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Replace latching relay with Mosfet

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igeorge688

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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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I presume that you don't actually mean latching relays.

It's not O.K. to use MOSFETs with ground conncted source. This way you are shorting one of the bridge rectifier diodes. It's hard to implement this or a similar circuit without opto couplers.

Using triacs with triac opto couplers would be a standard solution for the problem. They can work quite well for voltages down to 24 VAC.
 

FvM said:
I presume that you don't actually mean latching relays.

It's not O.K. to use MOSFETs with ground conncted source. This way you are shorting one of the bridge rectifier diodes. It's hard to implement this or a similar circuit without opto couplers.

Using triacs with triac opto couplers would be a standard solution for the problem. They can work quite well for voltages down to 24 VAC.
Click to expand...
The existing relays in the house thermostat are latching relay to minimize the current and extend battery life
For me , analog is like the Amazonian jungle. I got lost there.
Opto coupler i cannot use on this circuit as they consume too much current ≈ 20 mA. As in this case , the triac will stay on during the heat or cool request from thermostat, my battery will be killed in a week or so.
The battery needs to last a year or more.
Can you suggest a practical example to look at it ?
 

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Why do you need a battery when you have 24 VAC supply?
 

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I think, in this case you better stay with the latching relays.
 

Are latching relays used for the three boxes in your control unit diagram? If so, why are they latching? I see no battery or intermittent coil control that would necessitate latching relays.
 
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Because my whole system run on 3XAA batteries, and i need to save power
 

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