# Christmas lights tester

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#### jumpjack

##### New Member
I'd like to test bulbs for my Christmas tree without detaching them from the line. How big should an electromagnetic field be to generate in the bulb such a current that it would shine?

Sorry, my english is too poor for such a technical question... Can anybody kindly correct the question in such a way any English people can understand it without needing a graduation in foreign-english?...

#### ljcox

##### Well-Known Member
jumpjack said:
I'd like to test bulbs for my Christmas tree without detaching them from the line. How big should an electromagnetic field be to generate in the bulb such a current that it would shine?

Sorry, my english is too poor for such a technical question... Can anybody kindly correct the question in such a way any English people can understand it without needing a graduation in foreign-english?...

However, I think you mean magnetic field not electromagnetic field.
I don't think it is possible since, to induce a current into the wire, you would need a closed circuit.

Normally the globes in Xmas tree lights are in series.

Therefore you would have to short circuit the wires at the power plug. So if you induce a current into the loop to make the globes glow, it would only work if all globes were intact. If any globe was faulty, no globes would glow thus negating the point of the exercise.

#### jumpjack

##### New Member
ljcox said:

However, I think you mean magnetic field not electromagnetic field.
I don't think it is possible since, to induce a current into the wire, you would need a closed circuit.

Normally the globes in Xmas tree lights are in series.

Therefore you would have to short circuit the wires at the power plug. So if you induce a current into the loop to make the globes glow, it would only work if all globes were intact. If any globe was faulty, no globes would glow thus negating the point of the exercise.
I did a little home-made test... with the microwave oven!
Good news: it worked!
Bad news: now ALL bulbs are broken
A big, colorful FLASH demonstrated that a variable EM field can light bulbs even in an open circuit. Funny... but quite "smelly"....

Anyway, now we should "only" understand the maximum strength the field must have to light the bulbs without burning out them...

#### philba

##### New Member
I've seen a tester circuit. It used an xor gate and capacitively couples the two input pins to the light's wires. each pin goes to a probe that is hook shaped. You powered the light string and put the two probes over the wires on either side of the suspect bulb. The probes couple with the wire. If the bulb is good the two wires are in phase and thus the output of the xor gate is high. If the bulb is bad, the two sides are out of phase and the output is low. the circuit is powered directly off the mains AC so it's got safety issues.

one disadvantage is that it doesn't work for multiple burnt out bulbs. I think there is a straightforward enhancement to fix that. I think you could also run this off of an AC transformer for isolation.

sorry, I don't have a schematic. the guy sketched it out for me. I've been meaning to reconstruct it. maybe some day.

#### Hero999

##### Banned
Some mains meters have a non-contact mains voltage detector.

Also if you hold one leg of a nen lamp and touch the other leg on the plastic insulation there might be enough capacitive coupling to make it glow very dimly. I've used this technique before to find breaks in cables.

#### audioguru

##### Well-Known Member
Our electrical utility is giving away strings of LED Christmas lights when you turn in a string of old incandescent ones. I didn't trade any this year because I ran out of the old ones last year.

All the LED ones they gave away this year are recalled because they are a fire hazzard. They are Chinese ones.

#### RODALCO

##### Well-Known Member
I use a voltage detector pen to check which bulb is blown of the string.
This only works for the mains voltage strings.

Turn the string on and follow the lamps till the detector stops beeping. that is where your blown bulb is.
Turn the string off, replace bulb and repeat same again from where you replaced bulb.

This is a very simple and fast method of checking where the fault is.

Audioguru ? Where these LED lights mains or transformer ones.??

#### Hero999

##### Banned
RODALCO said:
I use a voltage detector pen to check which bulb is blown of the string.
This only works for the mains voltage strings.
It will work for low voltage strings too, providing you're careful.

onnect one end of the string to the phase conductor of the mains in series with an HV 1Mhm: resistor then use you voltage sniffer pen to find the break.

I wouldn't recommend this for LEDs chains though as they can be sensitive to ESD and that very tiny current causes by capacitive coupling might be enough to zap some of the more sensitive LEDs. Red LED chains will probably be alright because they're much tougher than the green, blue and white varities.

#### audioguru

##### Well-Known Member
RODALCO said:
Audioguru ? Where these LED lights mains or transformer ones.??
The LED Christmas lights plug into the 120VAC mains. There are about 35 LEDs in series and I don't see a current-limiting resistor (they aren't bright), don't see a capacitor and don't see a rectifier. They seem to flicker when you move your head near them. I have so many sets of free ones that I haven't used some.

My electrical utility is giving away LED Christmas lights because my area had a very high peak consumption of electricity just before Christmas in the last few years, about the same as the peak in summer when air conditioners are being used. Incandescent lights waste a lot of power, LEDs use hardly any power.

#### Hero999

##### Banned
Perhaps each LED has two chips in reverse parallel and a built in capacitor.

#### philba

##### New Member
I tried this with a non-contact voltage detector. The problem was that I had to seperate the lamp-series wire by quite a bit (around 1.5") from the return and pass-through wires. This was way too much work for the larger twisted strings with the "add-a-string" plug at the end. Maybe my cheapo detector is too sensitive.

#### Hero999

##### Banned
jumpjack said:
I found two possible solutions:
http://www.nextag.com/christmas-light-tester/search-html

Unfortunately second one appears not to be available in Italy. I wonder if schematic in first link is good for 12V powered lines too.
No, it probably won't work for the 12V variety especially if it's DC.

However if you don't want to mess around with high voltages you could build an astable running at a relatively high frequency (100kHz will probably do) from the unused gates and connect its output it to one of the Christmas tree light's power lines, then use the other scope to sniff out the power.

#### Gayan Soyza

##### Active Member
jumpjack said:
I'd like to test bulbs for my Christmas tree without detaching them from the line. How big should an electromagnetic field be to generate in the bulb such a current that it would shine?

Sorry, my english is too poor for such a technical question... Can anybody kindly correct the question in such a way any English people can understand it without needing a graduation in foreign-english?...

Already u got the answer..here is another idea same i did in my last xmas...

There is a tester designed to do that..normal tester comes with a small ne bulb.I found a tester its having a LED bulb inside....so sensitive same price comparing with a normal tester..may be you have seen that.it does the job.just move the tester parallel with the series light wires...u'll find the LED bulb lighting in the tester....when the broken or burnt bulbs meets with the tester LED will not light...simple.....

but i got a series light circuit (pretty expensive) if a bulb or more bulbs burnt it will pass the current as earlier.......in this case u cannot do that mehod....

#### HiTech

##### Well-Known Member
Wally Mart sells a tester that will illuminate an individual lamp, test fuses, locate a faulty lamp, and remove stubborn lamp bases..... best part is it only costs about \$1.00 (US) especially now since they are already clearing out some of their Xmas decoration stock. I have one and I have repaired many nice looking outdoor decorations that folks discard to the curb because of their laziness and lack of effort to troubleshoot with this simple tester.

#### RetiredHAL

##### New Member
Hi, All,
Its Christmas time again and yes my Christmas Tree lights did not work.
I made up the little tester as per **broken link removed** and it works very nicely.
The gadget that HiTech bought last year is not available locally, nor anything like it.
I did make one little addition and that was another LED and On/Off switch to show that power is on and that the gadget is in "detect mode" If the "detect" Led is not and the Power On Led is lit it means that there is no power radiation present and the gadget is working as designed.
Cheers
RH

#### audioguru

##### Well-Known Member
jumpjack said:
I wonder if schematic in first link is good for 12V powered lines too.
A 4069 is an ordinary Cmos logic IC. It works from a supply voltage from 3V to 18V.

EDIT:
The link to alert me that there has been a reply to this thread brought me to the 1st page, instead of to this last page. So I just answered a question that is more than 1 year old.

Last edited:

#### bobledoux

##### Member
As a side note, I always wondered how a series wired light string could continue to light when a bulb burns out.

My lights have a wire that holds each end of the filament. Beside one wire is a little metal bar. If the filament burns out the wire springs out making contact with the bar. This creates a short circuit through the bulb. The lost voltage drop in the bulb is shared by the remainder of the light string.

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