12V 1.25A Power supply S-15-12


Official specifications: I got it from ebay dealer lucky-bird2011


This cheap power supply arrived in two envelopes inside each other.

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It has screw terminals for input and output connections and a trimpot for small voltage adjustments. There is also a led to show output voltage is present.


The model number and ratings are clearly marked on the supply.


On the top is holes for accessing the screw terminals, the trimpot and the mounting holes.


There is two mounting holes/slits at the bottom.



At 120VAC the over current limit is at slightly above 1.3A, this looks fine.


At 230VAC the current limit is slightly above 1.5A.

Running the power supply with 1.25A current for one hour worked, but it got hot.
The temperature photos below are taken between 30 minutes and 60 minutes into the one hour test.


M1: 48.1C, M2: 27.8C, HS1: 109.3C
The white squares are black tape, to better show the temperature (Shiny metal works as a mirror).


M1: 49.6C, M2: 117.0C, M3: 79.3C, HS1: 131.8C
The transformer inside is very hot.


M1: 66.9C, HS1: 124.0C


At 0.6A the noise is 25mV rms and 1068mVpp.


At 0.8A the noise is 25mV rms and 1244mVpp.


At 1.2A the noise is 24mV rms and 1100mVpp. The RMS value is low, the peaks is probably due to missing filtering and is common mode noise.

Tear down


The box is very easy to open, the lid can just be removed, there are no screws. This also means it must be mounted in a protected location.

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Everything is on a single sided circuit board.


One screw and the circuit board could be removed, the is a piece of isolation plastic between the circuit board the the box. The screw connected the earth terminal to the box, a toothed washer would have been nice here.


The mains input has a small glass fuse (F1) followed by 4 diode as a bridge rectifier. The switcher (DK1203: 12W) is a slightly underpowered for the job. There is a safety capacitor and optical feedback. On the output side is a large rectifier diode (D7), two capacitors (C8, C?) with a inductor (L1) between and the voltage reference (IC3: TL471).
This is a very typical switcher design, but without input filtering.


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This side do not have any parts.



Isolation distance is only 3mm here.


And 2mm with the slot.

The power supply passed the 2830 volt test, but failed the 4242 volt test between mains and low volt side and between mains and enclosure.


This power supply cannot really deliver the rated current for a long time, but will probably work fine with a lower current level. The safety requires that the earth connection is used also on the connected equipment, it is not safe enough to work without.


The power supply was supplied by a reader for review.

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