Best solution to use of 6" 6-digit 7-segment display the clock without multiplex

lara's picture

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Best solution to use of 6" 6-digit 7-segment display the clock without multiplex
June 1, 2019 - 12:15pm

Hi,

What is other options to get display for the subject arrangement other that multiplexing.

As the big (like 6"/12") display using ULN IC alongwith Transistors for anode/cathode site each segment is of 2 stringS of 6 LEDS ) with multiplexing technique is not a solution of getting proper brightness. 

Is there other technique to implement for this arrangemt??

tnx

Aswinth Raj's picture

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check you driver source current
June 1, 2019 - 1:04pm

I am confused as to why you have brightness problem with 7-segment display when multiplexing. It is very common method and I have personelly not seen the bringtness going down when multiplexed. May be you are driving the display module directly with I/O pins without a driver network. How are you using the ULN module? Sine the modules are big did you check their current consumption and will ULN source enough of that? Are the displays CA or CC.? Sharing your circuit might help us find the problem. Some times speeding up the refresh rate will help!

You can refer to this article to know how it is normally done, the article shows 4-digit display but is very similar for 6-digit as well. 

https://circuitdigest.com/microcontroller-projects/7-segment-display-interfacing-with-pic16f877a

 

I belive the problem is with your driver part and not with multiplexing. Becasue when switching from one digit to another the LEDs are turnign on and off fast so current cunsupmtion will be high and hence your driver should be able to source high current. However if you want to escape from multiplexing the only other way is to use I/O pins for all 6 digits. Whcih will be like 42 I/O pins or you can try I/O expander ICs or 7-segment driver ICs. 

lara's picture

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Hi raj,
June 1, 2019 - 7:03pm

Hi raj,

Used this string for each digit (2 string/segment), If-20ma, Vf-12 volts. and peak-30 ma.

As an Anode driver- ULN2003A (o/p current 500ma per driver pin) and BC327(collector current of 500mA).

As Cathod Driver- ULN2803(collector current 500ma).

Used 15v power supply.

7SEG_CA.jpg

For a 6 digit clock, the average current in the LED's will only be 30 mA / 6 = 5 mA

The peak current for one digit is then 7*2*30 mA + 1 * 30 mA = 450 mA

5 mA average in x6 mux may be too low for bright ambient light conditions. 

Thus I believe this display will be best operated in non-mux.

Note: The complete ckt not in my hand right nw. I will pase hare early.

7-segment driver ICs. 

>>Are u asking bcd-7segment driver max7219. that means 6 max7219 is req. One for each digit. Is there any link for ckt  diagram combination with my reqirement and can u provide that link for my knowledge gain. Although I am not going to implement my case with max7219 ic.

Tnx/

 

Aswinth Raj's picture

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Average current should be more
June 2, 2019 - 11:26am

I do not understand this part

For a 6 digit clock, the average current in the LED's will only be 30 mA / 6 = 5 mA

Because even a normal red colour 5mm LED will need aroud 20mA, so your assumpion of 5mA per segment could be wrong. 

From your statement I consider that the peak current of a single segment in your display is 50mA. So when you are using all 7 segments (ignoring the the dot) the current consumtion will be 4*50 which is 200mA for one digit. Now, since you are multiplexing the digits at any given time only one digit will be active and hence your ULN driver should be able to driver all 6 digits without any dimming problem. 

However if you are still facing the problem the reason couls either be your mis interpretetion of current values or your update interval is long. 

If you are looking for driver IC you can consider 4511, the detials of the same can be found in link below

https://circuitdigest.com/electronic-circuits/7-segment-display-driver-using-ic-cd4511

lara's picture

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Power supply rating selection
June 15, 2019 - 3:34pm

According to the datasheet, the decimal point uses one string of  LEDS, and all the other segments use two parallel strings.
I think this means that the peak current for the segments is 60 mA, 30 mA in each string.
The peak current for one digit is then 7*2*30 mA + 1 * 30 mA = 450 mA

As a power supply unit to drive all the 6 digits, I want my own unit which is combination of Bridge rect, Filter cap, ic LM7815 arrangement only. 
But what should be transformer (for 230/15v) amp rating for driving all 6 digits:

For testing purpose, i tested 2 digits with 230/15v 1 amp transformer but the trf. got damaged, where as a single digit worked correctly with the same 1 amp trf.

Question No: 1))>> Why trf got damaged while used 2 digit. As were requred 450*2 =900mA.

Question no 2:>> will be that for 6 digit, 450 mA * 6 Digits = 2700 mA (+ additional 20%) = 3240 mA
So Should I need to use the TRF of MORE THAN 3240 mA for sec 15V for all 6 digit (as this CURRENT will not available at market with 15v trf. , but with this calculation I wanted to drive 2 digits with 230/15v of 1000 mA trf only for testing purpose and same trf pr. side got burnt/open

OR, 15V 4A SMPS - 60W - DC Metal Power Supply, and if not available in local market then should i 
18V 4A SMPS - 72W - DC Metal Power Supply.

TNX..

Jayant's picture

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Use an SMPS circuit
June 17, 2019 - 12:09pm

Are you sure with your calculation that a single 7-segment display uses 450mA. If yes then I guess it should be a big display board. So yes, for 6 digits you will need about 2.7A which quite a big value. 

I would suggest you to buy an SMPS with 15V 5A rating to power your entire circuit. Use a transformer to power your project is not a good idea beacuse of the large current value and abviously SMPS will be cheaper than buying such a bulky transformer and designing the regulator on your own. 

 I want my own unit which is combination of Bridge rect, Filter cap, ic LM7815 arrangement only. 

Any particular reason to built your own circuit? Also microcontrollers are noise sensitive, so if you are builting your own power regulator make sure it is noise free else you will get weired results.

For testing purpose, i tested 2 digits with 230/15v 1 amp transformer but the trf. got damaged, where as a single digit worked correctly with the same 1 amp trf.

There could be multiple reasons for this. But when you draw too much current (overload) from a transformer it likely will get hot and will fail only after prolonged use. Did you notice any of these heating issues. If not the reason for failure could be something else. Also most transformer that we get from hobby store does not live up to the original current rating mentioned on their spec sheet. 

Also what is operating voltage of your design? Is it 15V, if yes why? 

 

lara's picture

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Are you sure with your
June 17, 2019 - 3:31pm

Are you sure with your calculation that a single 7-segment display uses 450mA. If yes then I guess it should be a big display board. So yes, for 6 digits you will need about 2.7A which quite a big value.

Yes it is big like 6 inch disp. and refer to leds' string in my post here on "June 1, 2019 - 7:03pm" datasheet mentioned forward current 30ma and volt 12v.

so my calculation refer to that,  The peak current for one digit is then 7*2*30 mA + 1 * 30 mA = 450 mA.

Also what is operating voltage of your design? Is it 15V, if yes why? 

As the drop is 12 v, for current regulation the design volt is 15v with current limit resistor.

I would suggest you to buy an SMPS with 15V 5A rating to power your entire circuit. Use a transformer to power your project is not a good idea beacuse of the large current value and abviously SMPS will be cheaper than buying such a bulky transformer and designing the regulator on your own. 

I tried to make all the things with my own including power supply.

But why 5Amp you are recommending here? why not 3.2Amp OR above 2.7Amp as calculation revealed? >> Any resons please.......

There could be multiple reasons for this. But when you draw too much current (overload) from a transformer it likely will get hot and will fail only after prolonged use. Did you notice any of these heating issues. If not the reason for failure could be something else. Also most transformer that we get from hobby store does not live up to the original current rating mentioned on their spec sheet. 

Yes may be spec issue, as i required 2*450=900mA (2 digit) but spec is 1000mA and may also below rated.

The heat was not noticed while damaged. both digit stand only a second with blinking. What could be the cause??

So what volt and current will fit for it??

tnx