8mA output to drive LED seven segment

Thread Starter

azngeek

Joined Jul 22, 2008
20
Would just like a recommendation on what should be used to drive a seven segment display. Any chips in particular, I'm eyeing up darlington arrays but man they output big voltages. I think if I can get it up to about 16mA shoudl be okay. Hmm
 

SgtWookie

Joined Jul 17, 2007
22,210
You'll need to use a resistor for each input transistor to limit current, otherwise you risk overloading whatever you're using to drive the ULN2001 with.

But for 5V TTL/CMOS, you could use ULN2003's.
For 6v and higher CMOS, you could use ULN2004's.
They both have built-in current limiting resistors.
If you want to drive the period too, look at ULN2803 and ULN2804 IC's; they have 8 drivers instead of 7.
 

mindmapper

Joined Aug 17, 2008
34
Also don't forget the resistors that limit the current through each segment on the display. The ULN-circuits doesn't really output voltage, instead they sink the current delivered by the power supply to ground. So you have to limit that current to a value safe both to the segment and the transistor in the IC.
 

Thread Starter

azngeek

Joined Jul 22, 2008
20
Hi just a bit confused on the common pin. The internal circuit shows a diode pointing into the common pin direction. What is the point of the common pin?
 

gotumal

Joined Mar 24, 2008
99
Hi,

You need not bother about this pin as long as you are not using any realy or other inductive load at the output. You have to connect it to +V supply in case of inductuive loads to make use of internal free wheeling diodes.
 

SgtWookie

Joined Jul 17, 2007
22,210
From a PM by azngeek:
I was wondering if you could show me an example on how to wire the ULN2002, I think I wired mine wrong.
I don't help people via PM, as that defeats the purpose of the Board.

Have a look at ST Microelectronics' datasheet for ULN2001A-ULN2004A that I've attached.

Notice on the 1st page that ULN2002's are for 14V-25V PMOS IC's.
Look on the 2nd page at the schematic diagram for the ULN2002A. You will notice that between the IN terminal and the base of the 1st transistor is a Zener diode and a 10.5k Ohm resistor. The Zener has a breakdown voltage of somewhere around 12V-13v. That means in order to turn ON one of these Darlington channels in a ULN2002A, you will need to have somewhere between 14v and 25v on the input, otherwise it will be off.

You have not mentioned the type of 7-segment displays that you are attempting to drive; whether they are common cathode or common anode. Common cathode requires a source driver, common anode a sink driver.

ULN200x are sink drivers. There is not a practical way to make them source current. If you need source drivers, you could look at UDN2981, UDN2983 or UDN2984, which are 8 channel Source Drivers.

You also haven't said what you're using to control the drivers with, either. I have no clue if you're using CMOS, TTL, a uC, or a battery and a pushbutton.
 

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