What is simple way to check Vf?
- Thread starter Russki
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Mr Happy
Flashlight Enthusiast
Light up the LED with the desired DC current and then measure the voltage across the LED terminals with a voltmeter.
Note that the actual value of Vf varies as the current varies, and it also varies between LED samples, and with changes in operating temperature of the LED.
Note that the actual value of Vf varies as the current varies, and it also varies between LED samples, and with changes in operating temperature of the LED.
Steve K
Flashlight Enthusiast
simple?
Ask someone else to do it for you.
otherwise, you need to put various currents through the LED and measure the voltage across it. And of course, the LED needs to be mounted on a heatsink.
If you don't have a power supply with an adjustable current limit, then you can add a resistor in series, and just vary the output voltage.
If you don't have two meters (one to measure voltage and one to measure current), you can use a single voltmeter. This does require that you have a resistor in series with the LED. Measure the voltage across the resistor in order to then calculate the current.
To get any more simple, you'd have to be using some batteries instead of a power supply. You would still want to have a series resistor, or multiple values of resistor. To get different currents, either use different resistor values, or let the batteries discharge gradually.
Did I miss an easier method? other than checking the LED's datasheet, of course...
Steve K.
Ask someone else to do it for you.
otherwise, you need to put various currents through the LED and measure the voltage across it. And of course, the LED needs to be mounted on a heatsink.
If you don't have a power supply with an adjustable current limit, then you can add a resistor in series, and just vary the output voltage.
If you don't have two meters (one to measure voltage and one to measure current), you can use a single voltmeter. This does require that you have a resistor in series with the LED. Measure the voltage across the resistor in order to then calculate the current.
To get any more simple, you'd have to be using some batteries instead of a power supply. You would still want to have a series resistor, or multiple values of resistor. To get different currents, either use different resistor values, or let the batteries discharge gradually.
Did I miss an easier method? other than checking the LED's datasheet, of course...
Steve K.
an ammeter 3 or 4 AAs and a variable resistor able to take what current your Vf is for at the approx voltage drop. start high resistance crank current up to match desired current then take ammeter out and use voltmeter across the LED
2xTrinity
Flashlight Enthusiast
Also, keep in mind -- the way you will need to determine what the current is in this case is use a resistor of known resistance, and measure the voltage drop across that:
I = V / R
So say you have a 2 ohm resistor, a voltage of 0.5V across the resistor will correspond to a current of 1A through it (and the LED in series). Once that is known, you can measure the voltage across the LED, and find the Vf at 1A.
I might be wrong here but the simplest way is with a variable power supply, with the LED hooked up and suitable heatsink I start with the power supply as low as it will go and slowly increase the voltage until the current reads 350mA and read the Volts and that is the Vf. P7 and M-CEs 2.8A.
Norm
Norm
The question was the simplest way, no mention of available resources. :devil:
chances are someone that has a variable power supply may have more knowledge of how to determine Vf than someone that did not.
I tend to think those that don't know how to do things usually are less equipped to do them also.
I tend to think those that don't know how to do things usually are less equipped to do them also.
Last edited:
znomit
Enlightened
Simplest way, look it up online! then :devil: :devil:
LEDs vary, so it still would only give you a range of Voltages not your LEDs precise Vf. :nana:
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balou
Enlightened
thread resurrection time...
stumbled upon this question today myself... don't have a variable power supply, and twiddling with a pot until amps match and then measuring seems a bit unpractical if you gonna test a batch of 40 leds.
One solution is a LM317 based constant current source. You just need an old 9v (7.5v might work, anything above 9v seems ok too) power supply or a 9v battery, an LM317 voltage regulator, a resistor and a small junkbox power filter cap. or leave the cap out alltogether...
See here for schematics... most LM317 datasheets have the circuit too
Most likely the simplest LED driver ever And the most inefficient one too
stumbled upon this question today myself... don't have a variable power supply, and twiddling with a pot until amps match and then measuring seems a bit unpractical if you gonna test a batch of 40 leds.
One solution is a LM317 based constant current source. You just need an old 9v (7.5v might work, anything above 9v seems ok too) power supply or a 9v battery, an LM317 voltage regulator, a resistor and a small junkbox power filter cap. or leave the cap out alltogether...
See here for schematics... most LM317 datasheets have the circuit too
Most likely the simplest LED driver ever
And the most inefficient one too 
