| Electronics > Beginners |
| Operational Amplifier driving MOSFET |
| << < (13/15) > >> |
| Ian.M:
That proves the 100Hz ripple at pin 2 was from the room lighting. However there's still something odd about the 50Hz mains hum you are getting at the LDR. Exactly what point on the above schematic are you probing, and where's the scope ground clip connected to? |
| mike_mike:
The oscilloscope probe is connected on the terminal block marked with arrows. The ground clip is on the upper arrow. And I am using the crocodile clip of the probe. That 50 Hz sine wave (or approximate sine wave) as in the DS0360.png is present on the LDR even if the power of the circuit is off. |
| Ian.M:
You can't do that unless you are using a self-contained battery powered scopemeter. You are lucky not to have blown the OPAMP, and if you'd put the ground clip on the other end of R3, if the PSU had a grounded output it would have damaged your scope. As you are using a mains PSU, even if its fully isolated, the scope ground clip *MUST* go on the 0V (circuit Gnd) rail as otherwise anything you are trying to measure will be swamped by the line frequency leakage current through the PSU's pri-sec class Y EMI suppression capacitor. The only way of making differential measurements safely with a conventional mains powered DSO is to either use two channels and use waveform arithmetic to display the difference (low accuracy if the common mode signal is large with respect to the difference signal, and no good at HF) or to use a differential probe. Forum topic: https://www.eevblog.com/forum/blog/eevblog-279-how-not-to-blow-up-your-oscilloscope!/ |
| mike_mike:
Thanks for the link, it is giving 404 error. I measured again, with the ground clip on the gnd of the circuit and the probe on pin 1 and pin 2 of the J4 connector. On pin 1, it gives me the following results: DS0363 in completely darkness DS0364 in direct natural light On pin 2: DS0366 in completely darkness DS0365 in direct natural light Please have a look and tell me what you think. |
| Ian.M:
Yep. I was right. Mains hum due to incorrect probe ground connection. I've fixed the links above. You *SHOULD* watch the video above . . . |
| Navigation |
| Message Index |
| Next page |
| Previous page |