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Operational Amplifier driving MOSFET
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mike_mike:
Dear Forum,

I just built the attached schematic and it is working.
I tested the circuit using a 12V/20W light bulb (halogen) and a 27R/5W resistor, separately, connected on the LED_load connector. One test with the light bulb and one test with the resistor.
When the mosfet is OFF, when I put the oscilloscope probe on the terminals of the resistor (27R/5W) on the oscilloscope screen appears the following image:
--- End quote ---
. Why ?
I powered the circuit using a LM317 + PNP variable power supply, set at 12V. The maximum current that can be draw from the power supply is 3A.
The circuit is a switch which is turning ON the mosfet when the night comes. I added a hysterezis and two leds, with 1.5k series resistor.
When I used the 12v/20W light bulb and I put the oscilloscope probe on the bulb terminals, on the oscilloscope screen appeared only noise, with 5mV amplitude.
On the pin 1, pin 6, pin 7 (LM358) and on the output of the LM317 power supply, whith respect to the GND, is only noise (about 5mV).
schmitt trigger:
Looks like 50 Hz hum.
Benta:
Yep, 50 Hz. I'd suspect your 'scope probe ground connection.
mike_mike:
Is that 50 Hz hum dangerous for my circuit ?
Or it is just a hum captured by the oscilloscope probe ?
Benta:

--- Quote from: mike_mike on April 24, 2018, 03:19:53 pm ---Is that 50 Hz hum dangerous for my circuit ?
Or it is just a hum captured by the oscilloscope probe ?

--- End quote ---

It's only 20 mVpp, Dangerous, no, I'd say it's noise captured by the probe. Place the probe point and probe ground directly on the resistor.

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