Question about multiple leds in parallel

[jorrit]

Hi,

In many circuits you can see that when a lot of leds are needed in parallel (for example ten leds connected to the outputs of a 4017) a separate resistor is used for every led. I wonder why that is. Isn't it possible to just use a single resistor which is connected from ground to all the 10 leds and then each other end of the led to an output of the 4017?

Greetings,

 

[Nigel Goodwin]

Hi,

In many circuits you can see that when a lot of leds are needed in parallel (for example ten leds connected to the outputs of a 4017) a separate resistor is used for every led. I wonder why that is. Isn't it possible to just use a single resistor which is connected from ground to all the 10 leds and then each other end of the led to an output of the 4017?

Best not to, as you would be reverse biasing the unlit LED's, which they don't like.

 

[jorrit]

Best not to, as you would be reverse biasing the unlit LED's, which they don't like.

Can you explain what this means?

Thanks

 

[Nigel Goodwin]

Can you explain what this means?

Thanks

When one LED is lit, the others would be connected the wrong way round - LED's don't like that!.

 

[audioguru]

With a 9V supply, the output current of a CD4017 is only about 10mA so current-limiting resistors aren't needed for LEDs.

Only a single output of a CD4017 goes high at one time. So a single current-limiting resistor can be used if the supply voltage is high enough to cause too much current without a current-limiting resistor. The resistor can have its value selected so that the LEDs turned off don't have more than their 5V max rated reverse voltage across them.

 

[Overclocked]

Hi,

In many circuits you can see that when a lot of leds are needed in parallel (for example ten leds connected to the outputs of a 4017) a separate resistor is used for every led. I wonder why that is. Isn't it possible to just use a single resistor which is connected from ground to all the 10 leds and then each other end of the led to an output of the 4017?

Greetings,

In some circuits you will see a low ohmage resistor Somewhere along the cathode of the LEDs. The Outputs of the 4017 Arent in parallel. They are all Seperate.

 

[jorrit]

In some circuits you will see a low ohmage resistor Somewhere along the cathode of the LEDs. The Outputs of the 4017 Arent in parallel. They are all Seperate.

Yes I know that. But I was talking about the difference between using one resistor for many leds versus several resistors (i.e. every led has its own resistor). And apparently (as I could see from the answers here) it is not good to use a single resistor here.

Greetings,