ULN2803APG question

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rascupanamuha

Member

• Apr 1, 2013

• #1

I have connected 8x8 dot matrix to the breadbord with 2x 2803, and with 2x hc74595. That didnt work

And i have fiigured out why. When input on ULN2803APG is 5V, output is only 0.65V, and for matrix it should be 3V i think.
When i tried to connect only single ULN2803APG this way: PIN 1 and 10 to +5V, and PIN 9 to ground, output is also 0.65. Isnt it too low?

 

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rascupanamuha

Member

• Apr 1, 2013

• #2

Shouldn't it be like a normal NPN transistor?

When base (input) is 1, output is +5V, and when input is 0, output is 0V

 

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kubeek

Well-Known Member

Most Helpful Member

• Apr 1, 2013

• #3

You have it mixed up, look at the schematic digaram in the datasheet. It is a low side NPN switch, so when input is 5V the output is grounded, otherwise it´s high impedance.

 

geko

Active Member

• Apr 1, 2013

• #4

rascupanamuha said:

Shouldn't it be like a normal NPN transistor?

When base (input) is 1, output is +5V, and when input is 0, output is 0V

Click to expand...

An NPN transistor doesn't do that.

With the ULN2803 when it turns on the output is connected to ground. 1 in, 0 out effectively.

Take a look at the datasheet for it and you will see the internal schematic, it's an NPN Darlington transistor with resistors so it can interface directly with a logic level ouput.

 

audioguru

Well-Known Member

Most Helpful Member

• Apr 1, 2013

• #5

The ULN2803 has darlinton connected NPN transistors so its output cannot go to ground, instead it goes to +0.7V to +1.6V when its input is high.

 

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rascupanamuha

Member

• Apr 2, 2013

• #6

Thank you all. I thought it is different. Now i have connected ULN2803 to catodes of my LED matrix, and it is working now. I guess i would need some other chip that can connect to +5V, like this drives to ground