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7 Segment Display Multiplexing timing info please ?

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UroBoros

UroBoros

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multiplexing display mcu

Hello

I am doing a system with 8 numbers of 7 segment digits. common cathod ,directly driven from PIC port pins. Usually for systems with 4 digits what I do is a use a 300R resistor series with MCU port pins and timing I adjust by trial and error and it used to be ok.

But here what ever I do with timing I am not getting a bright display. I am doing display on intrrupts. The series resistor I tried by reducing to 220. No substantial improvement.

Can somebody help me with series resistence calculations and timing requirements on 7 segment disaplays? Currents to be incresed to compensate multiplexing. But how much with a 25ma PIC pin? please.

There must be a relation between supply voltage , Ontime ,offtime etc.

Thanks

Picstudent
 

mutliplexing 7 segment but numbers

I use modern HE or Super RED displays, even with 6 digits it's very bright with 470ohm resistors. But your design is driving the cathodes directly from the PIC and that's not enough for most typical displays. Add a 1.2K resistor and a 2N2222 will allow you to properly drive most displays. Check the Dragonfly schematic on my site for details and a program to test it.
 
seven segment dispaly multiplexing current

Try this:

Segment current limiting resistance = [1/N] * [Vsupply-Vled] / Isegment

N = number of digits

For example:
Vsup=5V
Vled=2V
N=8
Iseg=5mA

Rsegment ≈75Ω

In other words, for 1/8 mark-space ratio, you will have to allow 40mA pulse current per segment and, in the worst case, when all 7 segments are on, the cathode driver has to sink pulse current of 7*40mA ..

Regards,
IanP
 

    UroBoros

    UroBoros

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7 segments display multiplexing current

blueroomelectronics said:
...with 470ohm resistors. But your design is driving the cathodes directly from the PIC and that's not enough for most typical displays. Add a 1.2K resistor and a 2N2222 will allow you to properly drive most displays. Check the Dragonfly schematic on my site for details and a program to test it.
Click to expand...

Hello

I am using almost similer circuit ,only difference is instead of 470R I am using 330, but disply may be low quality.

Any going to try using a driver for segments.
IanP

Thanks for the calculations. But to provide that 40 ma current , PIC cant do that. So I should use a driver like ULN2003, right?
by switching over to Common anode displays and a NPN PNP arrangemt in digit selecting anode line.

The problem is I have 4 digits large displys (around 12v) and 4 digits small normal 2 volts. i am trying different arrangements.

Now I plan to drive all 8 (Common Anode) through a ULN2003. segments.

Anode to 14V through a NPn PNP arrangement to switch 14 volt anode with 5 volt MCU out.


Thanks a lot for the help

picstudent
 

uln2003 cathode

Picstudent said:
Hello

I am doing a system with 8 numbers of 7 segment digits. common cathod ,directly driven from PIC port pins. Usually for systems with 4 digits what I do is a use a 300R resistor series with MCU port pins and timing I adjust by trial and error and it used to be ok.
<snip>
Click to expand...
1. 330 ohms resistor is ok.
2. The intensity of the LED depends on the ON time of the segment, current through it and also on the quality of the displays.
3. Keep a multiplexing rate of 2mS to 4 mS depending on the no. of display you use.

Cheers

Ravi
 

    UroBoros

    UroBoros

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uln2003 common cathode

I faced quite similar kind of problem....using at89c51 ......
to me its better, if u use npn transistor...that is current not from ur MCU pin directly. make the transistor forward biased by make high ur MCU pin.

or u can use decoder like cd4511;if ur circuit isnt fixed, to me,u should use decoder.it will also decrese ur MCU output pin.decocer input is just 4, for seven segment.and independent of input current.
 

    UroBoros

    UroBoros

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12v 7 segment common anode pic

You don't need base resistors, if you use transistors as emitter-follower, like here: **broken link removed**

For common cathode dislays use pnp transistors, and connect it's collectors to gnd.
 

    UroBoros

    UroBoros

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What you need to do is compute the proper timing, you should get the duty cycle in the right way without affecting the frequency to avoid blinking.
 

IanP said:
Try this:

In other words, for 1/8 mark-space ratio, you will have to allow 40mA pulse current per segment and, in the worst case, when all 7 segments are on, the cathode driver has to sink pulse current of 7*40mA ..

Regards,
IanP
Click to expand...

Thanks to all. I have removed the direct drive from pins.

used a ULN2003 to drive segments and NPN PNP pair to drive digits.

Now I can increase current as required with MCU pin constraints.

it is ok now, interupt on every 2.5ms and 8 digits

thanks again

picstudent
 

Dont use any resistors.

As you are switching at higher speed you need lot of inrush current.

Regards
Nandhu
 

nandhu015 said:
Dont use any resistors.

As you are switching at higher speed you need lot of inrush current.
Click to expand...
This is not a good idea at all. You will be damaging the drivers.

Cheers

Ravi
 

    UroBoros

    UroBoros

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nandhu015 said:
Dont use any resistors.

.... you need lot of inrush current.

Regards
Nandhu
Click to expand...

Still a limit is to be fixed depending on the size (which determins the voltage) of display.

I am using limiting resistors, not yet tried without them.

picstudent
 

Picstudent said:
Still a limit is to be fixed depending on the size (which determins the voltage) of display.

I am using limiting resistors, not yet tried without them.
Click to expand...
If LED's can take a steady current of 20 mA, then they can take a peak current of about 100 mA (depends on the LED and also on the duty cycle). Current limiting resistor is a must.

Cheers

Ravi
 

    UroBoros

    UroBoros

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I also want to know that how to calculate current of scan led with duty cycle??
 

sn_burki said:
I also want to know that how to calculate current of scan led with duty cycle??
Click to expand...

see what IanP mentiones in the third post.

picstudent
 

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