Heating Element Control

[cwood]

What about a Router Speed Controller....? I believe they are just HD rheostats, not the preferred variac.However, because they are designed to regulates the speed of a router motor, probably up to 3 HP at say 10-15 amps, wouldn't that do just as well...? Easy to plug into wall, and plug heating element into Speed controller....??? Not too pricey either..... Anybody ever done that??? Feed back............??

Thanks,

CW

[masonjar]

What's an HD rheostat?

I can't say much without knowing more about the type of motor and speed controller the router uses. If the motor is an AC brushless, then the control would have no effect at all on the heat output. If it's a DC motor, then it might work great - or it might blow up the speed controller, depending on how it's designed. I'll spare you the details unless you really want an explanation.

Don't use a rheostat, they waste too much power. Variacs are too expensive and bulky. I would use a triac. I almost did this recently, but found a free electric stove that someone was throwing out, so there was no point anymore. But, if you're any good with a soldering iron, you can build a very cheap, very simple controller that would work great with a heating element. It's the same principle as a light dimmer switch. The triac connects and disconnects the element to the power source on and off at 60 times a second, and varies the amount of power by changing the ratio between time spent on and off. I think a lot of electric stove-top burners use this method. If you're interested, I can show you the plans that I was going to use. The parts were going to only cost a few bucks.

[Tater]

No elect cook stoves dont use those type switches .Most use a switch called a infinite switch.That are a on and off type Hi being on all time med being on 50 percent of the times cycle they are set for.

[tonkyman]

Hey Masonjar! I'd love to see your plan if its for a 115-120volt circuit. All I can find are for 220-240 volt circuits and it's been way to long since I had electronics in school. Most of the triac circuits look simple it's just knowing what part numbers to put together.

Thanks a bunch,
Tony T

[masonjar]

Wow Tater, you're right. I thought infinite switches made a clicking noise when they cycled on and off and since my burners don't make any noises, I assumed they were triac controlled. But I just removed a heating element and connected the oscilloscope in its place and saw a plain 60 Hz, 165 volt sine wave no matter where the control was set, which means it isn't triac controlled. Interesting.

Tony, what are the power requirements for your switch?

[tonkyman]

Hi Masonjar,
My heating element is a 120 volt 1500 watt element. My home power will vary from 115V to 125V or so.

If you can help me out can you also let me know where to find the parts. The last circuit I built I hunted IC's from about 5 different places and the good old Radio Shacks don't really carry "electronic parts" anymore.

Thanks for your help,
Tony T

[cwood]

So does this mean somebody is gonna post a schematic, etc. for a good temperature control circuit...? I have access to all the parts.....I wasn't really looking to do a PID controller or anything, just a turn-up..turn-down sort of item...but who knows...I build tube amps and they work great.....I mean really, all we need is a "variable voltage regulator"....

Seems to be an iatem a lot of home distillers would be interested in.

Thanks,

CW

[Hackers]

If you do a search around this site I posted schematics for both a Triac and a twin heat triac controller modification about a month ago. Only problem for you is it was 230V but you should be able to adapt the parts list if you know your stuff.

[masonjar]

I need to find a web host before I can post a schematic.

Hackers, do have 50 Hz power where you're at?

I buy parts from www.digikey.com

[Hackers]

Yes here in New Zealand our voltage is 230V 50Hz (+/- 6%)

[tonkyman]

Hey Masonjar,
Send it to me and I'll get it posted and then post a link to it here.

We have 60 hz power here in the land of the not so free, but I'm sure you know that.

Later,
Tony T

[masonjar]

I still need to run some numbers to tweak the capacitor for 120 volts, but I haven't forgotten.

If you're in a hurry, you could use Hacker's circuit and reduce the values of C2 and C3 by about half. That should get it pretty close. There's also a trim pot in his circuit for tweaking. If you have tons of capacitors laying around, as I would expect for someone building tube amps, then you can find the proper values experimentally (but don't use polarized ones). The idea is that you slowly charge the capacitor through a potentiometer and when the voltage of the capacitor reaches the trigger voltage for the triac, it turns on and stays on until the next AC zero crossing. If the capacitor is too large, it will never charge up enough to trigger the triac, making the burner never turn on. If the capacitor is too small, it will charge up right away and turn the burner on prematurely. The perfectly sized capacitor is the one that turns the burner on immediately when the potentiometer is set to high and it never turns the burner on when the potentiometer is set to the lowest setting. You need an oscilloscope to really see that it's doing what you want.

The problems with this very simple circuit are that the control response is not linear (the control is the most sensitive in the middle range) and that it can generate some high frequency noise (possibly make some funny lines show up on your TV). Hacker's circuit may correct these problems some since it is a bit more complicated than just one resistor and one capacitor, but I can't tell just by looking at it.

A better way to do this would be to rectify the AC into DC and put that on a somewhat large storage capacitor, then a MOSFET could be used to switch the burner on and off. This would give a perfectly linear control. The next addition/complication would be to put a temperature sensor in your still head and read that with a microcontroller which controls the MOSFET and you could have a digitally tunable PID and regulate the still head temperature to exactly where you want it. Then you could get even more thorough and have the microcontroller also operate a servo on your reflux valve. But that would take all the fun out of it, wouldn't it?