| Electronics > Beginners |
| Operational Amplifier driving MOSFET |
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| Ian.M:
While we are optimising the existing design, the potential divider biasing in+ of the spare OPAMP in the LM358 is pointless. As its common mode input voltage range includes ground, the other half is already driving rail-to-rail, and the ON LED will be continuously drawing about 7mA, the tiny improvement in quiescent current consumption from biassing the spare OPAMP at Vcc/2 isn't worth the cost of the resistors. Even without the LED, if quiescent current consumption was critical, the spare OPAMP could be biased from the in+ of the active OPAMP, saving two resistors, with negligible effect on its operation. |
| mike_mike:
Is there any relation between the ON LED and the spare opamp ? Could you please be more explicit ? |
| Ian.M:
Only that the LED is using more than an order of magnitude more current than the spare OPAMP, so any small improvement in quiescent current by biasing the spare OPAMP at Vcc/2 will have negligible effect on the circuit's total off-state current consumption. If you needed to save power, using a high efficiency LED so you could reduce its current consumption to 1mA would be a good start. |
| mike_mike:
Please confirm that the modifications to be made are: 1. add two resistor, each 330 ohms, one resistor in series with the LDR and one resistor in series with the 10k trimmer. 2. connect the non inverting input (pin 5) of the spare op amp to the pin 3 of the used op amp. Beginner questions: 3. can I use IRF530 or IRF630 in the place of IRF830 ? 4. the mosfet gate resistor should be left at 100 ohms or it should be increased ? |
| Ian.M:
1. Yes, that will work. As an ORP12 in direct daylight can have a rather low resistance, if you need to distinguish between the lower light levels typical of artificial light and direct daylight, its better to add an equal resistance to the preset and the ORP12. If you are only interested in lower light levels where the ORP12 resistance >1K,, a single 680R resistor in series with the preset would be sufficient 2. That would work, but it adds unnecessary complexity, to save at most a small fraction of a mA, and there's no good reason not to simply ground the spare OPAMP non-inverting input. 3. *ANY* power N-MOSFET* with an adequate Vds voltage rating would work as long as its Vgs threshold voltage falls in the range 2V-6V. If lower, it might not turn off reliably. If higher, there would be insufficent gate drive to get the datasheet nominal Rds_on resistance. 4. There's no benefit in increasing it, but even if you made it several K the circuit would still function. * Except modules, 'hocky pucks' and other extremely high current devices with far far too much gate capacitance vs the limited drive current available from the OPAMP |
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