handy for finding out the mA/h of an led. I think this would be especially useful since many sellers on ali and eBay do not write the ratings of leds anywhere.
now several people did mention that the readings may become invalid once the 9v battery starts dying. I'm wondering, can the readings become more reliable if I solder the connector and power the thing from a 9v power supply?
Also I've seen some testers for capacitors made with an Arduino and a display with some circuitry that allowed you to get some specs of the cap. Anything like that for leds?
I don't understand what "the amperage of an led" means. I thought Vf was the thing one needed to know*, and you get whatever current the series resistor allows given Vs and Vf.
(* Alas, although my Fluke has a diode setting, it only gives Vf for ordinary diodes not LEDs which is a pity.)
manor_royal:
I don't understand what "the amperage of an led" means. I thought Vf was the thing one needed to know*, and you get whatever current the series resistor allows given Vs and Vf.
(* Alas, although my Fluke has a diode setting, it only gives Vf for ordinary diodes not LEDs which is a pity.)
right. I meant mA/h. Clearly I don't even know how to use that cheap tester.
From the look of it, that device has a number of (probably very crude accuracy) current source terminals. That's basically it. You can plug a led into any point in the jumper block and get some approximate given current.
It doesn't tell you anything about the maximum ratings of the LED. When you hold the button you get to look at how bright the LED will be for a given (approximate) current. Nothing more.
The "current sources" are probably just resistors from 9V with an assumption of approx 2V for the LED.
stuart0:
From the look of it, that device has a number of (probably very crude accuracy) current source terminals. That's basically it. You can plug a led into any point in the jumper block and get some approximate given current.
It doesn't tell you anything about the maximum ratings of the LED. When you hold the button you get to look at how bright the LED will be for a given (approximate) current. Nothing more.
The "current sources" are probably just resistors from 9V with an assumption of approx 2V for the LED.
So not good. Any good testers out there? I know you can get your numbers using a voltmeter, I'm just looking for a quick solution as I'm buying lots and lots of various dip and smd leds and noone wants to post the god damn ratings or label the packages to let you know who it came from... But heck, for a thousand leds for 5 bucks, I'm not complaining...
manor_royal:
That makes even less sense....
The link shows a current scale anyway.
Come on, man. Even google knows what I mean by mA/h:
TobiasRipper:
Come on, man. Even google knows what I mean by mA/h:
Um no it doesn't: that says mAh not mA/h (times not divide). And if you had written mAh, that would make no sense either since mAh is a measure of capacity, as in a battery, and LEDs don't store energy.
manor_royal:
Um no it doesn't: that says mAh not mA/h (times not divide). And if you had written mAh, that would make no sense either since mAh is a measure of capacity, as in a battery, and LEDs don't store energy.
Well thanks for correcting me right away, here I thought this I was speaking in a foreign language.
Well without knowing anything about the led, not current, not voltage, how does one with only a multi-meter go about getting the proper ratings for leds things. I have lots and lots of different kinds and definitely don't want to do trial and error with these things. Better get it right from the get go. There is absolutely no information on these. They could be 1.5v could be 2.5v could be 3, absolutely no idea.
stuart0:
It doesn't tell you anything about the maximum ratings of the LED. When you hold the button you get to look at how bright the LED will be for a given (approximate) current.
You're probably right. Some LEDs I have can probably be seen from the moon at under 10mA, so it's useful to know what visual effect you get for what current when you plan your series resistor.
I suppose the only way to get the absolute current rating of an LED is to pop one?
manor_royal:
You're probably right. Some LEDs I have can probably be seen from the moon at under 10mA, so it's useful to know what visual effect you get for what current when you plan your series resistor.
I suppose the only way to get the absolute current rating of an LED is to pop one?
That doesn't sounds like a good idea because I can pop an led in a second or it can seem to be fine but really be slowly cooking on the inside. Same outcome just slower. And when you think it's all good, get your project done and the led dies in an hour, that's pretty much not much of a solution.
TobiasRipper:
They could be 1.5v could be 2.5v could be 3, absolutely no idea.
For your purposes it won't matter. You never power an LED with its rated voltage anyway. You always apply more (yes) and then drop the balance over a resistor.
So if you have a 2V LED and a 5V supply, you have to lose 5-2=3V over the resistor. Then you take a guess at the current that will give you a suitable visual effect- that's what that meter does- say 10mA
Then divide 3 / 0.03 and get a resistor of 100 ohms. But I'll bet you can use 200 ohms or even much more: all that will happen is the LED will be dimmer and the current less. No harm at all and you save a tiny bit of energy.
I routinely use resistors up in the 500+ ohmage: some resistors are so bloody bright they look fine with tiny currents. If it's for playing around on breadboard I find some LEDs are blinding; to see from across the room as a warning, well that's different.
TobiasRipper:
That doesn't sounds like a good idea because I can pop an led in a second or it can seem to be fine but really be slowly cooking on the inside. Same outcome just slower. And when you think it's all good, get your project done and the led dies in an hour, that's pretty much not much of a solution.
Yeah I was being facetious but there's no smiley for that.
But whack a big enough resistor in there to stop it blinding you and you'll be waaaaay lower than the maximum current anyway.
TobiasRipper:
Well without knowing anything about the led, not current, not voltage, how does one with only a multi-meter go about getting the proper ratings for leds things.
You can only easily get Vf, the forward voltage: the 2-3 or whatever that you mention.
Stick the LED in series with a resistor of say 500 ohms and put 5V across them. Measure the drop across the LED: that's your Vf and will remain constant regardless.
But the recommended working current, absolute maximum current, and the absolute maximum reverse voltage are only in a datasheet... which is where you came in so to speak.
If one needs to operate an LED (or any electronic component) near its extremes, then one should probably buy that LED from someone who will specify the performance limits of the device. Whatever one might have saved on the purchase price is going to be wasted on screening every device for performance.
Most indicator applications aren't critical. Knowing that indicator LEDs are rated to 20 mA I'd be pretty comfortable using any generic device at 4 mA and it would be plenty bright.
All this said, I buy plenty of generic poorly specified stuff on eBay with the full expectation that I'm going to destroy some of it. In a hobbyist context, I can live with this.
With respect to the specific tester you linked, its utility would be to determine
Does a particular LED work?
What color is it? (e.g. for colored LEDs in a clear package)
How (subjectively) bright is it at some (approximate) current?
These are useful parameters, and I've wired up a voltage source, a resistor, and an LED countless times to do just that. It doesn't really tell anything about the maximum ratings of the device unless you exceed them by enough that the device fails.
