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September 23rd 06, 08:17 PM
posted to rec.radio.amateur.homebrew
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What's in a "wall wart" so-called "transformer"?
W3JDR wrote:
There are also some transformer-input types that produce well-regulated DC outputs using linear regulator technology, and a growing number of units that use switching regulator technology to make the transfoprmer so small that it's hard to tell from the outside that there's even a transformer in it.. The latter are easily distinguishable because they put out quite a bit of power, yet are smaller and much lighter in weight than traditional units with equivalent power capability. Joe W3JDR wrote in message ... wrote: I have a collection of "wall wart" (others may call them "transformers") and I'd like to use one to cobble a little power supply. I've never opened any to see what's really in them (and suspect it'll be quite a chore), but I'm quite sure they are not ALL "transformers", because some claim AC and some claim DC output, yet the first I grabbed out of the junkbox claimed DC AND ALSO provided a HEALTHY AC output on the same two wires! Furthermore, NONE of them seem to be voltage regulated! (But if one says "X volts at Y milliamps", then loading it to Y mils usually gives an output close to X volts.) So what IS in them? Do the really light-weight ones, for example, use a capacitor for relatively low-loss voltage dropping? And why would one give *BOTH* AC and DC? Bad diode(s)? -- --Myron A. Calhoun. Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge NRA Life Member and Rifle, Pistol, & Home Firearm Safety Certified Instructor Certified Instructor for the Kansas Concealed-Carry Handgun license It depends... The AC ones are just a transformer. The DC ones contain a transformer and diode(s). The cheapies usually don't contain any capacitors or regulators, hence there is significant AC on the DC. -- Jim Pennino Remove .spam.sux to reply. Such units actually have a bridge rectifier across the AC mains and supply DC to a switching regulator running at 20-50khz. There is an isolation transformer running at this frequency (it's core is just a small bit of lightweight ferrite). The output of the isolation transformer feeds another bridge rectifier and filter (at ~40khz not much of a filter is needed, small caps and chokes!). There may then be a linear or switching regulator. If the later, it may actually regulate by changing the switching frequency/duty cycle of the original switcher at the AC mains input. In this case the feedback is by means of an opto-isolator for safety. |
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