Calculating Current Limiting Resistance of the segment Diaplay

[mahender1987]

Dears,



Can anybody tell me how to calculate the current limiting resistance for 7 segment display. In an exercise it was shown that 330Ω resister is used as current limiting resisters and MC 14511 decoder used. As per my observation the output drive current of the decoder is 20mA. And the Forward voltage of the Display is 3.45v @ 20mA. So My problem is how to determine the value of the current limiting resister.

Thankns

 

[BradtheRad]

It's a question of how far you dare to push the led brightness. You need to see the digits but you don't want to burn out any led's.

Do you intend to use the device in sunlight? You'll need to boost the brightness to max.

All you can do is experiment. Install different value resistors between the 14511 and the digit display. Do not go over 20mA however.

An easy tactic is to put one resistor in the common wire to ground. Some numbers will be bright (such as 1, others dim (such as 8). From this you'll get an idea what level of brightness you need.

 

[hemnath]

resistor value = (Input source - Drop across each segment in display)/current to flow through the segment to glow the LED

eg: R = (5V-3.45V)/20mA => 775 Ohms.

Best wishes

 

[mahender1987]

Thanks for UR reply. I'm using Multisim 11.0. to try my interests. coming to the problem as shown in the pic if i connect output of decoder and input of the display directly with out any resistance that mean if I short i can observe more than 200A of current between the decode and display. if that much current flows through the ckt its should get breakdown. Instead of that its working properly in multisim 11.0.. So how far multisim is reliable. did i misunderstand the concept?

Thanks

 

[BradtheRad]

with out any resistance that mean if I short i can observe more than 200A of current between the decode and display. if that much current flows through the ckt its should get breakdown.

Yes, the simulator ought to show a flash and a bang, and smoke.

We know that 200A is an unrealistic amount of current, but the simulator can only calculate robotically. It is typical for a simulator to assume the supply is able to provide unlimited power.

When we spot a figure that is way too high, it is up to us to interpret that as an alert. At such times we should look around to see if we left out something important.

If you had omitted the resistors in real life, you would have gotten a high current reading, AND the led display would light up brightly, AND it probably would break.

So a simulator is a helpful assistant, if we know its limitations.

 

[rfredel]

Hello mahender1987,

in your circuit diagram I don't see any current limiting resistors. So it's clear, that your A-Meter shows more than 200A. The simulation don't say, that their is something wrong and your circuit will burn out.

Put a resistor of 150Ω in every output line of CD4511 and MULTISIM will show you ~20mA.

If you use your schematic in real circuit, the IC and your LED will really burn.

I hope it helps

Regards
Rainer

 

[mahender1987]

I'm so thank full to all name by name.

@ Rainer its not a problem. Thanks 4 ur reply

 

[mahender1987]

Can anybody find out the logic diagram for the above code conversion ?

 

[BradtheRad]

View attachment 88120

Can anybody find out the logic diagram for the above code conversion ?

It looks as though the total quantity of ones in column A-B-C-D, is converted to a binary value and displayed at K-L-M-N.