Beginner question

[jcferguson]

I am trying to make a simple circuit to light up the inside of a box. I have two 6.3V, .25A light bulbs and have them wired in parallel to a little 9V battery with a switch, but the bulbs go dim very quickly.

Is this problem the result of the mismatch in battery and light bulb voltage? It "seems" like the bulbs go dim once they get pretty hot.

Another silly question: does the light bulb have a polarity? (should I be wiring negative to the side or tip of the bulb)?

Thanks for any help!
Carlos

 

[JSWrightOC]

A 6.3V 250mA lamp is definitely not a match for a 9V transistor-type battery, especially when you consider the fact you have two wired in parallel. That's half an amp at rated voltage--something a 9V isn't going to be able to sustain for very long and still output.

A better solution might involve LEDs, or if you want to use the lamps, I would go with either a higher-capacity battery supply (4xAA, 4xAAA) or perhaps a series circuit--maybe 3 lamps in series with two 9V batteries in series? That wouldn't be as good as 2 lamps wired in parallel to 4 AA cells, but it would work better than two wired in parallel to a single 9V.

To answer your polarity question....no. Incandescent lamps are not polarity sensitive (in fact, the units you have were probably intended to be driven with AC--6.3V is a common voltage for vacuum-tube filaments) though it is customary to wire the tip positive (or "hot") and the body negative (or "neutral"). Hot/neutral refers to AC wiring, if your 6.3V secondary line is ground referenced.

Hope this helps!

 

[kalengkong]

yes, i guess 9V battery could not deliver enough power to 2 bulb at 250mA.

how about try to put 5 AA's in series? it will create 7,5V and i think, it should be enough for those bulb /ubbthreads/images/graemlins/smile.gif

no polarity. it is the same.

 

[jcferguson]

Thanks! If AA batteries would work well, would D batteries work the same and just last longer, or are there differences even though they are the same voltage?

Thanks for the help, what a great resource! I had been looking around the web for quite some time.

 

[UK Owl]

A Few point to consider.....

1) PP3 batteries (the small oblong 9 Volt batteries) are not suitable for the high current demand you are asking for here; and due to their capacity would only last 5-10 minutes with the demand you are placing on them in this situation.

2) You should expect the lamp (light bulbs) to get hot. that is how it works. You pass current through a wire until it gets white hot, this gives off 'light' as well as heat.

3) 'Light Bulbs' do not have a polarity.

You should try using a few LED's in series with resistors, you can get some good effects as the colours are so definite.

e.g.

you have a Blue led that needs 3.6 Volt, a red that needs 1.9 volt, and a yellow that needs 1.8 volt. (note these are typical values but will vary from LED to LED, you need to check for each type of LED you use)

connected in series you need 3.6 + 1.9 + 1.8 = 7.3 volt to get it to work.

as your battery is 9 volt you have 9 - 7.3 = 1.7 volts spare.

you want 20 mA (0.020 Amps) to flow through your resistor

so to work out the resistor value use the formula

r = v / i

v = 1.7 volts
i = 20 mA = 0.020 Amps

r = 1.7 / 0.020 = 85 Ohms

so you need to get the nearest prefered value resistior to this.

Hope this helps you !

 

[Doug Owen]

[ QUOTE ]
jcferguson said:
Thanks! If AA batteries would work well, would D batteries work the same and just last longer, or are there differences even though they are the same voltage?


[/ QUOTE ]

Yes. Based on the specs for alkaline cells from Eveready, AAs (2800 mAh) would probably run four or so hours, the D cells (18,000 mAh)twenty five or so.

Basically, the half amp total current is typical for flashlights, you need "flashlight batteries".

Cheers.

Doug Owen