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?