7 segment displays

[KansaiRobot]

Hello and thanks always...

I have a project where I need to display something. Now I was thinking of using an LCD but I dont have it. Instead I have some 7 segment displays, so instead of spending money I think I would use them...

Now, these 7 segment are kinda mysterious in that I dont know their part number. I found the part number of one and it happens to be the LA-601VB.
So I learned that they are just like any other LED but arranged to have a common anode.. So far so well...

Just in case I searched for some examples and yes there was this

https://circuitdigest.com/fullimage?i=circuitdiagram_mic/Decimal-Counter-Circuit.gif

From the view of the circuit these 7 segments are just any other LEDs and all I have to do is write some driver so as to translate a char value into the appropriate combination of LEDs...

Now my question is:

Normally for a LED you have to put a resistor to limit the current. But in the above circuit there are no resistors for the 7 segment....

Is that ok? aint I gonna burn something?

Also I heard that common anode is preferable than common cathode... what do you think?


Thanks for the help

 

[KeepItSimpleStupid]

Display drivers sometimes have some magic in them. Generally resistors should be used.
Common anode might be a little easier to drive, but HIGH SIDE DRIVERS are available and some ports can source or sink currents.

 

[JimB]

You say that your 7segment LEDs are common anode, the 7seg LEDs in the circuit to which you linked are common cathode.
Your LEDs will not work in that circuit.

Normally for a LED you have to put a resistor to limit the current. But in the above circuit there are no resistors for the 7 segment....
Is that ok? aint I gonna burn something?

If we wanted to be absolutely certain that we were not going to over current the LEDs or the microcontroller I/O ports, we would use resistors to limit the current.
However, because the display is multiplexed, each display is only illuminated for part of the time, the average current is lower and we can run the circuit without current limiting resistors.

Some time ago there was a very heated discussion about display drivers like this here on ETO. If I remember correctly, it did not end well!

JimB

 

[KansaiRobot]

Tnank you for your reply

You say that your 7segment LEDs are common anode, the 7seg LEDs in the circuit to which you linked are common cathode.
Your LEDs will not work in that circuit.

Oh yes I noted that. In my case I would have to put 5V in the common and join each pin to ground (with a resistor I guess). I could then multiplex the anode.

I also experimented with the LEDs without partnumbers and they are common anode as well.

If we wanted to be absolutely certain that we were not going to over current the LEDs or the microcontroller I/O ports, we would use resistors to limit the current.
However, because the display is multiplexed, each display is only illuminated for part of the time, the average current is lower and we can run the circuit without current limiting resistors.

Some time ago there was a very heated discussion about display drivers like this here on ETO. If I remember correctly, it did not end well!

JimB

Oh I see, so they dont use resistors because they are multiplexing very rapidly? ..ummm in my case I think to be safe I would use resistors...
(in low voice) can you like to that discussion?

 

[KansaiRobot]

One more question though

Instead of putting 8 resistors for each pin of the LED, can I put only one resistor of say 1K in the common pin and the others go directly ( to ..say a microcontroller??)

 

[ikelectro]

One more question though

Instead of putting 8 resistors for each pin of the LED, can I put only one resistor of say 1K in the common pin and the others go directly ( to ..say a microcontroller??)

In that case if you are use 5V as supply and put a Current limiting resistor in the common pin then the glow of the LED will be very low. If the intensity of the LEDs are not matter of fact you can consider it, but depending on your circuit choose the resistor carefully as I think 1KΩ in that matter is too high.

 

[KeepItSimpleStupid]

Not really.

Take a look here https://www.adafruit.com/products/453 for some other options.

LED multiplexing is a real pain.

 

[KansaiRobot]

Wait! I just realized.....
In that example the current is being limited by the transistor, isnt it???! I mean there is a resistor to the base which limits the current to the base therefore the current in the collector emiter will be limited...ergo not needing a resistor!
am I wrong?

 

[KeepItSimpleStupid]

But you don't want the brightness to be different depending on the number of segments that are on do you?

 

[Ian Rogers]

Wait! I just realized.....
In that example the current is being limited by the transistor, isnt it???! I mean there is a resistor to the base which limits the current to the base therefore the current in the collector emiter will be limited...ergo not needing a resistor!
am I wrong?

The transistor is just a switch in the example... Resistors will be needed or bye bye micro... The current on a micro pin is less than the max current of an LED!!!

 

[KansaiRobot]

Ok I put 1.2K resistance to each pin and a 220Ω to the transistor base.
Results.. they are ok but the brightness of the LEDs are low...

 

[Ian Rogers]

Try 220 ohn on each segment and a 470 ohm on the base.. That way you'll see it... The multiplexing will dim the LED's quite a bit and a 1.2k will resist the current a little too much...

Have you considered using 74HC595 drivers??? that way you can give the segments the brightness they deserve..
Are you using the AVR as in the diagram??

 

[KansaiRobot]

Try 220 ohn on each segment and a 470 ohm on the base.. That way you'll see it... The multiplexing will dim the LED's quite a bit and a 1.2k will resist the current a little too much...

Have you considered using 74HC595 drivers??? that way you can give the segments the brightness they deserve..
Are you using the AVR as in the diagram??

Thank you for your advice. Ill follow it
I haven't being able to check the driver... sorry.I will. And I am planning to use my usual PIC for this. (18F2550)

 

[Ian Rogers]

I have done this on the pic16f887 both multiplexing and direct drive... I prefer direct drive as the serial loading is a easy and the LED's are very readable...

 

[JimB]

I once made a 7seg LED display for a project of mine.
I hated the dim display due to the multiplexing.
On later projects using 7seg LEDs, I always used a direct drive where the LED was always on, not multiplexed.
I always use current limiting resistors, even in the multiplex situation.

LCD modules are often so much easier and sometimes cheaper to use.

JimB

 

[AnalogKid]

Instead of putting 8 resistors for each pin of the LED, can I put only one resistor of say 1K in the common pin and the others go directly ( to ..say a microcontroller??)

The schematic in post #1 is not well drawn, is missing some omponents, and has the wrong resistor value for the two 2N2222 base resistors. It is possible that either the displays have current limiting built-in, or that the schematic is more of a system diagram, lacking some details. Either way, the kinds of LEDs in most standard 7-segment displays *always* require current limiting. Sometimes that is built into the segment driver drvice, but that is not the case for an ATMEGA. When the displays are multiplexed, you can increase the current to get more brightness by reducing the value of the current limiting resistors. This is safe because none of the displays are on 100% of the time. The display datasheet lists the maximum pulse current value separately from the max. continuous current value. Generally speaking, it you are multiplexing 3 displays you can pulse them with 3 times their normal current.

In a multiplex situation as in your schematic, a single current limiting resistor in series with each display's common terminal will not work well. The brightness of each display will change depending on how many segments are energized, and the segment brightnesses will not be even. Yes, you need only one set of current limiting resistors even though there are 3 or 4 displays.

ak

 

[Tony Stewart]

Using Common Anode 7seg LED's, a high side driver (p-ch logic level FET or PNP) can Mux each digit but average current goes down with duty cycle.

As I recall, The ATMEGA CMOS drivers hav an internal resistance of about 25 Ohms and the LED has an ESR determined by size or power rating, often in the 15 Ohm range. This combined ESR will current into a 1.6~2.2V Red LED when driven from a 5V supply and dependent on actual wavelength of Red and Vf @ rated current. If you choose LEDs rated at 2V @ x mA then Ohm's Law estimates current at 3V/40 Ohm or 75mA.

This is ok for LED's for each digit only on for 25% duty cycle but is bad for CPU heat loss and may exceed max power dissipation of chip with 8outputs = 1.125Wmax increase based on above assumptions.

Reducing the voltage to CPU or adding 30 to 50 Ohms in series with each segment Vol driver will lower its power dissipation accordingly and brightness. The **broken link removed** is assumed to have an RdsOn or ESR ~ 1 Ohm otherwise total ESR is increased and current reduced and is assumed to be biased properly such as a logic level FET.

 

[Ian Rogers]

As I recall, The ATMEGA CMOS drivers hav an internal resistance of about 25 Ohms and the LED has an ESR determined by size or power rating, often in the 15 Ohm range. This combined ESR will current into a 1.6~2.2V Red LED when driven from a 5V supply and dependent on actual wavelength of Red and Vf @ rated current. If you choose LEDs rated at 2V @ x mA then Ohm's Law estimates current at 3V/40 Ohm or 75mA.

That's why I asked what processor... He's using a pic..

 

[audioguru]

He was planning to limit the LED current by limiting the base current of the driver transistor without realising that every transistor will have a different current because they are not all the same. Then some LEDs will barely light and other LEDs will burn out.

 

[Tony Stewart]

Using a Common Anode Display (CA) with FET drive for the digit and port drive for the segment from 3.3V you can drive the LEDs direct
With a 5V supply the ESR drops to 25 Ohms at 25'C but doubles at 85'C so a small value series R (10~22) for each segment is required for VoL