Limiting Resistor for 7 segment LED display

[PRAVEEN K]

7 segment display resistors

Hi,

Why do I need 7 seperate current limiting resistors for 7 segment LED display?
Is there any problem in connecting a single resistor of high wattage at the cathode(in case of common cathode)?

 

[mosfets.bjt]

resistor socket

Actually there comes out to be a little problem

when u connect one common resistor, what actually happens is, all led's now appear in parallel, parallel connectin of led's is usually not recommanded.

this is because the led with lowest voltage turning on capability will turn on and most of otherz will turn off.

even in led torches they are not recommanded to be connected in parallel



OR IN SHORT WORDZ
IT HAS BEEN EXPERIENCED THAT CONNECTING LEDS IN PARALLEL WILL CAUSE ALL LEDS NOT TO TURN ON WITH EQUAL INTENSITY OR NOT EVEN TURN ON


more over if any u have connected 5 led's in parallel and 4 of otherz r burnt out forexample, then last one will automatically burn out due to ur circuit design i-e. u have connected one low value high power resistor.



last thing,
if u wanted to get ur work as short as possible, then use 7 resistor socket, that is usually used in microcontroller circuits in which we have to connect 8 led's all wid resistors, we use 8 resistor socket.

hope u can understand,
itried my best to help u
regards
mosfets.bjt

 

[klystron]

current drawn by 7-segment display

The LED needs 3 V to switch on. With a 5 V supply and R = 470 om ; a current of 4.3 mA will flow through the resistor. If more Leds are connected in parallel less and less voltage will be availble for the LEDs and a point the LEDS will not switch on

 

[PRAVEEN K]

led 1k resistor

IT HAS BEEN EXPERIENCED THAT CONNECTING LEDS IN PARALLEL WILL CAUSE ALL LEDS NOT TO TURN ON WITH EQUAL INTENSITY OR NOT EVEN TURN ON

This will happen if I am connecting LED to a common driving point. If I use a D latch for switching the LEDs, there will be 7 separate driving points. So it is similar to, driving each LED from different sources. And I think, there will not be issue related to current also. Am I correct??

if u wanted to get ur work as short as possible, then use 7 resistor socket, that is usually used in microcontroller circuits in which we have to connect 8 led's all wid resistors, we use 8 resistor socket.

Actually I need to reduce the circuit, but I didnt get what you mean exactly using resistor packs.

 

[mosfets.bjt]

7 segment dim

by means of latch, if i am not wrong u may b using it in digital circuit.

ya no doubt that will act as non parallel circuitry,

from my experience what i can tell u that will happen if u do this,

for example u connect one 1000 ohm high power resistor,

then if we check worst case scenarios, that is when one led is on and when 7 leds are on what will happen...

when one led is on, 1000 ohm resistor may have correct value that is required to limit the current for one led.

when 7 led's are on there may be the possibility that sum of currents passing through that single resistor is so high that would not let led's tun on.

for example if all leds are turned on:
there might be 70mA current passing through the resistor, ultimately the total voltage drop across resistor would be 70mA * 1000 ohm = 7 volts. if u have 9 volt battery or something remaing voltage may not let ur led turn on.

resistor socket meanz, there is a kind of integrated resistor that r avialable specially for bunch of led's. i have used it but dont remeber its name.
let me search it....

ya they are known as resistor packs
check it out **broken link removed**

hope that u can understand
regards
mosfets.bjt

 

[PRAVEEN K]

7 led display

Thanks for reply.....!!
I will test it practically and let u know.....

 

[mosfets.bjt]

how to connect led display

no prob.
if u let me know. reply me on pm for fast reply

 

[Audioguru]

7 segment led resistor

Don't be ridiculous.
If you have a 9V supply and a 1k resistor then you will not have 70mA. If the LED is 3V then you will have (9V - 3V)/1k= 6mA.

If you have 6 LEDs in series with one 1k resistor then each will look dim because each will have only 1mA. The brightness will change depending on how many segments are turned on.

 

[trekkytekky]

7 segment display resistor

I've seen circuits with a common anode resistor feeding a multiplexed 7-segment display where the cathodes were sequentially pwm switched. The resistor gave a total possible current in the vicinity of 80mA, but net flow to each would be about 10mA as any segment was on for only 1/7 of the time.

A lot of IR remotes also use this same technique to maximize power transmission (range) and battery life. A typical 40kHz driven IR LED can have several hundred mA pulsed through it, even though net draw is only a few mA.

 

[mosfets.bjt]

current limiting resistors in seven segment

Don't be ridiculous.
If you have a 9V supply and a 1k resistor then you will not have 70mA. If the LED is 3V then you will have (9V - 3V)/1k= 6mA.

If you have 6 LEDs in series with one 1k resistor then each will look dim because each will have only 1mA. The brightness will change depending on how many segments are turned on.

excuse me sir
but i have experimented that one led can sink upto 20 mA

moreover
LED's are not in series,
they are in parallel

 

[Audioguru]

big led

Most LEDs can pass 20mA. But how much current is your current-limiting resistor giving the LEDs?

If you have a 9V supply and one 3V LED with one 1k current-limiting resistor then the current is only 6mA.
If you have two LEDs with only one resistor then the current in each LED is only 3mA and they will look dim.
If you have six LEDs with only one resistor then the current in each LED is only 1mA and they will look very dim.

If each LED has its own current-limiting resistor then the current in each LED is not changed by the other LEDs.

 

[thomaslg]

big led common cathode

If you have a voltage source V connected to a LED (Turn On Voltage as V(LED)) through a resistor R, you will have a current of I(LED) pass through the LED and produce certain brightness.

I(LED)=(V-V(LED))/R

Now if you connect 2 LEDs parallel to the same voltage source with the same resistor
you will only get half the current pass through each LED and the brightness will drop.
(assume voltage drop on LEDs are the same! )

In extreme case where difference in V(LED) between LEDs is big you may end up with only LEDs that have lower V(LED) being on and all others are off.


Regards.

 

[PRAVEEN K]

common cathode

Hi,

I tested the display with a single resistor at common cathode... I connected each anode to the output of D latch and it worked fine.... and I did'nt find any difference in brightness...

 

[thomaslg]

adjust brightness on 7 segment circuit

Hi,

I tested the display with a single resistor at common cathode... I connected each anode to the output of D latch and it worked fine.... and I did'nt find any difference in brightness...

Try to see if there is any difference in brightness between a "1" and a "8" displayed on the 7-Segment LED.

Increase the current through a LED segment over its rated value will not increase its brightness a lot further. I suspect a "1" is hotter then a "8" (for the light-up segments)


Regards.