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Those AC/DC modules: my, how you have shrunk!

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bcarso
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re: Those AC/DC modules: my, how you have shrunk!
bcarso   1/26/2011 5:40:38 PM
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Inefficient they are, certainly---but as well as generating almost zero HF noise (if it is a simple transformer) or just diode reverse-recovery switching spikes (if it is a d.c. output one), the "linear" adapters generally have much lower input-output coupling capacitance. So your downstream load, until otherwise referenced to something, has a few tens of picofarads couping to roughly half of the mains voltage. This is still sufficient to wiggle the load around a bit, and if it has sensitive nodes exposed to the electric fields/references (mainly earth potential) of the outside world will induce noise into the system. Local shielding will often be required, unless impedances and gains are very low. But---compare this to a switcher, which is usually very much worse---not only are the transformers poorer for isolation, but as well, to meet conducted noise compliance there is often an additional capacitor coupling from input to output! When a client company began to migrate to switcher adapters as mandated, they often had to go and redesign the products that they powered---too much hum/buzz in the audio.

BicycleBill
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re: Those AC/DC modules: my, how you have shrunk!
BicycleBill   1/13/2011 2:00:15 PM
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It would be a great project-if I didn't have my day job! Yes, I had some "idle time" on my hands to do what I did--but that was as much as I had, unfortunately (or is it "fortunately"?)

Loring
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re: Those AC/DC modules: my, how you have shrunk!
Loring   1/13/2011 1:44:47 PM
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So, Bill how about a follow-up on electrical performance chart: Efficiency at load of .5, 1, 2Amps Power used at 0 load Noise (PARD) at loads? Regulation (%) at load

sharps_eng
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re: Those AC/DC modules: my, how you have shrunk!
sharps_eng   1/6/2011 10:36:26 PM
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Quantifying the RF EMI you may well find that it is instantaneously narrowband, but because small switchers and LED lamps often have variable-frequency or pulse-skip behaviour, coupled with undersized electrolytics and indifferent filtering, you get a kind of 'wandering' wideband noise across the spectrum. This is bad news because it can add and subtract with other noise sources and create intermittent blanking of RF control devices and transmissions. They also don't all switch off when unloaded. Actually this is handy, my phone charger is clearly audible off-load, and chirps away at night to tell me I forgot to put my phone on charge!

BicycleBill
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re: Those AC/DC modules: my, how you have shrunk!
BicycleBill   1/5/2011 9:53:50 PM
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My informal testing using an AM radio near the power modules shows that the switches do, indeed, pump out a lot of RF. The linear units produce none. Maybe not scientific--but a good first-order indication of RF. Same problem with my HP printer, and mnay other devices. And then there's the light dimmer--it radiates RF plus it turns the AC wiring it is on into a big RF carrie system, causing EMI far away!

sharps_eng
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re: Those AC/DC modules: my, how you have shrunk!
sharps_eng   1/5/2011 6:39:36 PM
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When I designed a linear wallwart I was shocked to find out how the transformer design world worked: while equalizing the copper and core losses and ignoring coupling efficiency, you fit the smallest core you can get away with (temp rise being the determinant) while employing a pre-designed bobbin which ensured you met the safety reqts. If I had used anything I learned in college it would have been huge overkill, but by cutting every corner it ran hot but was cheap enough to sell. Several lessons to be learned there. BTW what was the noise level on your various wallwarts and how well did they filter RF and transients?

Duane Benson
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re: Those AC/DC modules: my, how you have shrunk!
Duane Benson   1/5/2011 4:46:34 PM
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One of the nice things about my collection of old linear wall warts is the variety of voltages available. I'm sure there are switchers with other than 5V, but my empirical study (looking through my box of stuff) says that anything other than 5V in a switcher wall wart is rare. I have all variety of linear units, from 3V regulated DC up to 24V AC. That gives me a lot of versatility in supplying power to my projects without having to buy something new or build something. If I want to build, I do have a set of old bare transformers too. On the other hand, it's getting rarer to find an application that requires other than 5V or lower. So though my collection may last until 2013, they may never get any use anymore.

Sanborn84
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re: Those AC/DC modules: my, how you have shrunk!
Sanborn84   1/4/2011 11:03:01 PM
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The really sad part is that your decade+ old bricks are the only ones that will last until 2013. I've had two bricks physically identical to your 2A USB switcher die within the last year. One of them was also so poorly designed/constructed that it's death could be predicted almost to the month that it would no longer allow my little Roku box to access Netflix.

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