Yet another HVAC monitor question

[Timoh]

Glad you found a solution. As someone pointed out to me on another board, a hall effect sensor output would need to be rectified as well.

http://www.appdigusers.com/forums/ubbthrea...3279/#Post19421

I'm still look at hall effect sensors for my "intelligent" power bar. Using a current transformer would be tough because I need to be able to measure from a fraction of an amp up to 10A+. I gather this is hard to do with a current transformer since you need to tune your resistor and wraps to the current to get the voltage you desire on your output.

Tim

 

[SethP]

I ordered a Hall effect sensor as well, but it hasn't arrived yet. I didn't think about the fact that we're monitoring A/C, so I'm not sure if what I ordered will even work. Definitely let me know how your setup works out.

Seth

Glad you found a solution. As someone pointed out to me on another board, a hall effect sensor output would need to be rectified as well.

http://www.appdigusers.com/forums/ubbthrea...3279/#Post19421

I'm still look at hall effect sensors for my "intelligent" power bar. Using a current transformer would be tough because I need to be able to measure from a fraction of an amp up to 10A+. I gather this is hard to do with a current transformer since you need to tune your resistor and wraps to the current to get the voltage you desire on your output.

Tim

 

[Timoh]

Will be ordering today...
It looks like the ACS713, which is unidirectional version of ACS712 (bidirectional/AC), will work better. Since it only senses current in 1-direction, it will only output 1/2 the AC waveform, so no rectification needed. The other advantage is it looks like you get a greater range on the unidirectional/DC version.

On the AC version, you get 2.5v at 0A. ~0.5 at -10A and ~4.5 at +10A. Since we're only measuring forward current, we have 2.5v - 4.5v as a usuable range.

On the DC version, the range is still 20A, but instead of -10A - +10A, it goes from 0A - 20A, with 0A being ~0.5v. So you you get a much better resolution.

For simple on-off, it might not be an issue.

Will keep you posted.

Tim

 

[sda]

Yet another way to monitor 24VAC ... Tap the 24VAC signal wire (i.e heat/cool/fan). The voltage between the signal wire and the common wire will be 24VAC. Pass that through a full wave rectifier, 100uf cap, 7824 regulator, and a 24VDC relay. This keeps the HVAC voltage on the coil side isolated from whatever needs to know the state on the contact side. For 1wire, on the contact side of the relay you could use a 10k pullup to +5 sinked to ground when energized and feed that into a DS2405/DS2406. The relay I used was a Hamlin HE721A2400 from Digikey, its in a 14DIP package and has a built in diode. If anyone wants a circuit diagram let me know.

 

[Timoh]

What about lack of access to the common?
As was the case here. You essentially have a "runner" leg to the thermostat. With one side 24vac and the other going to the device/relay. The 24vac tranformers are not grounded so you can't use the ground as a common.
Tim

 

[SethP]

Exactly right. Plus, I think that the current transformer is a much simpler circuit. I picked up round bridge rectifiers at Radio Shack with long legs, and soldered the capacitor and resistor inside the legs, and put heat shrink tubing over it, so it's a very compact package. You could replace the resistor with a multi-turn 50k or 100k pot and have total flexibility.

Here's a pic:

That's on the hot water furnace. The white & black wires come off the CR3110, and at the end is the heat shrink with the components, and a connection lead. Follow that to the two red crimp connectors, and the wire runs directly to the DS2406 input board:

The wiring is a work in progress, but it's all working very nicely.

Seth

What about lack of access to the common?
As was the case here. You essentially have a "runner" leg to the thermostat. With one side 24vac and the other going to the device/relay. The 24vac tranformers are not grounded so you can't use the ground as a common.
Tim

 

[sda]

I use the Hobby Boards HVAC monitor for my 1wire HVAC monitoring and its been working great, but I have easy access to a common. I was just pointing out a different way to do it. This project is for a PICAXE but could be easily adapted to 1wire. The cost of one input using the HB HVAC monitor is about $7.50 which includes the 1wire interface, the cost of one input using a relay is less than $10 w/o the interface, and you've stated the cost of one input using the CR3110 is about $17 ($15 for just the CR3110) w/o the interface. The CT and relay methods are essentially doing the same thing, converting AC to DC, with almost the same components. You ran a wire to the remote stat to send the signal back to your main board. You could just have easily sent the 24vac on that wire (a "runner" back to your panel) pulled a common from your transformer and centralized everything. You used a split core CR3110 while you could have used a smaller and cheaper solid core like a CR8401 or CR8410. Just remember that electronics projects are like politicans, everybody's thinks they've got the best answer for you and we all know that's poo.


Board is 2"x3". Left side terminals are from HVAC, right side terminals go to PICAXE. Top 3 rows are HVAC input circuits. Rectifier, 100uf cap, 7824 regulator, 24vdc relay, LED. Resistors for LED and +5 pullup are on back side of board. Bottom row is PICAXE power supply, which will be fed off the HVAC xformer. Rectifier, 100uf cap, 7824 regulator, resistors for voltage divider.

 

[SethP]

You are right -- I didn't need the split core, and could have saved some money. However, I figured they are a bit more flexible, so I can monitor other things with them as well without having to cut wires. Figure it's around $20 for a wired up CR3110, plus the cost of the 1-wire input. I'm using both DS2406 inputs and the Hobby Boards HVAC monitor.

I further agree that there are many different solutions out there. I tend to like simplicity and flexibility. I found the CR3110 easy to work with. It turns out that CR Magnetics makes a version with a built in siwtching transistor, which would have further simplified things. About $30 each, but 15 fewer minutes of soldering & heat shrinking.

As long as I'm having fun, I know I'm doing it the right way

The bigger issue: I'm gathering a lot of data, and now I need to do something with it. When I have some free time...

Seth

I use the Hobby Boards HVAC monitor for my 1wire HVAC monitoring and its been working great, but I have easy access to a common. I was just pointing out a different way to do it. This project is for a PICAXE but could be easily adapted to 1wire. The cost of one input using the HB HVAC monitor is about $7.50 which includes the 1wire interface, the cost of one input using a relay is less than $10 w/o the interface, and you've stated the cost of one input using the CR3110 is about $17 ($15 for just the CR3110) w/o the interface. The CT and relay methods are essentially doing the same thing, converting AC to DC, with almost the same components. You ran a wire to the remote stat to send the signal back to your main board. You could just have easily sent the 24vac on that wire (a "runner" back to your panel) pulled a common from your transformer and centralized everything. You used a split core CR3110 while you could have used a smaller and cheaper solid core like a CR8401 or CR8410. Just remember that electronics projects are like politicans, everybody's thinks they've got the best answer for you and we all know that's poo.

View attachment 1466
Board is 2"x3". Left side terminals are from HVAC, right side terminals go to PICAXE. Top 3 rows are HVAC input circuits. Rectifier, 100uf cap, 7824 regulator, 24vdc relay, LED. Resistors for LED and +5 pullup are on back side of board. Bottom row is PICAXE power supply, which will be fed off the HVAC xformer. Rectifier, 100uf cap, 7824 regulator, resistors for voltage divider.