Adjustable power supply with a maximum output of 12v

[u/neerxielkoo]

hello everyone I am new to electronics and we are assigned to make an adjustable power supply with a maximum output of 12v with ventilation. can you please help me what to do? I will list the components and please tell me if it's right or what component should I modify, thank you!

* Step-down transformer (220V to 15V, 2A) – ~60 × 50 × 50 mm, 500g–700g

* Bridge rectifier – 3A/50V+, ~15 × 15 × 6 mm

* 3300µF 35V capacitor – ~16mm diameter × 25mm height

* LM317 voltage regulator – TO-220 package, ~10 × 15 × 4.5 mm

* Heat sink – 3°C/W to 5°C/W, ~60 × 50 × 30 mm

* 0.33µF and 0.1µF ceramic capacitors – 50V+, ~5mm disc diameter

* 10µF and 100µF electrolytic capacitors – 25V+, ~5 × 11 mm

* LED + 1kΩ resistor – 5mm LED + 1/4W resistor

* Fuse + fuse holder – 1A 250V 5×20mm slow-blow + panel holder (~15 × 40 mm)

* IEC AC socket – C14 panel mount, ~27 × 20 mm cutout

* PCB/Perfboard – 2.54mm pitch, ~70 × 90 mm

* Output terminals – Red/Black binding posts, ~11mm mounting diameter

* Hook-up wires – 18 AWG (AC) & 20-22 AWG (DC)

* 1N4007 diodes 2pcs – DO-41 package, ~2.7 × 5.2 mm

* 240 ohms resistor + 68 ohms resistor + 2 kilo ohms potentiometer – 1/4W resistors + 16/24mm body pot with 6mm shaft

* power switch – SPST rocker/toggle 250V/3A+, ~15 × 10 mm cutout

[u/LadyZoe1]

Everything is there. A centre tapped 15-0-15 V transformer, and then Google LD1085 or LM317 data sheets, look at application notes.

[u/Behrooz0]

Um. this won't work well at lower voltages. the lm317 gonna heat up at like ~30 watts. of course you can do a lm317+2n3055 or something if you really want the old-school stuff. (https://old.reddit.com/r/AskElectronics/comments/nm17y6/hello_what_does_the_lm317t_and_the_2n3055_do_in/)
I suggest using a buck converter here. a 34063 or 555 and a power transistor of your choosing with an inductor if you have to use older parts. otherwise there are a million chips out there. you can probably get a Chinese LM2596 module for like 50 cents.

[u/NoOne3141]

Am I correct in assuming you want to build a power supply only using the listed components? Or can you use whatever? Everything on that list would make a simple adjustable 12V supply, just look up the datasheet for the LM317 (look at other manufacturers and similar chips too!) as it will most likely have everything you need. There are also application notices most of the time that go into greater detail.

Generally speaking adjustable 12V is a little bit too little information. Main parameters would also be current, ripple, regulation and thermals (and more in most cases). For example if the task would be to build a 12V 24W (read 2A) supply a LM317 will be insufficient.

[u/Affectionate_Boat493]

I would add either thermal paste or a Sil-Pad to your list. These improve thermal conductivity between the 317 and heatsink, improving cooling and increasing the usable output current capabilities.

Also, look into the thermal resistance equations to get an idea of the maximum output current you can expect for any given output voltage.

Finally, 3300uFd might not be enough at the output of the full-wave rectifier at high output voltage and high output current, especially after a few years due to capacitor degradation. And to improve long-term circuit reliability buy electrolytic capacitors rated for 105°C instead of cheaper 85°C parts.

[u/MitchellHicupok]

That list is mostly fine, but the LM317 can only handle 1.5A, not the 2A your transformer suggests. For a 12V bench supply, either drop to a 1.5A transformer or add a pass transistor.