Convert to LED, transformer trips

[grasfulls]

I changed out nine 12V landscape fixtures with 25watt incandescent lamps to ten 12V fixtures using 1.5W LED lamps. There were 4 different groups of lights on the original installation, three runs of #8 feeding two fixtures, one #8 feeding three fixtures. The transformer overcurent protection tripped.

I took the larger transformer out and put a smaller one in (900 watts down to 250, 'cuz that was all I had with me). I figured the low wattage LED's looked almost like a direct short to the transformer.

Anyone run into this before?

gare

 

[PetrosA]

Are the LEDs rated for 12V AC current? And are both transformers tripping, or just the larger one?

 

[grasfulls]

No longer tripping

No longer tripping

Are the LEDs rated for 12V AC current? And are both transformers tripping, or just the larger one?

The retrofit bulbs are rated 12VAC and the lesser rated transformer is not tripping.

 

[PetrosA]

I doubt the overload switch in the larger transformer is "seeing" a short between hot and neutral - I rather suspect that it's a very sensitive magnetic breaker that can't handle the inrush of the LED circuit. Where you able to make it work with any number of LEDs? Does it say anything about being for incandescent only?

 

[TwinCitySparky]

I doubt the overload switch in the larger transformer is "seeing" a short between hot and neutral - I rather suspect that it's a very sensitive magnetic breaker that can't handle the inrush of the LED circuit. Where you able to make it work with any number of LEDs? Does it say anything about being for incandescent only?

Inrush with an LED circuit operating in the milliamp range?

 

[jrannis]

If you still had it setup, I would suggest leaving one 25 watt lamp connected to the circuit with the LEDs and see what happens

 

[SG-1]

Will the larger transformer trip the protection if energized completely unloaded ?

 

[PetrosA]

Will the larger transformer trip the protection if energized completely unloaded ?

Good question. I was wondering whether maybe the 900W transformer had a minimum load rating. I only remember seeing that on electronic transformers, but who knows?

 

[PetrosA]

Inrush with an LED circuit operating in the milliamp range?

I know for a fact that Phillips LED light bars will trip some GFI breakers. Wattage-wise, they're way under but evidently the caps in the circuitry must be pulling a lot of juice to get the DC side working. IOW, they're using more than the 5mA needed to trip the GFI while charging up. Just how much they pull I don't know. My meter at the time wasn't good enough to get that kind of reading. The unknown here is the overload built into the transformer. How sensitive is it, what is it designed to react to etc.

 

[SG-1]

Through Current ?

Through Current ?

There is a thread that may explain why some GFIs seem to trip on inrush current. GFIs use a differential circuit to meter current in vs current out. If one of the baby CTs or monitoring devices should saturate before the other due to manufacturing varations then a trip that does not involve actual ground current should be possible.

How much through current would it take ?

Would a phase shift between the voltage & current matter ?

I am interested in hearing what some of the forum engineers think of this "theory".

Here is the other thread.

http://forums.mikeholt.com/showthread.php?t=124484

 

[ELA]

I know for a fact that Phillips LED light bars will trip some GFI breakers. Wattage-wise, they're way under but evidently the caps in the circuitry must be pulling a lot of juice to get the DC side working. IOW, they're using more than the 5mA needed to trip the GFI while charging up. Just how much they pull I don't know. My meter at the time wasn't good enough to get that kind of reading. The unknown here is the overload built into the transformer. How sensitive is it, what is it designed to react to etc.

I had not responded to this part of the question earlier because the OP indicated his issue was solved by a smaller transformer if I understood his post correctly?

Many LED circuits utilize a PWM converter to drive the LEDs. A single LED can be pulse width driven with as much as 1 amp. On power up the inverter circuit takes a little while to begin oscillating and during that time period the "inrush" can be several amps for a string of parallel LED drivers.
This can overload the transformer at power up. I did not understand why the smaller transformer worked better without more information from the OP.

 

[SG-1]

Out of curiosity & I had a LED bulb, I took a floodlight style LED bulb and measured the peak inrush current. To make sure I was closing the circuit at the top of the wave for maximum effect I monitored it on the second channel.

The bulb normally draws .016 amps RMS at 120VAC RMS.
I managed to capture a 4 Amp inrush.
This is 250 times normal current, not 250%, 250 times.
The main limitation to my test is that I used a 10A varable supply, which tripped out once. So, the inrush could be even higher from line current.