Problem with Electronics - Grounds and Osciloscope.

[arreamoro]

Hi,

I think that i have a general problem with the Oscilloscope and the ground with different devices.

My problem is that this is the second time that one microcontroller of my circuit starts to burn when i tried to see the signal over one pin, and I don’t know what happen.

I use a PC power supply to feed with 5v my circuit, the circuit have usb connection with the laptop and I connect the oscilloscope ground to the ground circuit, and when I connect the oscilloscope ground I think that the problem start.

I checked that the grounds of the devices are connected to the wall connector, and I realize that both chassis/ground are connected (oscilloscope and PC power supply).

I think that the problem it is on the circuits ground, because it is not logical that If you touch with the oscilloscope probe, this give any current to the circuit.

Could you help me? I don’t know what to do, may be the problem it is also use a PC power supply to feed my circuit, don’t know…

Some of you have had similar problem…¿?¿?¿?¿?¿?

[vintage1]

The PC power supply is not usually grounded to anything.

But you can check with the scope on DC by touching a probe to the ground of your circuit before you connect the oscilloscope ground.

I doubt that this would burnout a chip though. More likely you shorted the power to a pin that didn't like it with the probe..

[jh2rc97]

Current always finds the hidden path. Use one of the little plug deals that prevents the ground of the scope power from connecting to the ground in the wall. I have seen several people fry scope channels on Tek scopes because of grounding issues.

[Hugh P]

Old guy who scoped 150KV devices and things that switched 1000V at 1000 Amp as short as a half millisecond for a living and never blew a Tek scope and never ever ungrounded or floated one.

Never float a scope, learn how to do differential inputs with two scope probes with invert and add.

Having one of those thingies to unground a scope in your tool bag was grounds for automatic termination.

If you have to float a scope you probably can not rely on the waveform you are seeing.

Hugh

[Rodney]

Hugh P has given you some very good advice. Never use a plug insert that isolates the ground on the scope unless you want to flirt with a serious health problem (electrical shock).

[arreamoro]

Hi thanks to all...

First i checked that my laptod doesnt have the ground cable to the wall... so the laptop is tecnically float from the main power, and second with the multimeter i check if the chasis of the pc power supply is connected to the chasis oscilloscospe and also to the ground of the probes (I have made this without connecto to the main power), and i notice that both are connected ( cero resistance), so the ground-chasis of both are connected to earth pin in the wall connector.

So i guess that the problem can it be generated by the power supply in the laptop... because i have connected all the grounds (oscilloscope, pc power supply, and laptop), and first i turn on the laptop, then pc power supply and later the oscilloscope... so... i dont know...

And also i didnt touch any pin with the probe, It started to burn only with the ground connected... leaving the ground connected for 1 min... more or less

Now i have soldered a new cable for the laptop power supply, connecting the eath cable to the ground in the 19v... let's see what happend....

I have to receive more microcontrollers, i will solder it again, and i will try... i want to find the error and learn from my mistakes...

kind regards...

[arreamoro]

Hi,

Today i checked a really curious thing. I have a radiator from a central heater in front of me, and sometimes i am touching it with my feet without any shoes.. jejeje.. nice... :p
And i checked that when i am touching the heater and later i touch some circuit i feel some "little current" in my fingers...

So the question is that i connect the oscilloscope and if i touch the probe, the oscilloscope tells me that i have in my finger 400volts at 50Hz, nice... And this without touching the heater with my body... i don’t know why... And the funny part is when i connect the heater to ground in the wall connector, this voltage drops to 1v or something simillar...

I guess my problem is about groundings, but 400v... this can be really dangerous...

Now i have connected the heater to ground... and I will leave it like that…

I guess that now my circuits will not burn…