AC vs DC adapters

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duke
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AC vs DC adapters

Post by duke » Thu Nov 29, 2012 4:11 pm

ok so most routers / modems have dc powersupply

some require ac but internally they always(?) convert it back to DC using rectifiers /diodes
has anyone ever fried a device with dc in place of ac

i have always put dc powersupply/adapter (smps based) on these router which require AC voltage

for eg 9V 1 Amp ac gets 10V 1 amp dc
15v 1 Amp ac to 12 v 1amp dc
basically trying to keep wattage in limited range

{limited range as in if you deviate too much then excess heat EMI radiation pcb overheat will shorten life of components like capacitors and since devices are on 24X7 life expectancy is in months if not cooled}

Code: Select all

[b]watts = amps X volts[/b]
also when giving dc supply to ac device it helps it keep it cooler as diodes /rectifier generate heat

and ac supply is done mostly by cheap manufacturers by using a transformer , makes adapters heavy
dc adapters are light because they use smps based components and run cooler



has any one any input on this
mstombs
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Re: AC vs DC adapters

Post by mstombs » Thu Nov 29, 2012 5:43 pm

Use the correct one or you may find you have invalidated your house insurance and they won't pay out when it causes a fire and burns your house down!

AC or DC and the correct voltage rating essential, excess current rating not a functional problem but may invalidate safety protection and CE mark etc. Have definitely heard of fried routers with wrong polarity plugs, or using 12V on 5V device.

PSUs often quoted with "VA" rating not watts, may mean max voltage * max current, as there's no guarantee you can get both at same time - and the power equation requires a cos(phi) term for AC.

Rare to see transformers in routers, guess they tend to use efficient but noisy DC to DC convertors or simple inefficient linear drop for 5V to 3.3V.

In my experience the PSU is usually the weakest link and it is electrolytic capacitors in them that fail, but I do test with router with replacement 5V/3.3 chip.
duke
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true

Post by duke » Thu Nov 29, 2012 6:54 pm

fortune favors the brave
wish it was true
better safe than sorry is the motto

anyway i always make sure if the AC device is actually using DC or not
if a board converts ac to dc it will have diodes (but newer one dont{for them you might have to look at component names near power barrel jack and google them})
AC board.jpg
board which needs AC but can be fed DC
dc_board.jpg
board which requires dc supply
and of course tombs is right you will be at risk, and of course disclaimer
i can be held responsible
mstombs
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Re: AC vs DC adapters

Post by mstombs » Fri Nov 30, 2012 6:07 pm

Sometimes cheap devices have diodes for polarity protection - they just short out the supply if not connected right way round.

There's been discussion on RaspberryPi forums about simple chips/boards that can replace linear regulators - i.e. use a DC-DC convertor (goes via high frequency and tiny transformer) to generate 3.3 or 1.8V etc from >6V supply, aparently much more efficient than just lowering voltage by generating heat - but presumably the noise not good for ADSL or WIFI, but these are so cheap I couldn't resist... but not used yet!

http://www.ebay.co.uk/itm/LM2596-DC-DC- ... 0900704967
duke
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Re: AC vs DC adapters

Post by duke » Wed Dec 05, 2012 7:33 pm

you mean buck converters
which can be as efficient as 95 %

hmm... how about a emi shield/faraday cage for the components for noise suppression

http://www.talkingelectronics.com/proje ... plyP1.html

http://www.talkingelectronics.com/proje ... plyP2.html :shock:
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