# 7-Segment display w/ 8V forward voltage

Discussion in 'General Electronics Chat' started by tres, Mar 3, 2009.

1. ### tres Thread Starter New Member

Mar 3, 2009
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0
Hello guys,

I'm a new member here and would like to say hi first to all the members...

Need suggestions in driving my 7-segment display with an 8V forward voltage. I want to multiplex them and I need help on how to do that. I'm using a PIC and a 7447 decoder.

Thx.

2. ### beenthere Retired Moderator

Apr 20, 2004
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282
You will need to add resistors in each segment line to control current. 620 ohms should be about good.

Just one display, or just one 7447?

3. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
Well, you say that the 7-segment displays have a Vf of 8v, but what's the current rating?
Also, what voltage are you planning on sourcing them from?

I hope your 7-segment displays are common anode; if not, you will need a source-type driver array, which are getting to be very difficult (and expensive) to find.

You will need current limiting resistors between each segment's cathode (pins a through g) and the 7447 output lines, calculated as:
Rlimit = (Vsource - VfLED) / DesiredCurrent
If you're planning on sourcing from a 12v supply, and you wanted 15mA current, then you'd calculate:
Rlimit = (12v - 8v) / 15mA = 4v / 0.015A = 266.666... Ohms.
The closest standard value is 270 Ohms.

Note that the decimal point will very likely have a much lower Vf. If you're planning on using it, you'll need a different value for the current limiting resistor.

Last edited: Mar 3, 2009
4. ### tres Thread Starter New Member

Mar 3, 2009
5
0
Hi,

thank u for replying... here is the model of the 7-segment display I'm using. The current per segment is 20mA... by the way it's common anode.

I've attached the datasheet.

I will be using 12 of these 7-segment display and will only be using a two(2) 74LS47.

Any help would do.

Thank you.

File size:
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5. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
OK, just checked the specs in the sheet.
MAXIMUM is 8V@20mA.
Typical Vf is 7.2 @ 20mA (segments a-g) - you want to use the typical Vf @ current for the calculations.
That means the decimal point has a typical Vf of 3.6.
Recommended current is 12mA. Running them at a higher current will shorten their life.

So, plug those numbers into the formulas I gave you above:
Rlimit >= (Vsource - 7.2v) / 12mA [segments a-g]
Rlimit >= (Vsource - 3.6v) / 12mA [decimal point]

6. ### tres Thread Starter New Member

Mar 3, 2009
5
0
Ok,

Now my problem is how to multiplex them using a transistor or something....

Here is my main circuit, just a simulation but I will be using the 7-segment display I've mentioned above... My problem is driving each of them (turning on and off).

File size:
26.8 KB
Views:
43
7. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
So, you're planning on driving 12 displays?

You will need source drivers, then. Your PIC16F877A is not capable of supplying the voltage and current required.

UDN2981A's should do it for you. Digikey has them in stock for \$1.78/ea:
http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=620-1120-ND
Download the datasheet. You will need two of them, and you'll need to use six of the eight available channels on each IC.

8. ### tres Thread Starter New Member

Mar 3, 2009
5
0
Thank you very much SgtWookie,

Is there any alternative for the UDN2981 or alternative way? Because the availability of that IC here is rare. And the shipment of that device from digi-Key is very expensive.

What about this post I've seen, attached in the post is a 12V circuit... Can I use a transistors?

hope there is an alternative way. thx

9. ### tres Thread Starter New Member

Mar 3, 2009
5
0
thx Sgtwookie,

I've found a the same source driver IC... Manufactured by toshiba... here is the link of its datasheet http://www.datasheetcatalog.org/datasheets/320/377826_DS.pdf

Could you take a look at it, some parameters are the same and it's application. Hope to make it work this week.

I'll try to test it first. the program is finish already having problem with the hardware. I'll let you know when it work. Thx for the help guys.

thank you,
tres