Within the last couple weeks, my 50A Sorensen instrument power supply has been giving up the ghost. If I would draw anything more than a 19A load, the radio would start to output hash all over the band, generating a recklessly wide signal on CW. I must say, not very good amateur practice. Since then, I’ve been looking around for a solid power supply, even finding a nice 35A rack mount Duracom at a local hamfest for $75 which I ended up kicking myself days afterwards for not buying.

My search ended while browsing some eBay auctions and was instantly introduced to the Chinese OEM-type switching supplies used in various devices and appliances. The prices were very reasonable, 30A supplies for approximately $50. How good could they really be?

I remembered hearing something early on about switching power supplies causing interference in the shack, or generating hash so loud, it would cover-up the reception of weak DX stations. I immediately started my search on eHam to see if anyone else had attempted using similar supplies and expected to hear the usual horrific war stories. I was surprised to hear something positive on the subject with many hams having gone the same direction.

Here’s a link to one of such discussion threads:
http://www.eham.net/ehamforum/smf/index.php?topic=71829.15

With that in mind I found an eBay auction for 2 units and purchased both.

View at Picasa Chinese Power Supply (S-350-12)26-Oct-2011 22:30, Apple iPhone 4, 2.8, 3.85mm, 0.067 sec, ISO 160

View at Picasa Chinese Power Supply (S-350-12)26-Oct-2011 22:30, Apple iPhone 4, 2.8, 3.85mm, 0.067 sec, ISO 200

Upon wiring up one of the supplies with Anderson power-poles leads for output DC and the stripped back end of an extra computer power cord for the input AC, I was ready to unleash my abuse. Below you will see my findings tested with a power meter used for troubleshooting electric RC airplanes. The meter displays voltage, current draw, watts and amp hours and has been a very sought after tool in my shack. I highly recommend picking one up.

The first photo shows current draw on one supply leg of a 200w Kenwood TS-480HX at 100% duty-cycle, full output. Current draw measured was 19.5A.
The second photo captures nominal output voltage of 13.24V at minimal load. 1.2A was measured with the radio on and idle. This was the maximum voltage I could get out of this supply.
The third photo indicates the voltage drop with a 19.5A load. Output only dropped .25V.

View at Picasa TS480HX Single leg current draw at 200 watts26-Oct-2011 22:16, Apple iPhone 4, 2.8, 3.85mm, 0.067 sec, ISO 200

View at Picasa Voltage, minimal load, 1.2A idle26-Oct-2011 22:16, Apple iPhone 4, 2.8, 3.85mm, 0.067 sec, ISO 160

View at Picasa Approximate .25 voltage drop at 19.5 amp load26-Oct-2011 22:16, Apple iPhone 4, 2.8, 3.85mm, 0.067 sec, ISO 200

OK so it looks like the supply will do 20A fairly easy. But will it output it’s rated 29A? I was able to draw the radio’s maximum current draw from just one supply. This is probably a bad idea but it does seem to do it.

View at Picasa 26-Oct-2011 22:56, Apple iPhone 4, 2.8, 3.85mm, 0.067 sec, ISO 160

In addition to testing the supply on the bench with an oscilloscope (which output a clean DC trace), I also tested it with the 480HX at full output on CW. Not a hint of RF hash was detected.

I’m only a couple days on the new supplies and I’m very happy with my decision to go this route. The RF hash issues with these types of supplies seems to be a thing of the past. Long gone as the technology has gotten better. However I did notice a voltage output difference between the two units I bought. One of them outputs 14.8V max and the other 13.6V. This doesn’t seem to be all that critical for this application, but could indicate a QC issue during manufacturing. I don’t know who made mine, since I can’t read Chinese, but similar supplies can be found on-line under the name Mega-Watt or other makes with the similar red lettering in the upper left-hand corner of the printed label.

One of the other things to keep in mind is that the terminals are exposed, a concern especially for the AC input terminals. I plan to mount mine in a 2U rack enclosure with Power-pole panel mounts, an AC switch and volt meter. That should address the electrocution any nosy cats who may exploring the shack.

Until next time, 73!
de N3QO
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