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| Operational Amplifier driving MOSFET |
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| mike_mike:
Thanks for the reply. The last uploaded schematic (reply #47) is correct ? Sorry for asking again. I just want to be sure. |
| Zero999:
--- Quote from: Ian.M on May 02, 2018, 02:34:17 am ---Use a Polyfuse, they are much more compact, and self-reset when the power is switched off for a few minutes. @Hero999: the whole point of the second LED connected to the first stage OPAMP output is it should *ALWAYS* match the one switched by the MOSFET. If it doesn't it means the MOSFET's blown - which is difficult to avoid in such a simple circuit if its output is shorted, without incurring excessive losses or complexity protecting the MOSFET, so its useful to have an indication that there is a fault. --- End quote --- Good point. --- Quote from: mike_mike on May 02, 2018, 05:27:49 am ---Thanks for the reply. The last uploaded schematic (reply #47) is correct ? Sorry for asking again. I just want to be sure. --- End quote --- It looks good to me. |
| mike_mike:
I built the circuit and I tested it. Everything looked OK, but when I put the scope on pin 2, there appears an oscillation (I don't know what is it, but I suppose it is a oscillation). The first image is from pin 2 of LM358 (the probe between GND and pin 2), and the second image is from the terminals of LDR. On the pin 6 and 7 is only noise, about 2-3mV. On pin 1 is the same with pin 6 and 7 (DS0262.png). I tried to remove the two 10 nF capacitors but it behaves the same. I tried with another LDR and it behaves the same. I do not know what other details I need to show. It is normal or my circuit does not work correctly ? |
| Ian.M:
So you've got 35mV of 100Hz ripple at the first OPAMP input. That could be noise on your +12V supply - try probing the +12V rail with the scope input on AC coupling and see if its worse there, or run it from a 12V battery and see if the ripple at pin 2 goes away. Alternatively it may just be the LDR responding to the room lights flickering at twice mains frequency. Test that by turning off the lights and illuminating the LDR either with natural daylight, a LED keyring torch that doesn't have any electronic controls or fancy modes, or an torch with an incandescent bulb. As the circuit has >0.5V hysterisis, 35mV of ripple isn't going to cause any problems. As for what you measured at the LDR - unless its well lit, its fairly high impedance and that looks a lot like its picking up mains hum on the wiring. |
| mike_mike:
I made some tests, with the light turned off in the room. The light of the room consist of 1 LED 230V light bulb. Natural light, load off DS0355.png. Natural light, load on DS0358.png. Output of the 12V power supply: DS0356.png. The probe on the LDR with the circuit turned off and the LDR connected directly to the PCB: DS0360.png. Please have a look at the measurement images and tell me what you think. |
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