Cooling fan thermostat

[lanceh5]

I want to install a thermostat that will turn on a small fan to blow air to the downstairs. I want to put the thermostat at the top of the stair well near the ceiling. This will be for winter time use and turn on a fan to blow the warm air back down stairs. This is an inline fan, 0.5 amps, in a 6" duct. there are two of these at each end of the house. <p>I was thinking of using a cheap cooling thermostats, under $20, but then would need a transformer to use 24 volts AC needed for the thermostat. <p>The door bell transformer is only 16 volts, I believe. so could not use that voltage. <p>Any ideas? I can get the thermostats at the hardware store but where to get the transformers? I searched the internet but not sure what type would be needed.

[lanceh5]

I did not mention that some type of relay would be needed. I saw some inline thermostats on the internet but they were for heating not cooling.

[Mike6158]

I've been struggling with a circuit that uses a MAX6675 and a PIC12C508 that would fit this application perfectly. The 6675 is a TC-to-digital converter. It talks to the PIC serially. You could add a voltage regulator and a couple of diodes to the board (I'm pretty sure that the doorbell voltage is AC so you would need to rectify it) and run the board off of your doorbell voltage. You could then program it to turn an output on at X degrees and back off at X - Deadband. Connect the output to a small relay.<p>Now to the struggle part The 6675 comes in an SO8 case... I'm completely clueless as to how to use this on a single sided board. At least from the standpoint of making it work in Eagle... Once I clear that hurdle I should be able to prototype this thing and put it to work. <p>BTW- My application is different but it would "convert" to yours pretty easily. You could even use a 6642 and do away with the need for using a thermocouple.<p>[ November 25, 2004: Message edited by: NE5U ]</p>

[Chris Smith]

"All Electronics" should have everything you need.

[terri]

As long as you're going to use a relay anyhow, stop at any plumbing or furnace shop and beg for an old used 24 volt thermostat transformer. Wear old clothes and look poor. Chances are they'll just give you one.<p>Incidentally, it's amazing how many 110 VAC relays work just fine on 12 VDC with no ill effect --but check it out first. Most of the time, since there's no reactance involved with the DC, the relay coil pulls enough current to operate on the lower voltage DC. <p>I'd also bet that 16 VAC will operate the 24 VDC relay directly.<p>As with any home-brew system do not let the finished product run unattended for the first couple of tries --no matter how simple or "foolproof" it looks. And always check casework voltages to earth ground to avoid possible shock hazards --again, no matter how simple and foolproof it looks.

[jwax]

Skip all the complications! Use a snap-action thermal switch. It closes its contacts at your selected high temperature, and opens when cooling is done. Do you really need to change the temp setpoint?
http://www.pepiusa.com/
John

[Mike6158]

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Skip all the complications!<hr></blockquote><p> But where is the fun in that

[k7elp60]

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>I'd also bet that 16 VAC will operate the 24 VDC relay directly. <hr></blockquote>
AC relays will work on DC, But DC relays will not work on AC because they don't have the shader ring. The DC relays on AC will sound like a buzzer as they vibrate.

[terri]

Sorry, I meant to type 16 VAC relay. You are right, of course.

[terri]

Sheesh! In both cases above, I meant to type 24 VAC.... I must've had too much turkey.<p>Coffee! Give me coffee! More! More!

[jollyrgr]

Why is everybody fixated on using 24 volt relays and transformers? This is the "standard" voltage used by most furnace and air handler units but it does NOT mean you have to use this voltge to control 24 volt relays to activate fans in a different application. <p>A thermostat is nothing more than a low current switch. You could use a 12 volt DC power supply and 12 volt relays (available at Radio Shack) and the thermostat would not care. A simple 12 volt DC wall wart and relays will do nicely. The thermostats are only a switch closing a circuit to activate a relay in the furnace. In a furnace application the "standard" is 24 volts.<p>It does not matter if it is 5, 12, or 24 volts if you are developing your own application. Most of the digital thermostats I've seen are battery powered and simply drive small relays inside the wall unit. These relays in turn provide 24 volts to the coils in the furnace to activate the burner. (In some more complex furnaces there is a controller board to activate electronic pilots, power vents etc.) In summer the cooling relay activates a relay in the furnace to turn on the blower only and to operate a contactor (read two pole heavy duty relay) in the condensing unit outside. There are some very rare instances where you will have digital thermostats that sneak their power from the 24 volts from the furnace. But any of the mechanical or digital thermostats that run on batteries (of which most are of this design) will work in this application. As jwax points out a snap action thermostat would work. A bi-metalic furnace thermostat would work, a mercury based (if you can find them anymore) thermostat would work. What it boils down to is that a furnace thermostat is nothing more than a low voltage, low current SWITCH. <p>Now, don't go trying to drive the fans directly with a thermostat! But driving a couple of coils of a relay will not be a problem.<p>[ November 26, 2004: Message edited by: Jolly Roger ]</p>

[terri]

'Cause he happened to mention 24 VAC transformers. <p>Not exactly a design parameter, but that's what lh mentioned. <p>There dozens of ways to do it, of course. Designing from the function to the hardware is one thing, but most of the time these home projects end up being designed from parts available in the junk box or for free. Let's face it, walking up the stairs and turning on a manual fan switch would work, too.<p>But, like lh, I would be leery of using an old 24 volt thermostat to switch a 110 volt appliance directly --whence the relay.<p>Nothing magic about 24 VAC, but if that's the way he wants to do it, that is, with a relay, the xfrmrs are readily available.

[Edd]

When this thread started with about 3 add ons, intention being to get the brand of a heating/cooling thermostat that I have been well satisfied with. Needless to say I kept forgetting to write it down.
It’s widely used by contractors, being of Far East origin and is also labelled as a Robertshaw 3GD27 or Honeywell 1RC47, White Rodgers 3MU49, et al…?

<a href="http://www.grainger.com/Grainger/produc ... 93&ccitem=" target="_blank">http://www.grainger.com/Grainger/produc ... 93&ccitem=[/URL]</a><p>
<a href="http://www.grainger.com/Grainger/produc ... 21&ccitem=" target="_blank">http://www.grainger.com/Grainger/produc ... 21&ccitem=[/URL]</a> <p>http://www.grainger.com/Grainger/produc ... 81&ccitem= <p>or the name I couldn’t remember, was Maple/Chase, which I got for ~$20.
The advantage of this/these units is the simplicity of set up , as all my prior units required a manual referencing to set in all of the different times. This one is so simple that it can be done by memory, or instructions are so short that they are posted inside the front cover.
All of the other help posts referencing aptly inferred the importance of the utilization of an isolated additional power relay so as to not tax the contacts small internal relay ( its about ¾ in long by 3/8 in square) of the wall thermo unit.
What I can supply, is the load specs on this unit that my equipment imparts upon it, as I typically research normal operating states on equipment BEFORE experiencing problems.
In the heating mode , utilizing a 24Vac control transformer, only 100 ma of current is required by the natural gas valves control coil to be handled by the thermostats relay. The other case is in the cooling mode, where 360 ma is required for the 50A contactors coil of a 4 ton AC condenser.
The thermo control unit uses 2 AA alkaline batteries and I have received 1 ½ years of use before the battery logo was displayed.<p>73's de Edd
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<p>[ March 21, 2005: Message edited by: Edd Whatley ]</p>

[bodgy]

Shortly (End of the week) on my website will be a Climate controller circuit with temperature/humidity sensor.<p>This has 3 MOSFET DC switches for motors, solenoids etc and one AC switched ciruit for heaters and the like.<p>OUTPUTS are OVER TEMP, UNDER TEMP, UNDER HUMIDITY<p>www.btech-online.co.uk<p>Colin<p>[ November 29, 2004: Message edited by: bodgy ]</p>

[dacflyer]

Pffff if it was me, i'd connect the fans to the blower circuit of the furnace.. this way you can use everything with out any transformer...
just watch out.. some furnace blowers use a 220v blower..<p>this way when the furnace blower comes on so will the fans...simple...done.