Soldering a bulb

Postby maskedwarrior » Thu Feb 07, 2008 6:38 pm

HI - I'm making a small dimmer circuit for 4 2.2V bulbs. It's purpose is to illuminate a small heart-shaped crystal glass bowl from the inside and create the appearance of a luminous crystal heart when I turn the bowl upside down

I'm going to wire the bulbs in Parallel with two 1.5V batteries in series to create a 3V charge (I'm actually going to wire six batteries in two groups of three creating 3V). I'll then add a 0.8V resistor if I can find one.

I'm then going to add two 25ohm rheostats in parrallel - one set half way and the other to act as the dimmer control (if my maths is right this will halve the resistance of the rheostat giving me a more sensitive dimmer control)

Well, supposing all this stands up to the laws of physics I want to solder copper wire to the tips of the bulbs and wrap copper wire around the bodies to create a strip of bulbs.

The only concern I have is that soldering the bulbs will blow them. But if I solder them before I connect them to the circuit will it be ok? -my reasoning being the voltage from the iron will have nowhere to go so the bulbs will be safe.

Any feedback would be most welcome.
Thanks in advance,
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Simply Build It

Postby 333rocky333 » Thu Feb 07, 2008 7:14 pm

I would think your more likely to damage the bulb with the heat.

The filament is usually soldered in already, so you may be able to tag on to the exposed solder that is on the bulb already.

Have you thought of getting the matching bulbholders and soldering them in, it will also be easier to replace the bulbs in future
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Postby maskedwarrior » Thu Feb 07, 2008 8:16 pm

Hi rocky, it would be much easier with bulb holders I admit! But the orifice it's all going to fit into is only just big enough for the bulbs alone ... shame

ALso, do you happen know of a good glue to adhere to glass firmly but that will also peel off without leaving a mark????

Many thanks
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Postby 333rocky333 » Thu Feb 07, 2008 8:54 pm

I am a nightmare with glue, everything sticks apart from what I want to.

What about small lamps or even Led , some come prewired with a short lead, usually quite cheap from RS or Maplins etc
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Postby sparx » Thu Feb 07, 2008 9:09 pm

Hi soldering OK if low wattage iron used, but if putting batts in series - parallel as you indicate you will need to put a small diode in line with each series pair of cells or they will flatten each other quite quickly even with lights switched off. using say a 1N914 or 1N4001 type with anodes to pos. of cells & cathodes together should do the trick ,it will drop output volts by 0.6v for each pair but 5.4v should extend bulb life,
good luck, SPARX
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Postby maskedwarrior » Thu Feb 07, 2008 10:10 pm

Hi Sparx - what classes as a low voltage soldering iron, how high is too high??
Where should I put the diode? Between the two parrallel groups in the series chain?

The voltage I'm aiming for is 3V. I'm going to put 3 1.5V cells in a parallel arrangement, I'll call this unit 1, and then put this in series with another 3 parrallel 1.5V cells- unit 2.

SO should the diode go inbetween unit one and two in the series? ANd what about this anode/cathode thing??

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Postby maskedwarrior » Thu Feb 07, 2008 10:24 pm

Ahh I realise a littl more what you mean now. That's a really helpful suggestion sparks, a big thanks!

When you say cathodes together I get a little confused. So by putting a diode in line with the circuit I can stop a reverse current flattening the batteries? So this is the problem with wiring batteries in parrallel-series.

SO is the purpose if the diode to create a one way street between each group of parrallel batteries?

So if it was eg. -Parrallel-series-parrallel-series-parrallel-series- would I have to seperate each 'parrallel' with a diode so it looked like......
-parrallel-series-diode-parrallel-series-diode-parrallel-series-diode etc...??

Once again, many thanks
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Postby ericmark » Thu Feb 07, 2008 11:17 pm

Sparx said low wattage not low voltage iron. The Cathode is the line on the diode sign which will also be marked on the diode. If you connect the anode to battery positive terminal then you will get power from the cathode to the negative of battery which will be 0.6 volts lower the threshold voltage. This 0.6 volt is very handy as not dependent on current flow so if you connect couple of bulbs using these to drop volts if one blows the volts will not rise.
As Sparx says LEDs will most likely do better job less current so battery will last longer and handy tails so solder to. Using batteries in parallel not good idea in general as they can discharge through each other if drain is low. LED's do need current limiting resistors as if they blow they can go short circuit a white LED about 4 volt at 30ma so three batteries in series and 15 ohm resistors some LED's have built in current limiting resistors and there are tri-colour, flashing etc. I think a trip to Maplin would help you. Or try web site. Also they do battery holders etc. Hope you have fun and it all works well. Also bulbs available with tails designed to be soldered. Example is PG83E 12V 85mA 5.3x13mm White Maplin part number or again maplin Wire Lamp 12V BT43W 418 £1.09 also LES lampholder only 45p each then you can change a bulb if they blow.

All best Eric

Postby maskedwarrior » Fri Feb 08, 2008 4:00 am

thanks ericmark,
SIze constraints force me to improvise a bulb holder setup - I would like to use ptoper holders but they won;t fit!

Ahh, wattage, not voltage - my mistake. My iron is 20w - is this likely to blow my 2.2V / 0.5 watt bulbs do you think?

If I wire the bulbs in parrallel I won't need to be too worried about voltage increase when bulbs blow will I?

The very simple thought ocurred to me - why don't I use bigger batteries? I am currently trying to use AAs - but two Ds in series would last much longer and i won't have to worry about parrallel power discharge.
The item I'm making won't need to be lit for long - just turned on an off periododically for a few minutes at a time over two days (it's a prop for a short film I'm helping with), and I don't mind having to change batteries if need be.

I was going to use LEDs but I read online that it's hard to make them dim evenly as their voltage requirements aren't linear as you increase reisistance - causing brightness fluctuatations. I have read it's best to use 'driver' chips to dim LEDs - which is far too complex for me!!!

I want to be able to dial the brightness up and down very evenly to create a pulsing effect - this is why I chose to use old-fashioned bulbs.

I also read online that a good way to glue things onto glass is to wipe the surface with white spirit then put selotape good and firmly on the glass and glue onto that. I'll try it and see if it works!

Thanks once again to you all. My quest is not over by any means over so anymore suggestions would be invaluable.
Tony :)
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Postby ericmark » Fri Feb 08, 2008 12:54 pm

Your iron should be OK. And good point about dimming easier with bulbs. But still think bulbs with wires rather than caps would make life easy. Bigger cells should make it last better but is only for few days it really would not matter I had assumed a long lasting thing.

The iron will not blow a bulb with the type of iron you have I think there was some confusion there are irons with instant heat that put voltage through the tip and these could blow transistors but not bulbs they use very low voltage.

I did experiment with red LED's with light output and overdriving including using high frequency pulses and they do change is brightness up to there max but over driving caused them to become dimmer much to my lectures dismay I was only one to complete and others guessed the results I insisted on repeating the experiment when he tried to reduce my mark and he was not amused to find it was mine that was correct and he had to give me marks but by then could not reduce marks already given to others.

But to your project LED's when dimmed keep their colour but bulbs become more red as the voltage reduces as well as dimmer.

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