Review Request for Arc Volt Sensor Schematic

Hi all. I've been playing with a design for a board that will allow me to read arc voltage with a Raspberry Pi. I think I've got something that will plausibly work. I'd appreciate a review of the attached schematic because circuit design is not my core competency.

Here's a quick tour:
1. Inputs are expected to be connected to the arc volt port of a blowback-start plasma power supply. 
2. Input voltage is limited to 250V by a TVS diode
3. Voltage is divided 50:1
4. RCRC filter limits frequency to 14Hz
5. Raspberry Pi reads divided voltage via ADC

Thanks!
 

Arc voltage is DCEN, that is, between -400V and 0V.
The torch is negative, ground is positive.
Not saying that the below is correct. It has worked for me for many hours though.



github.com/MetalMusings/MyOwnEtherCATDev...in/Cards/THTIC/Kicad

My intention with the TVS was to limit input voltage to 250V. The ADC expects 0-5V range, and voltage division is 50:1, so anything above 250V is out of range.

In reality 250V is way high for a cutting voltage. I just came up with it by working backwards from the ADC voltage range.

Does that make sense?

Optoisolators are a good idea. I was planning on the TVS limiting the voltage, but I guess it's always possible that someone would hook this board up to a HF start torch.

Thanks for the schematic. I wish I had found this a while ago.

I like how you use isolated and regulated voltage. I need to see if I can find a single component that does that.

> The torch is negative, ground is positive.

Ah right. I forgot that. Is that why U4 is inverted?

I took a stab at dissecting your schematic. This is what I got:

Voltage source
5V: Common 5VDC power source
U1: 1W Isolated DC-DC 9V out; Isolation 1.5k VDC/min
C3: Decoupling
U5: 5V LDO voltage regulator
C3: Decoupling
VCC_ISO: Isolated 5VDC

Input stage
R7,R9: Voltage divider, 38:1 ratio
D1: Limit input to max 5V
R6: Current limiting
U4: Voltage buffer (inverted ?)
R8: ?
R5: Current limiting?
C4: Decoupling?
C5: Decoupling?
U3: ADC with 12C output
R4: Pull-up resistor
R5: Pull-up resistor
C2: Decoupling?

Output stage
U2: I2C isolator; Isolation 2.5-kVrms
C1: Decoupling?
R1: Pull-up resistor
R2: Pull-up resistor

Yes, inverting and decoupling. 
Wouldn't be surprised if there is a similar isolated SPI component.
Best if the ADC has some kind of free run mode. Otherwise the reading of a value can take too much time.
Was looking for a new value every millisecond, with as many bits of resolution as possible.
Have also used ADS1014 with similar good result.

I moved the voltage divider to the first stage and put a zener after it for overvoltage protection like Hakan.

@tommylight: Do you have a schematic you can share that shows how you use 3 resistors for voltage division. I'm having trouble visualizing your description.

Here's where I'm at right now. I need to add SPI isolation. That's going to be the biggest stage of all because there aren't any good through-hole all in one solutions like there are for SMD.

I also wonder about the 2 50k resistors.

Good find on the regulated TMR-0511.
I looked at the output of the isolated power supply I use. I wasn't terrible, almost ok but not totally stable.

Why the filter step? HF?