By way of comparison with those relays, Farnell lists a 200V 18A (max) NFET at ~$4, and a 400V 10A one at ~$6. Heatsinks might be necessary for those, depending on load.

 

Standard control voltage is 24vac rms minus the bridge.
Rarely over 40 va transformer. So 2 amps is conservative for fet.

Standard fan relay draws 350ma seated.
12 ohms dc resistance to estimate inrush.

Can't put my hands on an AC relay right now. Est. to be double the fan relay.

 

Hi inwo,
the chip looks good, but i have to contact linear Monday for a replacement
It is obsolete, and nobody sell it

Sorry, didn't research further as I thought a ma was too much for you.

 

Not sure I'm interpreting this right.

http://www.components.omron.com/components/web/pdflib.nsf/0/6E5CD26722AA549D86257AD000600DBC/$file/G3VM_41AY_DY_1212.pdf

This is an isolated 2amp mosfet. With typical trigger of .3ma.
Max is listed @ 2ma. however.

 

outch ..... 4 dollars at digikey in quantity of 400
good solution but very expensive
I noticed , that any product with low consumption is expensive
Same for relay.

 

i found this optocoupler at digikey for .85 dollars 4 channels, 1 mA per channel
http://www.digikey.ca/product-detail/en/LTV-847S/160-1371-5-ND/385841
transistor output, but now i have to figure out how to drive the fets from it
I saw some suggestion to put two fets in series for ac, but i have no clue how to do it with these opto
back to reading the web

 

I'm full of ideas, but mostly just full of it.

If you don't mind I'll keep throwing stuff on the wall.

Found my scr latching circuits.
Don't think you mentioned having extra io for discrete on-off control.

 

I'm getting lost.
Where are we now?

1ma per channel while on is ok?

Common ground is ok?

Here is some ac mosfet examples.

 

or this one at mouser at 10 µA forward current for diode
http://ca.mouser.com/ProductDetail/IXYS/LDA111/?qs=lQlU%252bwB40gu1I42JkIJeBg%3d%3d
Yes i do have I/O extra as the original circuit was done with latching relays

 

Hi inwo,
I will try Monday one of your circuits with a regular optocoupler
Using 2 fets in series i do not have to use the bridge rectifier, so will be no connection between the control to wires from furnace, nor the ground.
I can leave with 10µA constant current taken by optocoupler
I will redraw the schematic and post it later
Thank for help to you and Alec

 

Hi inwo,
I will try Monday one of your circuits with a regular optocoupler
Using 2 fets in series i do not have to use the bridge rectifier, so will be no connection between the control to wires from furnace, nor the ground. I don't think that will work. The gate drive will still have to come from battery side.
I can leave with 10µA constant current taken by optocoupler
I will redraw the schematic and post it later
Thank for help to you and Alec

From what I've read, the gate voltage requirements for the ac-fets are quite high. That's why they often drive with led thru optic device.

.............................................................................................
Re-designing normally-on load switches with zero-power
MOSFETs reduces power consumption

http://www.aldinc.com/pdf/ZeroPowerNormallyONSwitch.pdf

 

i am talking about your design sent before
The fets are just ordinary DC fets which in series can handle AC. Maybe i am wrong

 

I have to go to pick up my wife from the mall, but i will redo the schematic for one so you can see what i am talking

 

i wonder if this is correct ?
The opto i just pick up one from library, is not the one i want to test

 

Where is vdd+ coming from?

 

it is from my power supply for the system 3.3Vdc

 

if we make it, maybe we retire both of us on a tropical island, no internet, no FedEx, no IRS, no Canada revenue, and no digikey or radio shack

 

it is from my power supply for the system 3.3Vdc

That's what I thought.
Then no need for the isolator, as the battery side and ac side are connected.

Now you are back to the original circuit with grounds connected.
Better common ground than common supply.

Could put a diode in there, but still not "isolated". imho

As far as retiring. Haven't given an opinion on this being competitive.
It just fit with my HVAC side.

I buy electronic thermostats all day for less than $20.

 

like this ?
http://www.homedepot.ca/product/wi-fi-programmable-touchscreen-thermostat/807070
mine except the wifi which is separated has all the features and more

 

No, cheap ones.

You set you sights high.

My supply is limited.

Looks like you've done your research.

Have you reverse engineered any?