Half wave rectifier and GPIO expander

[SWAGGANEER]

Hey there.

I have a circuit where I need to get 120VAC down to 5VDC for use with a GPIO expander device, I am currently using an MCP23S17. I am using a half-wave rectifier, to a resistor network used to limit current and divide the voltage, then a zener diode to regulate the voltage to 5V. All debouncing is handled by software, so a half-wave is fine. The circuit is used as a digital input module for a PLC.

The problem I am getting is that I am getting negative voltages out of my half-wave rectifier. These should be equal to the fwd voltage drop of the diode (~600mV), but I am getting -1 to -2 VDC on my scope. I guess I have two questions:

1) How do I reduce this negative voltage? It occurs during the crest of the AC input waveform.

2) Will the negative voltage damage the GPIO port? There is no mention of it in the datasheet. If I can ignore it that would be the easisest and cheapest solution.

Thanks in advance for any input.

 

[KlausST]

Hi,

here is no mention of it in the datasheet.

Look at the section "Absolute maximum rating". I bet it is mentioned.

To the circuit. It would be helpful to show us...

Is the second line of the AC connected to your expander device gnd?

You are aware of the risk of electrical shock?

I reccomend to use an optocoupler to get galvanic isolation.
For sure you can do the debouncing in the ucontroller, but using an R, C and Hc14 is a clean way.

Klaus

 

[SWAGGANEER]

Klaus,

That information is not available in that section, or any section of the datasheet. Microchip doesn't have the customer support of TI, I don't even know how to reach someone.

AC Neutral and expander device ground are connected.

I am aware of electric shock. I am using MOVs and TVS diodes for surge protection.

All of my isolation is after the GPIO expander, this reduces the number of isolation channels: DI --> Serialize to SPI--> Digital Isolator --> uC

Debouncing is handled by software, since saving $0.10 on parts is a big deal for this application.


Anybody have similar experiences with half-wave rectifiers?