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Power Supply Evaluation
By Eric Lemmon WB6FLY
The Mega-Watt S-350-12 power supply (see photo below) is based upon the Motorola TL494 Switchmode® Pulse-Width Modulation controller chip. It is housed in a semi-open frame module measuring 8.5 inches (215 mm) long, 4.5 inches (113 mm) wide, and 1.9 inches (49 mm) high. It weighs 31.6 ounces (896 grams). All electrical connections are made on a 9-position terminal strip at one end that has a plastic barrier strip over the top to keep stray fingers away. This model is rated at 30 amperes continuous and 33 amperes peak. A fan is included to keep the innards from overheating. A recessed switch on one side allows the AC input voltage range to be set for 90-132 volts or 180-264 volts. The unit will accept any line frequency in the range of 47 to 63 Hz. A small pot adjacent to the terminal strip allows the DC output to be adjusted over a narrow range, specified as 10 to 13.8 VDC. There are no internal adjustments; the over-voltage and over-current trip points are fixed. A glass 5x20 mm fuse rated at 8 amperes is soldered into the PC board at the AC input terminal. A green LED indicates when power is applied to the unit. No schematic diagram was provided with the unit. Here's a photo of the working end.
The design of the power supply is typical of modern switching supplies, and appears to be well laid-out (see the inside photo below). All of the power-switching transistors are provided with appropriate heat sinks, and all were installed with heat sink compound and insulating film. The components that might be damaged by vibration were secured with epoxy. Although the power supply had no labels to indicate the country of origin, the cardboard used in the packaging appears typical of products from Asia. The fan is an ARX DC Brushless device, part number FD1260-S1112C. It is a 60 x 60 mm fan with sleeve bearings.
To minimize the possibility of burning up one of the DC output traces on the PC board, I suggest paralleling the three output terminals for each polarity, and using #10 wire or larger to feed the load (see photo below for the test cable configuration). I confirmed that the negative output is floating, and is not jumpered to chassis ground.
The Mega Watt S-350-12 power supply closely resembles the Mean Well S-350-12, although the specifications are slightly different. Coincidence? I think not! One difference between the two is that the terminal board projects beyond the cabinet on the Mean Well but is flush with the cabinet on the Mega Watt.
The range of the output voltage adjustment was found to be 8.73 to 15.12 VDC. The fan operates continuously, at a speed that varies with the voltage setting. A GenRad StroboTac® was used to measure the fan speed. At 8.73 VDC, the fan speed was about 1700 RPM, and at 15.12 VDC, it was about 3080 RPM. At 13.80 VDC, the fan speed was about 3000 RPM. As explained in the instruction sheet, the fan runs at a constant speed, and is no longer thermostatically controlled as in previous versions. Despite running at a fairly high speed, the sound of the fan was barely audible.
As shown in the Load Test Report, the power supply was very stable, and was able to supply 30 amperes continuously for more than six hours without any problems. The hottest spot on the case stabilized at 144 degrees Fahrenheit, where the two power switching transistors are clamped to the sheet metal as a heat sink. The load test was stopped at 33 amps, because the AC input current at 33 amps DC load is very close to the AC Mains fuse rating of 8 amps. The linear decrease in measured voltage as the load was increased is due to voltage drop in the #10 AWG cable. Refer to the Power Supply Tutorial for details on how the Load Test was performed.
The frequency of the switching oscillator was measured and found to be 27 kHz. The amplitude of the signal varied with load, as expected, but the frequency remained constant. Both the input and the output circuits of the power supply contain EMI filters, and I found no evidence of spurious emissions.
This power supply easily passed a severe test that many units fail: the ability to start under load. I repeatedly cold-started the unit with a 30 amp load connected, and it started every time.
Overall, the Mega Watt power supply performs as expected, and seems to be a good value for the money. The only negative comments I can make are these: I would rather the fan have ball bearings instead of sleeve bearings, and I found it difficult to set the output voltage to an exact value, due to the wide span of the small adjustment pot.
The power supply I tested was purchased from Marles Electric for $50, plus $5 for shipping and $4.50 for sales tax. Web site: www.12voltpowersupplies.us. A power cord with an inline switch was included at no extra charge - a nice touch.
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This page originally posted on Sunday 19-May-2013.
This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.