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Author Topic: I2C: Noise in the power source  (Read 636 times)
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South East USA
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I noticed my I2C LCD locks up when I turn a little space heater off.  I moved the space heater to an outlet on the other side of the room, and it still does it.  The 2 outlets must be on the same circuit.  I don't feel real comfortable installing something like this if it's that susceptible to noise.  Power for the LCD and my UNO comes from a variable voltage wall adapter from Walmart.  I don't think it's the transformer type, it's very light and when set on 12 volts, it puts out 12 volts, unlike the old transformer style wall warts.
My power supply board consists of a 0.1 uF & a 47 uF cap right next to the DC Jack.  A reverse polarity protection diode, and a 7805, My UNO gets 12 volts to Vin, and the 5 volts for the LCD comes from the 7805. 
I tried adding a 100uF, and then a 220uF cap next to the DC Jack, but the screen still flakes out, and locks up when I turn the little space heater off.  It wouldn't matter quite as much if it didn't lock up, but I have to power down/up to get it to work again. 
(it's not affected if I turn the heater off via the thermostat)
I kinda liked I2C, but this may be the determining factor to go with Serial, as my one in Serial mode isn't affected by the heater.
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It's more than likely the space heater's injecting a high frequency spike into the supply rails when it switches on and off.

With this in mind, and remembering the basics of capacitors, you'll need a lower value of C to suppress the higher frequency. Now you could use an algorithm to find the correct (closest) value of capacitor to use if you knew the frequency to suppress, an easy and quick way (assuming you have the parts to hand lol) would be to try a 10nF (0.01uF) in parallel to the 100nF and 47uF that's already on the board then, if necessary, go down to a 1nF (0.001uF).

If that fails then I'd be looking at getting the caps as close as possible to the supply pins on the ICs as the board traces could be the culprit in your situation.

All the best.
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Thanks.  That's interesting, so it'd be better to have the small cermamic caps close to the SDA & SCL pins.  Kinda like the H-bridge IC with the little motor it runs, I put one close to it.
Would this small cap be of usefullness to Serial pins, as well?
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The power pin, not the data pins.
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