Negative dc voltage on earth

[Giro]

HI,

I hacked into a device that contained rectifiers in it to temporarily power a wireless games console controller. (3v tapped from output of regulator legs to the controllers AA battery compartment). I thought this was pretty neat and/because it worked.
Polarity was confirmed correct with a multimeter. I noted the device (not controller) ran slightly hot.
My brother accidentally reversed the polarity.

Since then the device I tapped into is not fully functional. Its a usb device and the usb part isn't handshaking with PC anymore. I suspect the USB controller - this was measured, and it appears the various supply voltages are -3v thereabout to little. the Ground is -3v.

Given that AA batteries are 1.5v I thought the regulator (3.5v exact) would be suitable.

What could be the possible cause? I think there is an earth fault, perhaps shorted diode somewhere.

 

[KlausST]

Hi,

the Ground is -3v.

Usually GND is the reference for voltage measurement, so how can it be something else than "0V"?

Klaus

 

[c_mitra]

What could be the possible cause? I think there is an earth fault, perhaps shorted diode somewhere.

If you are referring to the mother earth, then you are wrong. The games console controller will also work in a plane (far away from the mother earth)

The earth and ground are often used interchangeably. They are also used for measuring the voltage (you can only measure voltage difference and never an actual voltage)

While measuring voltage, you connect the common lead to the ground and you will see the voltage of the red lead; that is the voltage difference between the two test leads.

 

[Giro]

:thumbsup:

Hi
The red lead was to a GND pin on the SMD usb controller & black lead to usb socket 'casing' (see silver/ALU block in picture top right) - This gives a negative value.
**broken link removed**

The main regulators and on/off light work so its probable the usb controller specifically has too little power. Compared to the application example seen the datasheet for the controller, the voltage readings are slightly under whats in the datasheet, but this could be a design choice. I may contact the company for schematic though its highly doubtfull they'll make it available. Before the hack the device shown worked.

Ps : admirable pcb layout to say the least; very professional.

 

[c_mitra]

Doing a decent post-mortem is always of great educational value. Your measurement appears to be correct.

If the ground pin of the microcontroller is at a lower potential compared to the metal shield of the USB jacket, then perhaps the metal cover of the USB IS NOT connected to the PCB ground.

The microcontroller is having beautiful legs like a centipede; you may perhaps need another microcontroller to position the red lead of the multimeter!!

How about the output of the voltage regulator (near the bottom right)? Use that IC as your voltage reference (ground pin connection).

Microcontrollers look delicate (because of their legs) but they are quite rugged (compared to other devices)...

If the USB is not being recognized and the power is present, then the usb controller is gone. Sometimes wrong polarity may only damage the electrolytic filter cap; that may be a point to test (just a suggestion)