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I need to replace relays on a Thermostat with Mosfet

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I know is just control signal because on the board they are some diodes on each return wire and a cap

If they rectify it at the end anyway, you probably can get away with sending them a half-wave only. Just put a diode on the COM wire and then treat it as DC. This will save lots of board space, but it won't be universal and won't work on old furnaces.

With AC, you need to reference your battery supply to something, say COM. The biggest problem is that when COM goes positive, your mosfets will be negative relative to COM, and consequently negative relative to their gates. So, they will start turning on. You need to prevent this somehow and also protect your controller from this negative voltage. I don't see a good way to do that with diodes. Perhaps the capacitors in the original design will charge so slow that the mosfet will not have enough time to turn on during this half of the cycle. In this case you just don't do anything extra and it simply works. However, with almost no load, mosfet will turn on very easily. You need to experiment with this.

Make it simple, i need the features on mine, without relays : contact 120 vac / 2 amps

The fan probably needs more than 2A, especially when starting. It would be tough to switch even if you had an access to the second AC line, yet alone under $6.
 
are you trying to make something that will be sold or just something for yourself, here's another try which is different form the others. not sure what will happen with more than one fet on at a time, could get a cross connect problem.
 

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last comment, gotta go visit
 

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NorthGuy said:
The fan probably needs more than 2A, especially when starting. It would be tough to switch even if you had an access to the second AC line, yet alone under $6.

Way back when, some 50-60 years ago, furnaces consisted of a "Fan Center" http://customer.honeywell.com/en-US/pages/product.aspx?cat=HonECC Catalog&pid=R8239A1052/U which is nothing but a "Definite Purpose Contactor" and a transformer.

The "Definate Purpose Contactor" would be the "Relay" that turned the fan on. The 120 VAC at 2A is a contact rating that might turn on a "contactor" such as this one: http://www.pexsupply.com/Honeywell-...ctor-30-amps?gclid=COer44HPs7sCFag7OgodB1QAXg

You CANNOT use a 2A rated relay to turn on a 1/3 to 1/2 HP motor. Modern controllers may just use an optoisolator to detect the presence of 24 VAC to the furnace microprocessor and then use some very wacky control signals to run a 3-phase motor on single phase. i.e. Variable Speed Drive.

There is a LOT of misconceptions here. On another forum, one of my titles was an HVAC expert.
 
Way back when, some 50-60 years ago.

I still have a "mechanical" furnace heating my garage, although it probably won't live much longer. The termostat only controls the gas valve. The furnace lights up when it is turned on, and the fire stops when the termostat turns off. I have a custom-made termostat which uses 5A relay to turn it on and off. There's a thermo-mechanical switch insie the furnace, which turns fan on when the furnace gets hot inside and turns it off when it cools down. You cannot control fan from the thermostat.

My home furnace, which is Lennox, is completely different - all electronics, two-level termostat control, support for air conditioning and dehumidifier. I don't think there ever be furnaces where thermostats are switching any meaningful current.
 
OK, this last post now seems like you understand what's going on. The post I quoted, was made by a different person or persona?
I sort of mis-understood the post I was responding too. OP was going to make his system universal, so that it could switch 2A@120VAC. I assumed that since it's 120VAC, he wants to be able to switch the fan directly.
 
Given that most furnace transformers are 40 VA and 24 V, 40/24 Amps is about 2 A. So, 3A is not a bad rating to use at all for a contact. The contact could basically switch a shorted relay winding and have the fuse blow with no damage to the relays.

40 VA was enough, and power to spare to activate a fan, heat relay and gas valve or a fan and compressor relay.

The OP is building a thermostat or should we say multiple thermostats in a zoned system and wants something small and cheap.

So, let's revisit the original problem:

A small and cheap replacement for a 24 VAC relay for use in a typical heating/cooling thermostat where the intention is building a thermostat from scratch.

I'd look at the OPTOMOS series of relays: https://www.digikey.com/product-search/en/relays/solid-state-relays/1048664?k=optomos

Now, they could be a little power hungry, but some require currents on the order of a mA to switch, so one might not be able to pass the power budget.

Call 4 batteries, $5.00 and the device to add a wire $25.00. Payback is 5 years. Without batteries (OK, maybe a lithium or supercap), the unit is smaller, but you won't have heat if you don't have an extra set of batteries. More savings could result if you could run the extra wire in some places.
 
Another idea would be to build everything inside the furnace. My Lennox has plenty of space, others should too. Then you can use thermostat wiring simply for communications. It'll be cheap and won't take space on the thermostat's PCB.
 
Easy to say but it cannot be done
This is a product i want to make and sell it, so user cannot go to install anything on the furnace :(
 
Easy to say but it cannot be done
This is a product i want to make and sell it, so user cannot go to install anything on the furnace :(

I see. This changes things. You must be wire-for-wire compatible with regular thermostats, and you cannot even do trick things like using electrical ground.

I would go with optoisolators, what KISS suggested or similar. It'll be less space and money that the original circuit.

I do think that the original circuit should work if capacitors are large enough. It is definitely cute. What I don't like is that large capacitors will also lead to long[ish] transition periods.
 
You are perfectly right
The KISS suggestion is great , but i cannot use it on this project
1. even at 0.11ma it is still a constant consumption
2.at 4.4 dollars per piece is 0.9$ higher than one coil latching from TE Connectivity
But they are beautiful and i intend to use them in other projects for future
 
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