Re: 12v adapter for lights

In <8670c280-affe-4fb3-8e4d-84985babf77d@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
JIMMIE wrote:

On Mar 8, 9:15 pm, d...@xxxxxxxxxxxxxx (Don Klipstein) wrote:
In article <rImdnWrYTeEe5y_UnZ2dnUVZ_tmWn...@xxxxxxxxxxxxx>, HeyBub wrote:
fredinstl wrote:
I bought a lighting system from Ikea ~2 years ago, made of 3 halogen
lamps, each being 12V 20W. It's mounted on electrical wires.
The adaptor died. It is actually a little scary as one wire feeding
the adaptor apparently get too hot, and almost melted the adaptor
plastic. Looks like a fire hazard to me.

On the adaptor, it says:
input: 120V, 60Hz, 0.6A
output: 11.5V, 5A, 57.5W

which kind of makes sense from what I remember from school: Watt = Amp
x Volt
I went to radioshack and the guy said they didn't carry anything like
this.

So I the plan now is to buy an adaptor from the internet but not sure
what to get:
- can I buy any 110V to 12V 5A adaptor ? Apparently, it is easy to
find as they are commonly used as a LCD adapters. e.g:
http://www.amazon.com/New-Power-Adapter-Monitors-Cable/dp/B000SJ7OMA/...
- what about the fire hazard: should I replace the whole lighting
system ? or 12V 5A should be fine ? or should I go maybe with more
Amps, such as 12V, 6Amp to be on the safe side?

Any feedback much appreciated.

If each lamp is 20 watts, you're pulling 60 watts from a transformer rated
at 57.5(!). No wonder it croaked.

Confine your replacement search to something rated around twice the
anticipated load. Go for 100 watts minimum.

  If the load is far below the load rating of the stepdown device, and the
stepdown device is an unregulated type, then such an underload can lead to
excessive output voltage - and shortened lamp life.

  I would go for either something known to be regulated or something
with load capacity only mildly exceeding the actual load.

 - Don Klipstein (d...@xxxxxxxxx)- Hide quoted text -

- Show quoted text -

Regulation depends on the quality of the transformer. On a lot of
cheap import transformers a 12 volt transformer may put out 18 volts
into a light load then drop to 10 when pushed to near there rated
capacity. Get a good transformer and you dont have to worry about
this. Good transformers arent cheap but neither is replacing cheap
ones. My 12.6 volt 10 amp Stancor transformer only varies about 1 volt
from no load to full output.

Besides quality, there are 2 other factors:

1. Voltage variation with change of load tends to be less with larger
transformers and more with smaller ones. Though my experience is that
upsizing the transformer tends to make load voltage more excessive, until
the transformer gets so big that open circuit voltage is only a little
higher than the nominal load voltage. A 12.6V 10 amp Stancor brand one is
a big as well as decent quality transformer as far as ones to find inside a
home go - though maybe only mildly oversized for 60 watts worth of halogen
lights, probably not especially bad for this job after all.
(If voltage runs half a volt high at 5 or slightly more amps, then 60
watts worth of halogen lamps will be supplied 13.1 volts. If the lamps
get that much voltage after wiring voltage drops, expect
incandescent/halogen lamps rated for 12 volts even to have life expectancy
reduced maybe 60%.)

2. Most wallwarts have DC output, and not only the transformer windings
but also the rectifier circuit (diodes and filter capacitor) has issues of
load voltage varying with load current.

- Don Klipstein (don@xxxxxxxxx)
.