# Electronic Circuits and Projects Forum

1. ## 119 Light Bulb Sign Project

Hi I am here to ask you to have a look at the project I am planning to do and to see if there are any glaring errors or mistakes that I will make. Also any hints or tips as the best way in your opinion of how to go about such a task will be much appreciated.

I am undertaking a project to create a sign. The letters cut will be cut from MDF Board and I will then want to mount light bulbs onto the letters. In total I have worked it out to be a total of 119 light bulbs and will be set up in a very similar way to this sign here http://artwednesday.com/wp/wp-conten...-Wednesday.jpg

The bulbs I will be using are 25 Watt. There are 11 Characters in the sign that will be lit, each with a different number of light bulbs. I live in the UK so am on a 230 Volt supply.

I think the best way of doing the circuitry is in parallel, I have done some calculations and believe that the current that will be drawn from the mains will be nearly 13 Amps.

I will also connect a dimmer switch to it to control the entire sign.

I am unsure of the thickness of the cable required from the plug supplying the sign and am concerned about over heating of the circuit.

Thank you for any help that you can give me.

Voltage Supply = 230V
Number of Light Bulbs = 119
Light Bulb Wattage = 25W

2. Yes, you would wire the lamps in parallel. The current would be as you figured about 13 amps. Here in the US I would use AWG 12 wire (copper) which is about 2.05mm diameter wire. That should handle about 20 amps so it leaves a nice cushion. The only problem I see is the dimmer. You are looking at a dimmer capable of handling over 3 KW. Did you plan on building or buying the dimmer? Also, if this is an outdoor sign I would consider a GFCI line.

Ron

3. Hi, Thanks for the quick reply. The dimmer is not essential, but would be nice to dial back the brightness of the sign. I was thinking of just buying a dimmer, but as you say 3kW is a lot for it to handle. It is only for indoor use as well.

Another question, which is completely elementary but, looking at the sign I linked, If the power cable runs behind one of the support beams as it reaches each letter the cable should be split (Suggested splitters?) to supply the individual bulbs. The cable then running behind the letters should be able to be a lot thinner, (more 1mm). Is this correct? or should I be safe and use the 2.05mm that would be used for the main trunk of the supply.

4. UK ring-main cable (2.5mm^2) is rated at 20A. 15A flexible extension cable is also available but may run a bit warm. UK lighting cable (1.5mm^2) is ok for 10A so would do for spurs.

5. Deleted

6. Thanks Alec for the UK cable info. I am really poor when away from AWG.

Also, for the spurs as Alec states will do just fine.

Ron

7. Thanks again,

Is there a problem with drawing 3kW from a wall socket for an extended period of time?

8. 3kW = ~ 13A (for 230V mains). A properly wired standard UK ring-main wall socket compliant with IEEE Regs is rated to take that, but poor wiring or tarnished contacts could result in the socket and plug heating up over a prolonged period. It would be better/safer to divide your lighting array into two roughly equal parts powered from two different sockets.

9. That depends on how the outlet is wired and the load it can support as well as the line circuit breaker. Hopefully Alec will be back with info on UK mains outlets.

<EDIT> As I type Alec post! </EDIT>

Ron

10. I would suggest that you split the lights into groups of 10 - 20, and fuse each group with a 3 A fuse. I would use a few domestic switch fuse units, such as http://www.screwfix.com/p/volex-13a-sw-dp-fcu/98744

That way, after the fuses, you can use 1 mm^2 cable (or even 0.5 mm^2 flex) which are a lot easier to wire into lamps. You are going to want to connect two cables into most of the lamp holders, (one cable from the supply, one to carry on to the next lamp holder) and two larger cables would be more difficult to get into the holders.

By the time you have wired 119 lamp holders, you will appreciate the advantage of a thinner cable. You will have to strip off nearly 500 wire ends.

The smaller fuses will also mean that a shorted bulb will be far easier to find and there will be a less impressive bang if something goes wrong.

As a rule, you should always have cables rated up to the fuse value that is feeding them, so if you don't have smaller fuses, your cables should be able to take the full 13A of the plug fuse, and if there is a fault the fuse should blow before the cable is damaged. I realise that a 13 A fuse feeding a 1.5 mm^2 cable is fine, but you shouldn't drop sizes down further without smaller fuses.

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