12V 5 digit 7 Segment LED driver circuit

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raffy0991

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Hi Guys,

I am making a LED driver circuit for a 12 V LED array 5 digit 7 segment display Common Cathode. My input is from an Arduino which is 5v. So I need to create a LED driver and I have some ideas like creating a darlington pair and also the ULN2803A. However, I have only basic idea on electronics especially in transistors and IC and stuff. I need your help here is my initial simulation from Proteus but it does not turn on the LED. I used 2n2222 (NPN) and 2n5401 (PNP) since these are the only available transistors I have and I have plenty of these. Please check if I have done wrong on this circuit. I am also planning to use ULN2803A but I still have no Idea yet how to use it on a common cathode display with 12v power supply and input of 5v. Is it possible?

Hopefully you could help me guys.

Thank you

 

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The first thing that jumps out at me is you’ve got 7 LEDS in series. If their forward voltage is 2 volts, they’ll never turn on. Are you saying the actual circuit doesn’t work or the simulation? The bottom of R4 says “V=FLT”, what does that mean?
 

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Design of a LED driver starts with a specfication of segment voltage and current. The circuit might work, depending on this parameters. As stated by barry, if the segement is made of 7 series connected LEDS which might be the case for very large (>= 3") displays, you need higher supply voltage than 12 V.
 

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Hi,

I don't also know why the R4 is V= FLT I am new to proteus and i just wanted to simulate the circuit here. I also checked the forward voltage it is 2.2 V for each LED. I reduced it to 3 but it still does not work.
 

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During my testing in real life the LED array 7 segment works with direct 12v power supply. I just don't know how to make it work here in proteus. My question is my transistor and resistance values are they correct?
 

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Start with the display. Lets assume segment current is 10 mA (remember to use
in following your specific display value). The 2N3904 wants to be saturated
as a switch. Now assume there is a test mode display does with all segments
on. Lets assume 8 segments.

So 10 mA X 8 = 80 ma in 2N3904 collector. Rule of thumb to sat the 2N3904
drive Ib = Ic(max) / 10 = 8 mA.

Now look at Vouthigh on Arduino with ~ 8 mA coming out of pin.



So we will use the 5V VCC, above values show for atmega328. So at 20 mA the
output will be at least 4.2V.

So Rbase (2N2904) = (4.2 - .7 Vbe) / 8 mA= ~ 440 ohms.

Segment R's : Rseg = (Voh - Vsegment - Vsat3904) / Isegment
Vsat3904 is ~ .35V, Voh =~ 4.2V, Vsegment you get from display datasheet.



Do your best to use worst case numbers over T and V to get adequate margin and minimize power.

Regards, Dana.
 
Last edited:

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raffy0991 said:
During my testing in real life the LED array 7 segment works with direct 12v power supply.
Click to expand...
What make's you think that your LED display is correctly represented by 7 series connected LED? Very unlikely.

However, you still didn't mention a display type or a specification. Come back with some info.
 

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Common anode would have let you use commonly available open-collector /
open drain low side switches. Probably could find an octal, certainly a quad,
meant for automotive use. There used to be ULN20xx part family, not sure if
anyone makes them anymore (20V - 40V bipolar parts). Must be some, given
the electronics load of modern cars.

There's probably then also quad / octal high side switches that will take a
"TTL" input and work OK off 3.3V - lower might be flaky at -40C industrial,
but who sits around in -40C conditions to read LED panels?

Simplest discrete solution is probably 2 resistors, NPN and PNP per channel.
You could pick off prices from distributors and decide whether a discrete or
an integrated driver fits your BOM cost. board size targets.
 

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Hi I have tried to create my circuit. this is the circuit i created in proteus using 2N2222 and 2N5401 (readily available to my desk). This works fine in proteus however I tried it in actual and it doesn't work. (The LED in proteus have forward voltage of 1.8V x 6 and 20mA)

The LED keeps turning on even if no 5v supply in the base of 2n2222. (note: all the resistors have change because I played with different resistor values because I thought it would solve the problem). I checked the 2n2222 circuit, without the 2n5401 circuit, the switching of 2n2222 has no problem it turns on the LED when i supplied 5V to its base. Now, I also tried 2n5401 circuit, without the 2n2222 circuit, before I could put the resistor to ground and touch the leg of the resistor the LED will turn on (as shown on the picture). I think there is a problem with my 2n5401 circuit and I don't know how to solve it.


 

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FvM said:
What make's you think that your LED display is correctly represented by 7 series connected LED? Very unlikely.

However, you still didn't mention a display type or a specification. Come back with some info.
Click to expand...
I do not have the specification of the LED array since it is an old display which will be repairing. But there is 6 LEDs in a segment (7 segments) and there are 5 digits. I am assuming that the forward voltage is 1.8 v and when I tested one digit with all the segment is on the current is 150 mA.
 

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I am sorry for the wrong information I have tested the LED display and it is a common cathode. The segment pins are positive while the digit pins are connected to negative when I tested it. I believe I can use udn2981 or something to drive the LED array but currently I am trying transistors 2n2222 and 2n5401 since these are readily available.
 

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I tried siming this circuit with various 2N5401 spice models, something is wacky,
part never turns on like it should. Almost like the models for it C couple the base.
With a generic PNP circuit runs fine as shown below.




Regards, Dana.
 

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Hi,

your last photo is is 2318 kBytes in file size.
How much of the 2318 kBytes are useful?

cut out: 80 kBytes (3.45%)
.. not missing anything useful


reduced pixel rate and color depth: 6k Bytes (0.26%)
still not missing anything useful
So wouldn´t you agree that 99.74% of your given information is useless?


Text: 0.036 kBytes (including stars and line feeds, 0.0016%)

***
BR
N5401
B z491
***

Now some claim not to have a fancy photo editing tool:

Especially for you I used the more than 20 years old "photo editor" coming free with Windows.
I takes about 15 seconds to get to the 6 kBytes small result.

You may use some tools like "irfanview" where you can use a command line to automatically reduce pixel rate and color depth. Takes just a button press and some milliseconds.

Or you may use online tools that also do the same without the need for installing software on your PC / cell phone.

Klaus
 

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But in proteus it can be simulated but on the actual implementation there is problem like the LEDs always turns on even if 0v is applied to 2n2222. I think my transistor 2n5401 have a problem or maybe wrong pin connection because it is BR N5401 not 2N5401 I don't know if they are the same.
--- Updated ---

sorry about that. this is noted.
 

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Hi,
Please be as exact as possible:
0V/5V is not applied to the base, but on the other side of the base_resistor.

To this "turns on problem" please show your simulation schematic with the voltages at all nodes.

Klaus
 

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