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Project 001 - Simple 12vdc 2amp Switching Power Supply

DISCLAIMER: This design is experimental, so if you decide to build one yourself then you are on your own, I can't be held responsible for any problems/issues/damage/injury that may occur if you decide to follow this build and make one yourself.


My Draytek DSL modem/router's replacement no-name 12vdc/2A PSU (cheap foreign rubbish!) has been playing up so for the past while I've been using an old Bench Power Supply.........not ideal!

According to Draytek the official PSU is a 12vdc @ 1.5A design and with the actual power requirements demanded by the 2820n router at just 10-15W. I also measured the actual current consumption to complete the picture and which gave approx. 700mA @ 12vdc.

However, with nothing readily available to hand and not really willing to buy a new one I reckoned I had enough parts in my workshop to build a simple switching PSU. So, looking through my bucket-of-bits I found a large 35VA 15v AC-AC Adaptor. In addition, it had enough space inside it to mount a pcb.

After assembly and testing it works just fine. One concern I did have was the VA rating of the transformer, i.e. would it cope? Well, the calculations said it would even taking the regulator volt drop into consideration. I am getting 12vdc @ 2amps output.
Also, as the regulator IC is actually a 3A switcher the heat dissipated by the heatsink is minimal, and with plenty vents in the ABS casing of the adaptor housing it seems to be ok at full 2A load.


Listing the specifications/calculations for the design:

35VA Transformer = 2.3A @ 15vac
Unregulated DC output off-load = Approx. 21vdc
Regulator drop-out voltage = Better than 1.5vdc
Bridge Rectifier = 3A average at full load output
Finally output power rating = 24W



1 * 35VA 15vac transformer (part of an AC-AC Adaptor)
1 * 6A bridge rectifier
2 * 20mm fuse holder
2 * 3A 20mm anti-surge fuse
2 * 1000uF 50v Electrolytic Capacitor
1 * 1000uF 25v Electrolytic Capacitor
1 * LM2596T-12 3A Step Down Switch Mode Regulator (TO220 Package)
1 * SB540 Schottky Diode
1 * 33uH 3A Inductor
1 * Aluminium Heatsink