[SOLVED] on/off triac at full power circuit

[igeorge]

Hello,
I need help to find a circuit to turn on a triac with just a pulse, and a way to turn it off with another pulse
I am using now some latching relays with two coil, but they take too much current -30 mA each time
This application is for a thermostat
From main HVAC i have 28VAC comming in one wire, the common for all ( Red wire) , and has to return to the control on 3 diferent way
1 for heat
2 for cool
3 for air on all the times.
I do not know what is in the HVAC control, but my house thermostat has 3 latching relays
I designed a thermostat, battery operated, so i need the minimum current consumtion.
The relays, take 30 mA each time are Set or Reset
It is two much, so i need the lowest current use from the battery. Moc optocouplers also take 30 mA , and the have to be on all the time, so out of question
Please sugest some reading or schematic
Thank you

 

[shatruddha]

I think you can go ahead with mosfets

 

[igeorge]

Can you please show me a schematic to understand.
I saw some schematics, but you must keep the controll line ON all the time , meaning it is consuming current
Others, they use optocoupler with mosfet, but again, it is a led which take 30 ma
Can you show me some schematic how to send just a pulse to the mosfet, and keep it on ?
How much is the gate current for a mosfet.
If it is under 1 miliamp , i maybe keep it on all the time
What should be the voltage on the gate to turn ON ?
My PIC send out 3.3 volts
Thank you

 

[shatruddha]

Well You have a triac right? So all yo u need to do is, control the 'G' of the triac. Can you tell me the voltage required on 'G' for control action?

 

[igeorge]

That is the problem. I do not have a triac
I just have the PIC and want to add something to control the 28VAC from the red wire to the others
Because it is a portable one, does not have a power suply
Just battery to run on it
The voltage comming from HVAC is just one wire. No ground
Can you post something with mosfets ?

 

[shatruddha]

I'm not sure, but may be with something like this you will be able to achieve your objective. I'm not sure though, this needs to be tried.

 

[igeorge]

Thanks, but what is controling the A and B ?
How much is the gate current ?
Do you have a better or full schematic ?
Thanks

 

[shatruddha]

A and B will be controlled by your microcontroller. Its your choice if you want to connect them together and have a single control or have control them seperately, But as per your application, I think a single control will do.
Mosfets are voltage controlled devices, so it will draw very less current, only the current required to charge the gate drain capacitance. This value depends upon the mosfet you choose.

 

[igeorge]

Will you be so kind and give me same sample schematics and a sugestion for mosfet ?
The voltage is 24 VAC and the current is under 3 amps
Thanks

 

[tpetar]

Hello,
I need help to find a circuit to turn on a triac with just a pulse, and a way to turn it off with another pulse
I am using now some latching relays with two coil, but they take too much current -30 mA each time
This application is for a thermostat
From main HVAC i have 28VAC comming in one wire, the common for all ( Red wire) , and has to return to the control on 3 diferent way
1 for heat
2 for cool
3 for air on all the times.
I do not know what is in the HVAC control, but my house thermostat has 3 latching relays
I designed a thermostat, battery operated, so i need the minimum current consumtion.
The relays, take 30 mA each time are Set or Reset
It is two much, so i need the lowest current use from the battery. Moc optocouplers also take 30 mA , and the have to be on all the time, so out of question
Please sugest some reading or schematic
Thank you

You can use impulse relay. This type of relay uses power only when changing state of contacts.

Existing EDABoard thread:
https://www.edaboard.com/threads/264375/



Best regards,
Peter

;-)

 

[igeorge]

Thank you Peter,
I did get some Latching relays, or as you call them impulse relay
It is still takeing a burst of 20 miliamps when it switch
I was looking to find something which can take less than ONE milliamp to switch
Any sugestion ?

 

[tpetar]

Yes, they use current only when switching, this is short moment and its small amount of energy. Enough energy is needed to move lever and disconnect contacts.

I'm not sure for triac, you will lose more energy on triac than on impulse relay switching.

Triac need current for gate to work. Low current triacs have 5-10mA and higher currents triacs needs 30-50-100-150mA current for gate. That gate current must be supplied whole time during triac work.

Plus triac dissipate power, and this can be from few watts to 60W.

Did I mention heatsink for triac cooling ?



Best regards,
Peter

;-)

 

[igeorge]

Thank you all for help
I went back to latching relays as sugested by Peter
I found 3 volts coil at 11 mA from panasonic
The time to close or open is 5 msec, so not too much energy lost
Thanks again