[SOLVED] Question regarding 4511 & 7-segment led display

[jarred23]

I'm currently building a circuit that has 4 sets, each set consist of



The datasheet for the 7-segment (CC) I used (ELS-511SURWA) http://www.autcomp.com.br/fichas/everlight/displays/displays/ELS-511SURWA-S530-A2.pdf

This is my understanding, please correct/advise me if I made any mistakes.
CD4511 output drive voltage is 4.5V, meaning each output to the 7 segment is 4.5V. I place 330ohm resistors on each output to limit the current. According to the 7 segment display datasheet, each led inside takes (max) 2.4v, so the current for each led is 6mA. Considering 1 set draws 42mA and I have 4 sets, the total current drawn is 168mA.

My concern is that the current drawn is too high for my liking, and I wish to reduce the current drawn as I'm using 5V voltage regulators to supply voltage to CD4511.
If I replace the 330ohm resistors to 1kohm, will it still work? Doing the maths, each led will draw 2.1mA, and the total current drawn is only 58.8mA. I know the led brightness will go down due to lesser current, but I think the 7 segment display datasheet does not show what is the optimal current required for it.

Thank you guys very much, I really appreciate any inputs on this.

 

[betwixt]

Your math is reasonable but is the one 4511 driving 4 displays simultaneously? I can't open the dropbox without an account to see what type of display you are using. Please post the data sheet here instead of using external file servers, remember that some are protected or not available in other countries.

A 4511 is only designed to drive one 7-segment display but you can use it as a segment decoder and driver to one DIGIT at a time in a multi-digit display by only enabling the CC of one of the digits at a time. Like this:
1. send segments data for digit 1
2. enable CC of digit 1
3. wait a few mS
4. disable CC of digit 1
5. send segments data for digit 2
6. enable CC of digit 2
7. ..... repeat for all digits
8. back to step 1.

It's called multiplexing and is very commonly used. If you repeat the process fast enough your persistence of vision doesn't notice that only one digit is actually lit at a time, it looks like they are all working all the time. The 4511 is only providing current to one digit though so it's current rating isn't exceeded.

Brian.

 

[jarred23]

Your math is reasonable but is the one 4511 driving 4 displays simultaneously? I can't open the dropbox without an account to see what type of display you are using. Please post the data sheet here instead of using external file servers, remember that some are protected or not available in other countries.

A 4511 is only designed to drive one 7-segment display but you can use it as a segment decoder and driver to one DIGIT at a time in a multi-digit display by only enabling the CC of one of the digits at a time. Like this:
1. send segments data for digit 1
2. enable CC of digit 1
3. wait a few mS
4. disable CC of digit 1
5. send segments data for digit 2
6. enable CC of digit 2
7. ..... repeat for all digits
8. back to step 1.

It's called multiplexing and is very commonly used. If you repeat the process fast enough your persistence of vision doesn't notice that only one digit is actually lit at a time, it looks like they are all working all the time. The 4511 is only providing current to one digit though so it's current rating isn't exceeded.

Brian.

Sorry about the dropbox thing, and not being clear in my post.

I have 4 7-segment displays. I have 4 4511s to run each individual 7-seg display.

Do you have any comments about the 'reducing current draw' part?

 

[betwixt]

Sorry for misunderstanding. The total current is the sum of all the segment currents so your only way to reduce it is to drop the current in each segment individually. Increasing the series resistor is the easiest way but you will sacrifice some brightness.

Alternative methods are:
1. convert it to multiplexed as I thought it in my first reply, it isn't complicated and it draws far less current.
2. join all the -BI pins together and drive them with a PWM signal, again simple and it will allow you to set the brightness/current compromise for best results.

Brian.

 

[Audioguru]

A multiplexed display draws far less current because it is much dimmer. Each digit is lighted for only part of the total time which is Pulse Width Modulation that is used as a light dimmer.

 

[betwixt]

Multiplexing isn't PWM because there is no 'WM' (Width Modulation), only sequential pulsing of each digit of the display. Subjectively, the brightness isn't dropped proportionately to the mux ratio, it appears brighter than math would suggest because of the persistence of vision 'remembering' each digit at full brightness. There is a bonus that to some extent you can overdrive the LEDs if that's an option becuase they have a longer 'cooling off' period than on time.

Brian.

 

[Audioguru]

If you multiplex four displays then each display is turned on for only 1/4 of the total time which is a narrow pulse. Then of course each display is dim due to the PWM.
Persistence Of Vision only "remembers" the brightness when a light is turned on for more than 30ms which does not happen with multiplexing because the on-duration is much less.
Daytime running lights on most cars are dimmed with PWM (except not on Fiat-Chrysler cars, they use high beams at full blast). There are many PWM LED dimming circuits in Google.