[SOLVED] Poweer transistor low "on" voltage

Power transistor low "on" voltage

Hi,

I'm using the 2SB507 as a regulator bypass transistor, and have a small concern about the specific two I am testing the circuit with which I wanted to use:

A few months ago, the same type of transistor (perhaps even the same batch, they came in the same bag of 10) had a fully on voltage drop of 0.65 - 0.7V, and a guesstimated hFE of ~100.
Yesterday, having problems with the bypass resistor calculation and reality not coinciding, I checked the hFE of the two I am using and the "on" voltage. The "on" voltage is only ~0.32V for either of the two (the gains are ~200 and ~250 respectively).

Is that normal for a transistor which in the datasheet only says for Vbe on saturation: max. 1.5V, but mentions no minimum, or could the transistors be damaged and not wise to use in a real circuit?

I must/may have used them for breadboarding at some point in the past, so may or may not have done something detrimental to them, but I doubt that, even if it is a possibility, and they both are working as per expected.

...Also, if it's okay to tag another question on here: Can I use 4 5V voltage regulators (3 x 7508, 1 x 73801 LDO) to separate four parts of the same circuit @ 9VDC supply voltage?
It's to power a CD4060, a 7135 ADC, a constant current source circuit, and lastly a CD4511 + 6 NPN + 5 x 7-segment displays - so I'd really like to keep each part of the circuit on it's own supply, but all connected on the same ground plane (best I can come up with for the ground problem).

If it helps, 2SB507 datasheet:



View attachment BJT PNP 2SB507.pdf

Thanks

Maybe I did something incorrect measuring, don't see how, but anyway: ...I measured the "on" voltage the same as the last time, using a resistor from +V in series with a trimpot to 0V as a voltage divider, and had the base at full supply voltage (from the trimpot wiper) then lowered it until the collector went from 0V (actually 3mV to 6mV) to 8.70V. 8.7V is what I get from the regulated DC 9V supply. I used a 100R in series with the 1K trim.

You didn't mention related Ic/Ib values for the Vce measurements. I can just guess that different Ic or Ib causes different Vce.

And no, it's no good idea to discuss unrelated topics in a single thread.

Hi,

Thanks.

With a DMM, how would I go about measuring Ic and Ib, please? Just using an ammeter from output to load and from input to base?

I imagine that for this purpose the valid method is to check the Vbe voltage on the circuit it will be used for?

With a DMM, how would I go about measuring Ic and Ib, please? Just using an ammeter from output to load and from input to base?

Click to expand...

Depends on the circuit which wasn't mentioned yet. In most cases, the currents are determined by external conditions, you don't necessarily need to measure it explicitily.

Measuring Vbe is probably the least accurate way to get operation point information.

So what is an accurate method of getting operation point information?

I'd like to know how to do that for future uses, as it seems very useful to know how to measure the actual device operating parameters for some applications and the corresponding calculation beforehand, not just for this circuit.

...Let me guess - to answer my own question: I shouldn't waste your time and be a lazy knowledge vampire asking you, but should use my own brain and time and Google/and learn a lot more about BJT transistors, then I would know.

Anyway, thanks FvM, your replies are very helpful.

Your Japanese transistor datasheet is missing important information. American and European datasheets list the maximum saturation voltage in writing and show the "typical" saturation voltage on a graph.

Note that the base current is 1/10th the collector current regardless of the hFE. There is a big difference from typical to maximum.