[Lab] AC adapter question

Richard Guy Briggs rgb at tricolour.net
Fri Jul 12 00:50:22 EDT 2013 

On Thu, Jul 11, 2013 at 08:56:28PM -0400, greg atcheson wrote:
> Your LEDs have a forward voltage of 1.6 volts and you are supplying 5
> volts,  leaving 3.4 volts for the 91 ohm resistor. Ohms law: current =
> volts / resistance. Plug in the numbers and you get 0.037 amps per LED,
> much less than the 0.250 amps that the resistors are rated for. The total
> current would be 7 amps, which is more than your supply is rated for.

Actually, the resistors are rated at 0.25W (a quarter watt), not 0.25A.
To get the power dissipation, P = V x I = 3.4V x 0.037A = 0.1258W.

Now that I see your piece, I would have put those LEDs in two strings of
94 each, reverse-bias one string and just plug them straight into the
wall!  Ok, so you would need to add one resistor in series with the
power cord, say, a 490 ohm, 1W resistor.  That might detract from the
visials by removing 188 91 ohm resistors though...  and save you
a whole lotta soldering...

If you are nervous about 120VAC going straight into your project (which
is an entirely rational concern), a reasonable in-between would be
strings of 20 with a 24V transformer, no rectification, and no
resistors.

> On Jul 11, 2013 7:59 PM, "Britta Evans-Fenton" <technical at artengine.ca>
> wrote:
> 
> > I'll check it out tonight or tomorrow morning, when I'm with my artwork
> > again to be sure, but I think it might be even more than the 9 amps Michael
> > suggests.
> >
> > How important do you think the other resistors would be. It would be a
> > huge undertaking to re-do this piece.
> >
> > Here is the work here:
> > http://brittaevansfenton.com/System_Failure.html
> >
> > Britta
> > *Technical Coordinator *
> > Artengine.ca
> >
> > Ottawa Mini Maker Faire
> > Aug. 31 & Sept. 1 2013
> > www.makerfaireottawa.com
> > @MakerFaireOTT
> > #MMFO
> >
> >
> > On Thu, Jul 11, 2013 at 4:49 PM, Darcy Whyte <darcy at siteware.com> wrote:
> >
> >> Why don't you check how many amps you are using?
> >>
> >>
> >>
> >> --
> >> Darcy Whyte
> >> Twitter: https://twitter.com/inventor_Artist
> >>
> >> Art+ inventorArtist.com | Aviation rubber-power.com | Software Since '88
> >> siteware.com | Contact: darcy at siteware.com | 613-563-3634 by appointment
> >> | Canada N 45? 25'03.1" W 75? 42'21.4"
> >>
> >>
> >>
> >>
> >> On Thu, Jul 11, 2013 at 7:41 PM, Britta Evans-Fenton <
> >> technical at artengine.ca> wrote:
> >>
> >>> Maybe I'm drawing too much Amps.  Would you recommend this AC converter
> >>> over the last one:
> >>> http://www.digikey.ca/product-detail/en/LS150-5/285-1817-ND/1918828
> >>>
> >>> I would have Amps to spare on this one.
> >>>
> >>> Britta
> >>> *Technical Coordinator *
> >>> Artengine.ca
> >>>
> >>> Ottawa Mini Maker Faire
> >>> Aug. 31 & Sept. 1 2013
> >>> www.makerfaireottawa.com
> >>> @MakerFaireOTT
> >>> #MMFO
> >>>
> >>>
> >>> On Thu, Jul 11, 2013 at 4:32 PM, Britta Evans-Fenton <
> >>> technical at artengine.ca> wrote:
> >>>
> >>>> There are 188 IR leds:
> >>>> http://www.digikey.ca/product-detail/en/QED233/QED233-ND/187410  All
> >>>> connected in parallel each with their own Resistor ( 91 OHM 1/4W 5% CARBON
> >>>> FILM)
> >>>>
> >>>> Britta
> >>>> *Technical Coordinator *
> >>>> Artengine.ca
> >>>>
> >>>> Ottawa Mini Maker Faire
> >>>> Aug. 31 & Sept. 1 2013
> >>>> www.makerfaireottawa.com
> >>>> @MakerFaireOTT
> >>>> #MMFO
> >>>>
> >>>>
> >>>> On Thu, Jul 11, 2013 at 4:00 PM, Ken <klmckinnon at rogers.com> wrote:
> >>>>
> >>>>>  It should power a LOT of LED's.  How many were you using and what
> >>>>> sort of current limiting resistor did you have....a quick circuit diagram
> >>>>> would tell a lot.
> >>>>>
> >>>>> Ken
> >>>>>
> >>>>>
> >>>>> On 7/11/2013 6:49 PM, Britta Evans-Fenton wrote:
> >>>>>
> >>>>>  A little question about AC adapters.
> >>>>>
> >>>>>  I have an artwork that I did that I used an AC adapter to get power.
> >>>>>  It was on for 4 days during a show, and I was surprised on how hot it got.
> >>>>>  (I almost burned myself).  I did have duct tape on it holding it to the
> >>>>> back of a wall, so that may have been half the problem.
> >>>>>
> >>>>>  I'm showing the work again, but I'm wanting to make sure everything
> >>>>> is safe.  I need to hide the adapter due to aesthetic reasons, so I wanted
> >>>>> to create a little white box around it with air holes, and I was suggested
> >>>>> a fan, although it if it getting hot due to over working, adding another
> >>>>> fan might not be a good idea.
> >>>>>
> >>>>>  It is only a bunch of IR leds that it is powering, so I'm surprised
> >>>>> at the power I am needing.
> >>>>>
> >>>>>  any help would be very well appreciated.
> >>>>>
> >>>>>
> >>>>>  Here is the Adapter I used :
> >>>>>
> >>>>> http://www.digikey.ca/product-detail/en/VGS-25-5/102-1928-ND/2045659
> >>>>>
> >>>>>
> >>>>>  Here are the IR LEDS:
> >>>>>
> >>>>> http://www.digikey.ca/product-detail/en/QED233/QED233-ND/187410
> >>>>>
> >>>>>
> >>>>>  And the Resistors I have on them were 91 OHM 1/4W 5% CARBON FILM.
> >>>>>
> >>>>>
> >>>>>   Britta
> >>>>>  *Technical Coordinator *
> >>>>>  Artengine.ca
> >>>>>
> >>>>> Ottawa Mini Maker Faire
> >>>>> Aug. 31 & Sept. 1 2013
> >>>>> www.makerfaireottawa.com
> >>>>> @MakerFaireOTT
> >>>>> #MMFO
> >>>>>
> >>>>>
> >>>>> _______________________________________________
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> >>>>>
> >>>>>
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> >>>>
> >>>>
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> >>
> >>
> >
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	slainte mhath, RGB

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