# Looking for review of circuit design - 24v AC through Optocoupler to 3.3v ESP8266

#### haukeg

Joined Dec 9, 2018
66
Hi, I am working on a circuit design for watching 5 24v AC lines out of my Taco ZVC-405-4 zone control unit. I have already worked on a single line design via another thread here, but am now working on a fully integrated circuit for all 5 zones.

I have found the impedance of each zone is different because of the coverage of the zones, so I have calculated what I believe are the proper power ratings for my resistors in the design. Please see the attached design with the 3 colored amperages and resistor power selections in my design.

Looking for any feedback or advice on making this design better before locking it in. Thanks so much!

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#### Sensacell

Joined Jun 19, 2012
2,598
Why the different current levels?

Aren't you just looking for an on / off signal from each valve?
Looking closely, I see the resistors are all the same? puzzled now...

#### Alec_t

Joined Sep 17, 2013
11,649
The resistor values are inconsistent with the loop currents shown.

#### haukeg

Joined Dec 9, 2018
66
In my testing for a single HCPL3700 on any of my 24v Valve loops, each leg off the 24vAC runs required 3.3k Ohms on each to sense the switch. However, of the 5 valves, I have 3 with equal current values and 2 higher ones as indicated.

Based on calculations I found for determining the power for the resistors, P = I * V I calculated the power rating for each valve. I also read that it is best to double the value at least for safety/heating. So I calculated the following for my 5 valves:
0.011 * 24 = 0.264 W (selected a 1 W in my circuit)
0.04 * 24 = 0.96 W (selected a 3 W in my circuit)
0.09 * 24 = 2.16 W (selected a 5 W in my circuit)

Am I overdoing it because I have on e resistor on each leg of my 24v circuit? Should I cut in 1/2?

#### Sensacell

Joined Jun 19, 2012
2,598
The power rating of the valve has NO EFFECT on the resistor power level.

Each resistor will dissipate about 43 milliwatts, assuming the supply voltage is 24 Volts.
Just use a 1/4 watt resistors

#### Alec_t

Joined Sep 17, 2013
11,649
Your optocoupler outputs should be taken from pin 6, not pin 7, and will need pull-ups.
Those particular optos seem OTT for your purpose. There are cheaper, simpler ones available.

Last edited:

#### haukeg

Joined Dec 9, 2018
66
@Alec_t - Yes, my mistake - should have been pin 6 (not 7), thanks.
Also, you should see my 10K Ohm pull ups between (what should be) pin 6 and pin 8.

#### haukeg

Joined Dec 9, 2018
66
@Sensacell - Sorry, I did not mean the power rating of my actual valve, but the power rating for the resistors I determined for each of my valve circuits.

Can you help me understand how you arrived at 43 milliwatts? My circuit design is the actual current I found testing each of my valve 24v circuits with my voltmeter. That must have been the wrong way to get current for resistor values?

If I calculate for current through the resistor by using I = V/R, I get
I = 24/3300 = 0.007272 (or 7.27 mA)
Then to determine the Wattage for the resistor, I got
0.007272 * 24 = 0.175W
In this case, the 1/4W resistor seems appropriate like you recommend. Just curious if I am calculating properly and thinking about this in the right way.

Thank you.

#### BobTPH

Joined Jun 5, 2013
2,541
Except you have 2 resistors in series, each of which will drop half the voltage:

I = 24 / 6600 = 3.6mA x 12 = 43 mW. And that is the max, the LED in the opto-coupler will drop part of the voltage.

BTW: there is no need for two resistors, use one calculated to give the desired current to the opto.

Also, is the opto coupler designed to take AC? If not, you need a diode as well.

Bob

#### haukeg

Joined Dec 9, 2018
66
@BobTPH that makes more sense - thank you for explaining. The main reason I am here is to learn.

Also appreciate the suggestion for dropping to one resistor, will definitely consider that. Yes, the opto is designed for both AC (pins 1 & 4 )& DC (pins 2 & 3) - which is also why I have the cap across 2 & 3 for reducing ripple.

Another question I had, I have found on my Taco ZVC405-4 that all of the leads to pin 1 on each opto in my design (the left leg on each 24V zone) all have continuity. I am assuming that is Common or Ground, but in general I was wondering if it would be better for me to connect all Pin 1's on all opto's to a single left leg on one of my 24V zones. Just curious about this.