I'm making a LED sign with 167 LEDs.

95 x Blue(20mA, 3.5V)
72 x Green(20mA, 2.2V)

I was hoping to run the sign on batteries, but the more I look into it the less feasible this seams. I ran the wizard here http://led.linear1.org/led.wiz with the following inputs.

12V Source Voltage(8 AA batteries)
3.5V Forward Voltage
20mA Forward Current
167 LEDs

The first result tells me that the array draws current of 1120 mA from the source. Now if a typical AA battery has a capacity of 2850mAh, this means I should be able to run the sign for about 2 1/2 hours.

If I run the wizard separately for 95@3.5V and 72@2.2V I get 640mA and 300mA respectively. 2850mAh/(640mA + 300mA) = 3 Hours.

I think I have a 15V, 8A power supply from an old laptop that I should probably use instead of batteries.

Can anyone confirm or correct my logic here? Thanks.

 

Batteries are given a mAh rating based on a 20-hour rate of discharge. If your batteries are rated for 2850mAh, that means a load of 142.5mA for 20 hours before being considered discharged.

You would be far better off using the power supply. However, check to make certain that it is regulated; if it is not regulated, the output voltage will be much higher at a light load.

 

That power supply should work, but you'll end up with a very inefficient design that needs to dump a lot of heat. Can't you get a 5V supply from somewhere?

Also, what's the usage that you expect from the LEDs? You might be able to use a smaller power supply if you can be certain that only half (for example) of the LEDs will ever be on at the same time.

 

Actually, using a 15v supply can be more efficient than a lower voltage supply.

Our OP can operate more LEDs in series, using a single current limiting resistor for the string. However, we don't know if the supply is regulated or not at the moment.

An unregulated supply output will vary depending upon the load. To determine the proper configuration, we'll first need to know the no-load voltage for the supply, then we can calculate what the voltage will be when the load of the LEDs is applied. From there, the individual string makeup can be determined.

 

You can read up on regulators as it applies to LEDs on Chapter 2.

LEDs, 555s, Flashers, and Light Chasers

 

Thanks for the responses. The power supply I'm planning on using is from an old Toshiba laptop. How can I tell if it is regulated? Would it be part of the specs or is there some kind of test I can run?

The sign I'm making will have all the LEDs on at once.

Thanks again for the help.

 

Use a multimeter to measure the output voltage of the power supply.

If the supply says 15V, 8A, and when you measure the output voltage it's 15v, then you can be pretty certain it's a regulated supply.

If it is not regulated, but rated for 15v @ 8A, the output voltage will be significantly higher when there is no load present.