Noise with pit thermcouple driving me bonkers


 
J

John Bostwick

TVWBB Wizard
Ok, I have a board I made that has a noise issue. I have resoldered, removed solder, checked every solder point over and over again. I have two other boards Identical, that have no noise at all. I should say these have a USB connector instead of a RJ45 jack, for a project Im doing.

I know power is fine, as the other two boards on same adapter and outlet work without noise
I thought it may have been related to the .1 caps I used, as they where different from mousers-I made another board with same caps and no noise issues with it.
I know its not the onboard thermocouple circuit, as I get the noise on an Thermocouple adapter that's connected. The same adapter works without noise on another board.
I have left the board running while I was at work, and the noise goes away, only to return, when I move the board or the temperature drops from when I turn on the AC unit.
The noise stays between 10-20

What components could be giving me noise?
 
Well... Components that could cause noise.... Bad 5v reg, bad 3.3v reg (or surrounding caps.) The diodes next.. Any semiconductor near it's conduction point will create noise. Any of these parts could cause noise. After verifying those I'd look at the rest of the caps. If nothing else, I'd swap out the largest group of semiconductors on the board, (the ATMega itself..) That's how I'd start looking if it were me...
 
Can I assume these boards have the traces for the probes running from the USB port you added to the probe jacks on the board (rather than jumpers)?
Otherwise the first thing I would check is the jumper wires (if present)
 
The probes are fine, its just the thermocouple on the board and on the adaptor. But if I take the adapter thermocouple and use the wires to attach it to one of the other probe connections on the heatermeter board, the noise follows it. So, it not a connection issue. I removed the dc/dc and still had noise.
 
OK, with that said I would think it has to be something related to your power being supplied to the TC amp, since the noise follows the TC amp to another probe input (which has no noise with a standard probe). Seeing that both the onboard and external TC amps are having the same noise issue it can't be the way the TC amp is laid out on the board. that really boils it down to the power supply as the source of the noise I think.
If you power the board from the rPi do you still get the noise?
 
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