Christmas lights

[Rekusu]

What ho one and all,

Want to buy some multi-coloured, mains powered string lights to put around a tree for Christmas. Built-in timer (8 on 16 off) essential. Having difficulty finding a 15m length of lights (may be 20m overall) but have found this https://www.robertdyas.co.uk/robert-dyas-mains-operated-led-string-lights-multicoloured.

Is there any reason why I should not cut to 15m LED length and re- attach the 10m from plug to bulb? Obviously, it would need to be waterproof.

Thanks

 

[ericmark]

There are basic two methods of powering lamps, series and parallel can't see which has been used from picture, and two ways to current limit LED's a electronic driver and a simple resistor again from picture can't see how done.

Also red uses less voltage to other colours, and so one would need to be really careful where cut, personally I would not cut them.

 

[winston1]

They are 3volt lamps running off a16 volt transformer. Presumably a number of series strings in parallel. Also LEDs require DC and the transformer outputs AC, not sure how they get round that. There is also a timer somewhere.

I wouldn't cut them either.

 

[Rekusu]

Thanks for the replies. I know it would be better to get the longer and just have a 'denser' display, which is probably what I will do.

Was thinking that the cable from the last bulb to the plug-in transformer is only a twin core so does not seem to be too challenging to reduce the length. But what do I know?

 

[RayCaister]

You're turning on Christmas Tree lights! You must be minted.

 

[Rekusu]

You bet! Have one of Rishi's magic money trees growing in the garden.

 

[pcaouolte]

Also LEDs require DC and the transformer outputs AC, not sure how they get round that.

I think that the clue might be in the D part of LED

 

[SUNRAY]

They are 3volt lamps running off a16 volt transformer. Presumably a number of series strings in parallel. Also LEDs require DC and the transformer outputs AC, not sure how they get round that.

Don't be so stupid. You know perfetly well LED's will happily work on AC.

And if an AC transformer is the item supplied with the unit then an AC transformer is surely the right type of transformer for the job.
Please don't start one of your rants.

 

[winston1]

Don't be so stupid. You know perfetly well LED's will happily work on AC.

Only if supplied through a diode. Reversing an LED is asking for it to break down.

 

[SUNRAY]

Only if supplied through a diode. Reversing an LED is asking for it to break down.

Do me a favour and don't insult us. Just look inside some electrical bits and pieces and see how many are running completely siccessfully on AC.

 

[robinbanks]

Reading the advert it seems like there is a plug, 10m of wire, and then (on the 100 led length) 10m of wire interspersed with leds

Sure cut the 1 section from plug to bulb down so it's only 5m..
..but I wouldn't bother; I'd just coil 5m up and cable tie it at the plug end?

 

[bernardgreen]

Are we talking about LEDs or LED lamps ?

"LED lamps" are seldom just an LED. In almost all "LED lamps" there is a driver and the LED device(s) where the driver controls the current and polarity of the DC supply to the LED device(s).

A basic single junction Light Emitting Diode will most likely be damaged / destroyed if the voltage polarity is reversed,

( junction = where two dissimilar semiconductor materials are in contact )

Modern LED elements are far more than a single junction device and protection against reverse polarity can be incorporated in the device by including in the semiconductor structure junctions that protect the actual LED junction.

LEDs that are connected directly to a 3 volt DC supply have a resistive element built into the semiconductor structure as well as the light emitting junction. Commonly these are used in strings of "fairy" lights,

Some LEDs are in reality two LED junctions, one that conducts current and emits light on one polarity and the other that conducts on the opposite polarity. These can have different colours such as red on one supply polarity and green on the other supply polarity


,

 

[winston1]

Do me a favour and don't insult us.

If you think you are being insulted perhaps it is time to leave the forum.

Just look inside some electrical bits and pieces and see how many are running completely siccessfully on AC.

Because they have a diode incorporated.

 

[winston1]

Are we talking about LEDs or LED lamps ?

"LED lamps" are seldom just an LED. In almost all "LED lamps" there is a driver and the LED device(s) where the driver controls the current and polarity of the DC supply to the LED device(s).

A basic single junction Light Emitting Diode will most likely be damaged / destroyed if the voltage polarity is reversed,

( junction = where two dissimilar semiconductor materials are in contact )

Modern LED elements are far more than a single junction device and protection against reverse polarity can be incorporated in the device by including in the semiconductor structure junctions that protect the actual LED junction.

Thankyou Bernard.

 

[FrodoOne]

What ho one and all,

Want to buy some multi-coloured, mains powered string lights to put around a tree for Christmas. Built-in timer (8 on 16 off) essential. Having difficulty finding a 15m length of lights (may be 20m overall) but have found this https://www.robertdyas.co.uk/robert-dyas-mains-operated-led-string-lights-multicoloured.

Is there any reason why I should not cut to 15m LED length and re- attach the 10m from plug to bulb? Obviously, it would need to be waterproof.

Thanks

You will find that these "strings" of LEDs are `actually multiple "sub-strings" of (possibly) 10 Green - Blue LEDs in series paired with 10 Red - Yellow LEDs in series (plus a current limiting resistor in each half sub-string) - so that the voltage drop adds up to the supply voltage.
Each of these series strings are connected in reverse parallel and supplied by the "adaptor" with about 32 V DC.
The "Adaptor" contains a "Switch Mode Power Supply" {SMPS) which derives this voltage from the "mains" and uses "electronics" to supply the voltage concerned in "pulses" - each pulse being in the reverse direction to the previous pulse - and having "pulse width modulation"..

Thus, when it appears that all LEDs are "Full On" they are NOT, but each half sub-string is turned ON and OFF so rapidly that it appears to human eye that they are "On" at all times.

When it is required to make the LEDs brighten or dim, the length of the full-on pulse is adjusted accordingly, so that it appears that the LEDs in each half sub-string are brightening or dimming.

Because of this, the complete "string" comes in sets of "sub=strings" of (say) 20 series/ parallel LEDs.
Each of these sub-strings (of 2 series strings plus the "return" wire) is fed by only a "pair" of wires, similar to the "feed" from the "adaptor" to the first sub-string.

Thus, one can cut the "string" only at a point where one sub=string connects with the next, via a 2-wire connection.
If such a cut is made elsewhere (to 3 wires), the LEDs beyond the 2-wire connection point will no longed function - unless suitable connections to the "Return" wire are made and appropriate additional current limiting resistors are supplied..