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Simple/ and or cheap variable thermostat?

Well I have a friend who is looking into the purchase of a snake, and meanwhile picking out the snake and the heating element isn't the hard part for him (he's looking at a 23 watt 120vac heating pad for it). He was also looking at a thermostat designed for snake tanks, however they want close to $150 for it, and it doesn't even have many features, it's basically a black box with an 3 digit lcd and a programable timer. I know it wouldn't be hard to put something similar together from a pic and a few different parts, however it's been a long few years since I've designed a full schematic of something, let alone look at one. So I was wondering if someone so kind might aid me in making of a schematic so my friend can go order the parts he needs and I can build it for him (don't have a problem assembying circuits from raw schematics, I find it fun). However, there is a couple things he wants out of it. Number one most importantly, he wants it to be able to adjust the output voltage so he doesn't fry his snake (most simple thermostats simply turn on until temperature is reached, then off), no he wants something more dynamic than that. Second, he needs to have an external temperature probe so that he can keep the thermostat outside of the tank whilst keeping a handy eye on the temp and also obviously so that the thermostat knows when to come on (is there a limit to the length of wire you can put on a thermocouple?). Third and last, we need a simple timer circuit that keeps track of the time (what's that involve? a 13mhz crystal and a few other parts?) so that at night the thermostat can change it's "heating schedual", I assume so that it tries to maintain a temperature say 3-4C above the programmed temp. (night mode anyone?) However the thermostat he was looking at buying wanted you to buy a second cord, which connected to an external appliance timer (yes you heard me, a $9 wall timer) so that it knows what time it was. My friend really needs this as he's looking into getting this snake within the month. I appreciate anything anyone has to offer. Thanks guys! -Punkguyta

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How about the thermostat out of a heating system? They can have timers, pretty good temp controls and lots of other gizmos...
Punkguyta (author)  killerjackalope9 years ago
This is what me and my friend were thinking, however a simple thermostat (digital or not) simply turns a signal wire to the furnace on or off, which I think runs at a lower voltage than 120V, I could be wrong. He wanted to be able to adjust the voltage also along with timer features, in order to not be just turning the heating pad on or off, but rather lower the temperature so that it's more acceptable to the snake.
Well you could find an acceptable temperature and either use PWM or resistors to calm the heating pad down...
Punkguyta (author)  killerjackalope9 years ago
ooo I have always wanted to hold a giant resistor in my hand (they do exist mate), but I think maybe something like an adjustable wall dimmer?
my toaster is a giant resistor some cheap back massagers have heating in them with controllers mine is for the car and runs off 12 vdc
Could do the job, many heating pads are resistance based anyway, though a thermostat wouldn't ever have the too hot pad problem, give the cage a day or two to find it's balance then the thermostat will be turning the pad on for very little time...
. You are correct - most t'stats use 12-24V. You would need to use an interposing relay to handle the power to the heating element. And a transformer to drive the t'stat.
. HVAC thermostat outputs are on/off. I think you're looking for a temp controller (eg, kj's PWM). Probably going to be much more expensive than a t'stat, but most of them use external sensors, so you can drop your RTD/tc into the tank and mount the controller where ever you like.
. If the temp in the room is stable, you could use a rheostat or variable transformer to adjust the voltage to the heater.
Punkguyta (author)  NachoMahma9 years ago
>You would need to use an interposing relay to handle the power to the heating element. And a transformer to drive the t'stat. . Really, are relays that hard to wire together? I think it's more a problem of finding relays appropriate for the job. The transformer end, pff that'd be a chinch, usually they're just battery operated (well depending on the relay circuit, could stay as battery operated?) but otherwise I suppose a 9V power wart would handle the task. (I've only seen 9v batteries in electronic thermostats) > HVAC thermostat outputs are on/off. I think you're looking for a temp controller (eg, kj's PWM). Probably going to be much more expensive than a t'stat. .Well you'll see in my other comment to you, we'll scrap the variable temp control idea. As far as rheostat or variable trans. to have a more fixed voltage control, VARIAC comes to mind, remember the days.. Regardless, I'm sure a 500W wall dimmer would do him justice as he was planning on a 23W heating pad. -Punk
The transformer end, pff that'd be a chinch, usually they're just battery operated (well depending on the relay circuit, could stay as battery operated?) but otherwise I suppose a 9V power wart would handle the task. (I've only seen 9v batteries in electronic thermostats)
How it works: The 24V transformer in the controller on the furnace supplies voltage to the switch in the thermostat, and that voltage then operates the relay(s) that turn on the circulation (pump for water, fan for air). That transformer also operates the relays that keep the furnace at a usable temp.

The thermostat itself is just a switch. If the thermostat uses a battery, it's because it's some sort of programmable one and needs it for the clock mechanism and similar, although more advanced ones could have a relay-like action, probably solid state with a MOSFET or similar that uses practically no current.

There are line voltage thermostats used for heat pumps, but you'd have to see what the amperage ratings are, maybe by themselves they can switch a 23W resistive load, 0.19A ain't much. But don't try it with a normal low voltage thermostat. The mechanical thermostats, like the old trusty round ones, often have a small heating coil for the anticipator, when the current is flowing it slightly heats the main bimetal coil so the circulation shuts off before the set temperature is actually reached, since things like heated radiators will continue to heat some more after the circulation stops. Pump 120VAC thru that normally 24V coil, and if you still have a thermostat it might not work right. Likewise, if the voltage is too low there may also be problems.

Regardless, I'm sure a 500W wall dimmer would do him justice as he was planning on a 23W heating pad.
Well both a heating element and an incandescent bulb are resistive loads, an old cheap rotary one should work. However, as NachoMahma was pointing out, a newer electronic one might have issues.

So simplest construction, if it works, is line from wall plug to line voltage thermostat, thermostat to dimmer, dimmer to socket (outlet), outlet back to wall plug. Then plug the heater into the assembled unit. You can test it with a table lamp with a small appliance bulb if you want.
. Yikes! I forgot all about the anticipator. Be kinda fun to see what happens with 120V. heehee . . I like your idea of combining a 'stat and dimmer. . I didn't know they made a home t'stat with 120V contacts - much more simple than an interposing relay. ;)
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