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Operational Amplifier driving MOSFET
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Ian.M:
That's fairly sucky.   The power in connector, load connector, MOSFET, 1N4007 diode and 470uF cap all need to be as close as possible to each other with minimal trace length between them so the MOSFET and diode should be right next to the connectors at the board edge.  The power trace *MUST* go direct from the power connector to the 470uF cap and from there one branch back to the load connector and one off to the rest of the board.   Same goes for Ground if you aren't using a ground plane. All high current traces need to be beefed up - doubling their width where you can wouldn't go amiss.   The 100nF cap needs to be asw close to the OPAMP as possible with minimal trace length between them. The ORP12 needs a connector in parallel for off-board mounting.   Otherwise,it doesn't look *TOO* bad for a minimal jumpers single sided layout, though you could probably do better making the traces round the OPAMP apart from its power and ground pins skinny enough that you can route one between its pins.
mike_mike:
I uploaded new files.
Please have a look and tell me if it is ok.
I am not sure if I understood correctly what you said.
Ian.M:
I've hilighted the critical high current traces that need to be shortened and separated from the low current traces.  They should be grouped in one corner of the board, with the power and load connectors next to each other, then the MOSFET beside them, then the 470uf cap.  The 1N4007 diode and the gate resistor should be near the MOSFET.  They can go between the MOSFET and the cap for a bit of space so the MOSFET doesn't cook the cap if its ever loaded enough to run hot.   Apart from the diode, *NOTHING* I haven't hilighted should join a highlighted power or ground trace anywhere other than at the 470uF cap pins.
Zero999:
You don't need big, fat, traces everywhere, although it doesn't cause any harm. Only the ones which carry current, to the load, i.e. the ones from, J2, to the MOSFET's drain, source, the 470μF capacitor and J1 need to be much larger, than the other traces, which can be relatively thin. Ideally the heavy current carrying traces should be solid copper pours, if possible.  I don't know what the exact scale is. The traces look thick enough for 3A, but use the calculator, linked below, to be sure. Consider the trace length, the voltage drop, as well as the temperature rise.
http://www.4pcb.com/trace-width-calculator.html

By the way, please avoid using jpg format for drawings. It results in large files and fuzzy edges, which can make editing difficult. Please stick to PNG for schematics and PCB layouts. Look at the size of the files I posted and how sharp they are, compared to the JPGs. I also reduced the colour depth to 4-bit, to make the files size, as small as possible, but this can only be done with images which don''t contain many colours, without loss of quality: 8-bit colour will be fine for most drawings.
mike_mike:

--- Quote from: Ian.M on April 29, 2018, 08:43:06 am ---It should work as-is, but Hero999 has proposed a small improvement (top schematic in reply#30) If you want to adopt his improvement, as you need to drive both a LED and a MOSFET, move the connection to R4 for his output LED from pin 7 to 1 to drive it off the first (comparator) OPAMP in the chip, and drive the MOSFET gate resistor from pin 7 of the second (buffer) OPAMP.

--- End quote ---
You said to drive the led from pin 1 of LM358. I drive it using the mosfet (like in the top schematic from reply #36). It is a mistake ?
I already made the top schematic from reply #36 and it is working. Also, the layout was made for the top schematic from reply #36.
It is a problem if I drive the led as shown in the top schematic from reply #36, using the mosfet ? (That led is connected at J3)
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