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ericmar
Joined: 14 Apr 2004
Posts: 279
Location: Singapore
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 27 Aug 2004 8:08 How much voltage/current should I drive to 7-segments LED? |
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| tags: led voltage drive led segments |
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How much voltage/current should I drive to 7-segments LED's CC/CA pin in order to enable the digit? 
Is it the same for most of the 7-segments LED?
I got a big 7-segments LED with 11 pins and I found out the it has 11 pins with 3 pins for Common Cathode, 7 pins for 7 segments, 1 pin for the decimal point. But I couldnt get the datasheet for this 7 segments LED.
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sapaku
Joined: 17 Dec 2003
Posts: 5
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| 27 Aug 2004 8:12 How much voltage/current should I drive to 7-segments LED? |
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| By using 5V supply just enough to drive ur LED.
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ericmar
Joined: 14 Apr 2004
Posts: 279
Location: Singapore
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| 27 Aug 2004 8:21 Re: How much voltage/current should I drive to 7-segments LE |
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| tags: led segments |
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| sapaku wrote: |
| By using 5V supply just enough to drive ur LED. |
Wouldnt 5V ruins the 7 segments LED?
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emmos
Joined: 30 Dec 2003
Posts: 60
Helped: 2
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| 27 Aug 2004 11:54 Re: How much voltage/current should I drive to 7-segments LE |
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hi everybody
i had alot of projects with seven segment displays and burned one or 2 before i reached that its volatge limit is approximetly between 2.5 and 3.5 voltes.
u can see the brighteness as u icrease the voltage. u can test that by and ordenary power supply see wether it's common anode or cathode test it.
C U
Emmos
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balaji280283
Joined: 26 Aug 2004
Posts: 7
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| 27 Aug 2004 13:01 How much voltage/current should I drive to 7-segments LED? |
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| to prevent burnouts, simply included a 470 ohm resistor when u connect the Supply pin to 5V. That wud protect all the 7 segments
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samcheetah
Joined: 25 May 2004
Posts: 672
Helped: 31
Location: Pakistan
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| 27 Aug 2004 14:31 Re: How much voltage/current should I drive to 7-segments LE |
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actually its the current that should be controlled. an LED is basically just a diode with a special function. after the barrier voltage the current rises exponentially. so u dont have to control the voltage to control the brightness of the LED display. its the current that has to be controlled by adding a resistor in series. just keep the voltage above the barrier voltage and then control the current
i hope that helps
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jorgito
Joined: 26 Mar 2003
Posts: 164
Helped: 11
Location: Argentina
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| 27 Aug 2004 14:47 How much voltage/current should I drive to 7-segments LED? |
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Hello
As samcheetah said, you must control current.
The ligth output is not linear with current, so if you are under maximum average current, you could pulse something like 1 Amp with 5% duty cycle and get more ligth than 20 mA DC drive.
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glenjoy
Joined: 01 Jan 2004
Posts: 1021
Helped: 47
Location: Philippines
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| 27 Aug 2004 14:50 Re: How much voltage/current should I drive to 7-segments LE |
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| If you will be using it as a multiplexed display, I think you can use 5V, just check your dutycycle that it is at most or not more than 50%, it is by PWM.
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ericmar
Joined: 14 Apr 2004
Posts: 279
Location: Singapore
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| 27 Aug 2004 15:02 How much voltage/current should I drive to 7-segments LED? |
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| Sorry but I hv no idea on the dutycycle that u guys were talking about! How do I check the dutycycle?
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glenjoy
Joined: 01 Jan 2004
Posts: 1021
Helped: 47
Location: Philippines
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| 27 Aug 2004 15:31 How much voltage/current should I drive to 7-segments LED? |
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| Are you multiplexing the LEDs or the LEDS are TURN ON all of the time?
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jorgito
Joined: 26 Mar 2003
Posts: 164
Helped: 11
Location: Argentina
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| 27 Aug 2004 15:33 How much voltage/current should I drive to 7-segments LED? |
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| tags: led voltage drive led segments |
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If you only need to ligth the segments, use any DC supply and a resistor in series with each segment.
To calculate the resistor value, take the DC voltage of your supply, substract the forward voltage of your LED segment and divide by the maximum segment current. Forward voltage and segment current can be taken from the data sheet.
You could use a resistor 20 to 30% greater to get some safety margin.
Multiplexing is a method to drive displays using less wiring. If you have, lets say, four digits to drive, you drive each one 1/4 of the time, one after the other. If you repeat the procedure at a frecuency greater than 25 to 30 Hz, the human eye will not notice any flicker or blinking.
You only need to concern with the "duty cycle" if you are multiplexing the display or you need the most ligth output you can get from your display.
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Big Boy
Joined: 20 Jan 2004
Posts: 253
Helped: 10
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| 27 Aug 2004 18:15 Re: How much voltage/current should I drive to 7-segments LE |
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| tags: led voltage drive led segments |
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Normally, 10mA should be about right.
So, if you have common cathode, you need to:
- Connect the CC to GND
- Connect a resistor to each of the segments (including the decimal point).
- Connect the output of your driver to the resistors.
Normally, a typycal led take 1.7V dropout voltage. So, if you use 5V to switch on the individual segments, usinh 330 ohm resistors will give 10mA. Though, you could use 470 ohm too.
| Code: |
+---|<|---\/\/\----- 5V
| 330 ohm
+---|<|---\/\/\----- 5V
| 330 ohm
...
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GND |
Note that since each segments use 10mA, this can give a total of 80mA (when all segments are turned ON, with the decimal point).
In this case, wou should make sure that the device that drive the leds can give the required current output. Use a buffer/driver chip if you're not sure.
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M!k
Joined: 22 Apr 2002
Posts: 815
Helped: 69
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| 27 Aug 2004 21:36 Re: How much voltage/current should I drive to 7-segments LE |
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| jorgito wrote: |
Hello
As samcheetah said, you must control current.
The ligth output is not linear with current, so if you are under maximum average current, you could pulse something like 1 Amp with 5% duty cycle and get more ligth than 20 mA DC drive. |
seems to be logic because a pulsed current of 1A @ 5% duty cycle conforms to a DC current of 50mA. But isn't this a little bit too much?
How long can the maximum pulse length ("on time") be at such a high current?
Mik
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jorgito
Joined: 26 Mar 2003
Posts: 164
Helped: 11
Location: Argentina
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| 27 Aug 2004 21:45 How much voltage/current should I drive to 7-segments LED? |
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Ooops!
My fault! read 2% where it says 5%.
I have pulsed leds at that current, with some 500 microsecond ON time.
best regards and excuse me for my poor math.
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ericmar
Joined: 14 Apr 2004
Posts: 279
Location: Singapore
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| 28 Aug 2004 7:52 Re: How much voltage/current should I drive to 7-segments LE |
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| tags: led segments |
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I'm able to enable the digit now but the output current from LS48 is way too small and the 7 -segments LED is very dim! How can I solve this problem without using 7 transistors to amplifies the current?
Here is my module schematic:
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M!k
Joined: 22 Apr 2002
Posts: 815
Helped: 69
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| 28 Aug 2004 8:59 Re: How much voltage/current should I drive to 7-segments LE |
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sorry, but your circuit can't work:
take a look at the datasheet of the 74LS48. The outputs are active high with internal pull-ups and the maximum high level output current is 1.3mA ! So the 74LS48 should be used with 7 additional NPN driver transistors (which you wanted to avoid  ). Another possibility would be using a 74LS47 with open collector outputs. But this means that you have to use common anode 7-segment displays and the 4 NPN transistors must be exchanged to PNP types. Problem: the maximum low level current of the 74LS47 is 24mA. I don't know if this is enough to drive the 4 displays in a multiplexed mode (see discussion above).
Btw. your schematic shows 7-segment displays with Common Anode. It's only cosmetic, but they are Common Cathode.
Mik
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ericmar
Joined: 14 Apr 2004
Posts: 279
Location: Singapore
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| 28 Aug 2004 9:23 Re: How much voltage/current should I drive to 7-segments LE |
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| M!k wrote: |
sorry, but your circuit can't work:
take a look at the datasheet of the 74LS48. The outputs are active high with internal pull-ups and the maximum high level output current is 1.3mA ! So the 74LS48 should be used with 7 additional NPN driver transistors (which you wanted to avoid ). Another possibility would be using a 74LS47 with open collector outputs. But this means that you have to use common anode 7-segment displays and the 4 NPN transistors must be exchanged to PNP types. Problem: the maximum low level current of the 74LS47 is 24mA. I don't know if this is enough to drive the 4 displays in a multiplexed mode (see discussion above).
Btw. your schematic shows 7-segment displays with Common Anode. It's only cosmetic, but they are Common Cathode.
Mik |
Thanks for ur explaination. In fact, I'm still a bit confusing about how to differentiate Common Anode and Common Cathode?  Can anyone give me a short tutorial here? I tried to look at those online tutorial but still...
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M!k
Joined: 22 Apr 2002
Posts: 815
Helped: 69
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| 28 Aug 2004 10:06 Re: How much voltage/current should I drive to 7-segments LE |
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| tags: led segments |
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that's simple:
in a display with a common anode all 7 (with Decimal Point 8 ) anodes of the LEDs are connected to one common terminal (CA). Connect the CA to Vcc, the segments to Gnd.
Common cathode: all cathodes are connected in one point (CC). Connect CC to Gnd, the segments to Vcc.
Take a look at this datasheet (page 5), where both possibilities are shown:
http://www.vishay.com/docs/83179/83179.pdf
Please note: the upper wiring is a common anode (TDCG1050), the lower type is a common cathode ((TDCG1060). For the comprehension ignore that there are 4 digits inside.
Mik
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ericmar
Joined: 14 Apr 2004
Posts: 279
Location: Singapore
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| 29 Aug 2004 10:51 Re: How much voltage/current should I drive to 7-segments LE |
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Is there any software that I can use to simulate this circuit?
Last edited by ericmar on 29 Aug 2004 11:38; edited 1 time in total |
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madjid
Joined: 29 Aug 2004
Posts: 36
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| 29 Aug 2004 11:16 Re: How much voltage/current should I drive to 7-segments LE |
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Hi,
Different 7-seg may have different turn-on voltages form 1.5v to 5v, but the common is about 1.6v-1.8v, applying this voltage will cause the 7-seg to illuminate. But about the currnt, it depends on the driving methode you have implemented and of course the size of 7-seg, more current will cause more illumination.
Bye.
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glenjoy
Joined: 01 Jan 2004
Posts: 1021
Helped: 47
Location: Philippines
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| 29 Aug 2004 16:37 How much voltage/current should I drive to 7-segments LED? |
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Hi, even you meet the voltage requirements if your driver cannot meet the required turn on current, it will be still not light up at the desired level.
Better use a transistor or another high side buffer driver.
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