Transistor Gain Test Circuit. I'm about to give up on all this.

[digitalzombie]

I've tried to find the right way to simply test for silicon NPN gain on a breadboard using a multimeter and I CAN'T DO IT! Nothing I try seems to work. It doesn't help that every guide I've found is for PNP transistors - so I don't have the benefit of the correct diagram for an NPN right in front of me - but nothing is presented practically to a novice like myself, explaining the real-world application of where the pieces should go.

I've found this (section 24): http://diy.smallbearelec.com/HowTos/FuzzFaceFAQ/FFFAQ.htm
And this: http://sound.westhost.com/project31.htm
And of course the impossible to link to Geofex page for picking Fuzz Face transistors.

I even went and bought a brand new multimeter that reads mV, uA, mA, AND hFE - because I thought it'd be fun to plug a transistor directly in, then compare my results from the breadboard circuit.

NOPE. NOT HAPPENING.

Maybe it's the cheap DMM, but it's got one of those adapters that you plug into the front and you plug your tranny legs in there, but that doesn't even seem to work.

Can someone please tell me in plain English: HOW DO I TEST NPN TRANSISTOR GAIN ON A BREADBOARD WITH A MULTIMETER?

[bluebunny]

And of course the impossible to link to Geofex page for picking Fuzz Face transistors.

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

[smallbearelec]

Hi--

Take a deep breath and let's proceed step-by-step. Silicon devices can be tested accurately on your multimeter Hfe scale. It may be that you have not correctly identified C, B and E, and that's why you are not seeing a reading. What transistor type/part number are you testing?

[digitalzombie]

Thanks for replying, smallbearelec. I love all the work you do and I see you helping out a lot on other's posts.

I understand why you'd suggest I might have the parts oriented incorrectly but I'm not that green. The parts are oriented correctly. Here's an Imgur gallery showing trying to test transistors right on the DMM. http://imgur.com/a/tXZbi

The last picture in that gallery is another breadboarding tutorial I found that actually (sorta) made sense to me: http://www.buildinggadgets.com/Transistor_beta_gain_measurement.pdf

I just did the first test in there. So when I probe ground and transistor-side lead of the 1M going to the base I get 11.9uA.
When I probe ground and transistor-side lead of the 1k carbon I get 1.3 mA
So according to that pdf, Ic/Ib=Beta – 1.3/11.9=0.109... erm, I'm guessing I'm supposed to move the decimal place over three times? I guess the numbers are all in uA, correct? So 1300 uA divided by 11.9 uA = 109.24! That fits in the tolerance of a 2N2222A so I'm guessing this is the hFE of this particular device.

Again, total novice here, had to work that out myself. No one's trying to make it easy on me. Yeesh.

Unfortunately the control of my experiment didn't work out. I put that 2N3904 in there and while I get 0.91 mA @ Ic, I get 0.0 for Ib, so now I don't know what to think.

[smallbearelec]

I don't know why the adapter for your meter is not working. As for the textbook, the author is showing how to measure a current as a voltage drop across a resistor. This is fine if you understand that that's what he's doing. I will set up on breadboard to measure collector current directly, take a pic and post again later.

[digitalzombie]

I don't know why the adapter for your meter is not working. As for the textbook, the author is showing how to measure a current as a voltage drop across a resistor. This is fine if you understand that that's what he's doing.

I'm sure I'll understand it one day. For now though, no, I don't.

I will set up on breadboard to measure collector current directly, take a pic and post again later.

Very much appreciated. I brought my breadboard & DMM with me to work in anticipation. 

[wilrecar77]

Just to make sure, when you say "probe ground and transistor side lead of the 1M", do you mean the RED probe is at the base of the transistor and the BLACK probe is at ground? If so, and you have your multimeter in current-measuring mode, that will not work. To measure current, the meter has to be in series with whatever current you are trying to measure. To measure the base current, you need to break the connection between the 1M and the transistor's base and then complete that connection with the multimeter (in current measuring mode).

If you are instead measuring the voltage at the base of the transistor to find the voltage across the 1M, and then derive the current through the 1M using V/R=I, you're good to go.

[digitalzombie]

Just to make sure, when you say "probe ground and transistor side lead of the 1M", do you mean the RED probe is at the base of the transistor and the BLACK probe is at ground? If so, and you have your multimeter in current-measuring mode, that will not work. To measure current, the meter has to be in series with whatever current you are trying to measure. To measure the base current, you need to break the connection between the 1M and the transistor's base and then complete that connection with the multimeter (in current measuring mode).

If you are instead measuring the voltage at the base of the transistor to find the voltage across the 1M, and then derive the current through the 1M using V/R=I, you're good to go.

Interesting. I guess that makes sense, but I guess I don't understand yet why it needs to be in the circuit rather than just taking readings at points. Again, I haven't found a "beginner level" tutorial on this. I built it out as you described, does this look right?
http://m.imgur.com/V0q4bRZ

[Fast Pistoleros]

testing a transistor on a Bread board you need to look at the tranny as two diodes or a BJT, Bi-polar Junction - base/collector junction and base/emitter junction, junction type does not matter at all ( npn,pnp).

good diodes read open or extremely high R in reverse bias will show very low R in forward - defect will show high/open for both

use the diode test feature to get accurate forward and revers bias. you need an Oscope to look at trace curve characteristics and get important characteristics of the device itself.

a typical NPN will read between .5 and .9 forward bias. open circuit in reverse. do this on both junctions. Kirchoff's current (I) law states: total current entering a junction must be equal to the total current leaving the junction.:

Ie = Ic + Ib

Vb = Ve + Vbe

Vc = Vcc - IcRc

Dc Beta  - Bdc = Ic/Ib

cut off and saturation = Ic = Vcc/Rc

I believe a DC load line is the points in between the Ic and the cutoff Vce


BJT base bias is Ic=BdcIb

or Ic = Bdc (Vcc-Vbe/Rb)

Collector feedback Bias = Ic = (Vcc-Vbe/Rc+Rb/Bdc)

Large B is Beta or known as Hfe  - for example a collector current of 1000uA / base current of 10uA is a Hfe of 100 or Bdc=100. Current gain must occur for the device to be used as an amplifier.

then the basic voltage divider is Vb= (R2/R1+R2)Vcc

Ve=Vb-Vbe
Ie=Ve/Re
Ic approx or equal to Ie
Vc=Vcc-IcRc
Vce=Vc-Ve

Emitter Bias

Vc=Vcc-IcRc

you can use Ohms law to find any missing variable with two of the unknown variables

* I am currently enrolled in Analog 121 - analog fundamentals, a second semester course in the EE program right after analog 101.  So this is basic as it gets for looking at any PN junction or similar device.

[digitalzombie]

Is there a glossary around here somewhere? I don't know half of what that means. I'm feeling in over my head.

[Fast Pistoleros]

you can google search most of it to read,all the number crunching formulas were out of the analog 121 book from the college. you can get all these basic formulas from almost any electrical engineering website.

if you like to learn through watching videos, which is like watching a profession engineer work, go here:


pin out and identification - AAFR - he will answer any email questions as well in a very timely fashion:

https://www.youtube.com/watch?v=EcfGXHoWiZ4

Mr Carlson is a genius lol:
https://www.youtube.com/watch?v=KGcoOETCaEQ

all around great yt channel and basic b2b guide

https://www.youtube.com/watch?v=qM7_h8my8eA

[mth5044]

I'm assuming you read the GEOFEX article about PNP measurment, so perhaps this thread would help.

http://www.diystompboxes.com/smfforum/index.php?topic=87008.0

[Transmogrifox]

I would ignore the parts about an oscilloscope and curve tracing.  Fast Pistoleres probably went far beyond you in the first sentence   You will get there some day, but I can springboard off some of that to give you a glossary of some of the terms you will commonly see:

Starting with the very most basic electrical concept, this will help you understand why you want to make current measurements in-line with the circuit, and voltage measurements between points.
Ohm's Law:  Voltage = Current X Resistance, symbols, V=IR
https://en.wikipedia.org/wiki/Ohm's_law
Anecdotally  I was told "I" was used to represent "Current" to avoid confusion with "C" for "Capacitor".  Ironically, now we engineers use "j" to represent the square root of minus 1, while all the mathematicians and physicists use "i".  The letter "j" replaces "i" to avoid confusion with "i" for current, which uses "i" to avoid confusion with "c" for capacitor.  Now I wonder why you're having a hard time getting this stuff all sorted out

Kirchoff's Current Law (KCL):  The sum of all currents entering a node is equal to zero.
https://en.wikipedia.org/wiki/Kirchhoff's_circuit_laws

A simple way to look at KCL is if you have a "Y" splitter on a hose and the faucet connected to one side, then the total amount of water going into the Y splitter is equal to the total amount going out.  If you add "gallons per minute IN" to "gallons per minute OUT" you get ZERO, because OUT is exactly the same as IN except OUT has a negative sign (IN positive, OUT negative).  Following that analogy, Voltage is like the water pressure, resistance is like the diameter of the hose.

If you have a really small hole at the end of the hose, you will have high pressure, high resistance, and relatively small current (gallons per minute).  You can increase the gallons per minute by either increasing pressure or by making the hole bigger, or both.

Now a transistor, by that analogy is like having 2 faucets feeding into a single Y splitter, and the 3rd connection on the Y splitter goes out into the grass, or into a sprinkler or something.  The 2 faucet connections are at Collector and Base.  The emitter is the end that goes out into the grass.

If you want to measure flow of current going into the base, you need to stick a flow meter in series with the hose, which would mean you disconnect the hose from the base connection, insert the flow meter, then a hose from the other end of the flow meter into the Y splitter base.  Then it will tell you how much current goes in.

If you want to measure pressures, all you need to do is poke a hole in the hose and screw a pressure gauge in it.  The pressure gauge doesn't care what volume of water is flowing past, it only cares how its pressure compares to atmospheric pressure (ground by analogy in an electric circuit).

NPN and PNP transistors work according to the the same principles, but everything is reversed between the two.  A NPN by analogy expects to have water injected into the base and collector.  A PNP expects water to be sucked out of the collector and base.  A normal usage of an NPN is a base voltage that is positive with respect to emitter, and collector is positive with respect to base and emitter.

A PNP is exactly opposite.  If you make a circuit to measure hFE on an NPN transistor, then to measure a PNP just swap the power supply + and - leads and be ready to observe that all the numbers that measured positive on the NPN will measure negative on the PNP. 

I hope I haven't just added to your confusion.  It certainly wasn't a clear step-by-step how to measure transistor gain, but you will it will start to make sense as you spend more time poking around with a meter on your breadboard.

I think when Steve (Smallbear) posts again he will give photos of a very practical way to do this.



 

[Ben Lyman]

Sometimes our photos show us things we can't easily see. Check to make sure your emitter is connecting in that adapter. Looks like the base socket could be an issue too.

[smallbearelec]

To get down to the Bare Bones, set your meter measure a low CURRENT and then set up what's shown in these two pics:





The second pic shows a little more detail of the breadboard. The device is a 2N2222A. Maybe I should first repeat what Transmografix said, with this emphasis: To measure the flow of current directly, the current has to flow Through your instrument. The meter does not show current flow from Collector to Emitter yet, because there is no current going into the transistor Base. Now connect a 1 meg resistor from the Battery + to the transistor Base:



The meter now shows a current flow of about 1.6 ma (1600 microamps) from Collector to Emitter. You don't need a meter in the Base circuit to get a very close estimate of the current flow going into the Base, because the resistance from Base to Emitter is tiny relative to 1 Meg. So by Ohm's Law, the Base current I is 9 Volts (E) / 1,000,000 ohms (R) or .000009 amps, most easily expressed as 9 microamps. If you are not clear about Ohm's Law, find a basic reference and get straight with it.

The gain of the device is now 1600 / 9 or 177. I would expect a 2N3904 to run in the low 200s.

I hope this has made sense.

[digitalzombie]

Awesome, SBE. I'm going to give it a shot right now.

[Transmogrifox]

Adding to what Steve (smallbearelec) said, you can get a little bit closer to reality if you make 2 adjustments to the base current calculation:
1) Measure your actual battery voltage since they are rarely exactly 9.0V
2) Subtract 0.65 volts from the battery voltage.

With a 1Meg base resistor that will be your base current in microamps.

To summarize:
Base Current = [ (Real Battery Voltage) - (0.65V) ]/ 1Meg = [(Real Battery Voltage) - (0.65V)] microamps

Steve's method will get you pretty close so please ignore this if it confuses you.

[digitalzombie]

Thanks guys. Transmogrifox, where'd you get 0.65V from if I may ask? That's the only part I don't understand.

Quick update: I'm returning this multimeter and ordered a new one that should be here Thursday.

Long version; I set up my breadboard pretty much exactly as Steve did, with my Vout connecting to the red lead and the black lead connected to the wire going to the transistor's collector, and I got a reading of -0.2mA Then I tried popping the red lead plug into the mA socket thinking that was it, but that stays at 0.00. I even checked continuity on the fuse in the battery door and it checked out.

I don't know if it's defective or if it needs to be calibrated, but the manual is ripe with some of the worst Engrish I've seen, so it's got to go. Hopefully this next one's better. I'll check in again after I've tried it out, but until then I've got some reading/watching material thanks to you kind folks. Anything else educational you'd like to throw my way I promise to eat it up.

[Transmogrifox]

The 0.65 is an estimation of the voltage drop from base to emitter.  This makes that part of the error less.  You can expect it to be somewhere between 0.55 to 0.75 under most conditions, so by guessing 0.65 you remove a large portion of the predictable amount of error, and it makes your gain measurement just a little bit closer to reality.

[digitalzombie]

Alright. Got the new DMM in the mail today. I'm hoping this one works. Feels a little sturdier than the last one, but has a similar adapter style thingy for the front. I'll try out the posted methods tonight and report back.