# Constant low charge in PCB even when disconnected

I have a very strange issue that I can't explain but maybe someone can shed light.

I cut a homemade PCB on my CNC that basically just connects a couple Arduinos to a Raspberry Pi using i2c. It's mostly breakout board stuff. I actually have two variants of it that connect via a board connector. Both PCB's have the same issue:

There is a small voltage of exactly 0.37V on the circuit - EVEN when disconnected. But what even spookier is that the +5v voltage line for the Arduinos pull 2.7V even when there is no direct connection between the Raspberry Pi voltage powerline.

I thought first that I soldered a short circuit or something like that but I couldn't find anything - all traces are clean. I have a feeling that somehow there is some flux under the connectors for the Arduino and they short circuit some of the stuff.

But the 0.37V on both boards makes no sense... I tested with 3x different multi readers and they all show the same.

I know it's Halloween pretty soon but that's spooky.

How can be there Ba constant charge without electricity?

• There is nowhere close to enough information to address this. Pictures or diagrams would be a huge help. Commented Oct 16, 2015 at 6:26
• You don't have a USB cable plugged in do you? Commented Oct 16, 2015 at 6:38
• No nothing is connected. I will upload some pictures in the morning. Commented Oct 16, 2015 at 6:54
• Between what two points are you measuring the voltage?
– CL.
Commented Oct 16, 2015 at 8:35
• Multiple points. But mainly between GND and +5V Commented Oct 16, 2015 at 14:51

If you're using a digital multimeter on DC range the input impedance will typically be 10MΩ. This makes the meter sensitive enough to give ghost readings when connected to isolated circuits.

Try connecting a 10kΩ resistor between the probes and repeat the reading. You should find that any difference in potential is discharged fairly quickly through the resistor and the meter will read close to zero.

Interestingly, Fluke have a range of meters with a Lo-Z range to avoid this problem on DC and AC circuits. The input impedance is about 38kΩ (from memory). This is low enough to give a zero reading when measuring voltage on an open circuit even in the presence of 'noise' on adjacent lines.

Electrical charges naturally want to rearrange themselves to minimize voltage if they can, so if there's nothing connected anywhere and you still measure a voltage, there are only a few possibilities:

1. your measurement equipment is defective
2. there's some stored energy in the board somewhere
3. the board is coupling to some other energy source in a non-obvious way

Sounds like you've ruled out defective measurement equipment, though I'd check it again. This, or operator error, seems like the most likely explanation to me.

So it's possible there's stored energy in the board. It could be a battery, or a capacitor. It could even be stray capacitance between the traces on the board. Remember that a capacitor is just two conductors separated by an insulator. The conductors can be the traces and the insulator can be the board. It's a very small capacitance, but some voltage meters have a very high impedance, and so the voltage across even this small capacitance can persist for a while. You can eliminate this possibility by shorting the two measurement points with a wire first, which will discharge the capacitance, and then measuring the voltage. But I think this is pretty unlikely.

The other possibility is that the energy is coming from somewhere else. Like maybe AC electromagnetic fields from nearby electronics, the principle on which transformers and antennas operate. Good meters shouldn't be very sensitive to this sort of stuff (at least when set to DC), but a cheap meter from China might be. A cheap meter from China might just be off by 0.37V, too. 0.37V would be a lot of voltage to pick up on a little circuit board from stray electromagnetic fields unless you are sitting right next to a commercial broadcast station or a magnetic degausser. So this seems pretty unlikely to me also.