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Old 08-30-2014, 04:35 PM   #1
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Understanding Low Voltage Lighting Issue


OK Guys.... Think alot of you know I'm a GC, not an electrician, but really like learning more.

Ran into a low voltage lighting issue, which has me perplexed.

Good friend called as he could not fix a LV pendent light that went out.

These pendents have individual transformers, size of a 9V batterry. The transformers are line voltage to 12 (11.75)V and 6.25 A for 75W. ... with a label "short protected"

Just one light out.... other 5 on switch working fine. Light had been operating fine for 6 years.

Upon inspection.... one wire nut on transformer-to-light lead was pretty well melted.

Friend had 2 new/never used/extra trasformers... he swore they came new from manufacturer 6 years ago.

Connected new transformer.... no light. Checked 120 feed, wiggled it etc, and 120 to transformer confirmed no question. No DC voltage from transformer leads.

Hooked up second "new" transformer. Exactly the same result as first new one (and old original one).

Went ahead and checked the light fixture wiring, continuity check with bulb out showed no short. Ohm check with bulb in showed 1.3 OHM.

Several questions and trying to understand low coltage lighting....

1) Seems highly implausable 3 bad transformers... have I missed something??

2) In the case of original transformer with the melted wirenut (at low voltage feed to light) could a bad splice have caused the transformer failure.???? (In absense of a short, which I could find none)??

Alternatively, what causes a transformer to go out.?

Is there any test with my MM (apart from checking output) to check the transformer.?

3) As far as the melted wire nut, is it amperage that causes heat, and is it reasonable that 12 volts at 6 amps thru a bad splice would create enough heat to melt the wirenut.???

4) Could the bulb have been defective and overpulled the transformer.
Supposedly I had 12 volts, and I read 1.3 OHMS with the bulb in (not lit obviously) which would indicate an amperage of 9+. (I realize that bulb ohms change when in operation, but does this make any sense, or am I mixed up somewhere ... maybe with DC current)

5) The transformer was labeled "short protected". There was no reset or any way to open transformer. Does "short protected" just mean the transformer quits.

6) Any ideas of what is happening here... have I missed something?

TIA for any ideas/explanation/ideas/correction.



PS: Mike is just going ahead and ordering new transformers.

PPS: We thought of swapping in an operating transformer to test, but the lights are very inaccesable and a hugh PIA to take down/access.

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Old 08-30-2014, 05:00 PM   #2
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Understanding Low Voltage Lighting Issue


When you swapped out the transformer did you just try to read voltage with a MM or did you connect the low voltage output to the lamp circuit and test it with lamps installed ?

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Old 08-30-2014, 05:16 PM   #3
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Understanding Low Voltage Lighting Issue


Electronic transformers cannot be accuratly checked with cheap ditital meters !
As there output is not pure DC,
Its not even a pure AC.
Its a chopped up AC.
Most cheap meters will have trouble reading it properly.
Only good trade quality meters can work in this situation.

PS - a lot of electronic type transformers will not work
With little or no load on them.

Last edited by dmxtothemax; 08-30-2014 at 05:20 PM.
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Old 08-30-2014, 07:18 PM   #4
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Understanding Low Voltage Lighting Issue


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When you swapped out the transformer did you just try to read voltage with a MM or did you connect the low voltage output to the lamp circuit and test it with lamps installed ?
Brick... Thank ya..... Let me stay with ya....

Mike, my friend did wire it up (w/o a MM) to the light and no light lit.

(Edit... Just called Mike to confirm.... he had wired up only the first new transformer to the light with no light lighting. So my test on the second new one may have been false as I never wired it to the light. (The bulb is good, and had been confirmed)

Actually, with a sun pro MM, I did NOT have it wired to the light.... ( I assume/guess ? my voltage test when wired to the light would have been from the low voltage lead to to the egc?????) Is that the correct V test method. ... or do I test it in series.?????

This DC has me confused.

I happened to be using my SunPro MM. I had no idea that an electronic transformer required a certain load.

Do you think my Fluke 79 would of placed enough load on it.???

Thanks again
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Old 08-30-2014, 07:32 PM   #5
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Understanding Low Voltage Lighting Issue


Quote:
Originally Posted by dmxtothemax View Post
Electronic transformers cannot be accuratly checked with cheap ditital meters !
As there output is not pure DC,
Its not even a pure AC.
Its a chopped up AC.
Most cheap meters will have trouble reading it properly.
Only good trade quality meters can work in this situation.

PS - a lot of electronic type transformers will not work
With little or no load on them.
DMX.... Thank ya...... Please see my response above that was also an explanation to you.

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Old 08-30-2014, 10:11 PM   #6
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Understanding Low Voltage Lighting Issue


Quote:
Originally Posted by MTN REMODEL LLC View Post

These pendents have individual transformers, size of a 9V batterry. The transformers are line voltage to 12 (11.75)V and 6.25 A for 75W. ... with a label "short protected"


Like these? I'm guessing you had a bad splice causing the wire nut to self destruct, also, i find the lamp sockets never really last when the lamps overheat the socket, they are just a poor design from the get go....

And i'm also going to guess the output was 12v AC, not DC... so you probably had your meter set to the wrong setting.

Last edited by stickboy1375; 08-30-2014 at 10:16 PM.
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Old 08-30-2014, 10:15 PM   #7
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Understanding Low Voltage Lighting Issue


is transformer/light being fed a full 120 volts? test amp draw on first light in line and compare it to last from this problem child. longer down the line, more amps a load requires.

i'm interested in result, please tell.
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Old 08-30-2014, 10:21 PM   #8
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Understanding Low Voltage Lighting Issue


and yes, ac voltage, not dc. dc comes from battery.
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Old 08-30-2014, 10:21 PM   #9
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Understanding Low Voltage Lighting Issue


Quote:
Originally Posted by MTN REMODEL LLC View Post
Actually, with a sun pro MM, I did NOT have it wired to the light.... ( I assume/guess ? my voltage test when wired to the light would have been from the low voltage lead to to the egc?????) Is that the correct V test method. ... or do I test it in series.?????
You are dealing with a transformer, the only way to test the secondary voltage is by measuring the secondary conductors only.
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Old 08-30-2014, 10:23 PM   #10
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Understanding Low Voltage Lighting Issue


Quote:
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and yes, ac voltage, not dc. dc comes from battery.
They make DC output transformers, but my bet is on an AC unit.
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Old 08-30-2014, 10:28 PM   #11
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Understanding Low Voltage Lighting Issue


inverters change output, not transformers.
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Old 08-30-2014, 10:31 PM   #12
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Understanding Low Voltage Lighting Issue


Quote:
Originally Posted by stickboy1375 View Post


Like these? I'm guessing you had a bad splice causing the wire nut to self destruct, also, i find the lamp sockets never really last when the lamps overheat the socket, they are just a poor design from the get go....

And i'm also going to guess the output was 12v AC, not DC... so you probably had your meter set to the wrong setting.
Stick ..... thanks.... That looks almost the very same......... (not the name necesarily..... but the body and leads.

I figured the bad splice, although it took 6 years to show....

Actually, I did not have my glasses with me when I tried to read the transformer.... Mikes wife had to read it to me.... I just assumed it was a normal (to me) transformer that produced DC,... I'll recheck tomorrow.

But as to the light socket going bad, I had an OHM reading when testing just the fixture with bulb in it....?????

Could we have blown the other two "new" transformers with any of the testing I did?????


Best

Peter
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Old 08-30-2014, 10:39 PM   #13
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Understanding Low Voltage Lighting Issue


Quote:
Originally Posted by Doc Holliday View Post
is transformer/light being fed a full 120 volts? test amp draw on first light in line and compare it to last from this problem child. longer down the line, more amps a load requires.

i'm interested in result, please tell.
Doc... Thanks... but I'm getting lost.......

1) Yes..... the transformer is getting full 120V.

2) Well, I have no idea which LV light is first or last on circuit, but it is a parrellel circuit.... should not my amp draw be the same for each light????

3) The problem child should have no amp draw.... it does not light????

4) I can not practically get to other pendents reasonably to check their A draw.
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Old 08-30-2014, 10:43 PM   #14
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Understanding Low Voltage Lighting Issue


the longer electricity has to travel to any machine which may perform work from said electrical feed, the lower the voltage and higher the amps required.

More amps due to distance results in higher heat, especially upon in rush/start up.

The only time there are no amps involved is when the device is off.

Amps= measurement of electrical work. If it runs on electricity, it pull amps, no if and or butts.
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Old 08-30-2014, 10:44 PM   #15
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Quote:
Originally Posted by stickboy1375 View Post
You are dealing with a transformer, the only way to test the secondary voltage is by measuring the secondary conductors only.
Thanks Stick.... Not that I fully understand or anything.... but that makes better sense to me.

(My inquiry/question was predicated on earlier advice to test the voltage with the lighting load on it.... and I did not know how to do that)

Best

Peter

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