The first reasonable compromise is to accept 30°C temperature rise in the track. The second, which is often not an option with low-cost fast-turn prototype board houses is to use 2 ounce (0.0028", 0.07 mm) copper. Even 4 ounce copper begins to impose limitations on trace & space width. Anything over four ounce is going to cost substantially more because it ties up the board fab's plating line for a long long time.

Again, a 4N25 has a specified current transfer ratio (CTR) of nominally 0.2. That means for each mA through the input emitter, you can expect only 200 µA through the transistor. For 77 mA for the relay, you would have to put 385 milliamps - over a third of an ampere - through the input emitter. It would be instantly destroyed.

Thank you very much

 

I have just seen an "indestructables" article where they actually use de-soldering braid to repair tracks. I guess that as the braid is coated with some sort of flux makes it easier to solder. Never tried it, but might be worth a try.

I've used the braid several times to save window air conditioner boards.

 

R2 and R3 are to make a voltage divider to achieve the Opto-Couplers operating voltage
U1 is the Opto-Coupler itself

i assure you that i know that those are resistors and optocouplers.
my question was what is the purpose of R2,R3 and U1 in THIS circuit?

there is no need for any of them...

 

I prefer standard 12 or 14 AWG solid wire to beef up a power trace.
Got scraps from house wiring projects.
I use a Weller 100/150W gun, since it does take some wattage.

 

I've used the braid several times to save window air conditioner boards.

I too traditionally use the Dry-Wick braid to repair oven controllers where the relay terminal and foil meet and usually gets blown.
Max.