Wiring Query: 24V 3W Light Bulbs for Christmas Decoration

[John Readman]

Hi all,

This will be very basic I am sure. I am wiring 10 number light bulbs together in series to be used as a Christmas decoration. They are 24V and 3W. What are the minimum wire properties I should be using 240V and ?? watt?

I am also interested in internal shunts, so that if one bulb were to blow then the whole string would not cut out. Could anyone one me in the direction of something which would describe this as Google searches have been little use. Any help would be great!
Johnny

 

[sophiecentaur]

DIY mains voltage systems like this are not really to be recommended - particularly if you need to be asking these questions. Bear in mind that 24V fittings are not designed for 240V use and that your home (fire) or life (shock) could be at stake here.
You may well find that it's just as cheap to buy a ready made set, in any case - particularly at this time of year.

This is not just an old spoilsport fart talking!

 

[russ_watters]

The only possibility here for DiY is in buying a 24V transformer (assuming the lights take 24VAC), then wiring them in parallel. But it seems like an awful lot of work for something you can probably buy pretty cheap.

 

[sophiecentaur]

Yes. A mains transformer - suitably fitted for legit, 'stand-alone', mains operation with all the correct sockets etc. will cost a fortune - several sets of fairly lights worth.

 

[John Readman]

Hi all,
These are a new idea I am working on and not just normal fair lights. I have a set made and wired in series, they work fine. However, prior to leaving them running I would like to find out the way in which the number of watts and amps are totalled up. Could someone give me any advice?
Johnny

 

[sophiecentaur]

I repeat: if you know as little as you appear to (from the very elementary questions you are asking) then I should steer clear of this project. The regulations for consumer equipment are very stringent (with good reason) and I feel that you need far more knowledge before you start doing something like this for yourself.
As for internal shunts to keep 9 out of 10 alight when one blows - that will be putting a 10% excess voltage on the remaining bulbs which is at the maximum for normal operation. If your supply volts are over-spec for a start then they could all have very short lives.
I'm afraid I just can't encourage the project at all - unless you use a pukka 24V transformer with propriatary connections and connect them all in parallel.

 

[Noesis]

Sophiecentaur and Russ: I am curious as to exactly why this is a bad idea, and how one could go forward with the concept, if at all, if the bulbs were to be wired in series from a 240V line. I don't plan on doing this, and from the comments it seems neither should John at this point in time, but this still seems to be a great opportunity to learn.

The bulbs will be wired in series so they will all have the same current, say I. Each bulb will cause a 24V drop along the line, so 240V is needed across it. The maximum power for each bulb is 3W, so this will restrict the current to 240*I = 3W, and so I=.0125A=12.5mA. This means my wire must have a resistance value of 240/12.5 = 19.2 k$$\Omega$$.

So if all is well at this point, I would search for a wire capable of withstanding 240V and a resistance value of ~40k$$\Omega$$, where I arbitrarily (please comment on this) chose to double the value 'just to be safe.' I understand this will impact bulb brightness due to current being halved.

Now this seems way too simple so I strongly believe I'm missing something. Is it possible for there to be voltage spikes that would cause a fire as mentioned? Is this fire specifically due to the current exceeding its calculated maximum? Could one put a fuse into the circuit to prevent this?

Is there some transient response that would occur upon first connecting it that would also cause problems?

 

[Integral]

Sophiecentaur and Russ: I am curious as to exactly why this is a bad idea, and how one could go forward with the concept, if at all, if the bulbs were to be wired in series from a 240V line. I don't plan on doing this, and from the comments it seems neither should John at this point in time, but this still seems to be a great opportunity to learn.

The bulbs will be wired in series so they will all have the same current, say I. Each bulb will cause a 24V drop along the line, so 240V is needed across it. The maximum power for each bulb is 3W, so this will restrict the current to 240*I = 3W, and so I=.0125A=12.5mA. This means my wire must have a resistance value of 240/12.5 = 19.2 k$$\Omega$$.

Each bulb produces 3W, the wattage for the string is the sum, so your string of 10 bulbs will consume 30W. Your current is off by a factor of 10.

So if all is well at this point, I would search for a wire capable of withstanding 240V and a resistance value of ~40k$$\Omega$$, where I arbitrarily (please comment on this) chose to double the value 'just to be safe.' I understand this will impact bulb brightness due to current being halved.

Wires are usually considered to have zero resistance. All of the circuit resistance is from the bulbs, at what I compute to be 192 Ohms each. With a circuit current of .125A a standard 12 or 14 Ga wire will be fine.

Now this seems way too simple so I strongly believe I'm missing something. Is it possible for there to be voltage spikes that would cause a fire as mentioned? Is this fire specifically due to the current exceeding its calculated maximum? Could one put a fuse into the circuit to prevent this?

Is there some transient response that would occur upon first connecting it that would also cause problems?

The trouble is that working with 240AC is hazardous for someone who KNOWS what they are doing. For the questions you are having to ask it is pretty clear that you are not experienced in these things. We cannot recommend that you proceed simply based on your lack of knowledge.

 

[sophiecentaur]

There was a time when they used to import cheap products from the 'Far East' (this is not the case, these days, of course), which were made in this way. They used to cause injury and fires because, statistically, they were a greater risk than equipment that was produced to our internal standards. The cheap imports and the legit equipment all followed Ohm's Law and Kirchoff's Laws and a lot of other Scientific Laws. The difference was in the extra risks associated with using sub-standard components. What would you do if one of the screw terminals on the LV bulb holder came adrift because it was not a crimped and moulded connection?

Of course, you could make up your circuit - you could even wire up the bulbs with solder joints and insulating tape and they would probably light up and stay lit without setting fire to your house or giving you a shock. It would just be very bad practice and you would not be safe to leave the room with the arrangement running.

My point is that, from some of the comments you have made (such as the way you would deal with a blown bulb) lead (not only) me to think that, maybe, you may be better off not getting involved at this stage in your knowledge of the subject and standard practice (your calculations are seriously adrift, aamof, which rather proves my point).
Familiarity can breed contempt. Your next project - or the next or the next might let you down if you do only just enough to get away with it. Better to follow standard practice and do things the low-risk way. You could then remain safe even on your hundredth project.