www.TK560.com

[drcrash]

I think I figured out some things about dimmers and stove-top-burner "infinite switches."

It turns out that infinite switches look like they give continuous control from low to high, but really they don't. According to some guy from GE, infinite switches generally go from off to about 10 percent, up to about 60 percent, and after that they jump to 100 percent.

(They turn the power on and off, and the percentage is a rough percentage of time that the power is on.)

That's pretty annoying if you want precise heat control, and especially if you want it most on the high end. A dimmer switch isn't much better than a three-position off-medium-high switch.

I suspect that light dimmers have a different problem for this application, which works out to similarly crude control. I suspect that light dimmers don't use a "linear taper"---turning the knob 10 percent of the way doesn't increase the power by 10 percent of the max power.

Instead, they probably use a "log taper," where turning the knob 10 percent of the way increases the power by a constant factor, like 1.5 or 2. If so you'd have to turn the knob most of the way up to get half power, and most of the rest of the way to get three-quarters power, and so on. At the low end, turning the knob a few degrees would add very few watts, and at the high end, turning the knob the same amount would add a bunch of watts.

(The human eye's perceptions of light levels are logarithmic, not linear. Going from 10 to 20 watts seems like about the same increase in light as going from 100 to 200. That's why I suspect that light dimmers are logarithmic----if they weren't, turning the knob the first quarter of the way would make a huge difference, but turning it the last quarter of the way would make a barely perceptible difference.)

If that's right, it means that you'd need to mark positions on your dimmer switch, and be very careful about small angles on the high end. In practice, I don't think you'd do much better than a off-medium-high switch, or an off-low-medium-high switch. (All the way down, half-way up, almost all the way up, and all the way up.)

One of the implications of this is that my double-spiral design may be better than I thought, for different reasons than I expected. If you have very coarse-resolution control of the heat, you're better off applying it only to half the coils, and leaving the other coils on all the time. That way, instead of having a jump from 60 to 100 percent for all the coils, you have a jump from 60 to 100 for half the coils, which amounts to an effective jump from 80 to 100 percent. Not too bad.

I suspect that having lowish, medium, and highish settings would accommodate almost all plastics, if the low is not too low. For most plastics, you could just leave it on medium and vary the exposure time, like with the TJ oven or Doug Walsh's designs. For thick, low-melting point plastics, you could put it on low. For thin high-melting-point plastics, you could just use high. And for thick, high-melting-point plastics, you could put it on high for a while, and then switch it to medium while the heat soaks in.

(That's what a lot of industrial thermoformers usually do. They hit the plastic with a lot of IR at first, but once it's kinda warm, turn down the heat to avoid burning the surface. You can use the heat/soak strategy for tricky plastics, or just to heat plastic faster without burning it.)
_________________
Paul (a.k.a. Dr. Crash)

Tired of buying cheap plastic crap? Now you can make your own! www.VacuumFormerPlans.com

[drcrash]

An update on the cheap heat control front...

I don't much like the way the infinite switches on my two-burner hot plate oven behave. It's hard to get the same halfway-down heat on both burners. I end up leaving the infinite switches all the way up, and just using exposure time---or for thick plastic, switching the whole oven on and off, or flipping the plastic over now and then, to keep it from burning one side.

That generally works fine, but it's a bit tedious, and I wouldn't want to do that on the 2 x 2 foot machine. I'd rather be able to turn the heat down.

I figured out a different kind of heat control, which an electrician friend says should work fine.

My plan is to use power diodes (a.k.a. "rectifiers") to cut the power in half. A diode is just a one-way valve for electricity, so if you run regular alternating current through it, the forward half of the waveform will go through, and the backward half will be blocked. What you get out is pulsating DC at half power.

My plan is to set things up so that the two variable-heat segments of my four-segment oven have diodes in series with them, and a switch that can open the circuit for zero power, close it with the diode in series for half power, or bypass the diode for full power.

Given that half my coils will work that way, that will give me overall power levels of half, three quarters, and full for the oven as a whole. (And since one of the variable-heat coils covers the inner part of the oven, and the other covers the outer part, it'll give me the ability to boost heat around the edges.)

Anyway, I got a couple of suitable power diodes at Fry's for about $6.50 each. The ones I got are way overkill, and I think you could find suitable ones on the web for half that. With a switch costing a few dollars, each temperature control should come in under $10. (Actually, I get light switches for 50 cents at the Habitat for Humanity re-store, so this is a really cheap solution. If I shopped around for diodes, I could probably do both heat controls for $10 or so.)

I could also put on-off switches on the other 2 coils, which would let me run the oven at a quarter power using only the variable-heat coils. I don't know if I will bother with that, but I may so that I can use the oven as a hot box for pre-drying.

If anybody else wants to try this, figure out the amps that your coil draws and multiply it by 1.4. Also multiply the AC voltage by 1.4. That gives you the instantaneous peak amps and volts (at the peak of an AC half-cycle). You need a diode rated for at least that, because they're rated in DC terms, not AC (RMS average) terms, and a bit more would be good. So a diode with a PIV ("peak inverse voltage") rating of 200 is probably fine for 120VAC.

I also picked up a diode for $7.50 that should be able to handle an entire portable grill element, so that I can turn my little over-and-under down to half power. I think that'll be good, because it runs pretty hot. It's fine for thin plastic, but I end up switching it on and off to avoid burning 1/4" plastic before the heat soaks through. With a diode, it will probably work fine to use it full-on for a little while, then turn it down to half before the plastic burns. (If may still have to turn it up and down again before thick plastic is done, but at least won't have to do it every 30 seconds.)
_________________
Paul (a.k.a. Dr. Crash)

Tired of buying cheap plastic crap? Now you can make your own! www.VacuumFormerPlans.com