Electronics > Beginners
Operational Amplifier driving MOSFET
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mike_mike:
Thanks for the reply.

I modified the R4 to 1.5K if Ian.M said that it will be OK to increase the R4 value.
I was advised (not on the forum) to add a capacitor (100nF) in paralel with the 10K trimmer, so I added that capacitor.

I attached the modified schematic. Please have a look at the schematic and tell me if it is OK.
Ian.M:
C1 may give you more trouble than it prevents.   Without it both the +in and -in voltages are purely dependent on the resistance ratios  of their respective dividers and the instantaneous supply voltage.   With it, the -in voltage is delayed, which means that if the supply voltage transiently drops more than about 1.5V as the load switches on (and that 20W bulb has a cold resistance under 1 ohm so pulls a very large current spike at turn-on), the -in will be above the + in and it will switch back off resulting in oscillation.   If you *MUST* put a capacitor there , make it a lot smaller and distribute it evenly from the input pin to both supply rails e.g. 10nF to each rail from the pin.

1.5K will do for the gate resistor - it slows down the switching time to the order of 10us, which will increase the dissipation in the MOSFET, but its still well within the S.O.A for any of the MOSFETs you have listed, and unless it toggles rapidly and continuously (see issue in previous paragraph) the dissipation should be low enough that it wont need heatsinking.
mike_mike:
Thanks for the reply.
If I will remove that 100nF capacitor and I will use the circuit without it. It will be ok ?
Ian.M:
Yes, as long as you keep the photocell wiring as short as possible.  If its mounted off-board use twisted pair.
mike_mike:
I am planning to use for the photoresistor 1 meter long wires.
1. What should I do in this case ? It is sufficient to use twisted pair in this case ?
2. Or should I use a capacitor between GND and pin 2 and one capacitor between +12V and pin 2 ? 10 nF is sufficient ? Please confirm if I am correct.
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