Help me understand my AC translation circuit

Thread Starter

robotDR

Joined Mar 17, 2020
90
I have a PCB that converts 24VAC to 5VDC and 3.3VDC for use with a micro controller and other circuitry.

I need the microcontroller to read a conventional irrigation rain sensor. It is my understanding The rain sensor is basically a switch so my thought was to give it 24VAC and when it switched on (Rain) it would pass that 24VAC to my bridge rectifier which then gave power to a 3.3v regulator and the output of that went to my micro controller so it would know when a digital high was happennig. Brute force and not elegent I'm sure.

Anyways, As you can see, one side (of the AC side) of my bridge rectifier is always connected to AC gnd/neutral. The other side of the primary winding goes to a connector which I was going to connect to the rain sensor. so currently the primary is just a stub on the nuetral.

My problem is that even with one side of the primary just floating, and other tied to ac neutral/gnd, I am getting 36v out of the secondary and 3.3v out of my regulator when I don't want it.

Can someone help me understand and fix this. I appreciate the help and insight.
 

Thread Starter

robotDR

Joined Mar 17, 2020
90
The more I think about it. If it works this way, I can just cut the trace leading to the bridge. So when the rain sensor goes high and closes its switch, it will put 24VAC on the bridge (with one end of the primary floating) and it will turn on like it is now.

But to better understand what is happening, I suppose a changing voltage which is totally fine on a floating wire, will induce a current or voltage on the secondary right? But can I pull current from that? apparently I can pull some because the regulator has some quiescent current and the led has maybe 3mA DC.
Only if you post a schematic of what you have. :rolleyes:
Sorry doing a million things tonight. had it in my clipboard, forgot to attach. It's above..
 

MrChips

Joined Oct 2, 2009
30,790
Looks like a Rube Goldberg to me.
Why not just connect the rain sensor directly to the MCU input with an appropriate pullup resistor?

If you would prefer to bias the sensor with AC, you can connect the sensor to two MCU inputs.
 

Thread Starter

robotDR

Joined Mar 17, 2020
90
Looks like a Rube Goldberg to me.
Why not just connect the rain sensor directly to the MCU input with an appropriate pullup resistor?

If you would prefer to bias the sensor with AC, you can connect the sensor to two MCU inputs.
Because it was my understanding it's not exactly like a switch and that it needs 24VAC input for power, and at the same time only outputs 24VAC.

This would be for an industry standard irrigation rain sensor. Is my understanding incorrect? I can't find much info.
 

MrChips

Joined Oct 2, 2009
30,790
Post a link and photograph of the sensor.
Rain sensors are usually just two electrodes made of platinum or carbon or just interleaved finger grids on a PCB.
 
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