# Power supply

Discussion in 'The Projects Forum' started by ansbridge, Dec 18, 2009.

1. ### ansbridge Thread Starter New Member

Dec 18, 2009
1
0
Hello,

I am trying to create a hollywood style dressing table mirror as part of my G/fs xmas prez, Unfortunately they don't seem to be readily available so ...

I have 17 small bulbs & sockets running in a series circuit.
I asked at maplin what a suitable powersupply would be and they have given me a 6v ac to dc supply. However once all wired up it seems that the circuit does not work - thought originally it must be a loose connection but I used a multimeter which shows that the circuit is complete - however I think it must be that the powersupply doesn't provide enough amps for the number of lightbulbs ?? - Although here I'm not sure! ...

Part details are below (with links to the parts on the maplin website)

Any help you could give me would be great & will save alot of wasted time and money!

J

Parts:
http://www.maplin.co.uk/module.aspx?moduleno=1938 (Lamp holder)
http://www.maplin.co.uk/module.aspx?moduleno=1977 (Bulbs)
http://www.maplin.co.uk/module.aspx?moduleno=96962 (Power supply )

2. ### steinar96 Active Member

Apr 18, 2009
239
4
In series you will need 102V across the chain of bulbs if each bulb is supposedly dropping 6V over itself, that is a voltage which is hard to aquire with a cheap dc power supply. So what you need to do is connect them in parallel to fix this. Then your 6V power supply will do unless it's not capable of providing the proper amperage.

The problem you are facing is the when a single light bulb is connected across your power supply the current going trough it is decided by the resistance of the bulb. The current will be the voltage divided by the resistance.

Now if you connect 17 of them in series, the total resistance will be 17x(resistance of each bulb). And if you connect your power supply across the chain the current will be so small that virtually no light is emitted from the bulbs.

So you want to change your wiring to parallel form for 2 reasons. 1: i just stated above, and 2:If 1 bulb fails the whole thing goes off because it will act as a open circuit. And you wont know which light bulb failed because you will have to test each one whether they are ok or not. By plugging them in parallel the bad ones will simply not lit up while those that are fine will stay lit.

But in short if you connect 2 bulbs in series the voltage will be "divided" between them assuming they are perfectly alike. If you hook 17 of these in series you can imagine that the voltage across each one will be VERY small if you only have a 6V supply. But as i said if you switch to parallel form then you can keep your 6V supply assuming it is rated to supply enough current for all 17 bulbs (MAKE SURE IT DOES) because these wall warts can easily overheat and melt down if you push em too hard.

Last edited: Dec 18, 2009
3. ### eng1ne Member

Dec 4, 2009
97
3
Steinar makes one of the most fundamental considerations for you - the configuration of the bulbs, i.e. series or parallel; as was explained, parallel is the most desirable, and pretty imperative!

Which is the specific bulb that you are using? There are several listed on the page that you gave. No one can give much more advice without knowing that.

4. ### eblc1388 Senior Member

Nov 28, 2008
1,542
102
With the 17 bulbs (depends on his selection) each ranging from 0.3W to 2W, the regulated 2.4W power supply the OP had bought would have no chance to power them up and these paralleled bulbs would appear to be a short circuit to the power supply.

The OP would need a bigger power supply.

5. ### Audioguru New Member

Dec 20, 2007
9,411
896
Aren't the little bulbs too dim to light a dressing table mirror?