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open collector chip

I'm refering to the the chip SN7407 form texas instruments.

Why is it open collector? What are the situations where it is useful?

I need to advice you that I did many electronic courses at university but I have no working experience.

thank you!
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Top Expert 2007

Oper collector is normally used for sourcing larger amounts of current and for driving other devices ( not TTL ).
I think it can source 20-40 Milliamps or more, which why it is used t interface to external devices.

I  hope this helps !

Open collector means that you connect your load (LED and current limit resistor) between the power (+5V for example) and open collector pin.
Open Collector or OC is quite useful for many situations:

1) They can support WIRE-OR.  So two or more outputs can be wired together and connected to +V through a resistor.  If any of the outputs go low, they all go low.

2) Open collector will SINK current only, it cannot SOURCE current.  This can be quite useful if you are wanting to drive another output circuit that is based on something other than the +5V of the 7407.  Say you want to drive a +12V circuit.  The 7407 will tolerate +12V on the collector of its output.

3) Interface to CMOS or non-TTL logic circuits.  A common problem in interfaceing to non-TTL level logic is the Voh spec of push-pull TTL outputs.  The spec'd HIGH output voltage is usually 2.4V.  This is NOT enough to drive CMOS which usually requires +Vcc - 1.5V or so.  So even +5V CMOS will need 3.5V.  If you use an OC driver, it will go almost all the way up to +5V when the OC output is off.

4) Current driving applications vs. voltage.  In the case where you need to drive a current instead of a voltage, the 7407 is useful.  Here, a "1" is the flowing of current and a "0" is no current.  Older serial devices used a scheme called "current loop" as an alternative to RS-232.  There are still good reasons for this.  Current driving interfaces are less prone to noise interference than voltage driving interfaces.


Great jhance!

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