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

duemilanove

Apr 24, 2012, 10:39 pm Last 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

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

The LED array is just connected to the battery

Grumpy_Mike

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

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.
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duemilanove

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


but they have been used a bit


Dead batteries have less current capacity as, well, not dead batteries.
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duemilanove

With all new AA batteries, I get a reading of ~680mA. Still not what is specified...

James C4S

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

I'm using 150 ohm resistors and it is wired exactly as the diagram shows and standard alkaline AA batteries

James C4S

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

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

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

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

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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