7408 quick question

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BioniC187

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Hey all,

Just a quick question. I want to use a 7408 logic Ic in my car.
I have a 5V supply voltage for the IC.

With regards to the logic inputs, can I connect it directly to the 13V? I have used 15V once as an input to test(it worked), but i wonder if the life of the IC will be shortened by using it like this

Has anyone used an IC with high input logic voltages with no IC death?

Also will the spikes from the car's V+ damage the inputs?

Thanks
 

did you bother to google the datasheet ?
it would answer all your questions
and you would discover that the life of the chip would probably be counted in milliseconds

DAve
 
davenn said:
did you bother to google the datasheet ?
it would answer all your questions
and you would discover that the life of the chip would probably be counted in milliseconds

DAve
Click to expand...
Yes I have bothered to read the datasheet
try counting in days instead of milliseconds, I have used it 10V input with no failure for a few days to test a 10V sensor I have. Anyways, I would only post here if I needed help. Meaning datasheet etc was not enough.

But thank you, you are indeed helpful
 
BioniC187 said:
Yes I have bothered to read the datasheet
Click to expand...

hi B,
The problem with vehicle battery supplies is that they are VERY subject to electrical noise.

Positive/Negative transients and voltage pull down while engine starting.

So you would need heavy filtering, which should include resistive dividers to reduce the 12V/14V input signals you want to use, down to +5Vmax
 

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Hmmm, alright. I will look into this. A transistor circuit to drop down voltage to 5-6V range will work. I just made an tested the circuit.

Thanks eric
 
BioniC187 said:
Yes I have bothered to read the datasheet
.......................
Click to expand...
Then you didn't read the right part. If you look at the "Absolute Maximum Ratings" section you will see that the maximum input voltage is listed as 5.5V. So if yours worked at a higher input than that, you were lucky.
 
A high voltage input will lift the supply voltage on the chip via its protection diodes. Any voltage regulator for the 5V supply will go reversed biased and not regulate to 5V.
As stated, I think it would last millisecs.

I look forward to your next post about fixing ICs that have been overvoltaged. You are on borrowed time doing this.
 
Okay, let me clear this up. I read the datasheet prior to posting. I only asked because, like i stated already, I have used a 10V signal for days without failure, even though the datasheet suggests otherwise. On more than one 7408 IC btw. Hence leading me to ask such a question.

Anyways, thanks
 

then as was said up the page .... you were lucky!
manufacturers dont put ratings in their datasheets for no reason
they are given to try and ensure a long and reliable operating life for the device

Dave
.... wanders off shaking his head ;(
 
The Absolute Maximum Ratings are not suggestions, they are limits you should never exceed. If you want to exceed them don't expect anyone here to say it's ok to do so.
 
I expect that the source impedance of the signal was high enough that the input structure of the 7408 were clamping it. Check the voltage at the gate input pins to see if that is the case.
 
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