Beginner: measuring maximum load current

[mneary]

For some, saturation is the Holy Grail of turning a transistor "on". Most data sheets show the forced beta at 10 and sometimes 20.

But saturation is just a point on the operational curve. The beta of a transistor begins rising very rapidly when it comes out of saturation. Beta is high, even at 1 volt, and some data sheets specify this performance. It is possible, often desirable, to design a circuit that uses this fact. It's true that dissipation is higher, and this must be taken into account.

 

[nyoo]

THe dissipation discussion does draw us dangerously.

For me, the harder-to-understand issue is: how to get the voltage at the comparator's output pin lower than the Vbe(sat) of .65 volts of the 2N3904.

As an exercise, I tried to change the comparator to an LM211, to see if I could reproduce the calculations. There was no Vsat, on any LM211 manufacturer's datasheet. But I did find a V(OL), low-level output voltage, same 0.4V as the LM2903, optimal at 8 mA. Can I use this? In which case, the pulldown resistor would be 1450 (say 1.5K) ohms. Is this correct?

 

[MikeMl]

If you ask the comparator to sink less current than 8mA, then you can reasonably assume that the Vol at the comparator pin will be even lower than 0.4V, which means that the transistor will be turned off slightly more... Still sounds like 3mA through the pull-up is a pretty good compromise