furnace fan timer

adub

New Member
This is an attempt to make a circuit using what's in the shoebox to control the fan on my furnace. <a>http://freewebs.com/adub/furnace%20circuit.gif</a>

We heat our small house with a pellet stove in the living room.
I'd like to control the on/off of the HVAC fan to circulate heat to all the rooms.

The program is not the problem. Simple timer on for say 10 minutes off for 50 minutes with two trimmers on pin1 and 2 to adjust the times for on/off using ADC.

Problem is: I have a 12 volt dc relay and the furnace is on 24VAC. What values should I use for the capacitors/resistors?

Do you see anything else wrong/missing?

Would usually just experiment but the evenings are starting to get cold! Winter is almost here in Missouri, USA.

Thanks for any/all help.
Arvin
 

adub

New Member
Oops try =&gt;<A href='http://freewebs.com/adub/furnace%20circuit.gif' Target=_Blank>External Web Link</a>
Used &lt;a&gt;&lt;/a&gt; html tags. I know cut and paste works but this is easier for everyone.

Arvin
 

adub

New Member
Well the <A href='http://' Target=_Blank>External Web Link</a> protocol didn't work.

Now I have another question. Why?
 

Michael 2727

Senior Member
You may find that the 7812 runs very hot,
in that case put a 47 to 68 Ohm 10 Watt ( or 2 x 5W @ half the resistance ea ) Wire wound resistor in front of your Bridge Rect.
Also use a heatsink, small to med should do.

Just keep in mind at full current the 7812 needs around 2 volts above (output) on the input side to prevent dropout.

Try a 1,000&#181;F Electro after the Bridge.
Also a small 0.1&#181;F Mono cap after that, then on the 7812 output adding a 10&#181;F electro may help dropout.

Sometimes the 24V AC rectified can almost exceede the manufacturers max rec input,
@ 37V.

good luck.


Edited by - Michael 2727 on 10/2/2005 3:48:37 PM
 

manuka

Senior Member
Use square brackets [ &amp; not normal html arrow heads &lt;! This editor URL technique auto adds a http:// . You'll hence usually need to trim out a 2nd one if pasting in your reference site. Stan

Edited by - Stan. Swan on 10/2/2005 5:56:27 PM
 

BeanieBots

Moderator
Without knowing the relay coil resistance it is hard to say. 47u is a little small and you should have at least 10u on the other side of the regulator. Putting a resistor in series with regultor input is a good idea if it does get too hot.
Alternatively, just use a series resistor. Its value should be about half that of the coil resistance. Once you know the total resistance, you will be able to calculate how much power it will dissipate.
 

Michael 2727

Senior Member
I did ad you might like to put a 0.1&#181;F mono cap on the 7805 input and a 10&#181;F on the output and 0.1&#181;F monocap on the picaxe supply rails.

Looks like the Edit didn't work.

This can be a very deceptive circuit to work with, I have experience.


 

adub

New Member
I think I have all the additions added to the schematic. See=&gt;<A href='http://freewebs.com/adub/furnace%20circuit.gif' Target=_Blank>External Web Link</a>

Anyting else?
Arvin
 

adub

New Member
My external link still doesn't work for me.
Could it be the %20 for the space in the name?
Begin to see why some people don't use this feature.

http://freewebs.com/adub/furnace%20circuit.gif

Arvin
 

premelec

Senior Member
The circuit looks OK to me - one thing to check that may or may not be a problem is if one side of the 24VAC transformer is electrically connected to 'ground' or anywhere else. The bridge rectifier configuration sort of implies a floating downstream circuit IF one side of the xfmr is connected elsewhere... in your case you could easily get around that using a single diode instead of a bridge rectifier as you have low current and way excess voltage...
 

adub

New Member
I'm sure the transformer is connected to ground. It will be the wire from the switch on the thermostat that's powered from the transformer that is used with the furnace.

A single diode changes 24vac into pulsating dc. I didn't think that would work for the pic.

It would be a simpler circuit if I had a 5v relay instead of the 12v that's in my box of goodies. Didn't want to spend two dollars more to test something that is possibly only an urban legend. Does the fan cycling on/off really spread the heat from a wood stove to the rest of the house?

Could probably do the same thing with a 555 timer too.

Arvin
 

Michael 2727

Senior Member
You may still need the 7812 even if you have
a 5V Relay, you can't get something for nothing.

when the voltage is reduced, a lot of heat is created it has to go somewhere.

The idea of the resistors in My suggestion
are there to split the heat difference
between the regulator and the resistors.

As I said this can be a very deceptive circuit. Basically 24V AC is not a very good voltage to work with using 7XXX regulators.

Good luck again.
 

adub

New Member
The project, if it gets to the soldered stage, will be housed in the closet with the furnace and use the power from the 24vac transformer that powers the thermostat.

I could probably scrounge up a 12 volt wall wart. Would that be the best way?

Arvin
 

Michael 2727

Senior Member
You will still need the capacitors even if you use a lower voltage Wallwart.

The 24V AC transformer can be successfully used as long as you always have 2-3 Volts
input on the regulator higher than the output at full load, (with some to spare).

Any heat dissipated by a resistor is heat the regulator doesn't have to.

Try a resistor in the range of 27 to 68 Ohms
10Watt or 2 x 5Watt wire wound which still
allows enough voltage and current at full load.
Try and split the heat 50/50 resistor/regulator

 

premelec

Senior Member
With the low current you need for the AXE and relay a half wave single diode into a 1000uf 35VDC [24x1.414 = 34 volts DC possible] capacitor then regulated by a 12 regulator from which a 5 volt regulator is connected [as in you diagram] should be fine.

You can add resistance to dissipate some of the voltage if necessary [and I'm not sure that the common 12v 3 term regulator takes 35 v input ok so check that - ] and possibly a zener diode shunt reglulator - a wall wart is fine but also consider what happens if your wall wart quits - does the house freeze? I've seen a lot of wall warts quit...
 

adub

New Member
Thought I'd give the external web link one more try. I am persistent.

<A href='http://www.freewebs.com/adub/furnace-circuit.GIF' Target=_Blank>External Web Link</a>

Arvin
 

BeanieBots

Moderator
Your link works and has let me know that all important relay resistance.
You should not have the 200R in series with the relay if you are going to use the 12v regulator. Its one OR the other. If you dispense with 12v regulator, the resistor is required to reduce the current in the relay to close to what it would see if driven from 12v. It will also need to dissipate close to 1W and will get quite hot.
Alternatively, keep the 12v regulator and put the resistor in series with its input (before the cap) to help dissipate the heat that would otherwise be generated soley within the regulator.
Other than that, all looks well.
 

adub

New Member
AAAhhh. Now I begin to understand. The diode would make pulsating DC, the capacitor smooths it out, the resistor drops the voltage. Am I close?

So I can eliminate the 12v reg by using a diode+cap+resistor. Sound right?

Glad the link worked. I, of course, was using the wrong url.

Makes my graphic look very strange. Now do I have a working schematic?
<A href='http://www.freewebs.com/adub/furnace-circuit-sans-recitifier.GIF' Target=_Blank>External Web Link</a>
Arvin

 

BeanieBots

Moderator
Yes, you've got it. I'd be very surprised if that circuit did not work. The only area of concern is the exact value of the 200R which depends on the exact voltage of your 24vAC and how much current your relay pulls when on. It is not a very critical value because neither the relay nor the 7805 are very fussy about what they are driven with. Once constructed, test the voltage across the 1000u cap. It should be around 30-35v with the realy off and about 12v with it on. If the 12v is low, decrease the 200R. If it is high, then increase the 200R. Anyware between about 10.5v and 14v should be OK.
 

Michael 2727

Senior Member
Getting close arvin.
I don't know what input voltage is produced
using only the single doide and 200R as I've
always used the bridge, but as long as you
have the 2 to 3 volts above the output on
the input of the regulator it should fine.

Try to keep the 0.1&#181;F and 10.0&#181;F cap very
close to the regulator for best results,
even the 1000&#181;F.

I would swap the diode and 200R positions, just my preference.
And the TIP 31 may be overkill, try BC337 etc. The 270R could be higher maybe.
Look at the &quot; picaxe_manual3.pdf &quot; for
inspiration.

As long as you can't burn your finger on
the regulator you may be in business.
 

adub

New Member
Looks like a go. I'll probably get to work on it this weekend. Will have to buy a diode for the input and the wire wound resistor.

The TIP31 was in the bottom of the shoebox with the 7805. Did find a 50ohm/10watt resistor in the bottom too. All at least 25 years old from a time when I was experimenting on a little robot I was working on. Now I'm back and things have changed. Built the robot using an oopic-R board. Thought I had destroyed it and bought another one before I found out about the great little picaxe!

The relay is new but I bought the 12v instead of the 5v. Coil current is supposed to be 200mA so could I use a 2n3904 or 2n3906 instead of the TIP31. Have them too.

The 24vac is more like 20.6vac.

Thanks all.
 

premelec

Senior Member
WOW! 200 ma relay coil current?? .2a x 12v = 2.4 watts. Is that a huge relay?? You could use a solid state relay at about .01A drive...
Anyhow it seems like you are getting to the answer!

BTW If the relay is actually .2a coil you need to do a heavy spike suppression diode and I'd use the hard core TIP not the little transistors.... unless you like 3 terminal resistors [shorted xistor] :) I haven't looked at the Safe Operating Area curves for those little units but it's worth checking...

Edited by - premelec on 10/6/2005 5:20:28 PM
 

BeanieBots

Moderator
I too thought 200mA was quite a beast!
Your diagram shows coil R=358 ohm.
This would pull about 30mA @ 12v.
Where does 200mA come from?

If it really does pull 200mA then you stand no chance of driving it with that circuit. You would need to loose the 200R series resistor and replace it with 50R rated for at least 5W. The net DC through the transformer might also be an issue with a half wave rectifier.
 

Michael 2727

Senior Member
Yes w0w @ 200mA.
Most of the common PCB type relays only draw
20 to 40 mA.
If you are using a motor starter type relay
or just a big thumper of a relay this changes everything.

Solid State Relays are good value.
They can be driven directly from a Picaxe and a rurrent limiting resistor (say 330 to 470 Ohm) 3Amp PBC models are available for small loads right up to 40Amp Zero switching
models, which I use on a 240V 10A fan heater.


They said Rome wasn't built in a day.
Wonder how long it took to get the central heating just right,?


 

adub

New Member
Make that coil current of 33.3 mA

Don't know where that 200 came from. The 200 was why I had the TIP31 in there. So I could use a much lower rated transistor like the 2n3904.

Woke every one up on that one, didn't I.

Thanks
 
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