can I burn out an LED without burning out the resistor

tjohnson

Joined Dec 23, 2014
611
By applying a voltage without a current limiting resistor you are allowing high currents to flow through the LED -- provided the voltage source's internal resistance doesn't prevent it -- and it is the high current that does the damage. The resistor is there not to limit the voltage, but to limit the current. Of course, like in a resistor, the voltage and current in an LED is an invertible function (at a given temperature). If you tell me the voltage across it I can tell you the current through it and vice-versa (assuming I have the voltage-current characteristic for that LED).
I don't understand why this project has given rise to so much discussion. Wouldn't using a 9V battery:
  1. with an ~10kΩ resistor
  2. with 330/470Ω
  3. with no resistor
do what the OP wants?
 

WBahn

Joined Mar 31, 2012
29,979
I don't understand why this project has given rise to so much discussion. Wouldn't using a 9V battery:
  1. with an ~10kΩ resistor
  2. with 330/470Ω
  3. with no resistor
do what the OP wants?
If the 9V battery can deliver enough current to destroy the LED. I think a fresh alkaline probably should be able to. The TS is not finding this to be the case, but I don't know how fresh their battery is by the time they get to this point. It's not surprising that most typical LEDs will tolerate a hundred milliamps or even significantly more for at least a short period of time. A current draw of 100 mA is a very heavy draw on a 9V battery that will drain it down to 6V or less in just three or four hours.

The TS needs to set some bounds on the expectations. A simple way to do that is to take a fresh battery and put it across an LED and see what happens.
 

Thread Starter

opeets

Joined Mar 16, 2015
103
I don't understand why this project has given rise to so much discussion.
Because my son and I haven't met our original objective (or it could very well be that the case that our objective is unattainable given our knowledge and resources). Add to that the fact that my experience in electronic circuitry is a distant memory.

What it comes down to is simple...he is learning about Ohm's Law...we are trying to create a simple experiment for his 5th grade science project to prove the value of Ohm's Law by using it to computer resistor values for operating an LED.
Our assumptions were that too low of a current would result in an LED not lighting up and that too high a current would burn the LED out. Both of these are proving to be difficult tasks because the LEDs that we have been using can tolerate a far greater range of currents than we anticipated.
 

Thread Starter

opeets

Joined Mar 16, 2015
103
I'm wondering at this point if I should just invest in a cheap regulated power supply to eliminate the issue of battery resistance. Would this help me better achieve our goal?
Here's an Elenco power supply I'm considering or maybe something even cheaper like this model.
 

WBahn

Joined Mar 31, 2012
29,979
Because my son and I haven't met our original objective (or it could very well be that the case that our objective is unattainable given our knowledge and resources). Add to that the fact that my experience in electronic circuitry is a distant memory.

What it comes down to is simple...he is learning about Ohm's Law...we are trying to create a simple experiment for his 5th grade science project to prove the value of Ohm's Law by using it to computer resistor values for operating an LED.
Our assumptions were that too low of a current would result in an LED not lighting up and that too high a current would burn the LED out. Both of these are proving to be difficult tasks because the LEDs that we have been using can tolerate a far greater range of currents than we anticipated.
I always thought the clearest demonstration of the value of Ohm's Law was in being able to calculate the voltages and currents in a circuit and see that the actual voltages and currents were close to the calculated values. After all, if you use Ohm's Law to calculate that you need a 330Ω resistor to get 20mA of current in an LED using a 9V battery and then you measure the current when you use that resistor and it turns out to be 20.7mA, that's a pretty good indication that Ohm's Law is valuable -- it allowed you do design something and have it work as intended on the first attempt instead of having to just try a bunch of different resistors until you found one that happened to work.
 

WBahn

Joined Mar 31, 2012
29,979
I'm wondering at this point if I should just invest in a cheap regulated power supply to eliminate the issue of battery resistance. Would this help me better achieve our goal?
Here's an Elenco power supply I'm considering or maybe something even cheaper like this model.
Look at the specs. The second one can only deliver 300 mA at 12V. But you already know that isn't enough current to quickly burn out your LEDs. The first one looks like it would probably do it (based on the front panel labeling).

Elenco is, in general, a pretty crappy brand of equipment, but the price is correspondingly low and the quality is good enough for the kind of things that hobbyists just getting started need. Plus, building up kits is a very good way to learn things and you and your son can enjoy doing that together.
 

Thread Starter

opeets

Joined Mar 16, 2015
103
Look at the specs. The second one can only deliver 300 mA at 12V. But you already know that isn't enough current to quickly burn out your LEDs. The first one looks like it would probably do it (based on the front panel labeling).

Elenco is, in general, a pretty crappy brand of equipment, but the price is correspondingly low and the quality is good enough for the kind of things that hobbyists just getting started need. Plus, building up kits is a very good way to learn things and you and your son can enjoy doing that together.
I stopped in my local electronic shop a short while ago and got lucky with this find. It's a 2 amp regulated power supply with 6 preset voltages. Here are the specs. Got it for $39 brand new.
This is definitely a better fit for our needs. And having the presets beats having to measure the voltage when using a variable dial setting like on the Elenco.
On top of that the Elenco puts out 1A max (unless you use the 5V connector).

20150421_120210.jpg
 
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Thread Starter

opeets

Joined Mar 16, 2015
103
I always thought the clearest demonstration of the value of Ohm's Law was in being able to calculate the voltages and currents in a circuit and see that the actual voltages and currents were close to the calculated values.
Agreed however since this is a science project for a target audience where no one really knows about electronics so I felt it was necessary to provide evidence of the importance of Ohm's law in the form of a visual aid. My son has to create a tri-fold display with pictures, diagrams, etc...so we need to show photographs of meter readings and (hopefully) the brightness of the LEDs. He won't be able to perform a demonstration in front of an audience so we needed to produce forms of visual evidence to summarize his learning experience with this area of physical science. We may even go high tech (if the teacher allows it) and create a short video that shows how a particular resistor value affects an LED when the applied voltage changes. We could even have the video play on a loop with his tablet next to the tri-fold display.

After all, if you use Ohm's Law to calculate that you need a 330Ω resistor to get 20mA of current in an LED using a 9V battery and then you measure the current when you use that resistor and it turns out to be 20.7mA, that's a pretty good indication that Ohm's Law is valuable -- it allowed you do design something and have it work as intended on the first attempt instead of having to just try a bunch of different resistors until you found one that happened to work.
Being an engineer I've always been the pedantic type where I like to show a person (unfamiliar with a particular topic) the full spectrum of possibilities. If you're gonna do it, then do it right. I'm trying to get my son to have this type of attitude as well rather than just settling for "good enough."
 

blocco a spirale

Joined Jun 18, 2008
1,546
A variable voltage source, a selection of fixed load resistors a voltmeter and an ammeter would provide a much clearer demonstration of Ohms law.

I find the whole LED thing somewhat irrelevant and more likely to confuse than enlighten. You could more reliably smoke a resistor by exceeding its power rating.
 

upand_at_them

Joined May 15, 2010
940
I felt it was necessary to provide evidence of the importance of Ohm's law in the form of a visual aid.
You can only provide evidence of Ohm's Law. You can't provide evidence of its importance. (Unless you're trying to create your own universe from scratch and are wondering what rules you should apply.)

And, again, burning out the LED has nothing to do with Ohm's Law. It has everything to do with exceeding the device specification.

If you want some visual aid to show that current varies as resistance varies, with a fixed voltage, then you might want to get ahold of an analog multimeter. "With this value resistor the meter shows this value of current. With this value resistor..." And: "All these data points adhere to the formula of V=I x R."

And/or show each data point plotted on a graph. The graph represents the formula for Ohm's Law.
 

Thread Starter

opeets

Joined Mar 16, 2015
103
A variable voltage source, a selection of fixed load resistors a voltmeter and an ammeter would provide a much clearer demonstration of Ohms law.
All of which I have now....

I find the whole LED thing somewhat irrelevant and more likely to confuse than enlighten. You could more reliably smoke a resistor by exceeding its power rating.
Which is why I purchased some high wattage resistors. I have a pair of 100 Ohm 1-watt resistors and a 10 Ohm wirewound resistor.
In the worst case scenario I should be able to use the 12V setting on the new power supply and draw almost 0.9 amps using the 10 Ohm resistor which will most defintely burn out a 3V LED.
 

Thread Starter

opeets

Joined Mar 16, 2015
103
You can only provide evidence of Ohm's Law. You can't provide evidence of its importance. (Unless you're trying to create your own universe from scratch and are wondering what rules you should apply.)
Agreed. Maybe I'm just not saying what I want to say the correct way. All we want to show is that if you don't use Ohm's law you'll have no idea what to use for a resistor. You might get lucky and choose one that lights up the LED. But then after a short period of time it may burn out because you had no idea how much current was actually driving that LED. Or you may have picked out a resistor that is just way too high and it barely lights up at all. I think it would become evident (at least to a person with no knowledge of electronic theory) that it really is important to use Ohm's law to get that right resistor value. At least that's how I see it.

And, again, burning out the LED has nothing to do with Ohm's Law. It has everything to do with exceeding the device specification.
Also agree (to a certain extent) but what fun would this be to a 5th grader if he didn't get to do cool stuff like burn out an LED after spending hours on theory and calculations. You have to understand that I didn't start learning until about electrical circuits until I took it as an elective in 11th grade. Even at that age I (initially) found it boring until I realized the destructive things you could do unless you understood certain principles and concepts. That's when I started to gain respect for understanding things like Ohm's Law.
 

WBahn

Joined Mar 31, 2012
29,979
I stopped in my local electronic shop a short while ago and got lucky with this find. It's a 2 amp regulated power supply with 6 preset voltages. Here are the specs. Got it for $39 brand new.
This is definitely a better fit for our needs. And having the presets beats having to measure the voltage when using a variable dial setting like on the Elenco.
On top of that the Elenco puts out 1A max (unless you use the 5V connector).
PLEASE! Don't post such huge image files. There is no reason to post a 5+ MB file to show something that can be shown quite handily with something that is about 1% of that size:

20150421_120210.jpg

Resizing this took less than a minute using Paint.

Many members have slow internet connects. My home connections would have completely bogged down on this post.
 

Thread Starter

opeets

Joined Mar 16, 2015
103
PLEASE! Don't post such huge image files. There is no reason to post a 5+ MB file to show something that can be shown quite handily with something that is about 1% of that size:

View attachment 84368

Resizing this took less than a minute using Paint.

Many members have slow internet connects. My home connections would have completely bogged down on this post.
Apologies. I assumed this site re-sized images to fit certain specifications otherwise I would have expected it to reject it.
 

Thread Starter

opeets

Joined Mar 16, 2015
103
On the subject of this new power supply, I would to know how to verify the voltage and current values it is specified for. I believe that measuring the voltages is as simple as connecting my DMM leads across the two terminals and checking all the presets. Measuring the current requires a load IIRC. Can you suggest how I can verify that 2A can be drawn from this unit for each of the presets? I've searched for this information on the Internet for the past hour but haven't been able to find the exact answer I'm looking for.

Oh, and BTW....I was watching a YouTube video today on LEDs and learned that blinking LEDs do not require a resistor (because one is built into the LED). Is this really the case or a false claim? I only ask because we've spent several hours in the last week experimenting with a blinking LED. If I should go back to the solid LED then please let me know.
 

WBahn

Joined Mar 31, 2012
29,979
On the subject of this new power supply, I would to know how to verify the voltage and current values it is specified for. I believe that measuring the voltages is as simple as connecting my DMM leads across the two terminals and checking all the presets. Measuring the current requires a load IIRC. Can you suggest how I can verify that 2A can be drawn from this unit for each of the presets? I've searched for this information on the Internet for the past hour but haven't been able to find the exact answer I'm looking for.
Use your power resistors as a load and measure the voltage across them. This will generally give you better results than using your meter configured as a current meter, particularly at lower voltages.

Oh, and BTW....I was watching a YouTube video today on LEDs and learned that blinking LEDs do not require a resistor (because one is built into the LED). Is this really the case or a false claim? I only ask because we've spent several hours in the last week experimenting with a blinking LED. If I should go back to the solid LED then please let me know.
Blinking LEDs, in general, are going to require something more than just the LED in order to make them blink. This something is almost always going to include some form of current limiting since you can essentially get it for free along with whatever else is in there to make it blink. The switching is probably performed using a FET and so you just make the size of the FET such that it limits the current (has an appropriate ON resistance).
 

Thread Starter

opeets

Joined Mar 16, 2015
103
Measure the current by connecting an ammeter in series with the load.
Use Ohm's law to calculate the load resistors required to draw 2A at each voltage.
Assuming I had the resistors below, these would each draw the full 2A, right? But they would have to be able to dissipate anywhere from 6 watts to 24 watts.

3V = 1.5 Ohms
4.5V = 2.25 Ohms
6V = 3 Ohms
7.5V 3.25 Ohms
9V = 4.5 Ohms
12V = 6 Ohms

I do happen to have a 1-Ohm 10-Watt wirewound resistor.
At 3V I should probably draw the full 2A.
At 12V I should theoretically see 1.2A.

Does the above sound correct?
 

WBahn

Joined Mar 31, 2012
29,979
Assuming I had the resistors below, these would each draw the full 2A, right? But they would have to be able to dissipate anywhere from 6 watts to 24 watts.

3V = 1.5 Ohms
4.5V = 2.25 Ohms
6V = 3 Ohms
7.5V 3.25 Ohms
9V = 4.5 Ohms
12V = 6 Ohms

I do happen to have a 1-Ohm 10-Watt wirewound resistor.
At 3V I should probably draw the full 2A.
At 12V I should theoretically see 1.2A.

Does the above sound correct?
How are you getting the numbers in your last two statements?

How do you figure that at 12V with a 1Ω/10W resistor that you would see 1.2A of current?

Hint: Isn't the whole point to see the importance of applying Ohm's Law correctly?
 

Thread Starter

opeets

Joined Mar 16, 2015
103
Some basic questions about resistors while I'm on the subject....

1) How is it that a resistor draws current from a power supply when in fact it actually resists it? Is it more correct to say that the resistor accepts a certain amount of current (which is fixed based on the resistor value but limited by the resistor's power rating)?

2) Does it then follow to say that based on the maximum current a resistor can accept (e.g. 50mA for a 1/4 watt 100 Ohm resistor) that if the maximum voltage (5V in this case) is exceeded the resistor will (at least start to) burn up?

3) When a resistor burns up does the circuit short?
 
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