Sorry about the scratchy drawing. I don't like European boxes for resistors, I prefer the American zigzags.

The 0.1 microfarad peak detector capacitor has a reactance of around 30K at 50 Hz. So any resistor greater than 300k will not interfere with the working pulse but provide a discharge path when the 24 volts dc is removed.

We have already been told that this level of timing error is acceptable.

 

Bertus,

I have not received the parts yet, but I was reviewing your diagram and have a question. If I connect the + end of the thermostat voltage to the diode bridge and the - end to the other AC pin of the bridge, will that short out the thermostat?

The - end of the thermostat is the ground connection. The + end is the pin that actually turns on the heat. When there is +24VAC on the + pin, then the heat goes on. If I run another wire off the + pin of the thermostat and connect that through the diode bridge to the ground pin of the thermostat, will that short it out? Do I need a resistor or something in there? Will that effect the heat going on?

Thanks!
David

 

Hello David,

An AC source does not have an + and - connection.
It can be that the - sign is used to show the "cold" side, that is possibly grounded.
The + sign is used for the "hot" side in this case.

The circuit I have drawn can be connected to the AC voltage on the bridge input.
The optocoupler will isolate the heating system from the microcontroller.

Greetings,
Bertus

 

Hi Bertus,

I have built the circuit and in my testing it works perfectly! I do have a question though. Is it true that with this circuit it is bascially like connecting a wire from the +24 to the common of the thermostat? I spoke with the HVAC installer and he said that could short out the circuit board or damage the controls.

Is there a way to build this circuit without having to "close the loop" back to the thermostat? i.e. not use the common terminal on the thermostat. There should be no risk of short circuit or damage if I just read off the 24V AC line.

Thanks for your help!
David

 

Hello,

Do you know the specs of the heating system ?
How much current can the 24 Volts deliver ?
When I know this I can adapt the circuit for it.

Greetings,
Bertus

 

Hi Bertus. The heating system runs of 240VAC. I am pretty sure the 24 volt line just triggers a relay or something in the heating system. The HVAC installer was more concerned with any damage to the thermostat - which I am pretty sure would run on pretty low current.

I used to live in Amstelveen by the way. Belle Van Zuylenlaan off Zonenstein tram stop. I went to high scool at the International School of Amsterdam.

Thanks!
David

 

Hello,

The circuit I have drawn will take about 20 - 25 mA and not give back any voltage or current.
The thermostat will have a contact closure circuit that probably drives the relays you are mentioning.

Greetings,
Bertus

 

Hi Bertus,

I did not see a circuit diagram attached.

David

 

Hello,

I referred to the circuit you have build.

Greetings,
Bertus

 

Oh ok. So the circuit I built will not have "short out" the thermostat. Great. I will hook it up then.

Thanks for all your help!

 

I hope you bought a higher voltage capacitor than 63 volts, as I suggested back along, or it will soon fail.

 

Hi studiot. Indeed I did. I bought a 100v rated capacitor for the job. I am hooking it all up right now. Will let you know later today how it works. Thanks both of you for your help. This is an excellent forum for getting answers.

David

 

I'm glad Bertus' circuit worked out for you.

You should realise that if the 1k5 resistor failed short circuit this would place a dead short across your thermostat terminals, to earth if one is earthed.
For protection I would palce a 100 milliamp fuse in series with the resistor.

 

Hello,

I think the fuse can also be placed at the input (one of the AC sides of the bridge rectifier).
This would also protect the thermostat in case of a failure of the bridge.

Greetings,
Bertus

 

Yes indeed in series before the bridge.

If you have trouble sourcing a 100 mA fuse I expect a 250 would do.

 

Guys,
Why do you complicate things?
A simple relay with a 24VAC coil will do the job. Also, it offers isolation. A problem which may occur is the bouncing of the contacts when the relay turns on/off. This will be a problem because the OP wants to measure the times the thermostat energizes and bouncing of the contacts will fake the PIC that the thermostat has been energized for more times. This can easily be solved in software rather in hardware by the addition of little code.

 

Home depot have these devices cost $22 a PC cost what $200 [cheap] it uses 200watts even 100watts where is the cost savings. $22 forever or the cost of 100-200-watts forever.