Begginer question connecting IC output to ground

Thread Starter

Gsouza

Joined Apr 14, 2018
4

Maybe not the correct area to ask this, but: If i connect the output(y in this AND ttl) to GND will the chip get damaged? If so, shouldn't the resistors prevent it from happening like in (0)?
 

Alec_t

Joined Sep 17, 2013
12,906
Welcome to AAC!
Datasheets are the place to look for answers to questions like this.
According to this datasheet, no more than one output at a time can be shorted to ground, and that for 1 second maximum.
 
Last edited:

Deleted member 115935

Joined Dec 31, 1969
0
All outputs have a current limit before they blow.

if you have a short , it will blow.

its a heat thing in the output transistor.
 

DickCappels

Joined Aug 21, 2008
8,842
With TTL the datasheet will often specify the maximim number of outputs that can safely be shorted to ground. The main concern is how hot the IC die is likely to become.

At one time intentionally shorting outputs to ground was no big deal.
It made testing and troubleshooting easier and simplified some circuits.

The practice has fallen out of favor in recent decades.
 

crutschow

Joined Mar 14, 2008
30,131
Like I said, shouldn't the resistors prevent a short circuit to the point it burns the transistor?
And like was said, it depends.
It depends upon the value of the resistor, the max current rating of the transistor, and the heat dissipation ability of the package housing the IC.
That can't be determined from just a schematic.
 

Papabravo

Joined Feb 24, 2006
18,834
What does Ohms law tell you about power dissipation in the 130 Ω resistor assuming 0.2V of Vce drop on Q5 and 0.6V drop on D2 with Vcc at +5V DC?
 

Thread Starter

Gsouza

Joined Apr 14, 2018
4
And like was said, it depends.
It depends upon the value of the resistor, the max current rating of the transistor, and the heat dissipation ability of the package housing the IC.
That can't be determined from just a schematic.
Hmm, i get it
What does Ohms law tell you about power dissipation in the 130 Ω resistor assuming 0.2V of Vce drop on Q5 and 0.6V drop on D2 with Vcc at +5V DC?
30mA?
 

Papabravo

Joined Feb 24, 2006
18,834
Oh, sure. So around 135mW is the answer? enough to blow the transistor?
No, that 135 mA in the resistor will generate heat which will cause heating on the die. It may or may not be enough to cause damage. You have to look at the thermal resistance to estimate the temperature rise. For example if the thermal resistance from "junction" to "ambient" is 75 °C/watt then your 135 mW times 75 °C/watt gives about 10 °C. If the ambient temperature is 20 °C, then a 10 °C rise won't do much harm. Put the device inside an enclosure with no airflow and that 10 °C rise becomes more problematic. Do it for multiple gates in a package and it becomes more problematic.
 
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