I would appreciate some help with this seemingly simple problem.

I was given an LED desk lamp that didn't work. The first thing I did
was test the transformer (Input 120 VAC .15A and output 12 VDC 500ma)
which seemed dead so I dug up transformer #2 (Input 120 VAC 10W and
output 12 VDC 500ma) which seemed to work fine until my wife yelled
for me an hour later. The lamp went out, there was a burning smell and
the transformer was very hot. Seems to be fried.

I had THOUGHT that since it was 12 VDC 500ma like the first, it would
be fine. I now know W=V*I which means my replacement is .083A and the
original was .15A. I do not understand why my replacement was not up
to the task.

Moving on, I found another transformer (Input 120 VAC 17W and output
12 VDC 800ma) which makes it 17W. Testing this one with a tester
showed almost no output but I know there are transformers that do not
produce output unless under load so I plugged it into the lamp and it
worked. It seemed a little dimmer than #2 and I noticed a slight
flicker.

After about 5 minutes, I smelled burning and smoke started coming out
from the circuit board where the LED array is attached but the
transformer was cool.

Now I'm confused and can't explain what is going wrong unless the lamp
is defective.

Any explanations would be appreciated!

Thanks,

Mike

On Tue, 30 Nov 2010 07:55:30 -0800 (PST), wrote:

... I know there are transformers that do not
produce output unless under load


Now that's funny! Must be describing those new chinese intelegent
transformers, right?

On Nov 30, 10:55*am, wrote:
I would appreciate some help with this seemingly simple problem.

I was given an LED desk lamp that didn't work. The first thing I did
was test the transformer (Input 120 VAC .15A and output 12 VDC 500ma)
which seemed dead so I dug up transformer #2 (Input 120 VAC 10W and
output 12 VDC 500ma) which seemed to work fine until my wife yelled
for me an hour later. The lamp went out, there was a burning smell and
the transformer was very hot. Seems to be fried.

I had THOUGHT that since it was 12 VDC 500ma like the first, it would
be fine. I now know W=V*I which means my replacement is .083A and the
original was .15A. I do not understand why my replacement was not up
to the task.

Moving on, I found another transformer (Input 120 VAC 17W and output
12 VDC 800ma) which makes it 17W. Testing this one with a tester
showed almost no output but I know there are transformers that do not
produce output unless under load so I plugged it into the lamp and it
worked. It seemed a little dimmer than #2 and I noticed a slight
flicker.

After about 5 minutes, I smelled burning and smoke started coming out
from the circuit board where the LED array is attached but the
transformer was cool.

Now I'm confused and can't explain what is going wrong unless the lamp
is defective.

Any explanations would be appreciated!

Thanks,

Mike

A guess would be that there is a problem on the LED board... drawing
too much current. That blew the first transformer, and then the
second, then the last 'beefy' transformer, could source the current
without failing ... and the board smoked.

George H.

On Dec 1, 8:48*am, PeterD wrote:
On Tue, 30 Nov 2010 07:55:30 -0800 (PST), wrote:
... I know there are transformers that do not
produce output unless under load

Now that's funny! Must be describing those new chinese intelegent
transformers, right?

I'm not sure if you are joking. There really are such things and I
found out the embarrassing way.
I was installing a fancy pendant light and there was a problem in the
wiring. Long story short, I checked the transformer and got almost no
output so I was sure it was defective. I called the supplier and he
told me the transformer didn't output current unless under load. I was
sure he was wrong never having heard of such a thing but after finding
and fixing the short which caused the light not to go on, the
transformer worked perfectly. Always something new to learn just when
I think I have a full grasp of something!

Mycroft

On Dec 3, 10:17*am, George Herold wrote:
On Nov 30, 10:55*am, wrote:





I would appreciate some help with this seemingly simple problem.

I was given an LED desk lamp that didn't work. The first thing I did
was test the transformer (Input 120 VAC .15A and output 12 VDC 500ma)
which seemed dead so I dug up transformer #2 (Input 120 VAC 10W and
output 12 VDC 500ma) which seemed to work fine until my wife yelled
for me an hour later. The lamp went out, there was a burning smell and
the transformer was very hot. Seems to be fried.

I had THOUGHT that since it was 12 VDC 500ma like the first, it would
be fine. I now know W=V*I which means my replacement is .083A and the
original was .15A. I do not understand why my replacement was not up
to the task.

Moving on, I found another transformer (Input 120 VAC 17W and output
12 VDC 800ma) which makes it 17W. Testing this one with a tester
showed almost no output but I know there are transformers that do not
produce output unless under load so I plugged it into the lamp and it
worked. It seemed a little dimmer than #2 and I noticed a slight
flicker.

After about 5 minutes, I smelled burning and smoke started coming out
from the circuit board where the LED array is attached but the
transformer was cool.

Now I'm confused and can't explain what is going wrong unless the lamp
is defective.

Any explanations would be appreciated!

Thanks,

Mike

A guess would be that there is a problem on the LED board... drawing
too much current. *That blew the first transformer, and then the
second, then the last 'beefy' transformer, could source the current
without failing ... and the board smoked.

George H.

George, thanks for the reply. I didn't rule out a defect in the light
and, since nobody had a different idea, yours sounds the most
reasonable and explains everything.

Now if someone could explain the flicker, I would be REALLY happy.

Mycroft

On Fri, 3 Dec 2010 08:01:53 -0800 (PST), Mycroft668
wrote:

On Dec 1, 8:48*am, PeterD wrote:
On Tue, 30 Nov 2010 07:55:30 -0800 (PST), wrote:
... I know there are transformers that do not
produce output unless under load

Now that's funny! Must be describing those new chinese intelegent
transformers, right?

I'm not sure if you are joking. There really are such things and I
found out the embarrassing way.
I was installing a fancy pendant light and there was a problem in the
wiring. Long story short, I checked the transformer and got almost no
output so I was sure it was defective. I called the supplier and he
told me the transformer didn't output current unless under load. I was
sure he was wrong never having heard of such a thing but after finding
and fixing the short which caused the light not to go on, the
transformer worked perfectly. Always something new to learn just when
I think I have a full grasp of something!

Mycroft

You sure that light fixture wasn't using a switching power supply? A
transformer is a passive device, it doesn't know whether there is a
load or not, it just transforms one voltage to another.

On Dec 3, 3:35*pm, PeterD wrote:
On Fri, 3 Dec 2010 08:01:53 -0800 (PST), Mycroft668





wrote:
On Dec 1, 8:48 am, PeterD wrote:
On Tue, 30 Nov 2010 07:55:30 -0800 (PST), wrote:
... I know there are transformers that do not
produce output unless under load

Now that's funny! Must be describing those new chinese intelegent
transformers, right?

I'm not sure if you are joking. There really are such things and I
found out the embarrassing way.
I was installing a fancy pendant light and there was a problem in the
wiring. Long story short, I checked the transformer and got almost no
output so I was sure it was defective. I called the supplier and he
told me the transformer didn't output current unless under load. I was
sure he was wrong never having heard of such a thing but after finding
and fixing the short which caused the light not to go on, the
transformer worked perfectly. *Always something new to learn just when
I think I have a full grasp of something!

Mycroft

You sure that light fixture wasn't using a switching power supply? A
transformer is a passive device, it doesn't know whether there is a
load or not, it just transforms one voltage to another.

No, I'm not sure. There was a little transformer-like box that went in
the electrical box and the supplier said he gets calls like mine all
the time. (If the directions were better, he wouldn't be bothered so
much but that's another story)

I'm sure you know more than I and sorry if I was wrong. I DO know I
put a tester on one of the transformers I have (output 12VDC 2amp) and
there was a near zero reading, but it worked the light just fine
except for the slight flicker. I ALSO am sure he told me it wouldn't
have an output unless under load. Come to think of it, they are
halogen lights. I don't know why they would need a unit in the
electrical box.

Thanks,
Mycroft

It could just be the effect of the last power supply (it clearly was
something much more complicated than just a transformer if it needed a load
before it outputted the right voltage) struggling to output a high current.

--

Brian Gregory. (In the UK)

To email me remove the letter vee.

On Fri, 3 Dec 2010 15:45:16 -0800 (PST), Mycroft668
wrote:

... Come to think of it, they are
halogen lights. I don't know why they would need a unit in the
electrical box.


Probably they are 12V bulbs, which is a common configuration for
Halogen lamps.

On Fri, 3 Dec 2010 15:45:16 -0800 (PST), Mycroft668
wrote:

On Dec 3, 3:35*pm, PeterD wrote:
On Fri, 3 Dec 2010 08:01:53 -0800 (PST), Mycroft668





wrote:
On Dec 1, 8:48 am, PeterD wrote:
On Tue, 30 Nov 2010 07:55:30 -0800 (PST), wrote:
... I know there are transformers that do not
produce output unless under load

Now that's funny! Must be describing those new chinese intelegent
transformers, right?

I'm not sure if you are joking. There really are such things and I
found out the embarrassing way.
I was installing a fancy pendant light and there was a problem in the
wiring. Long story short, I checked the transformer and got almost no
output so I was sure it was defective. I called the supplier and he
told me the transformer didn't output current unless under load. I was
sure he was wrong never having heard of such a thing but after finding
and fixing the short which caused the light not to go on, the
transformer worked perfectly. *Always something new to learn just when
I think I have a full grasp of something!

Mycroft

You sure that light fixture wasn't using a switching power supply? A
transformer is a passive device, it doesn't know whether there is a
load or not, it just transforms one voltage to another.

No, I'm not sure. There was a little transformer-like box that went in
the electrical box

---
By "electrical box" do you mean a mains receptacle?
---

and the supplier said he gets calls like mine all
the time. (If the directions were better, he wouldn't be bothered so
much but that's another story)

I'm sure you know more than I and sorry if I was wrong. I DO know I
put a tester on one of the transformers I have (output 12VDC 2amp) and
there was a near zero reading,

---
Since the output of a transformer is AC, if you mistakenly thought it
was DC and used one of the DC ranges of your tester to try to measure
its output voltage, near-zero is what you'd read.
---

but it worked the light just fine
except for the slight flicker. I ALSO am sure he told me it wouldn't
have an output unless under load.

---
It wouldn't output _current_ unless it was loaded because it would be
driving an open circuit.

However, if you were to measure the AC voltage across its output
terminals you'd read something.
---

Come to think of it, they are
halogen lights. I don't know why they would need a unit in the
electrical box.

---
Probably because they're designed to run on something other than the
raw mains voltage, which the transformer converts into what the lamp
needs.

---
JF