35mA led with 25mA pic output?

[duncan303]

I have no 5v 10mA led left and the only led i have is 2v 35mA and I have to use that with a pic which can output maximum 25mA per pin. What will happen??

Hi, can you say what you THINK might happen,


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[xnihilo]

Hi, can you say what you THINK might happen,


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Led will glow dimmer...?

[aratti]

Place a 220 ohms resistor in series to your led. If you want it brighter, then you should use a transistor. Your beeper can be connected directly to the pic output.

Al.

[J. Mark Wolf]

You'll compromise the integrity of the pin trying to draw too much current from it with your 35mA LED. Possibly damage the PIC.

Put a transistor in the circuit.

[Nicmus]

Hi Xnihilo,

If you want to keep the full LED brightness you must use some sort of buffer (a small NPN or PNP transistor or a low power N or P channel FET or MOSFET will be your best choices) combined with a 82 Ohm resistor for your 2V 35 mA LED.
If you don’t mind loosing some brightness a limiting resistor (in your case 120 Ohm will give you 25 mA or 150 Ohm will give you 20 mA).
All you have to do is trying and decide.

Regards,

Nick

[Acetronics2]

Hi,

Two paralled-trough-180 Ohms-resistors pins will do it ...

simple way.

Alain

[xnihilo]

Hi Xnihilo,

If you want to keep the full LED brightness you must use some sort of buffer (a small NPN or PNP transistor or a low power N or P channel FET or MOSFET will be your best choices) combined with a 82 Ohm resistor for your 2V 35 mA LED.
If you don’t mind loosing some brightness a limiting resistor (in your case 120 Ohm will give you 25 mA or 150 Ohm will give you 20 mA).
All you have to do is trying and decide.

Regards,

Nick

I understand i should use the 5v power supply with a 82 ohms limiter and switch led with a transistor in order to have full brightness but i do not wish to add a transistor. I should then either choose an inline current limiter and chose the adequate value for let's say 20mA and have a dimmer led or get a 10mA led.
Okay, thanks all for your answers.

[skimask]

I understand i should use the 5v power supply with a 82 ohms limiter and switch led with a transistor in order to have full brightness but i do not wish to add a transistor. I should then either choose an inline current limiter and chose the adequate value for let's say 20mA and have a dimmer led or get a 10mA led.
Okay, thanks all for your answers.

Find the datasheet for your particular LED...
Look up 'Forward Voltage'...assume it's 1.5v for example.
Now look up 'Current'...assume in this case it's 35mA, 'cause that's what you said it was
And you're driving the LED from a PIC pin at 5V

The LED will 'drop' 1.5v across it 'cause it's a diode and that's just the way they are...
So, that leaves you with 3.5v to work with.
The LED wants 35mA (.035A)

Ohms Law - I=V/R, V=IR, R=V/I
R (ohms you need) = V (volts, 3.5V) / I (35mA)
R = 100 ohms...
But that'll allow up to 35mA per PIC pin, over the limit...but you're driving the LED from 2 pins (unless I misunderstood).
So, R = 200 ohm across each PIC pin will give you 17.5mA from each pin, paralleled to the LED.

But I gotta tell you, it'll work, but it's not 'the way to do it'. The right way to do it is to add an external transistor or MOSFET to turn the LED on or off.
Or you could just increase the resistor value and use one pin... Like you said, the LED will be dimmer.

[Nicmus]

Ski,

Don’t you mean two resistors of 200 Ohm each when driving the LED with two I/Os?

Nick

[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.

[xnihilo]

R (ohms you need) = V (volts, 3.5V) / I (35mA)
R = 100 ohms...

->Yes, I already know that. The led is a 2V, 35 mA so with 5V it will need a current limiter of 5-2 / 0.035 Ohms.

But that'll allow up to 35mA per PIC pin, over the limit...but you're driving the LED from 2 pins (unless I misunderstood).

->No, only from one pin (thus max 25 mA, according to Microchip DS).

So, R = 200 ohm across each PIC pin will give you 17.5mA from each pin, paralleled to the LED.

But I gotta tell you, it'll work, but it's not 'the way to do it'. The right way to do it is to add an external transistor or MOSFET to turn the LED on or off.

->Yes, this too I inderstand, that's the way I switch my IR leds (TSAL6100 that can eat up to 1A if used with pulsed current) from 9V source.
In this case I should not use a led that needs so much current as I drive it with an IC pin.
I used to order 5v Chicago red that need 10mA (from) MOUSER but as the shipping costs are too high, I order leds form FUTURLEC but these need 35mA and to keep my PCB simple I wish not to add an extra transistor. I guess I don't have the choice but to order the right led maybe from FARNELL if they have them... or use a transistor.

By the way, I read some info about BJT and MOSFET and still don't know what I should use in my design. MOSFETS are a little more expensive. I need to use a limiter transistor from IC pin to transistor base if I use BJT and choose the right value according to the Load switched by the transistor. I don't need to use such resistor with a mosfet (I think).
As I need to output a 40KHz PWM I could use both BJT and MOSFET even though MOSFET can swich faster but I found some IRL... mosfet wich are level-logic and can switch up to 16A and cost only .60usd versus .22usd for a BC182L for example. I just wonder if I could use such MOSFET to switch a motor or a nichrome wire instead of a bulky DIL transistor that needs a BJT, a resistor a diode,...


Or you could just increase the resistor value and use one pin... Like you said, the LED will be dimmer.

-> Right, as I don't need full brightness, I guess I will rather do that.
Skimask, thank you for your information.

[xnihilo]

You'll compromise the integrity of the pin trying to draw too much current from it with your 35mA LED. Possibly damage the PIC.

Put a transistor in the circuit.

I see, the led will try to draw the 35mA it needs. The Chicago red 5v 10mA i used before were better
Thanks

[xnihilo]

Place a 220 ohms resistor in series to your led. If you want it brighter, then you should use a transistor. Your beeper can be connected directly to the pic output.

Al.

Okay, thank you but tell me, how did you calculate this 220 ohms value?