Power coming from unknown source

Hi, I have a quite complex circuit with 5 DC/DC Converters all being powered by a 28V power supply and with a common ground. All DC/DC converters have a on/off feature and they all are controlled by a Raspberry PI. I'm also using three PanStamps (Arduino compatible device running a 8MHz, 3.3V with a RF chip C1101).

The Pi is connected to a screen using a HDMI - DVI cable and also to an Ethernet port.

My regulators are TracoPower TMR 6-2410 (3.3V), 3xTMR 6-2411 (5V) and another 12V.

One of the PanStamps is connected to the Pi using their RX/TX lines and also connected to a 12V analog pressure sensor using a voltage divider to sacale down the output to 0-3.3V. All PanStamps share the same +3.3V line

The problem is that when I turn off the 3.3V regulator I still reading 1.48V on its output.

The problem is that this 1.48V sometimes is enough to keep the PanStamp running (instead of being off) which leads my system to unknown states and some times out of control. When there is nothing connected to the regulator the off voltage is 0.09V.

I believe that this unknown power is coming from the Pi through the RX/TX lines passing the PanStamp and going to the power line.

Here is my power schematic:

enter image description here

My questions are: 1 - How can I stop it from happening? 2 - Can some device be damaged by low voltage/current?

  • \$\begingroup\$ If you test the 3.3V dc/dc separately, does it still read 1.48V when turned off? \$\endgroup\$
    – user36129
    Commented Aug 19, 2013 at 11:11
  • 12
    \$\begingroup\$ I would suggest you redraw your diagram with a white background and try and condence it in size so that it becomes readable. \$\endgroup\$
    – Andy aka
    Commented Aug 19, 2013 at 11:11
  • \$\begingroup\$ @user26129 the off voltage with nothing connected is 0.09V. Andyaka the schematic is too big to fit here, that's why I put a link to the full size image. \$\endgroup\$ Commented Aug 19, 2013 at 13:01
  • 2
    \$\begingroup\$ Many devices are not rated for having voltages on their I/O lines above that of their supply lines, or at least not above it by more than a diode drop. What tends to happen is that the I/O line then pulls up the power supply line through the protection diodes, but this can (amongst possible worse things) exceed the current rating of the protection diodes. It's not entirely clear if the pi is the source of the power here or merely a conduit for it. If it's the source, and you are talking about 3.3v serial signals from the MCU, you can probably software configure them as GPIOs and drive low. \$\endgroup\$ Commented Aug 19, 2013 at 14:10
  • \$\begingroup\$ @Andyaka I took the image and applied a negative filter and changed the hue so that it has a white background but still has the same colors. I will upload it. \$\endgroup\$ Commented Apr 25, 2017 at 14:41

1 Answer 1


Most of new ICs (analog or digital) have ESD protection diodes which are diodes connected between the input pin and the supply rails, their main job is to clamp the input pin voltage between the supply rails in the event of a voltage surge, see

How Diode Clamping Works

these diodes have low current rating so if you continuously load them with enough current they can be damaged,

Since you haven't supplied voltage to the circuit, your circuit can be turned on
through the ESD diodes on the data lines, the simplest way to protect these diodes and also fixing your problem is placing a resistor on your digital lines, if you don't have high baud rates 1kohm should be a good starting value


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