Pull up resistor doubt...

[fazan83]

Guys need your advise on circuit attached herewith:




Is it if we change R83 to such a low value like 100 ohm 330 ohm will cause Q24 is not strong enough to pull the signal low?

If yes why? Can you guys please explain this?

Please advise guys.

Thanks in advance.

 

[grittinjames]

i not found any resister labeled r83
i fmeant r89 . its due to transister turned resistance.
if ur using suitable power transister this can avoud
or u can use push pull configratiom instead for pull up resister

 

[e_eja]

transistor switch has tipicaly 0.2 - 1 volt turn on voltage between it's collector and emitter so you may found low offset voltage

 

[aledaniel]

Hello, there is no enought info to analize the circuit, but here you have some pointings.
Ussually logic circuits work in saturation and cutt region (i´m not sure this is the right word, in spanish its CORTE y SATURACION). In one mode the Vce is minimal and in the other is maximum. It depends on the Hfe, so it might be that when you lower Rc there is too much Ic to mantain the saturation, and then Ic lowers and Vc increases turning signal out of low state.
Hope this can help

 

[lomedva]

Maybe the problem is the load, which is not shown. In the ON state VCE should be near to 0.2V, but if the current the transistor is draining from the pull-up resistor and the load is too high then this voltage tends to increase resulting in a weak on state.

 

[Davood Amerion]

I think you must replace Q24 with Q27 and R83 with R89!
If you decrese the value of R89, then transistor Q27 cannot drive enough current to it. it is because of limitation on β of Q27