Noob question

[Jmsteele187]

I'm trying to figure out how to measure the hfe for si npn transistors.  Can anyone shed some light on this subject for me, please?

[Brisance]

do it by definition, measure the base and collector current and divide

[antonis]

do it by definition, measure the base and collector current and divide

Ahaaa....

Even if the circuit "restricts" full collector current..

[duck_arse]

a fun way to do it is to put the part into a collector biased amplifier circuit, with some emitter resistance, and then measure the voltages, work out all the currents. you'll find some numbers 'don't come out right', they'll give you your gain (for that circuit).




[disclaimers: this might be wrong. I use the word fun advisidly. ]

[Jmsteele187]

I feel like I really need a I emphasize the term noob.  I don't really know what either of you are talking about.  I thought there was a simple circuit and an equation.  I think that's what duck was talking about, but I don't know how to make the circuit or the equation.

[antonis]

It isn't something complicated but you better use a multimeter capable of hFE measurements..

Or use a circuit that drives BJT to saturation (full ON) and measure Base and Collector current..

[Brisance]

the circuit "restricts" full collector current..

should have specified, limit base current and measure collector current

[Jmsteele187]

It isn't something complicated but you better use a multimeter capable of hFE measurements..

Or use a circuit that drives BJT to saturation (full ON) and measure Base and Collector current..

My dmm doesn't measure hfe, which is why I'm asking.  The circuit to drive BJT's is what I think I'm looking for.  I'm just not sure how to put it together, or figure out hfe using it.

[antonis]

As Brisance said:

Collector to +9VDC... Base to 10k resistor... Resistor to +9VDC...

You should take a 835μA Base current (depending on actual Vbe and resistor's & PS tolerance..)

Measure Collector current (in μA also) and divide it by 835 (or by any other measured or calulated base current)

(you can put a 1R resistor between collector and PS and measure the voltage drop across it, if you feel inconvenient with Current measurement..)

[idy]

Geofex has an article with a schematic and step by step. The example also measures leakage, not an issue with si, but here is what the guys are talking about more or less.

http://www.geofex.com/Article_Folders/ffselect.htm

[Jmsteele187]

Thank you all.  I'm pretty sure I understand now.

[Jmsteele187]

Well, I ran out of time and didn't get to try R.G.'s method of testing.  I did however use this method, I found at small bear http://diy.smallbearelec.com/HowTos/FuzzFaceFAQ/FFFAQ.htm
Im not entirely sure I did the figuring right, though.  I measured as specified in the FAQ, but I couldn't get a reading with a mA setting.  Instead I used a DC voltage setting, like in R.G.'s article.  Then I got readings anywhere from .032 to .042.  Unfortunately the only 9v battery I could find measured at 8.56v.  Not optimal, I know.  But I used that measurement as my base current anyway.  If I did the math correctly, the hfe read somewhere in the 208-267 range. Can anyone confirm this for me?

[smallbearelec]

The method I show does the same job as R G's, but you are measuring current directly rather than measuring it as a voltage drop across a resistor. I realize that the last bit might not make sense to you yet, but it will in time.

First, my method will Not give you a correct reading with your meter set to measure voltage. Set it up to measure current on the lowest possible range. Consult the user's manual if you are not sure how to do this.

Second, The connections shown are for PNP devices. You Must Reverse The Battery And Meter Connections To Test NPN.

If connected correctly, a silicon transistor will show No leakage current. Then apply the Base bias and record the Collector current. Do the arithmetic and let us know what you find.

[Jmsteele187]

I originally started out trying to measure current, but couldn't get a reading on a 0-2mA setting at all.  I also reversed the polarity of the battery, and the test probes.  I'm really kinda wondering weather or not I put it on the breadboard correctly or not.

[smallbearelec]

I originally started out trying to measure current, but couldn't get a reading on a 0-2mA setting at all.

You would not get a reading for leakage current with silicon. NB: Some meters require moving a test probe to a different socket to measure current...Is this true of yours? If Everything is connected correctly, you'll see current when you apply the Base bias. Then the gain is just the current reading (in Microamps) divided by 9.2.

[Jmsteele187]

I wasn't measuring for leakage, because my BJT's are Si.  With my dmm I do not have to move the probe to a different socket, they're hard wired into the unit.  I just have to turn the dial.  I must have something wrong on my breadboard, or I need to try another power source.

[MaxPower]

Regarding your measurement of 208-267: Maybe I missed it but did you specify what transistor your trying to measure? Different transistors may have different hfe specs. Check the data sheets for the specific transistors you're trying to measure and compare your measurements to the given specs.

[Jmsteele187]

I was measuring a very small batch (6) of 2n2222a's.  My goal is to put together a matched set for a version of a fuzz face I've been working on.

[MaxPower]

Fairchild datasheet lists the 2n2222 as having an hfe in the range of 100-300.

[Jmsteele187]

Well, I cheated.  I went out and got a new dmm with an hfe setting.  I know it won't work properly with Ge transistors, but it does just fine with Si.  All of the 2n2222's I have are in the 230 range.  So, I'm assuming not that great for a fuzz face.  I think some of the bc108's I have will work well though.  I did find that a couple of the bc108's didn't give an hfe reading.  Does anyone have an explanation for this?