Buying ICs off of ebay - I think I finally learned my lesson

[Supercap2F]

Hello Everyone!

So recently I was trouble shooting one of my projects. And the LM2901Ns that I bought off of ebay, where triggering at really odd voltages. So I decided to do some testing. Here's what I found:

- Input	+ Input	Out

[TR1][TD]5.02V[/TD][TD1]Less then 3.95V[/TD1][TD]Off[/TD][/TR1][TR2][TD2]5.02V[/TD2][TD]More then 3.95V[/TD][TD]0V[/TD][/TR2][TR1][TD1]0V[/TD1][TD2]3.81V[/TD2][TD]0V[/TD][/TR1]

From my understanding the chart should look more like this:

- Input	+ Input	Out

[TR1][TD]4V[/TD][TD1]Less then 4V[/TD1][TD]0V[/TD][/TR1][TR2][TD2]4V[/TD2][TD]More then 4V[/TD][TD]Off[/TD][/TR2]

NOTE: The Off state on the output is the same as the + supply. But the LM2901N is a open-collector component (regards to Harald Kapp ).

Now is it safe to assume that I was slipped some mickeys? Or am I just looking at it wrong, again. A photo of my testing circuit is attached.

Thanks for your time!!
Dan

 

[BobK]

What was your supply voltage? The inputs can only go from 0 to Vcc - 2.0V, if you exceeded that range, anything might happen.

Also, these are open collector. Did you have a pull-up for the outputs?

Bob

 

[Supercap2F]

What was your supply voltage? The inputs can only go from 0 to Vcc - 2.0V, if you exceeded that range, anything might happen.

I was getting about 5.02 volts from a 7805.

Also, these are open collector. Did you have a pull-up for the outputs?

In the circuit that I was trouble shooting, yes. In the test circuit, no.

Thanks
Dan

 

[KrisBlueNZ]

As Bob said in post #2, the problem could be that the voltages you're feeding into the inputs are outside the allowable range. This range is 0V to (VCC - 1.5V) at 25°C, and 0V to (VCC - 2V) over the full temperature range. So at VCC = 5V, if either (or both) of the inputs are higher than 3V, the comparator's operation is not defined.

 

[Supercap2F]

AGGH!! I thought I may be missing something! Bob meant Vcc minus 2Volts. I wondered why he put that '-' on there. I will try to modify the circuit that I was trouble shooting to work with that. But is there any drop in replacement for the LM2901N, that does not have that 2Volt thing? Or do all comparators have that?

Thanks Bob and Kris!!
Dan

 

[BobK]

You need a rail-to-rail comparator. Microchip makes some good ones.

http://www.microchip.com/ParamChartSearch/chart.aspx?branchID=11011&mid=11&lang=en&pageId=79

Bob

 

[Supercap2F]

You need a rail-to-rail comparator. Microchip makes some good ones.

http://www.microchip.com/ParamChartSearch/chart.aspx?branchID=11011&mid=11&lang=en&pageId=79

Bob

OK, thanks Bob! Is the "Common mode input voltage range " on the data sheet what tells you the Vcc - 2V thing?

EDIT: After a slight alteration, the circuit I was trouble shooting WORKS!!! YAY!

Regards
Dan

 

[BobK]

Yes, that is the voltage range you can put on the + and - input pins.

Bob

 

[chopnhack]

You scared me Supercap, I just invested a whole buck in some v-regulators

 

[Supercap2F]

You scared me Supercap, I just invested a whole buck in some v-regulators

Well the title of this tread was just one of my lame attempts at trying to be funny. If those LM2901Ns really where duds, it wouldn't hurt that much. And if I did find a drop in replacement, then I would most likely buy it off of ebay too! I had some doubts about them being fake, because I bought them from some guy in Oklahoma!

Dan

 

[chopnhack]

I had some doubts about them being fake, because I bought them from some guy in Oklahoma!

Dan

LOL, never, ever trust some guy in Oklahoma!

 

[Supercap2F]

Well Blow me down and pick me up! It's a electronics component store in Oklahoma! Thanks Chopnhack, if it wasn't for you I would have never looked up the address of the guy I bought those from, and found out there's and electronics component store near by!

Dan