Resistor across LED

[Rajinder]

Hi all,
I have a question. There is a circuit of an LED and 680R resistor. The LED is powered from 3V, the 680R is driven from the I/O line of a PIC, so a logic 0 will light the LED.
However, across the LED anode and cathode is a 10K resistor. What is the use or idea for this?
Can anyone help?
Thanks in advance
Raj

 

[kellys_eye]

Is the port bi-directional and does the programming call for a logic high on that port, reconfigured as an input, during operation?

 

[Rajinder]

Hi,
It's a I/O line of a PIC. To turn the LED on the I/O line would need to be set low.
That's as much as I know.
Unless it is set as an input in that case you will have a 10K + 680R pull up, then reconfigured as an output to switch on the LED.
Does that seem reasonable?
Best regards,
Raj

 

[davenn]

However, across the LED anode and cathode is a 10K resistor. What is the use or idea for this?

Hi,
It's a I/O line of a PIC. To turn the LED on the I/O line would need to be set low.
That's as much as I know.
Unless it is set as an input in that case you will have a 10K + 680R pull up, then reconfigured as an output to switch on the LED.
Does that seem reasonable?
Best regards,
Raj

I know of no good reason for a resistor to be in parallel with a LED

please show a circuit

 

[Rajinder]

Yes me too. I can only think of the suggestion made earlier. Input with 10K + 680R, then output set to logic 0, so LED and 680R are in circuit.

 

[Rajinder]

Is the port bi-directional and does the programming call for a logic high on that port, reconfigured as an input, during operation?

It might be, as this seems plausible. Then the port may be configured as an output, where it is driven to a logic 0, then only the LED and 680R come into play. Not sure. That's all I can think.

 

[(*steve*)]

One reason for putting a resistor in parallel with a LED is too prevent it turning on (slightly) when leakage current flows.

In this case the resistor value is chosen to drop less than the Vf of the LED at the leakage current.

Another case is where there is also a capacitor across the LED and you don't want the LED to just get dimmer and dimmer and dimmer when power is removed. You want it to eventually go off. A resistor across the LED allows the voltage to fall below Vf by providing an alternate discharge path.

If you're driving a LED from an open collector or open drain output, or even directly from a transistor (typically with the LED in the collector or drain) then leakage can cause the LED to remain ever so slightly on when it should be off. Again, a resistor across the LED can be chosen to keep the LED off.

However, if you're using a normal microcontroller output, these are typically driven to the supply rails and this argument is unlikely to apply.

If this output is open collector then it could explain the resistor in parallel with the LED.

 

[davenn]

One reason for putting a resistor in parallel with a LED is too prevent it turning on (slightly) when leakage current flows.

In this case the resistor value is chosen to drop less than the Vf of the LED at the leakage current.

Ah hah .... I live and learn

 

[Rajinder]

One reason for putting a resistor in parallel with a LED is too prevent it turning on (slightly) when leakage current flows.

In this case the resistor value is chosen to drop less than the Vf of the LED at the leakage current.

Another case is where there is also a capacitor across the LED and you don't want the LED to just get dimmer and dimmer and dimmer when power is removed. You want it to eventually go off. A resistor across the LED allows the voltage to fall below Vf by providing an alternate discharge path.

If you're driving a LED from an open collector or open drain output, or even directly from a transistor (typically with the LED in the collector or drain) then leakage can cause the LED to remain ever so slightly on when it should be off. Again, a resistor across the LED can be chosen to keep the LED off.

However, if you're using a normal microcontroller output, these are typically driven to the supply rails and this argument is unlikely to apply.

If this output is open collector then it could explain the resistor in parallel with the LED.

Thanks for the explanation. I think it may be to keep the LED off due to any leakage current.
Brilliantly explained. I learnt something too.
Thanks

 

[Terry01]

I haven't got even the mildest of clues what you guys are talking about but I learned something too!
I just don't know how long it will be before I can make anything that even nearly sounds sensible to me out of it...but I'm on it! I'll give you a shout when I've got it

 

[(*steve*)]

Ok, a simple answer to your original question: it makes sure the LED goes completely off.