Morning,

I have 60 UV LED's, Vf = 2.8V and If = 20mA connected as follows;

+12V ---I>I----I>I----I>I----/\/\/\----0V I have an array of 20 of these connected in parallel. i.e. 20X3. I have some questions if I may.

1. I incorrectly calculated, bought and soldered in 120 Ohm resistors. By my calculations I would need to drop the 12V supply to 10.8V in order to have 20mA at each LED?

2. Secondly the data sheet quotes a Vf of 2.8V min to 3.6V max. I have used the 2.8V for my calculations, is this in fact correct or should I have used the nominal average of 3.2V?

3. Were I not to reduce the supply voltage and keep it at 12V this would mean;
a) I would have 4V across each LED ..... not good?
b) I would have 30mA at each LED ...... not good?

4. As "brightness / luminosity" is directly related to If it would be nice to have more than 20mA going through the LED's. The application is a UV scorpion torch and use is intermittent. What could one safely push to If to or is 20mA the number.

Thanks

 

Morning,

I have 60 UV LED's, Vf = 2.8V and If = 20mA connected as follows;

+12V ---I>I----I>I----I>I----/\/\/\----0V I have an array of 20 of these connected in parallel. i.e. 20X3. I have some questions if I may.

1. I incorrectly calculated, bought and soldered in 120 Ohm resistors. By my calculations I would need to drop the 12V supply to 10.8V in order to have 20mA at each LED?

2. Secondly the data sheet quotes a Vf of 2.8V min to 3.6V max. I have used the 2.8V for my calculations, is this in fact correct or should I have used the nominal average of 3.2V?

3. Were I not to reduce the supply voltage and keep it at 12V this would mean;
a) I would have 4V across each LED ..... not good?
b) I would have 30mA at each LED ...... not good?

4. As "brightness / luminosity" is directly related to If it would be nice to have more than 20mA going through the LED's. The application is a UV scorpion torch and use is intermittent. What could one safely push to If to or is 20mA the number.

Thanks

First, could you explain what a Scorpion Torch is? Then I can simplify my answer.

 

it would be nice to have more than 20mA going through the LEDs

But if they are rated for 20mA their life would be drastically shortened by an average current above 20mA .

 

Vf = 2.8V and If = 20mA

If so, 3 such LEDs drop 8.4v and the resistor should drop 3.6v. At 20ma, it should be a 180ohm resistor.

Driving then slightly over 20ma is OK if done so shortly, like in a ouksed fashion.

 

Morning,

I have 60 UV LED's, Vf = 2.8V and If = 20mA connected as follows;

+12V ---I>I----I>I----I>I----/\/\/\----0V I have an array of 20 of these connected in parallel. i.e. 20X3. I have some questions if I may.

1. I incorrectly calculated, bought and soldered in 120 Ohm resistors. By my calculations I would need to drop the 12V supply to 10.8V in order to have 20mA at each LED?

2. Secondly the data sheet quotes a Vf of 2.8V min to 3.6V max. I have used the 2.8V for my calculations, is this in fact correct or should I have used the nominal average of 3.2V?

3. Were I not to reduce the supply voltage and keep it at 12V this would mean;
a) I would have 4V across each LED ..... not good?
b) I would have 30mA at each LED ...... not good?

4. As "brightness / luminosity" is directly related to If it would be nice to have more than 20mA going through the LED's. The application is a UV scorpion torch and use is intermittent. What could one safely push to If to or is 20mA the number.

Thanks

What exact voltage do they have at 20 mA? Drive at a regulated current, not a specific voltage.

 

First, could you explain what a Scorpion Torch is? Then I can simplify my answer.

Apologies. Nothing special, it is simply a torch made with UV LED's at a wavelength of circa 395nm. When shone on the scorpion the scorpion fluoresces under the light.

Thanks

 

If so, 3 such LEDs drop 8.4v and the resistor should drop 3.6v. At 20ma, it should be a 180ohm resistor.

Driving then slightly over 20ma is OK if done so shortly, like in a ouksed fashion.

Thanks

 

What exact voltage do they have at 20 mA? Drive at a regulated current, not a specific voltage.

Clearly I know little about these matters. The reason for wanting to drop the voltage was that I had selected the incorrect resistor of 120R vs. the 180R it should have been. So in dropping the voltage to 10.8V I believed this would compensate for the incorrect resistor value and will stop me from driving too much current into the LED?

 

Clearly I know little about these matters. The reason for wanting to drop the voltage was that I had selected the incorrect resistor of 120R vs. the 180R it should have been. So in dropping the voltage to 10.8V I believed this would compensate for the incorrect resistor value and will stop me from driving too much current into the LED?

You'll be fine with the resistors you have. The UV will be a bit deeper UV but the bug will still fluoresce.

My biggest concern is that you have almost 5W of UV LED. Setting that down while it's on and a kid walks up to it - you could blind someone. If not blind, a severe retinal burn. Life-altering. Even for you, be very careful and a welding mask is not a bad idea while testing. People think
LEDs are toys and not dangerous like lasers but now that UV is available and you are stacking a huge number of them into an array, it becomes dangerous.

 

Thanks and thanks for the warning. Only I use it and it is kept out of reach of all others.

 

If dropping the supply voltage is easy, do that. Otherwise, you can't replace the resistors with the correct value?

You can also add a couple of diodes between the supply & the LEDs to drop some voltage.

 

If your LEDs are average, then they will have 3.2V drop. If they are not average, hopefully a group of three will be average and you'll have 3 X 3.2Vf or 9.6V.

Then, assuming your power supply is 12V, your 120 ohm resistor will be perfect to make 0.020 A per string.

If you're a little high or low, the only problem is you'll need more LEDs to replace some and then you can buy new resistors.

 

If your LEDs are average, then they will have 3.2V drop. If they are not average, hopefully a group of three will be average and you'll have 3 X 3.2Vf or 9.6V.

Then, assuming your power supply is 12V, your 120 ohm resistor will be perfect to make 0.020 A per string.

If you're a little high or low, the only problem is you'll need more LEDs to replace some and then you can buy new resistors.

Ah excellent. That was the answer I needed most. When I first calculated the resistor value it was on the average 3.2Vf based on the average from the manufacturers data sheet. When I checked the suppliers website they claimed 2.8Vf which then confused me.

Will measure the volt drop across three LED's for interest sake.

Thanks

 

Ah excellent. That was the answer I needed most. When I first calculated the resistor value it was on the average 3.2Vf based on the average from the manufacturers data sheet. When I checked the suppliers website they claimed 2.8Vf which then confused me.

Will measure the volt drop across three LED's for interest sake.

Thanks

Send some photos when you light up a scorpion.

 

Will do.

We will be in a unique desert national park in two week time. I am hoping to see and photograph some Parabuthus Granulatus (South Africa's most venomous scorpion).

 

Will do.

We will be in a unique desert national park in two week time. I am hoping to see and photograph some Parabuthus Granulatus (South Africa's most venomous scorpion).

The fact you have a "most venomous" scorpion implies you have other, less venomous types out there, too? I'll skip that tour and wait for the photos.

Good luck.

 

Will measure the volt drop across three LED's for interest sake.

Keep in mind that supply voltage may fluctuate so you want to make sure the current isn't too out of spec in all conditions.

Also, LEDs work well even if it is running under current - I often run them at 1/2 of rated current for portable applications. You may want to experiment a little to see if under rating the LEDs works for you.