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### Topic: LED Array current (Read 1 time)previous topic - next topic

#### duemilanove

##### Apr 24, 2012, 10:39 pmLast Edit: Apr 24, 2012, 10:41 pm by duemilanove Reason: 1
I have an array of 96 LEDs powered by 9v wired in the configuration below. When I measure the current draw of the entire array, I get a reading of ~190mA (my batteries are at 8v right now). The configuration below says that the LEDs should draw ~960mA. Do I need to lower the resistance to maximize the output of the LEDs? Why was the LED array wizard (or myself) inaccurate?

Thanks a lot!

#### Grumpy_Mike

#1
##### Apr 24, 2012, 10:56 pm
Is your array just connected to a battery or is it being multiplexed in some way?
If it is then a meter will not give you a correct reading and also all the LEDs are not on at the same time.

Use your meter and measure the voltage drop you are actually getting across the LED. Then put that back into your calculations to get the real current.

#### duemilanove

#2
##### Apr 24, 2012, 11:03 pm
The LED array is just connected to the battery

#### Grumpy_Mike

#3
##### Apr 24, 2012, 11:14 pm
I am not sure where that program got its information from but 960mA looks way out.

You say your voltage is actually 8V, if the LEDs have 3.2V forward volts drop you will have a voltage of:-
8 - 3.2 - 3.2 = 1.6V across the resistor.
As you have a 150R resistor this gives a current of 1.6 / 150 = 10.6mA
As you have 96 LEDs you should have 10.6 * 96 = 1.024 Amps

That is a lot of a miss match between what you measure. Are you sure you are measuring it correctly?

#### James C4S

#4
##### Apr 25, 2012, 02:39 am
Are you using a small rectangular 9V battery?

If so, this is a case where the battery can not supply the amount of current you are trying to draw.
Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com

#### duemilanove

#5
##### Apr 25, 2012, 03:21 am
Hmm... Still getting a reading of ~190mA. I tested some other parts with my multimeter and the current measuring function seems to be working fine.

I'm using 6 AA batteries, but they have been used a bit so they are outputting about 8v total. Would this cause the inaccurate reading?

#### James C4S

#6
##### Apr 25, 2012, 03:24 am

but they have been used a bit

Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com

#### duemilanove

#7
##### Apr 25, 2012, 03:49 am
With all new AA batteries, I get a reading of ~680mA. Still not what is specified...

#### James C4S

#8
##### Apr 25, 2012, 03:58 am
You're using the 150ohm resistor?

Are you using rechargeable AAs?
Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com

#### duemilanove

#9
##### Apr 25, 2012, 04:02 am
I'm using 150 ohm resistors and it is wired exactly as the diagram shows and standard alkaline AA batteries

#### James C4S

#10
##### Apr 25, 2012, 04:08 am
Put two rows of batteries in parallel.  And/Or

Measure the voltage while powering the LEDs.

I suspect you are still pulling more current than the batteries can provide.
Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com

#### duemilanove

#11
##### Apr 25, 2012, 04:26 am
With all of the LEDs off, the multimeter reads ~9.3v. With all of the LEDs on, the multimeter reads ~8.3v. Does this mean that I do not have enough power?

#### James C4S

#12
##### Apr 25, 2012, 03:43 pm
You might be interested in this Alkaline AA Spec Sheet:
http://data.energizer.com/PDFs/E91.pdf

Specifically, the graphs that show the max current out of the AA is around 300-500mA, for about an hour.

You're still drawing more current out of your batteries than they can provide, which you're seeing by the 1V drop (and lack of full current.)  You need to put 6 batteries in series to get 9V and then another 6 in parallel (at least) to get the amount of current you want.
Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com

#### Grumpy_Mike

#13
##### Apr 25, 2012, 06:31 pm
Can you measure the voltage across one of the resistors while powering them all. That will confirm your current reading.
What you are reporting does not add up.

#### duemilanove

#14
##### Apr 25, 2012, 09:53 pm
Quote
Can you measure the voltage across one of the resistors while powering them all. That will confirm your current reading.
What you are reporting does not add up.

~2.1v

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