Monday, February 18, 2013

Power Supply Pt. 2: Buying stuff!

August-2012 to near-present day and beyond…

In my last posting I reviewed some of the math involved to get our power supply components. 1500VA @ 58 VDC for the transformer, at least 26,666 uF for the capacitor bank, and 6 bridge rectifiers capable of handling the output from the transformer.

I can't stress enough that if you want to just go out and buy these things, there are a few different manufacturers making questionable grades of products, and few if any descent, objective and comparative reviews. I may write an article in the future on confirmation bias and compulsive forum posters, but it should go without saying that there are very few people who aren't in this professionally that will buy more than one version of essentially the same piece of hardware and the hardware to test it in order to review it for free for a limited audience. I am no different and just as cheap.

I originally decided I wanted to wind my own, and after deciding that was too much work, I settled on the idea of buying an Antek AN-15458, a 1500 VA @ 58 VAC toroid, with two 12v additional secondaries and split primaries allowing the use of 120 or 240V input voltage. That plan went away when I found a new Antek AN-15468 on eBay for less than half the cost of a new one, and bought it and some glass tape with the intentions of dewinding it until I got my intended voltage.

DSCF1147 About this same time I looked all over eBay to find some decently priced DC capacitors that would make a sufficient cap bank for my CNC. After poring over what must have been thousands of listings trying to find the best deal, I stumbled upon a set of 6 NOS, 200V, 56000 (!) uF electrolytic caps with mounting brackets for $5.75 each plus shipping, on an auction that showed them listed incorrectly as 5600 uF, and made a Buy-It-Now offer of $4.50 ea, and took the whole lot home for only $27 + SH! By comparison caps of similar size and voltage were looking to cost me in the range of $40-70 + SH/ea!

With the extra change rattling around my pockets I got a little cocky and bought a lot of a dozen 50A 600V bridge rectifiers for under $30. I figured it would be good to have a couple extras on the shelf if I needed them. I don't have a ton of semiconductor experience but my considerations for purchase required them to be US or Japanese made with a metal casing for better thermal conduction to a heatsink. I'm not sure what the rate of failure is (and don't care to find out myself), but beware that eBay is absolutely flooded with cheap rectifiers, it was easiest for me to identify them by the rough castings of the metal cases and sorting the location to only show items in North America. There are still American sellers of the poorly made imports but sorting by location reduced their numbers greatly.

Next up, a power supply build!

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