Fluke 732" DC Voltage Standard. The heated enclosure will, of course, be insulated from the outside world, will be tightly temperature controlled, and will contain nearly all of the circuitry of the standard. I'm using three LM399A references summed together and scaled up to 10V. Maybe also scaled down to 1.019V and/or other useful voltages. Thinking about using some LIon batteries to keep it powered up 100%. Anyway, I was just curtious as to how to glue the heaters to the enclosure. I've had these heaters in my junquebox for several years, and thought it might be a good project for them to be of use.
The LM339A has a build in temperature controller. That leaves any opamps. Those are small and easier to stabilize in a small box inside a bigger one. And that takes significantly less power, good for if you use lipos. I added a 11.1 V lipo to my experiment to cover long mains outages.
Yes, use what you have, but in the most effective way...
If using RTV, I suggest "neutral cure" type, not "acetic cure" that smells of vinegar and corrodes things sometimes.
If you really need a silicone that cures without requiring any moisture then there are two-part silicones, but they are harder to obtain and sometimes they won't cure in the presence of certain other materials, e.g. some masking tapes, and PVC grommets, etc.
After reading bunches of web pages related to bonding silicone with other substances, depending on the sustained temperatures involved, high temperature RTV is used. The 40 degree C max temperature of my project suggests that the high temperature RTV such as Permatex Red or similar products might not be required, since common household RTVs' service range is up to 200 degrees C. I'll try some GE Silicone 1 first, with a roughening of both the heater and the aluminum box, cleaning both with acetone and bonding with the RTV. If that doesn't work well, I'll get a small bottle of 3M AC79 primer and treat the silicone surface, followed by 2-3 drops of super glue. 3M's Youtube video
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describes using it to bond a piece of silicone to a plastic strip using one of their cyanoacrylate adhesives. It shows the rubber tearing but the bond holding firm. That's a pretty good bond. Not sure if it's absolutely necessary to use their CA adhesive with this primer, but nothing to lose by trying some of the dept store variety.
Anyway, that's the plan. Hoping one of the strategies works out.
The problem with using RTV to glue down a large sheet of something is that the water needed for the cure reaction has to diffuse in from the edges, and if you press the two items together firmly it has to diffuse sideways through a very thin film. Water diffuses through RTV silicone better than most people expect so it will work, as anyone who has left the cap off of a tube of RTV for a year and found the entire tube cured into a solid block, but it takes time. Apply the silicone, press down your heater, then put a plate on top of that and clamp everything together so the heater can't move (rubber bands, or cable ties, or c clamps, or whatever you have). Put all of that into a ziploc bag along with some paper towels dripping wet with water and seal it up. That gets the relative humidity up to 100% to speed the cure up, and put the bag in a warm spot. Maybe a sunny windowsill, or wrap it in a heating pad, or something. I'd go up to 150-180F if you can, and leave it for a few days. Hmm, maybe put it in a slow cooker filled with water and set on low? If you can, glue up a second assembly, doesn't have to be a real heater, just a sheet of rubber and a piece of aluminum to simulate the box, and test it after a few days to see how it's going. I'm guessing on the time, but just think how frustrated you will be if you peel it all apart too soon :-(. I've used RTV to make a mold where the silicone was essentially a 2" cube, and cured it overnight in a chamber held at 150F and
85% RH but that was when I worked for a company that made chambers that would do that :-). After 18 hours the RTV was rock hard all the way through, but it had a lot more surface area exposed than your setup will. Good luck.
Well, I took a chance and ordered a 2 oz. bottle of the AC79 primer. If that stuff works as well as it appeared to work in the video, that's probably what I'll use. Didn't see any reference to a shelf life on the primer, so that should be a lifetime supply. The cyanoacrylate super glue is only a buck at our local Dollar Tree store, Got a feeling that's the way to go with this.
As I said, I'll try the RTV first. The area to be bonded is only about
3"x4", so curing the RTV shouldn't be a big deal. No rush on the project, so it'll have plenty of time to cure. I'll try your suggestion about putting the stuff in a bag with a bit of water and heating I have a fair sized chunk of silicone somewhere that I can experiment with before commiting the heater strips to any abuse.
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