Using LEDs in a rectifier bridge in a suspended light?

[Adam Funk]

A kid's ceiling night-light, which shines planets, stars, &c., over
the walls & ceilings, has failed (after several years & changes of
tiny 12 V halogen lamps). I've been asked to see if it can be fixed.
The lamp-holder inside the globe has corroded & is no longer usable.
The whole thing consists of the following:

a wall-wart with a British plug & AC output, labelled

PRI 220-240V~ 50 Hz 0.1A
SEC 11.5V~ 10-20 W
SELV

a plug that fits into the wall-wart, cabled through an in-line
switch, to the lamp-holder, which clicks & twists into the globe

the globe, which is just a plastic sphere (more or less) with
colored disks to shine various shapes on the wall & ceiling


My first thought was to put a rectifier bridge on the SELV output &
then a few white LEDs with series resistors, but then I wondered ---
why not use either

1. a suitable resistor in series with a rectifier bridge arrangement
of LEDs,

or

2. a rectifier bridge arrangement of series resistors & LEDs

?


Thanks,
Adam

 

[Tom Biasi]

A kid's ceiling night-light, which shines planets, stars, &c., over
the walls & ceilings, has failed (after several years & changes of
tiny 12 V halogen lamps). I've been asked to see if it can be fixed.
The lamp-holder inside the globe has corroded & is no longer usable.
The whole thing consists of the following:

a wall-wart with a British plug & AC output, labelled

PRI 220-240V~ 50 Hz 0.1A
SEC 11.5V~ 10-20 W
SELV

a plug that fits into the wall-wart, cabled through an in-line
switch, to the lamp-holder, which clicks & twists into the globe

the globe, which is just a plastic sphere (more or less) with
colored disks to shine various shapes on the wall & ceiling


My first thought was to put a rectifier bridge on the SELV output &
then a few white LEDs with series resistors, but then I wondered ---
why not use either

1. a suitable resistor in series with a rectifier bridge arrangement
of LEDs,

or

2. a rectifier bridge arrangement of series resistors & LEDs

?


Thanks,
Adam

You can get this done but you may also be able to buy an LED replacement
for your bulb.

 

[Helmut Wabnig]

A kid's ceiling night-light, which shines planets, stars, &c., over
the walls & ceilings, has failed (after several years & changes of
tiny 12 V halogen lamps). I've been asked to see if it can be fixed.
The lamp-holder inside the globe has corroded & is no longer usable.
The whole thing consists of the following:

a wall-wart with a British plug & AC output, labelled

PRI 220-240V~ 50 Hz 0.1A
SEC 11.5V~ 10-20 W
SELV

a plug that fits into the wall-wart, cabled through an in-line
switch, to the lamp-holder, which clicks & twists into the globe

the globe, which is just a plastic sphere (more or less) with
colored disks to shine various shapes on the wall & ceiling


My first thought was to put a rectifier bridge on the SELV output &
then a few white LEDs with series resistors, but then I wondered ---
why not use either

1. a suitable resistor in series with a rectifier bridge arrangement
of LEDs,

or

2. a rectifier bridge arrangement of series resistors & LEDs

A LED is not a good rectifier, therefore you will always find
diodes in series to the LEDs.

w.

 

[Jasen Betts]

A kid's ceiling night-light, which shines planets, stars, &c., over
the walls & ceilings, has failed (after several years & changes of
tiny 12 V halogen lamps). I've been asked to see if it can be fixed.
The lamp-holder inside the globe has corroded & is no longer usable.
The whole thing consists of the following:

a wall-wart with a British plug & AC output, labelled

PRI 220-240V~ 50 Hz 0.1A
SEC 11.5V~ 10-20 W
SELV

a plug that fits into the wall-wart, cabled through an in-line
switch, to the lamp-holder, which clicks & twists into the globe

the globe, which is just a plastic sphere (more or less) with
colored disks to shine various shapes on the wall & ceiling


My first thought was to put a rectifier bridge on the SELV output &
then a few white LEDs with series resistors, but then I wondered ---
why not use either

1. a suitable resistor in series with a rectifier bridge arrangement
of LEDs,

or

2. a rectifier bridge arrangement of series resistors & LEDs

?

Or leds facing both ways,

the reason not to is it'll flicker at 50Hz if you do that, but at
100Hz (which is much less noticeable) if you use a real bridge
recitfier and keep all the LEDs pointing the same direction.

 

[Phil Allison]

"Helmut Wabnig"

A LED is not a good rectifier, therefore you will always find
diodes in series to the LEDs.

** Are you for real ???

Rhetorical question alert.

FYI:

LEDs are run from AC supplies all the time, with no additional diodes.

White LEDs normally have reverse breakdowns that exceed 20V so there is no
issue with 12VAC even when using just one.


.... Phil

 

[Phil Allison]

"Jasen Bleatts"

the reason not to is it'll flicker at 50Hz if you do that,

** Not true.

There is normally no visible flicker from a LED operating with 50Hz pulses.

Try it and see.

In the OP's night light app, it will be a complete non-issue.

But getting an array of LEDs to act like a point source ( for projecting
stars etc) may be one.



.... Phil

 

[Phil Allison]

"Tim Wescott"

A tiny halogen
bulb is a pretty small source of light, an LED isn't.

** You could have expressed that one better ...



.... Phil

 

[Adam Funk]

....
You can get this done but you may also be able to buy an LED replacement
for your bulb.

To clarify the "corroded" bit above: I'd only just replaced the lamp
again when it started going off intermittently. When I took it apart
& examined it closely, I found that the stranded copper wire connected
to the "squeezers" that the lamp legs get pushed into was falling
apart (with greenish bits), so I think I need to strip back to clean
wire & work from there.

I came up with this crazy idea as a more permanent solution --- if
used well (!), LEDs should last nearly forever. But maybe I could
splice a new lampholder in.


--
A recent study conducted by Harvard University found that the average
American walks about 900 miles a year. Another study by the AMA found
that Americans drink, on average, 22 gallons of alcohol a year. This
means, on average, Americans get about 41 miles to the gallon.
http://www.cartalk.com/content/average-americans-mpg

 

[Tom Biasi]

To clarify the "corroded" bit above: I'd only just replaced the lamp
again when it started going off intermittently. When I took it apart
& examined it closely, I found that the stranded copper wire connected
to the "squeezers" that the lamp legs get pushed into was falling
apart (with greenish bits), so I think I need to strip back to clean
wire & work from there.

I came up with this crazy idea as a more permanent solution --- if
used well (!), LEDs should last nearly forever. But maybe I could
splice a new lampholder in.

Well Adam you need to fix it. Maybe LEDs will do what you need and
maybe, as was mentioned, they won't give the same light characteristics.
Replace the socket and put in a new bulb, how long will the kid need
this thing? Play with the LEDs if that is fun to you. I might
Tom

 

[default]

A kid's ceiling night-light, which shines planets, stars, &c., over
the walls & ceilings, has failed (after several years & changes of
tiny 12 V halogen lamps). I've been asked to see if it can be fixed.
The lamp-holder inside the globe has corroded & is no longer usable.
The whole thing consists of the following:

a wall-wart with a British plug & AC output, labelled

PRI 220-240V~ 50 Hz 0.1A
SEC 11.5V~ 10-20 W
SELV

a plug that fits into the wall-wart, cabled through an in-line
switch, to the lamp-holder, which clicks & twists into the globe

the globe, which is just a plastic sphere (more or less) with
colored disks to shine various shapes on the wall & ceiling


My first thought was to put a rectifier bridge on the SELV output &
then a few white LEDs with series resistors, but then I wondered ---
why not use either

1. a suitable resistor in series with a rectifier bridge arrangement
of LEDs,

or

2. a rectifier bridge arrangement of series resistors & LEDs

I think the part where you say it "projects" the planets etc. makes
the led idea a non-starter. Assuming they just have some holes in the
sphere to "project" light from, the light must be a point source to
work. Using a large light emitter or several would make the
projection fuzzy at best, or in the case of focused leds, multiple
iterations of the same planet.

Halogen lamps in a plastic globe sounds like a fire hazard to me.

 

[Adam Funk]

A kid's ceiling night-light, which shines planets, stars, &c., over
the walls & ceilings, has failed (after several years & changes of
tiny 12 V halogen lamps). I've been asked to see if it can be fixed.
The lamp-holder inside the globe has corroded & is no longer usable. ....
My first thought was to put a rectifier bridge on the SELV output &
then a few white LEDs with series resistors, but then I wondered ---
why not use either

Thanks for the interesting replies. I'll play with the LED rectifier
bridge idea sometime soon just for fun.

What I hadn't realized when I thought of it was the simple solution:
strip back to clean wires & splice in another G4 lampholder (leaving
intact the plastic fitting that locks into the top of the sphere). It
hadn't occurred to me at first that I'd be able to find the part, but
it exists & is not expensive.

 

[Adam Funk]

"Tim Wescott"




** You could have expressed that one better ...

Well, I understood his point!

 

[Adam Funk]

I think the part where you say it "projects" the planets etc. makes
the led idea a non-starter. Assuming they just have some holes in the
sphere to "project" light from, the light must be a point source to
work. Using a large light emitter or several would make the
projection fuzzy at best, or in the case of focused leds, multiple
iterations of the same planet.

I'm going to splice in a new G4 lampholder instead.

Halogen lamps in a plastic globe sounds like a fire hazard to me.

It's just a G4 in the middle of an empty globe about 15 cm across,
with some vent holes in the top.

 

[Phil Allison]

"Adam Funk"

Well, I understood his point!

** He did not make one - smartarse.

Clearly, a LED is a smaller light source than the 12v lamp filament.



.... Phil