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Old 09-07-2009, 03:28 PM   #1
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9v DC to 3v DC


anyone know the easiest way to drop a 9v battery to 3v? (inline)
i need both voltages to come from one source.

thanks!

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Old 09-07-2009, 04:26 PM   #2
 
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I build a variety of small electrical gadgets (mostly guitar effects pedals and amps) and whenever I need to do this, I generally build a small circuit around a LM317 adjustable voltage regulator. You can get one at Radio Shack. The circuit itself is very simple, a few components, and a quick search of google will give you the info you need, or look at the data sheets for the chip.

You could also use a fixed voltage regulator; but the adjustable ones are often easier to get a hold of.

What are your current needs? You mention a battery, so I assume it's not an issue, but you can only get a little over 1A out of an LM317 with appropriate heatsinking.

You could also use a voltage divider, but depending on the application, you may have issues.

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Old 09-07-2009, 05:58 PM   #3
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was hoping someone would know what resistors to put in line to get it from 9v to 3v for two different light bulbs. 9v will blow the 3v bulb.
i may be able to find an old pot from something's power or volume control and experiment with that if need be.

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Old 09-07-2009, 06:13 PM   #4
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DangerMouse I dont know if this will help you any http://www.ngineering.com/Resistor_usage.htm
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Old 09-07-2009, 06:18 PM   #5
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DM,
A series dropping resistor is a very inefficient way to drop 9V to 3V.
The dropping resistor will use 2 times the power used by the 3V bulb.

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Old 09-07-2009, 06:31 PM   #6
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If it's an incandescent bulb, the current will need to be measured while 3 volts is applied. The resistance of an incandescent bulb changes a lot from cold to hot.

Once you know the current, it's easy math to figure the resistor needed. Since you want to drop 6 volts, simply take 6 divide by whatever current and you'll have ohms. For example, if the lamp current is 200 ma, you'd divide 6 by 0.2 and get 30 ohms.

Now, you'll need to figure heat dissipation of the resistor. Volts times amps equals watts. In the above case, we have 6 volts, X 0.2 amps = 1.2 watts. If this is too high for the resistors you have, you can connect them in series, or parallel. Just make sure they're all the same resistance. In the above example, I'd use 3-10 ohm 1/2 watt resistors connected in series. Each resistor would be dissipating 0.4 watts.

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Old 09-07-2009, 06:59 PM   #7
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Quote:
Originally Posted by PaliBob View Post
DM,
A series dropping resistor is a very inefficient way to drop 9V to 3V.
The dropping resistor will use 2 times the power used by the 3V bulb.

.
thanks everyone so far. what i'm doing is using one 9v battery to powert 2 separate bulbs on one of my boxes. a very bright 9v bulb like in Cherrywood Castle, but also 3v led BLACK LIGHT bulbs to light up the inside in flourescent colors! i have some cool things already painted in orange, pink, yellow, etc.
there will be no switches however, as the bolts on the outside will turn and the lights will come on that way, gravity will drop it from the connection when you put it down. i have some 1k ohm and some 3.9k ohm resistors here, will either do? my math sux.... and Bob, that shouldn't matter as the bulbs will be lit very short times.

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Last edited by DangerMouse; 09-07-2009 at 07:03 PM.
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Old 09-07-2009, 08:09 PM   #8
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ok, the 1k ohm seems to be doing the job. i'll stick with it and test a bit more, but the v is down to around 3 now.

thanks to all who helped.

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Old 09-07-2009, 10:09 PM   #9
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If you are always powering the 2 bulbs at low voltage you can just hook them up in series with the 9v. That should do it. Kinda like those 3v Christmas lights that plug into 120.
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Old 09-08-2009, 02:19 PM   #10
 
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You could just use three of the 3 volt bulbs in series, if that's not going to be too much light.
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Old 09-08-2009, 02:46 PM   #11
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i need a picture.... lol
3 leds might not be too bad being all black light... hmmmm

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Old 09-08-2009, 03:28 PM   #12
 
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Quote:
Originally Posted by DangerMouse View Post
i need a picture.... lol
You need a picture of three light bulbs wired in series? I suggest you stop doing anything that involves electricity
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Old 09-08-2009, 03:45 PM   #13
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funny.... no, i need a picture of the whole BOX wired up in my HEAD! there's a helluva lot more going on than just 3 light bulbs! lol
that was rhetorical i guess... i need the whole picture in my head before i can start wiring. everything has to be perfect before final gluing.

so far as stopping doing anything to do with electricity, i truly doubt i can do much harm with a 9v battery and some light bulbs....i also wired my whole house 110 and 220 and passed inspection, and will do so again next year as i finish building the home.

i only had one of the leds when i asked the original question, but now have 4 to play with, 3.6v, 3.8v max. ea. so running in series should be fine. but the 1k ohm resistor still worked ok. dropped it right down to around 3v.

thanks to all!

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Old 09-08-2009, 07:10 PM   #14
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Sorry I could not help it. Hope this helps.
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Old 09-08-2009, 07:37 PM   #15
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shouldn't that be 3 bulbs? lol
that would toast both of the 3v bulbs!

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