Hi all,

I am working on adding some controls to a commercial board, from a Sunbeam E85 electric blanket.



The board is controlled by some switches and a PIC16LF722 microchip.

There is a High Voltage (US 120V / 60Hz AC household mains) and a low-voltage ( ~5.0V DC) side of the board.

The low-voltage side of the board puts out 4.8-5.2V to drive the microcontroller, operate the transitors, LCD, and buttons.
Using a multimeter, I found that I can sink about 100mA of current from the points that appear to be the VDD on the circuit.
I'm trying to pull more power to power an ESP32 dev board, maybe 500mA?

I'm hoping you can help me figure out what components on the board provide the 5V DC power, and why they can only sink such a small amount of current?

Some photos attached show the +5V and the GND for the DC/low-voltage side of the board.

 

The 5V supply probably uses a capacitive dropper. They cannot provide much current.
I think the capacitor is C8. It’s value would tell you the current limit. I do not see any regulator or zener diode though, which I would expect.

I would not recommend trying to up the current to 500 mA.

Edited: On second look, CR1 might be the zener.

 

@BobTPH this is great! I'm checking out the formulas at https://ww1.microchip.com/downloads/en/appnotes/00954a.pdf to try to calculate some of the values in this circuit.

Is this interesting to note? With the circuit powered up, relative to the ground plane, here is the voltage at either side of CR2 and CR1, both of which connect to C2.



C2 and R1 are in series with each other.

R1 is a 150kOhm, large resistor


C2 has this labeled:

1 CL21
105K250V
www.sF-cap.com
and is proximal on the board to the mains input.

the ESP32's power consumption specs:

Mode Power Consumption
Wi-Fi Tx packet 13dBm~21dBm 160~260mA
Wi-Fi/BT Tx packet 0dBm 120mA
Wi-Fi/BT Rx and listening 80~90mA

Is it still likely a bad idea to try to drive this with a capacitive dropper?

 

Edit: it's 150Ohm not 150k

 

1uF at 60 Hz is 2652Ω. At 120V that is 45mA max current.

This type of supply is not good for much more than that. You will need a real isolated supply for your ESP32.

 

That type of supply used to be banned in this forum because they are dangerous. They cause the low voltage output to be connected to line voltage, thus it is unsafe to touch any part of the low voltage circuit.

I would recommend that you junk that control board and completely replace with with your ESP32 driving an SSR to switch the blanket heater.

 

The control board with the PIC does some other stuff, like connecting to a temperature sensor built into the blanket to prevent overheating. I don't necessarily want to replace all of its functions.

I'm inclined to just use the GPIO pins on the ESP32 to "press" the buttons on the existing control board, by tying the grounds together and using the GPIO to switch to digitalWrite(low) and pull down the PIC input that's connected to the button.

I'll cannibalize a USB power supply cube and share the line/neutral from the mains feed, and fit both the existing board, the dedicated 500mA USB 5V power supply, and the ESP32 into the case.

I'm curious for the opinions of folks on here, if any of these are a bad idea.

 

Yes, it is a bad idea. You need to use opto-isolators if you want to tie your ESP32 to that board.

I don’t think you have the knowledge to do this safely.

 

To design a circuit such as your electric blanket controller that is powered in that way requires much care to ensure it is safe, particularly in an appliance for home use. 'Hacking' it in the manner you describe is unwise.

 

Every design approach has a "sweet spot" where everything makes sense, the sweet spot for a capacitive dropper circuit is supplying a few mA of current. Trying to get 500mA pushes things into the realm of impractical.
Not to mention the danger of working with the non-isolated supply voltage- which is a terrible idea for hacking.

 

Thanks, everyone. I'll reconsider the project. At the very least, electrically isolating things with relays or optoisolators.

 

I am curious why you would use an ESP32 just to have a few GPIO lines. The ESP-01 (ESP8266) is smaller, cheaper, less power hungry and seems to be enough.