NPN saturation voltage

[andy257]

Hi all,

Can anyone recommend me a very low Vce saturation transistor.

I am using a NPN to switch the reset pin on a 555 timer. However the pin has to be taken lower than 0.4V (min) 1V (max) in order for it to work. Most NPN transistors cannot pull this low.

Any ideas of what i could use?

 

[crutschow]

Where did you get that idea? Are you confusing base-emitter voltage with the collector-emitter saturation voltage?

Just about any NPN transistor will have a saturation voltage below 0.1V at low collector currents. For example look at Figure 2 in this 2222A data sheet, where it's below 0.1V up to 100mA collector current at a forced Beta of 10.

 

[andy257]

Ive built the above and i measure 0.63V on the output of the above circuit. I assumed it was the transistor that could not pull any lower due to saturation.

I should have said the transistor i am currently using is one from a ULN2003 array.

 

[crutschow]

The problem is, that array has darlington transistors (two cascaded transistors) which by nature have a high saturation voltage (due to the base emitter voltage of the first transistor adding to the saturation voltage). You need a standard single transistor to achieve a low saturation voltage.

 

[andy257]

Thank you Crutschow

 

[audioguru]

Ive built the above and i measure 0.63V on the output of the above circuit. I assumed it was the transistor that could not pull any lower due to saturation.

I should have said the transistor i am currently using is one from a ULN2003 array.

If you look on the datasheet for a ULN2003 then it shows darlington circuits, not single transistors and it spec's a saturation voltage of 1.1V
to 1.6V max.

The datasheet of an ordinary 2N3904 transistor spec's a max saturation voltage of only 0.2V when its collector current is 10mA and its base current is 1mA. Its typical saturation voltage (shown on a graph) with these parameters is only 0.05V.