Author Topic: Measuring hfe of "P2N2222A" varies wildly  (Read 5046 times)

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Offline etiTopic starter

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Measuring hfe of "P2N2222A" varies wildly
« on: November 05, 2021, 03:33:13 am »
I've got a cheap Vichy VC97 dmm, and another one by "AstroAI", and when I shove a P2N2222A in the hfe tester, I get wildly different gain readings. On one I get 253, and the other is 150 - any idea what gives? And yes, the pins are correct, I've tested it 5x
(and I'm also aware that the *P*2N2222A* variant has a different pinout.


Thanks.
 

Offline IanB

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #1 on: November 05, 2021, 03:39:03 am »
Seems like hFE should be in the range of 50 to 300, so both indicated values are within the expected range.
 

Offline etiTopic starter

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #2 on: November 05, 2021, 03:40:20 am »
Seems like hFE should be in the range of 50 to 300, so both indicated values are within the expected range.

Ah okay, but why the vast difference - the device isn't going to change its gain between meters, is it. Also the sockets are horrendous on these cheap DMMs 😂
 

Offline Cerebus

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #3 on: November 05, 2021, 04:08:40 am »
It is going to change its HFE at different collector currents or different VCEs and you have no guarantee that those conditions are the same for both meters (and knowing the calibre of the VC97 I don't expect you'll find those conditions listed in the meter's data sheet/manual, or the data sheet/manual for the even more anonymous AstroAI which I've never even heard of before).

This is a question about pretty basic transistor theory, or rather a lack of sufficient grasp of it, and as such would have been more at home under "beginners" where people are generally more minded to be forthcoming with material of a tutorial nature - which is what it looks like is needed here.
Anybody got a syringe I can use to squeeze the magic smoke back into this?
 
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Online David Hess

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #4 on: November 05, 2021, 04:22:42 am »
Hfe is not well controlled between units, and it also varies significantly with collector current, with different transistor types optimized for different collector currents.  A lot of small signal transistors are optimized to operate with a collector current of about 10 milliamps, but low noise and high gain types will be optimized for 1 milliamp or even 100 microamps.  You can get a good idea of the design current from the datasheet by looking at the hfe specifications to see which ones are fully specified at a specific collector current.

 

Offline CatalinaWOW

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #5 on: November 05, 2021, 04:59:33 am »
Take a look at the data sheet.  https://www.onsemi.com/pdf/datasheet/p2n2222a-d.pdf

You could guess from the graph of hfe vs collector current that these testers are putting a few milliamps through the device in test.  You could even naively assume that one uses roughly 2 mA and the other about 15 mA.  But that would require assuming that the junction temperature was the same in both tests.  And that Vce was the same for both meters.

Some things you might do to investigate.  Measure the current (you have two meters so this is just a wiring issue.)  Measure the voltages.    Check repeatability on both meters.  Look for variation over time from self heating. 
 
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Offline etiTopic starter

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #6 on: November 05, 2021, 05:05:27 am »
Thanks everyone :))
 

Offline magic

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #7 on: November 05, 2021, 06:53:12 am »
Reverse engineer the hFE circuits in those meters :-/O
 

Offline tooki

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #8 on: November 05, 2021, 08:59:48 am »
I've got a cheap Vichy VC97 dmm, and another one by "AstroAI", and when I shove a P2N2222A in the hfe tester, I get wildly different gain readings. On one I get 253, and the other is 150 - any idea what gives? And yes, the pins are correct, I've tested it 5x
(and I'm also aware that the *P*2N2222A* variant has a different pinout.


Thanks.
In a nutshell, the hFE measurement of a multimeter is useful for binning transistors (sorting by similar gain), if you need specimens with similar values.
 

Online Zero999

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #9 on: November 05, 2021, 01:45:53 pm »
The hFE is unimportant for most applications. When a circuit calls for matched transistors, it's normally more critical to choose parts with the same VBE, at the same collector current. This can be determined using a diode test function. Connect the base and collector, with the positive probe on the collector & base and the negative probe on the emitter, for NPN transistors, or the opposite polarity for PNP transistors.
 
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Offline JoeyG

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #10 on: November 05, 2021, 01:49:21 pm »
Hfe  varies with tempreature. Put your finger on the transistor to see the warming effect
 

Online Benta

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #11 on: November 05, 2021, 07:00:25 pm »
Those cheap multimeter hFE measurements are a joke.
An almost correct way of doing it is testing the transistor in an op-amp current sink, where the emitter current is precisely controlled. This is not 100% correct (it should be the collector current), but  close enough. The emitter current is set to an appropriate value, and the base current is measured to compute hFE.
More difficult: it should be a pulse measurement to avoid heating of the transistor.

A fun project for spending time :)
 

Offline TimFox

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #12 on: November 05, 2021, 08:37:28 pm »
Usually, hFE is a stronger function of IC than of VCE, and typically it shows a maximum value for some reasonable value of IC.
 

Offline CatalinaWOW

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #13 on: November 05, 2021, 08:53:00 pm »
The hfe measurement on a DMM is a joke only if you try to make it more than it is.  It is a quick way to find if a transistor is dead, and gives an indication if the gain is enough to be useful on an unknown part and if it is generally what it should be for a part where you can get the datasheet.  Quite useful if you are away from the lab and have only basic tools at hand.

While you can get more information with enough effort and understanding (just as you can measure your latitude with time, a couple of sticks, and a tape measure) you are better off getting a better instrument if you need more than an "it is alive" indication. 
 

Offline magic

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #14 on: November 05, 2021, 10:29:40 pm »
On simple ICL7106 DMMs it tends to be reasonably accurate, maybe up to 10% off.
The measurement is performed at Ib=10µA (220k pullup to 2.9V IIRC) and the number displayed is simply 100x the collector/emitter current in mA. For a typical reading of 200 it's 2mA.

A 50% difference is somewhat surprising, it has to be significantly different circuits in these meters.
 

Online Benta

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #15 on: November 05, 2021, 10:40:02 pm »
The measurement is performed at Ib=10µA (220k pullup to 2.9V IIRC) and the number displayed is simply 100x the collector/emitter current in mA.

Which is totally useless. Collector current is the important controlling parameter. Just supplying some current to the base and measuring the IC is laughable.
But the feature looks nice in the sales brochure.
 

Offline magic

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #16 on: November 06, 2021, 06:38:27 am »
I'm telling you that the collector current is literally displayed on your meter during the measurement so you can correlate it with plots in the datasheet to estimate performance at other bias conditions ;)

And that the current used is fairly reasonable for typical small signal transistors with typical β of 100~300 (ends up being 1~3mA).
« Last Edit: November 06, 2021, 06:41:28 am by magic »
 

Online Benta

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #17 on: November 06, 2021, 09:53:46 am »
I understand completely what you're telling me.
I'm just saying that that's not an hFE measurement. It's just "something" that you can "correlate" with the data sheet.
The data sheet specifically lists the collector currents to be used for hFE measurement.
 

Offline CatalinaWOW

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #18 on: November 07, 2021, 03:19:10 am »
I understand completely what you're telling me.
I'm just saying that that's not an hFE measurement. It's just "something" that you can "correlate" with the data sheet.
The data sheet specifically lists the collector currents to be used for hFE measurement.

The data sheet tells the current at which the manufacturer specifies hfe.  But hfe is a parameter that exists at all currents and temperatures.  And what the DMM reports is a very good approximation of hfe at one operating point.  Whether that number is useful depends on your application.  I agree that it is nearly useless for evaluating specification compliance.  At best it can say maybe yes or almost surely not.
 

Offline armandine2

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #19 on: November 07, 2021, 07:47:32 pm »
regarding different subscripts  hFE and hfe - forward current gain CE config. V2 short circuited - I see from beginners video attached  that a large capacitor in parallel shorts the ac output, presumably for the dc gain measurement you would literally just short with a wire? (a beginner's question - tagged on here)


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Offline TimFox

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #20 on: November 07, 2021, 07:54:02 pm »
By definition, the “FE” subscript denotes the (DC) ratio of IC/IB, while the “fe” subscript denotes the (AC) derivative dIC/dIB, both evaluated at a given value of VC and IC.
« Last Edit: November 07, 2021, 07:55:43 pm by TimFox »
 
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Offline armandine2

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #21 on: November 07, 2021, 08:10:24 pm »
open circuit output? or something in between?
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Offline armandine2

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #22 on: November 07, 2021, 09:41:09 pm »
no reply -probably not a great question - but it looks like hFE matches up well with a simple test:


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Online Benta

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #23 on: November 07, 2021, 10:24:18 pm »
The data sheet tells the current at which the manufacturer specifies hfe.  But hfe is a parameter that exists at all currents and temperatures.  And what the DMM reports is a very good approximation of hfe at one operating point.

The DMM does NOT report hfe. That's an AC value.
It leads you to think it's reporting hFE (which is a DC value), but in the wrong way.

Uppercase and lowercase play a very large role in science and engineering.

The almost correct way of measuring hFE is this:

« Last Edit: November 07, 2021, 11:29:27 pm by Benta »
 

Offline CatalinaWOW

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Re: Measuring hfe of "P2N2222A" varies wildly
« Reply #24 on: November 08, 2021, 01:17:45 am »
The data sheet tells the current at which the manufacturer specifies hfe.  But hfe is a parameter that exists at all currents and temperatures.  And what the DMM reports is a very good approximation of hfe at one operating point.

The DMM does NOT report hfe. That's an AC value.
It leads you to think it's reporting hFE (which is a DC value), but in the wrong way.

Uppercase and lowercase play a very large role in science and engineering.

The almost correct way of measuring hFE is this:

You are absolutely and pedantically correct.  Though I don't know when this distinction was made.  When I was in university (during the transition from vacuum tubes to transistors) I don't recall both terms being identified.  And there is much useful (but approximate) analysis that can be accomplished without making this distinction.

If you check data sheets it is somewhat uncommon to find both specified.  And when they are it is common to find a wide range of spec values for both, with a very large overlap in the ranges.  So it would be unusual to make a production design that depended on a difference in the values. 

Of course, when it matters you need to be aware of the difference.  And you can then buy large numbers of transistors and select the ones that meet the needs of your circuit.  Or spend the time to measure each transistor and adjust the circuit to match the transistor.  Not the usual way to build things, but for certain applications might make sense.

 


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