PCB quote

Author Topic: Understanding my own design - moisture detection circuit  (Read 504 times)

0 Members and 1 Guest are viewing this topic.

Offline Tonkabot

  • Contributor
  • Posts: 16
  • Country: us
Understanding my own design - moisture detection circuit
« on: August 08, 2022, 04:01:16 am »
I have this circuit I desiged that tells how much resistance there is between the connector (both contacts are connected to one sense wire) and ground.
Then, if the test point (a brass elbow in a co2 gas stream) gets wet (blood, most likely), we know the machine has had a biological contamination.

So normally the ADC pin (A1_Fluid_Sense) reads about 32000 When J5 sensor is hooked up and open.   it reads 38000 or so when J5 is open.
it will read 0 if you dead short it to ground.

My problem is that D34 is getting blown.    it is just a sod-323 shottky.   I had thought that some transorb or something would be more appropriate there, but I never did figure out something that worked.   The intention of D34 was to be like a 5v transorb, but I couldn't get that to work.  We need to keep the ADC pin safe from any crap that might blow the processor.
D34 has not blown much (I think about 3, over a year of 200 machines running), but that is too much.

Oh, Vusb is 5.2volts.
[attachimg=1]

Not sure I am getting the schematic png attached...
« Last Edit: August 08, 2022, 04:41:03 am by Tonkabot »
 

Offline ConKbot

  • Super Contributor
  • ***
  • Posts: 1319
Re: Understanding my own design - moisture detection circuit
« Reply #1 on: August 08, 2022, 08:27:29 am »
Is ESD building up from a dry gas flowing? The diode should be sufficient for charge on the wire itself, but is a nearby sections of plastic tube charging up and then discharging into the sense wire?
 

Offline NiHaoMike

  • Super Contributor
  • ***
  • Posts: 8393
  • Country: us
  • "Don't turn it on - Take it apart!"
    • Facebook Page
Re: Understanding my own design - moisture detection circuit
« Reply #2 on: August 08, 2022, 12:39:01 pm »
Try a larger diode and add a 1k resistor in series with pin 1?
Cryptocurrency has taught me to love math and at the same time be baffled by it.

Cryptocurrency lesson 0: Altcoins and Bitcoin are not the same thing.
 

Offline Tonkabot

  • Contributor
  • Posts: 16
  • Country: us
Re: Understanding my own design - moisture detection circuit
« Reply #3 on: August 08, 2022, 02:44:19 pm »

I didn't think there could be a static build-up because there is only 75k to the 3.3v rail and 100k to the 5.1v supply.   I'm thinking that that should eliminate a static potential build up.

Brynn
 

Offline Tonkabot

  • Contributor
  • Posts: 16
  • Country: us
Re: Understanding my own design - moisture detection circuit
« Reply #4 on: August 08, 2022, 02:57:28 pm »
The client is pretty resistant to making board changes...   However it would be pretty simple to incorperate a series resistor in the sense wire harness, and I don't see any drawback to making such a change permanently.   I'll be trying that out, and it seems like it will help protect the diode on the board.

Brynn
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf