35mA led with 25mA pic output?

[RussMartin]

It may be that the 35mA rating for the LED you describe is a maximum, do-not-exceed rating. If you can find the data sheet for the LED, see if it also gives a typical operating current.

People sometimes forget that most LEDs will produce full brightness long before maximum rated current is reached.

A couple of replies have suggested a 220 ohm resistor; that or even a 180 ohm resistor would be a good starting point. Depending on the Vf of the LED, that will give you something between 14 and 19 mA. You may be surprised to find that the LED is quite bright.

The piezzo buzzer i use is 3-12v continuous tone, 9mA. 9mA for what voltage??

Piezo tone/buzzer devices are basically voltage-driven; the greater the voltage, the louder the sound. The 9 mA rating is probably for the 12 VDC maximum; at 5 volts, it will likely draw only 4 or 5 mA.

[skimask]

People sometimes forget that most LEDs will produce full brightness long before maximum rated current is reached.

As is the case with my parts box full of LEDs...good luck finding any sort of datasheet for any of them.
Just start with a high value resistor (1K-ish) and work your way down until it's good enough or you hit the current limit (47 ohm is my low limit and it's worked for me for years). Hit the current limit first? Put that LED in a different box (i.e. garbage can) and grab another LED.

[xnihilo]

As is the case with my parts box full of LEDs...good luck finding any sort of datasheet for any of them.
Just start with a high value resistor (1K-ish) and work your way down until it's good enough or you hit the current limit (47 ohm is my low limit and it's worked for me for years). Hit the current limit first? Put that LED in a different box (i.e. garbage can) and grab another LED.

Right, thanks.

[falingtrea]

Looks like the LED you are using can work as low as 12.5mA. So you have a broad range for the current. I would at least try running it at 18-20 mA and see if the light level is sufficient. One other thing you should remember about LEDs, is the higher the current you run them at, the more you reduce the life.

[mister_e]

You can still use a pair of I/O in parallel to drive up to 50mA. It's been widely used in the past. I'm not a fan of it... but sure it may work. You just need to play with the WHOLE TRIS register value.

[rmteo]

There are LEDs and then there are LEDs. They can vary widely in their brightness output. For example, these are 2 green LEDs in a 5mm, T-1 3/4, thru hole package. Both are rated at a test current of 20mA.

http://search.digikey.com/scripts/Dk...me=511-1191-ND
This one has a rated output of 100mcd. This is typical of commonly available LEDs, 50 -300mcd.

http://search.digikey.com/scripts/Dk...FPG1-15Q-A3-ND
And this one has a rated output of 34,000mcd.

The output of the 2nd. one is 340 times that of the 1st. And it is not even a matter of price - both of them are about the same. So if the LED is bright enough for you at 20-25mA, just go for it. If not, get a brighter one. No need to mess with paralleling outputs, using BJT's or MOSFETS etc.

[skimask]

The output of the 2nd. one is 340 times that of the 1st. And it is not even a matter of price - both of them are about the same.

Almost...
The 1st one, at 10 degrees off center still looks about 70% of it's total.
The 2nd one, at 10 degrees off center is only about 30% of total.
So while the one may be brighter, it's only brighter in the middle, it's got a bit more focus...if that makes any sense...

[xnihilo]

There are LEDs and then there are LEDs. They can vary widely in their brightness output. For example, these are 2 green LEDs in a 5mm, T-1 3/4, thru hole package. Both are rated at a test current of 20mA.

http://search.digikey.com/scripts/Dk...me=511-1191-ND
This one has a rated output of 100mcd. This is typical of commonly available LEDs, 50 -300mcd.

http://search.digikey.com/scripts/Dk...FPG1-15Q-A3-ND
And this one has a rated output of 34,000mcd.

The output of the 2nd. one is 340 times that of the 1st. And it is not even a matter of price - both of them are about the same. So if the LED is bright enough for you at 20-25mA, just go for it. If not, get a brighter one. No need to mess with paralleling outputs, using BJT's or MOSFETS etc.

34000mcd... awesome! I didn't even think such exist
What if I put one on my car ...

[xnihilo]

You can still use a pair of I/O in parallel to drive up to 50mA. It's been widely used in the past. I'm not a fan of it... but sure it may work. You just need to play with the WHOLE TRIS register value.

Unfortunately I don't have enough pins. One pin only can be dedicated to a led. I will use the led at lower voltage as many suggested in this thread.

[xnihilo]

It may be that the 35mA rating for the LED you describe is a maximum, do-not-exceed rating. If you can find the data sheet for the LED, see if it also gives a typical operating current.

People sometimes forget that most LEDs will produce full brightness long before maximum rated current is reached.

A couple of replies have suggested a 220 ohm resistor; that or even a 180 ohm resistor would be a good starting point. Depending on the Vf of the LED, that will give you something between 14 and 19 mA. You may be surprised to find that the LED is quite bright.



Piezo tone/buzzer devices are basically voltage-driven; the greater the voltage, the louder the sound. The 9 mA rating is probably for the 12 VDC maximum; at 5 volts, it will likely draw only 4 or 5 mA.

They say "max dc forward current" and "Typical" FC is 35 mA
See: http://www.futurlec.com/LEDR3R.shtml

About the buzzer, thanks for the info, I'm surprised it needs only 9mA (7mA in fact) at 12V. That's cool.

I lear so many things here.

[RussMartin]

They say "max dc forward current" and "Typical" FC is 35 mA
See: http://www.futurlec.com/LEDR3R.shtml

You are misreading the table.

The third line is saying that the maximum DC forward current is typically 35 mA. It means that for some of the LEDs, it may be a little more and for others a little less, but what it is telling you for sure is, "Don't exceed 35 mA."

The first line of the table is probably more indicative, where it shows a typical forward voltage (Vf) of 2 VDC when run at 12 mA. This sounds like a typical operating condition and is probably as bright as it needs to be.

So you need a series resistor of 220 ohms (13.6 mA), but you can probably use down to 150 ohms (20 mA).

[xnihilo]

You are misreading the table.

The third line is saying that the maximum DC forward current is typically 35 mA. It means that for some of the LEDs, it may be a little more and for others a little less, but what it is telling you for sure is, "Don't exceed 35 mA."

The first line of the table is probably more indicative, where it shows a typical forward voltage (Vf) of 2 VDC when run at 12 mA. This sounds like a typical operating condition and is probably as bright as it needs to be.

So you need a series resistor of 220 ohms (13.6 mA), but you can probably use down to 150 ohms (20 mA).

... Right. Thank you.
Stuff like "Vcc=Min,Iol=MAX,Vih=MAX" is not yet so obvious for me.

[skimask]

.....Information removed at the risk of annoying RussMartin......
See post below....

[RussMartin]

Most of those abbreviations are listed at the bottom of the pages of the datasheets.
If not there, usually a Google-search and/or Wiki will get you the answer on whatever the abbreviation means. In this case:
Vcc = voltage common-collector, generally positive voltage input
Iol = current, output, low (sinking current)
Vih = voltage, input, high

. . . and which have nothing to do with either his question or his LED. I believe he was conveying his thanks to me, not asking another question.

[skimask]

. . . and which have nothing to do with either his question or his LED. I believe he was conveying his thanks to me, not asking another question.

Well excuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuu uuuuuuuuuuuuuuuuuuse me