Oscilloscope measurement fail

[radoczi94]

Hi!

I just bought a scope and building a PWM dimmer for LED strips. A pic generates the pwm signal and mosfet just switches the leds on and off, the power supply is an unearthed power brick. I bulit the thing on breadboard and started to poke around with the scope probe if it's generating some unusual noise or ringing or something. The PWM signal is ok. I checked the signal across the LED strip, swapped around the probe, and noticed something: the waveform is not the same. I understand that the signal should be inverted, but it also should be the same shape. If I put the scope's ground on A and the tip on B, the signal is weird and jittery. When the gnd is on B and the hot is on A, the signal is acceptable. What happened?

[Zero999]

The ground on B or 0V and probe on A are the correct ways to 'scope the circuit, because both 0V and B are at a constant voltage.

The circuit will be capacitively coupled to the mains, via the inter-winding capacitance of the transformer inside the power supply and some of the mains frequency will be coupled to the input.

Even if the circuit were powered from a battery, it would still not be a good idea to connect the oscilloscope's ground to A because the parasitic capacitive coupling between the battery and ground, which will depend on the physical size of the battery and circuit.

Another thing to note is the voltage on the LEDs is never falling below about 7V, because the LEDs have a capacitance which stores the charge, when the MOSFET turns off.

[StillTrying]

It's normal. When the MOSFET's not conducting and the LED's not conducting point 'B'    'A' is just floating.

You could put about 3k in parallel with the LED to see square-ish waveforms there.

[radoczi94]

Thank you guys for the help! I had a feeling about the floating A point making my troubles, thanks for the proving!

[hussamaldean]

don't forget to add the pull down resistor between the gate of the mosfet and the ground unless a mosfet driver has been used

[Tom45]

Or probe ground to GND and channel 1 to B and channel 2 to A. Then display channel 1 - channel 2.

[rstofer]

Or probe ground to GND and channel 1 to B and channel 2 to A. Then display channel 1 - channel 2.

^^^ This!

Do not just clamp that ground lead willy-nilly into a circuit.  It's a real bad habit to get into!

Hook it up to the closest thing you have to Gnd (bottom side of the MOSFET, presumably the Gnd for the uC, and leave it there.

If you need a differential measurement (and you probably don't), use Ch1 - Ch2  Or, spend the big bucks and buy a differential probe (and the price will convince you that you don't need it).

Watch Dave's "How not to blow up your scope" video: