Dehydrator Thermostat

That was about the only thing I did find when I Binged the subject.

He doesn't go into wiring detail, but I suppose it would be simple enough to tinker with which wires turn voltage on and off.

Make sure all the switching is in the live conductor. Switched neutrals don't go down very well when fault-finding.

You are of course absolutely right.

Trouble is that especially when using mains plugs, some countries do not have any way of specifying where neutral and phase are, connector wise. Its really annoying. I almost rewired my house here with UK plugs and sockets, till I saw how much it would cost me!!

But t be safe, I added RCDs or what they are called nowadays, on each phase....almost as good.....

Equipment here needs to have dual pole on off switches to break BOTH lines, but seldom do....crazy!!

I responded to another of your posts before reading this article. This guy is switching the heater, not the fan, which makes more sense. How does your dehydrator work? I guess I was assuming it was a solar unit where the heat source was uncontrolled. If you turn off the fan and leave the heater running, you could have a big problem.

What's your idea of inexpensive?

If you want something accurate, it'll be cheaper to get something with fixed hysteresis, which will generally be only about 0.5% (i.e. less than one degree in the range you're looking for). So that'd be e.g. on at 150° and off at 149.25°.

Maybe a dumb question, but are you taking degrees Celsius or Fahrenheit?

Pulled out of an old water heater is my idea of inexpensive

But if I had to invest cash I'd say $20 maybe. The idea was to build a large, solid unit and keep the cost low without compromising too much. I've done that and it is very stable right around the range required, but it varies on the high side depending on ambient temperature of course.

That fixed hysteresis unit would be plenty accurate, it's just beef jerky and the idea is to keep it around 155 for a good part of the drying time but not overcook it. What is a unit like that called, made for, and do you know where would I can find one?

That was me. Don't know how I managed to post anon - didn't mean to. Finger must've twitched on the netbook touchpad .

Anyway, having posted anon, I couldn't delete my dumb comment about °C or °F - it had to be °F if you were considering using a water 'stat.

I made a very similar box to this about 20 years ago, with the same type of heat source, for heat-soaking bits of electronics before delivery to customers. I used the thermostat to switch the heater (as someone else has suggested) - it's much easier to control the temperature that way - the fan method is prone to overheating).

I used a controller like this: <click here>, which is almost certainly overkill for your application. It would definitely do the job, tho' and you may be able to pick up a cheap one (or similar controller) on e.g. e-bay or alibaba (other on-line retailers of cheap stuff are available).

Klixon is one brand that is economical. I don't know if they have one in the right temperature range.

This seems pretty straightforward. I found it on Amazon. It's a water heater thermostat that adjusts up to 160.

Looks like all I would have to do is interrupt the hot wire to the fan? (marked standard 120v element in the image)

That won't work if you want it "on at 155, off at 145" - in other words off with lowering temperature. The single water heater thermostat turns off with rising temperature. You'll need an upper thermostat from a dual element water heater. It has double throw action which switches from the upper element to the lower element when the temperature in the top of the tank reaches the desired temperature. It will have 3 terminals instead of 2. Here's a link to the schematic. You could just wire your fan like the upper element and eliminate the lower wiring, element and thermostat. Subtle but important difference. Your fan pulls less than 20 amps, I assume?

I was wondering how a single wire could do both, so it can not. Good info.

The fan is running at 5 volts dc off of a wall wort. I'm going to control it via its outlet. So, yes, it's way less than 20 amps. It's just an old computer fan.

Just joking about the U.L. approved thing

What kind of heat source are you using in this wooden box? Do you have a high limit safety switch, manual reset...?

He could salvage the one off the water heater and wire it to turn off all the power to the unit. I can't remember what they are set to, probably around 200 degF.

NOTE TO OP: if you do this, make sure you don't mix the high and low voltage wiring, and make sure you properly insulate or protect the high voltage terminals!

Maybe you could rig an electric skillet controller for the fan....cheap used...

http://www.thesmokering.com/forum/viewtopic.php?t=32266

SolarEagle,

Sweet!

A simpler way of obtaining a box for your dehydrator is to use an old refrigerator. It doesn't work so it is simple to use. It has the advantage of being better insulated so not as much heat source is needed. It is ready made with a hinged door, fan is easy to install, shelving and water washable interior surfaces. Simply install the fan, thermostat, wiring and your are ready to go.

This type of apparatus is frequently used for smoking meat. Salmon, whiting, turkey, beef, numerous other fish and meats are done in smoker adapted refrigerator boxes. A smoke generating smoldering fire can be put in a tray in the bottom to contain the smoldering fire. Many a fish caught in salmon runs have been smoked this way.

Many welders and especially welding shops use old refrigerators for keeping coated rod and other items moisture free. A small electric heater, a thermostat, are put in it to provide heat, and a vent in the top are all that are usually necessary. Much cheaper than the commercial units.

Good Luck, Old Salt

Most thermostats at that temperature will operate "backward" from what you want. The cheapest solution would be to get a cheap relay, which, if wired correctly, reverses the "logic" of the thermostat.

Cheapest solution: Insert

LT =140F=342812, or 342813 (Whirlpool part number for Clothes Dryer over temp cut off switch)

in series with the heating elements (light bulbs) that you are currently using. The heaters will cycle off when the air temperature exceeds 140 degrees. and turn on when it falls about 5 degrees cooler.

If you wish to operate about 10 degrees hotter, then use

MT=150F=341146, or 341147.

http://www.partsreadyonline.com/item.php?ItemID=78

$20 new, about $8 used from your local appliance house. No other parts needed.

Good luck,

Dan

Well done - nice to see a bit of practical stuff put to use, and working as required.

The hysteresis actually helps you as it stops energetic ON/OFF switching that can on its own cause problems and as you say, for the job its perfect.

I would say that you have a good setup with many practical advantages.

Well done.

Thanks for all the input. I looked into all the options offered and learned a lot.

For the record I must add that to push the temperature to the shut off point I did have to add a 3rd bulb, so it's 300 watts.

I'm very happy with the unit. It delivers a large quantity (8 sq ft) of very evenly dried food. The best part is that it cost me a fraction of what a commercial unit would. It's also 100% easily serviceable by me, and is much sturdier than any commercial unit.

One more point.

I originally wanted to control the fan since I figured that it would be better to switch it on and off repeatedly rather than the bulbs. Dumb idea because the controller would turn the fan on at the temperature set point, which would normally turn on the heating element in the skillet, not turn on a cooling fan! So the box would just keep cooling down while the controller waited for it to heat up in order to turn off. So of course I switched it to controlling the bulbs.