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3.2v LEDs 12v battery

Orion96

Dec 18, 2015
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I know this is a very basic concept but I can't rap my head around this one. I've got nine 3.2v blue LEDs rated for 20mA and I want to run them off of a 12v 7Ah battery. What I've done is I've put 3 LEDs in parallel then another 3 and another 3 so that's 3 sets put into series but that would make it 4 volts each (am I right?) so I put one extra LED in the series line to make the voltage 3 volts (I don't have the right resistors) my question is, is this the most effective way to wire it up for brightness, I'll get a resistor some day. And what would be the best way to wire it up? Also what would the power consumption be with how I've set it up?

Thanks in advance for your help.
 

BobK

Jan 5, 2010
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You have the right idea with series and parallel, but you have it backwards.

What you need is 3 strings of 3 in series with a resistor in each string. Then you parallel those three strings.

The resistor has to drop 12 - (3 * 3.2) = 2.4V at 20mA of current. Use Ohms law to calculate the resistor.

Bob
 

BobK

Jan 5, 2010
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Yes, buying those resistors will make it work, even if you do not use them in the circuit. :rolleyes:
 

Martaine2005

May 12, 2015
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Yes, It's a good read.
I thought it would be easier to read that first, and then ask questions later if needed..
But it's easy to understand.

Martin
 

Orion96

Dec 18, 2015
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You have the right idea with series and parallel, but you have it backwards.

What you need is 3 strings of 3 in series with a resistor in each string. Then you parallel those three strings.

The resistor has to drop 12 - (3 * 3.2) = 2.4V at 20mA of current. Use Ohms law to calculate the resistor.

Bob
Thanks for the help I'll do it that way, only problem is getting the resistors South Africa is a bit useless when trying to buy parts like that.
 

BobK

Jan 5, 2010
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Do the Chinese suppliers on Ebay ship there? They are a good source for parts like resistors, but it can take weeks for shipping.

Bob
 

Martaine2005

May 12, 2015
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Do the Chinese suppliers on Ebay ship there? They are a good source for parts like resistors, but it can take weeks for shipping.

Bob
I think the problem is with the postal system there. They "seem" to "lose" things very easily!!
Or at least the head honcho's think "finders keepers".....:p

Martin
 

Martaine2005

May 12, 2015
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They steal the sellotape the envelope is sealed with too!!
Corruption is awful in that country.
 

Orion96

Dec 18, 2015
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W
Buy
3 x brown-grey-brown-gold and
3 x brown-red-brown-gold[/QUOT]
Why two lots of resistors won't the 120 be alright?
Do the Chinese suppliers on Ebay ship there? They are a good source for parts like resistors, but it can take weeks for shipping.

Bob
They do, I bought the LEDs from Banggood to see if they are any good, prices are really good but I waited over a month for it.
 

Martaine2005

May 12, 2015
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Anyway, getting back to your original question, you said you don't have the right resistors?
You can series and/or parallel them to get what you need!

Martin
 
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