Thread: LEDs and Resistor Calculations

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I'm trying to use an LED resistor calculator for some LED's I'm looking at. The terminology doesn't match exactly and I want to make sure that I'm looking at this right. The two pics below show the LED's I'm looking at and the values entered into the calculator. The output of the wall wart I'm using I've measured at 8.67 volts. Did I enter the correct values into the calculator?

Yes, the numbers are correct. The LEDs wired in series will use ~ 6.8VDC of the wart's output, so the resistor will not have to dissipate much wattage. I'm guessing that your wart is a nominal 9VDC. Although a 1/8 watt resistor is called out, you should go with a 100 ohm 1/4 watt. They're much more common in places like R-S.

Originally Posted by Otaku

Yes, the numbers are correct. The LEDs wired in series will use ~ 6.8VDC of the wart's output, so the resistor will not have to dissipate much wattage. I'm guessing that your wart is a nominal 9VDC. Although a 1/8 watt resistor is called out, you should go with a 100 ohm 1/4 watt. They're much more common in places like R-S.

LOL!, actually the walwart is labeled as "6 volt DC". Last year I discovered that one of the 6 volt 8 amp wall warts we were using was actually putting out nearly 11 volts! I went through our collection of warts and measured all of them and found that these things are dreadfully unreliable at least with respect to their labeled voltage and actual voltage. When I measured them the actual voltage ranged from 5% to 30% higher than labeled voltage.

buckaneerdude,

Here's a link to an LED tutorial, if you'd like to learn more. Sparkfun has many more tutorials and primers, as well.

Regarding the wall warts, yeah, most of the older ones were unregulated. So, they only drop down to the advertized voltage if they're subjected to the proper load. However, many of the new supplies are regulated switching supplies. So, they'll remain the same voltage until the load is too great.

- Hook

How can you tell if a wart is regulated? I have lots of warts and I see none of them making mention of regulated.

Not sure, spinman. The surefire way is to hook up a voltmeter, and measure it with no load connected. That, and it possibly saying that it's regulated.

Beyond these, if it says it's a 'switching supply', it's regulated. Most new ones are nowadays. This lets them run cooler, and last longer. All Electronics seems to segregate their wall warts into Wall Transformers (unregulated) and Power Supplies (regulated).

- Hook

Seems logical. Hook to volt meter no load. If voltage matches label, then good chance regulated. Extra test is apply load below max rating and see if voltage remains at rating.

Yeah, any AC adapter you scrounged from something else or bought cheap is likely to be unregulated. It'll burn out an LED in a second since they use so little amperage. You can put a voltage regulator into you design but it really is easier to just buy a regulated supply, or convert an old PC power supply, which will put out nice clean 5v power. Regulated supplies can be expensive though, they're all $20 apiece at Radio Shack and usually about $12-15 + shipping online. Selectable voltage supplies tend to be well regulated and aren't much more than the fixed voltage ones so you may be better off buying one of those.

I use old cell phone chargers, you can find them at thrift stores for around a buck each. They put out regulated 5VDC, usually at 0.5A to 1A. If you want to be sure of the output, take a small DMM with probes to the store. Plug 'em in and test them. You can get a cheap DMM at Harbor Freight for ~$5:

http://www.harborfreight.com/7-funct...ter-90899.html