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Changing an LED - Page 2

post #16 of 25
4/22/10 at 2:30am
Why don't you use a multimeter and test the voltage with the positive lead in front of the LED and the negative behind the LED? Then test the supply voltage to the LED. That will let us find the resistor value. Or you could remove the resistor and measure directly.

Either way, you can control the brightness easily by tuning your value.
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post #17 of 25
4/22/10 at 11:16am
Thread Starter 
Thanks Punnisher. That is indeed something that I could look into, although I do not yet have a multimeter. Good idea, though! I will look into it and report.
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post #18 of 25
4/22/10 at 11:24am
Quote:
Originally Posted by sampson_smith View Post
Thanks Punnisher. That is indeed something that I could look into, although I do not yet have a multimeter. Good idea, though! I will look into it and report.
Highly recommend you get a multimeter before you continue.
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post #19 of 25
4/22/10 at 11:32am
Thread Starter 
I agree. I am starting to think that I eventually will want to build amps, etc. as DIY projects, and a decent multimeter would get some more use. There appears to be one available at a local hardware store (Canadian Tire) for $12.99. Sounds like a good deal.
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post #20 of 25
4/22/10 at 9:37pm
Thread Starter 
So, I tried to figure out the current running through the GLite amp LED. It appeared to be 3.61 mA, if that makes sense, connecting the +/-'s of the LED to the corresponding anode and cathode of the multimeter, respectively. Given that the Vf of the LED is 4.9 v (online spec's) and the resistor is 4.99k +/-1% ohms (actual color code: yellow/white/white/brown/brown), based on the simple calculation, the Vs is 22.9 v. Makes sense. Especially because, due to my ineptitude, while the amp was on I accidentally created a bridge around the resistor (with the current-measuring setting in place) and *pop* the LED burned out! I was irritated for a second, but, since this is my second snafu and I am aware that it is just the LED that is damaged (hopefully), what can you do? So... now I have an LED to match and replace in the GLite amp and a resistor to match in the GLite DPS. Oh. Joy.

First off, is there a better way to measure the Vs? I am ultimately trying to find out if the Vs of the DPS and amp are the same, so things hinge on the Vs. I measured the current going through the DPS LED as above and it appears that the current is actually greater (14.7 mA?!) than that of the brighter amp LED. The LED's are an exact match, so this is very odd. The DPS resistor is 51.1k ohms (color code: green/brown/brown/red/brown) making the Vs voltage 756 v! I understand that this is impossible, especially since we have a maximum of 110 v to work with in North America. If I read incorrectly and the reading for the DPS LED is actually 1.47 mA, then the Vs turns out to be 70.22 v, which is now just 2.5x greater than the amp Vs. Regardless, I am confused. Especially now that the GLite amp LED is not operational so that I can recheck my measurements against the DPS one. Any advice on what to do next would be, as per usual in this thread, a BIG help!
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post #21 of 25
4/24/10 at 11:24pm
Thread Starter 
Anybody?
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post #22 of 25
4/25/10 at 2:21am
Quote:
Originally Posted by sampson_smith View Post
Anybody?
To measure current, you have to sandwich the DMM in series into the actual circuit. The way you did it, you created a short circuit around the resistor. IOW, you bypassed the resistor. That allowed much higher current to flow to the LED and it burned out.

Always, always, measure current through a resistor by measuring voltage across the resistor. The DMM has no effect on the circuit when you do this. Once you have the voltage drop across the resistor, then Ohm's Law tells you that I = V/R. IOW, you divide the voltage you measured across the resistor by the ohms value of the resistor. That gives you the value of the current through the resistor ... and in a series circuit, that is the current through the entire circuit.

Actual amp readings in DC circuits are rarely done with a DMM, because you have to take the circuit apart to connect the DMM in series into the circuit. Simply read the voltage across a resistor and convert.
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post #23 of 25
4/26/10 at 1:32am
Thread Starter 
I appreciate the advice, tomb. I will certainly investigate based on your instruction, tomorrow. Thanks for the succinct, clear reply!
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post #24 of 25
4/26/10 at 9:22am
Quote:
Originally Posted by sampson_smith View Post
I appreciate the advice, tomb. I will certainly investigate based on your instruction, tomorrow. Thanks for the succinct, clear reply!
It wasn't your fault, really. We speak in terms of "measuring current" around here all the time, but in reality it means "measuring voltage around a resistor and converting." So, it's a bit misleading for a newbie.
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post #25 of 25
5/3/10 at 11:12am
Thread Starter 
Thanks again, tomb. I have now successfully changed the LED's (new blue, with 4.0 v forward voltages vs. the standard 4.9 v DigiKey variety) and resistors (both to 15k ohms, as I was able to determine that the supply voltages were the same). So, I finally have a perfect match for my DPS and amp, and I am plenty happy about it.

Or... is it just me, or are the LED's too bright? Perhaps 30k resistors would better do the trick... The battle continues!
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