Replace AC latching relay with Mosfet

Thread Starter

igeorge

Joined Jul 2, 2010
41
I want to replace the latching relays in a thermostat with mosfet
I found on the web a sugestion, but i am not sure if it is OK
At the furnace it is a 24VAC transformer
One wire come to the thermostat and return by closing the contact in one of the 3 relay
I am not sure if it is correct to connect the CPU ground with the wire coming from the thermostat
Please help
 

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inwo

Joined Nov 7, 2013
2,419
The original thermostat circuit is not referenced to ground.
It is an isolated circuit unless accidentally grounded.

The only connection to original wiring is thru bridge as shown.
 

Thread Starter

igeorge

Joined Jul 2, 2010
41
I agree, but how i can turn the mosfet on from PIC ?
Any suggestion ?
Using optocoupler is not a solution due to very large internal led current 20 mA. Will kill my battery in days.
Dedicated mosfet drivers are too expensive and no room left on the board
The whole budget for thermostat is 20 dollars, which include the hardware, lcd and pcb
Can i connect the ground of mosfets to the ground of cpu ?
If yes, how i can protect the cpu side ?
 

inwo

Joined Nov 7, 2013
2,419
How about a latching circuit with a MOC3023 (triac out) in stead of the bridge?

High pulse latches on.
Low pulse latches off.
Updated per your requirements.

Needs a few ma. But for a short duty cycle.

ps.
Are you sure you need "protection" on ground?
How about a high impedance ground connection?
 

inwo

Joined Nov 7, 2013
2,419
I agree, but how i can turn the mosfet on from PIC ?
Any suggestion ?
Using optocoupler is not a solution due to very large internal led current 20 mA. Will kill my battery in days.
Dedicated mosfet drivers are too expensive and no room left on the board
The whole budget for thermostat is 20 dollars, which include the hardware, lcd and pcb
Can i connect the ground of mosfets to the ground of cpu ? Yes
If yes, how i can protect the cpu side ? 10k enough isolation?
High value resistor maybe?:confused:
 

Thread Starter

igeorge

Joined Jul 2, 2010
41
Thank you Inwo for update
As you can see, analog it is a virgin territory for me
First i try to explain that the whole circuit, without the driving part is 2-3 µA.
mA it is a huge consumption. The system run on 3XAA batteries which needs to last at least a year. Then, when the main is gone, it is kept alive by a cr2032, which must last for at least 2 month ( assuming that is nobody around to see the LCD blank and the battery icon on).
On cr2032 , the system is in deep sleep and update itself 4 times per hours in the winter ( to keep the house warm and prevent the pipes freezing)
In the summer, it just stay in sleep, waiting for battery to be replaced.
As i was told, with mosfet is almost no consumption - but i am not sure as i do not know the.
Do you have a hand drawing with a proposed solution ?
Thanks
 

inwo

Joined Nov 7, 2013
2,419
Actually I like your idea.:)

I can't see a problem with a common ground can you?

Seems if you are concerned. Connect the driver thru the 10k as shown, and the grd thru a 10k also.

The .1 cap across the input as shown should be a good start. In another thread a protective zener from gate to ground was suggested.

If we keep this thread current someone who knows a lot more than I will opine.
 

Thread Starter

igeorge

Joined Jul 2, 2010
41
In fact is not my idea.
I did search and read so much lately, and i cannot remember were i saw it first time
All i did is to implement it instead of each relay
I hope too, that someone which is a guru on analog can help me
 

cravenhaven

Joined Nov 17, 2011
34
If you used another bridge from the 24VAC just to power the micro (filtered and conditioned to 5/3V) then forget the battery. It would be a very small power requirement.
 

Thread Starter

igeorge

Joined Jul 2, 2010
41
Thanks cravenheaven, but this is not possible.
I am looking to make a commercial product which does not require user rewiring the house.
Just use the existing wiring and replace an old thermostat
Sorry but i cannot use your suggestion even is excellent
 

inwo

Joined Nov 7, 2013
2,419
If you used another bridge from the 24VAC just to power the micro (filtered and conditioned to 5/3V) then forget the battery. It would be a very small power requirement.
That makes so much sense. Although:
Design may have to be universal. Not all thermostats have the "C" terminal brought to the wall.

What OP is showing as "C" is really "red". The transformer phase, returned to the furnace, to power relays. The other phase, common to the relays is not allways available.

Op should let us know what requirements are cut in stone.

Anyone see a problem with a common ground (common)?
Seems fine to me.
And of course with the above suggestion it will be "common ground" unless a 24vac transformer is in the picture.
 

Alec_t

Joined Sep 17, 2013
11,158
Sorry but i cannot use your suggestion even is excellent
You should be able to draw a small current via the FET drain terminals without activating the loads that would normally be switched by the thermostats. That current can charge a cap to provide a supply voltage for your electronics. A diode OR arrangement would enable a back-up battery to take over only in the event of mains failure.
I don't see a problem with a common ground either, if the transformer provides mains isolation.

Edit: If you want a battery-power-only circuit, bear in mind that when a load is switched on its FET will require a gate charge current pulse twice per mains cycle; which could add up to a significant current drain.

Edit: Scrub that edit.
 
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Thread Starter

igeorge

Joined Jul 2, 2010
41
Hi Alec,
On your post you developed 2 scenarios.
The first , is stealing the power from the wire going to drain . do you have a link, drawing or a hand sketch how to do it ?
The second you said about gate using power while fet is on
I was under the impression that this current is in the range of micro or nano Amps. Am i wrong in my assumptions ?
Maybe fet is not the right solutions and i have to go back to relays, which will kill the project due to cost.
Let's resume why i want to make a thermostat. Everybody make them
Even more advanced with WIFI and web enabled control from office or from Iphone. They are better than mine. The only advantage for mine is the cost. I use only 1 microprocessor , LCD, and few components around (r and c). My goal is to make it under 20 dollars my cost and lower if i manufacture it in China in mass production (maybe 10 to 13 dollars)
At this price i can try to sell it to home builder companies which can install it in new homes. Mine has through the software all the features of the top of the lines thermostat, but at a fraction of cost.
But i cannot ask the home builder to change in his project wiring, because they have standard approved drawing and any change will require new approval, time and cost. So i have to make it universal for standard or most common 4 wires system
 

inwo

Joined Nov 7, 2013
2,419
Do you have extra io available?

Getting back to my latching idea.
Output-1=off
Output-2=on
etc.

No I don't have a drawing. Only idea.
Latch is only thing not drawing current from supply.

Alec_t may be able to estimate fet gate current for you. I'm no fet guy! :(
Seems the right device could be really low.

If it is. Use common ground and you're in.:)

Here is lowest isolated fet driver I could find. http://cds.linear.com/docs/en/datasheet/1177f.pdf

Still a half ma. though.

On another thought:
Somewhere I have a drawing for a flip-flop scr latch. If it could stay latched thru full cycle of ac......................................
 

Thread Starter

igeorge

Joined Jul 2, 2010
41
Hi inwo,
the chip looks good, but i have to contact linear Monday for a replacement
It is obsolete, and nobody sell it
 

Alec_t

Joined Sep 17, 2013
11,158
Forget having to charge/discharge the FET gate capacitance twice per cycle: I didn't have my thinking cap on straight :rolleyes:.
What voltage does the FET have to switch/withstand? Finding a FET with a high voltage rating and low Rds_on may be difficult and/or expensive (more expensive than a latching relay).
 
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Alec_t

Joined Sep 17, 2013
11,158
110VAC has a peak voltage of approx 155V, so IMO you'd need a FET rated for >> 200V. You'd also have to allow for spikes/surges on the mains which could exceed that, so spike suppression components would have to be added. Relays look to be the cheaper option :).
 
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