Hex Inverter testing - has the chip failed or have I??

[TonZ]

Hi all, I wouldnt exactly call myself a beginner but this is a bit out of my comfort zone so chances are it's something simple which I am missing / doing wrong.

While repairing an old hi-fi system, initially damaged in a lightning strike, I came across a hex inverter chip (74LS05P) that seemed to have some unusual readings ie: inputs and outputs are not inverted.

I took the chip out of circuit and placed in a breadboard, then connected to a 5V DC supply with +ve to Vcc and -ve to Gnd pins. I then connected each input in turn to +5V and then to 0V, expecting to read the "opposite" voltage at the output, however the output (for all 6 inverters) remained very close to 0V despite what was connected at the input. With the inputs left floating they (inputs) each read somewhere between approximately 1.5V and 2.1V.

That seemed to me to be pretty conclusive that this chip had failed, so I ordered a replacement.

Today the replacement has arrived and so I tested it on the breadboard in the same way as before and found... EXACTLY the same results!

This leads me to think one of two things:

  - I damaged the new chip by handling it. I do not have a wrist strap or ESD safe board so it's a possibility although I did earth myself prior to handling, I know that's not ideal but its the best I can do right now.

 - I am misunderstanding how these chips operate and the test I devised is actually useless! Somehow this seems more likely as this is the first time I have encountered these and am not too familiar with logic circuits, although have studied the theory (Boolean algebra etc.)

Any thoughts?? Is there an accepted method for testing operation of these chips without anything fancy like a scope? The few examples I have found online don't seem to be accouting for anything I am not.

Any help is much appreciated!

[Wimberleytech]

Appears to me that the 74LS05 has open collector outputs.  Do you have pullup resistors on the output?

[TonZ]

Wow thanks for the fast reply, given that I dont know what you mean - it sounds like youve probably cracked it! 

Looking at the datasheet the ouput is a direct connection the collector, is that what you mean by the output being open?

So should these resistors go between output and ground to allow a voltage to develop?

Is 1Kohm a sensible value given 5V output and rated 8mA max output current?

Or am I way off??

[PA0PBZ]

You need resistors between the output and the 5 volt, so that the resistor pulls the output up and the transistor can pull it down, 4.7K would be a nice value.

[TonZ]

Right... so the inverter itself is only responsible for pulling the output down, and it needs some other element to keep it pulled up when the input is off.

Thanks for helping me understand, hopefully thats a step closer to getting this thing up and running!

[Gyro]

Note that the Open collector output is a 'special function' of the 74LS05. If you had used a 'normal' inverter like the 74LS04, you would have seen what you originally expected.

[Wimberleytech]

Note that the Open collector output is a 'special function' of the 74LS05. If you had used a 'normal' inverter like the 74LS04, you would have seen what you originally expected.

"If you had" read the original post you would have known that he is REPAIRING a piece of equipment that contains the device.

[Gyro]

I was merely explaining (maybe badly) to the OP that open collector is specific to certain logic ICs. ie. To prevent any possible misunderstanding that pullups might be needed on the outputs of all 74LS series parts (he said he was out of his comfort zone).

[NivagSwerdna]

Quite a good read to add to what has already been said... https://electronics.stackexchange.com/questions/159768/whats-the-difference-between-a-74ls05-hex-inverter-and-a-74ls04-hex-inverter