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WEP Power Supply PS-305D

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This compact power supply was advertised as a switched mode device with a rating of 30 Volts at 5 Amps. In the images below the actual size is 1024 px wide, download to see full size image.

Screenshot of top of advert.
The main display advertising showing the 30 Volt 5 Amp rating.
Power supply specifications from display advert.
Pages one and four of the instruction manual from the product box.
Pages two and three of the manual.
Instructions for use section enlarged.

The yellow highlight of section D is my addition. The restriction on use is clearly stated and even blames the operator if the 60% is exceeded and the supply fails.

Front of PS-305D with cover removed.
Photograph taken from above. Note the mains transformer!
Side view showing the specification of the mains transformer. On the left can be seen the three series pass transistors.

A 100 VA transformer from RS has laminations of 80 x 7 mm and the lamination stack is 30 mm thick. Here the laminations are 95 x 75 mm and 48 mm thick and so clearly the transformer is rated at around 150 VA. A vintage Farnell supply of 30 Volts and 5 amps has a much larger transformer to allow for the power lost in the four large finned heat-sinks.

Here we see the plain aluminium sheet that acts as heatsink for the three power transistors.

The small heatsink will struggle to dissipate much power, even with fan assist. In fact powering a valve heater at 4.0 Volts and 1.5 Amps causes the fan to run at maximum after a few minutes and that is only 6 Watts.

It is clear that this is a power supply designed for light duty applications. The build quality looks fine.

The circuit board has a pair of relays and only a small capacitor, not the large reservoir of classic designs. This suggests that the different tappings are switched into circuit as the voltage requirement alters - that is a cleaver bit of design. It is possible that the regulator is a hybrid of a switch mode design operating at the low voltage followed by the series pass transistors.

This looks a useful piece of equipment, but it is not a switch mode power supply as advertised nor is it capable of 150 Watts of continuous DC output as the advert leads you to believe.

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