How to properly test a transistor


I just set my first resistor on fire (not on purpose). I’m trying to apply what I’ve been reading about base and collector current relationships, hFE etc, in order to use a DC motor properly. However, I don’t want to burn the motor, so I’m testing different settings with resistors. I measure everything I can and see if I can predict the results.

I have a TIP41C (NPN) which gets +6V control voltage at the base (from a 1400mA max power supply) and a +9V supply voltage at the collector (from a 6A max supply). What I’m doing is basically trying different resistors at the base and at the collector, to see what I should do to control my 4A motor which isn’t connected yet.

The base resistor (Rb) drops a bit more than 5V most of the time, which seems to relate to the presumed 0.7V drop across the BE junction (i.e. both voltages add up to 6). With Rb = 1K and Rc = 10?, I got only 2.8V across Rc (which is the only load connected to a 9V supply), and I assume it is because the output current is limited by what shows up at the base (I measured Ib = 5mA). Ic was therefore 285mA.

I wanted to increase the current at the output further, so I replaced the base transistor with Rb = 100?. Ib became 52mA and the 10? resistor at the output burned immediately. Putting a 100? in its place I measured 92mA at the output (current gain less than 2). With a 1K at the output the gain actually became negative (Ic = 9.2mA).

What I’m trying to understand is how am I supposed to do these tests if the resistors burn when they get more than 2 amps… How can simulate a load such as a motor without actually connecting one (and without burning down my house).

Thanks a lot for your help.

Automotive lamps like headlights and brake lights draw 1 to as much as 10 amps and make ideal ballasts or loads for testing like you are trying to do. [u]You need to learn what the Hfe or Beta (Dc current gain really means) and how to apply it to your task[/u]. So you can do the calculations first and then prove them with your experimental setup. This is the way it is done when any transistor circuit is designed, Your calculations will tell you what to do and your [u]test setup[/u] is a [u]proving ground[/u] for your calculations... Not an empirical design tool as you are attempting to use it for. This is the proper way to test a transistor or circuit. Design first and test the design.


I agree with you on the designing and testing procedure, however since I've only laid my hands on a transistor for the first time last week, please indulge me... Testing stuff out is part of the learning process, I believe. I'm not even half a circuit designer, for now I'm just learning what circuits do :)

I'll try the loads you mentioned. Thanks a lot.

There are really 2 kinds of testers... 1) Go/No-Go testers which tell you if the transistor works at all 2) Gain testers, like the one commonly added to a multi-meter which will show a transistor working and how much (hfe) or forward current gain it can produce.

I have used this circuit for years to quickly sort PNP/NPN transistors acquired from "recycling" or grab bag purchases. It can also be used "in circuit", provided your hardware it belongs to is powered off.

I'm trying to apply what I've been reading about base and collector current relationships, hFE etc, in order to use a DC motor properly. However, I don't want to burn the motor, so I'm testing different settings with resistors. I measure everything I can and see if I can predict the results.

I kind of learned "circuit design" by playing with LTSpice. There's a difference between simulation and reality, agreed. (Simulation does not smell burnt) But there's also a difference between resistors and real motors.

I also learned that the actual value of hFE does not ( or should not ) really matter much. ( Same for resistor values -+ 25 % )

Well what surprised me was that hFE could be less than one, as in the last case I mentioned. I read that a transistor could limit a current at the collector if the base current was small enough (which is why I was getting low current compared to what I was expecting from the voltage, resistor and the datasheet hFE), but I didn't know Ic could actually be smaller than Ib.

I'll have a look at LTSpice, although I must say that the sight of a burning resistor has a certain down-to-Earth reality to it which is quite nice, it makes you more humble... :)

With 6v at the base you minus your. 7 drop, so 5.3 then you find how much current u need datasheet says minimum hfe @3a is 15 so @ 4amps/15=.26a so then 5.3v divided by that is a 20 ohm base resistor basically to ensure your getting full saturation

@winner10920: Thank you very much, I was almost getting there :slight_smile: But can I draw 260mA from the Arduino? For the moment I’m using two power supplies just so I understand the mechanics and have the settings in the general ballpark, but the idea is for the control voltage to come from Arduino Uno (which will be 5V, not 6V of course).

An arduino pin can only supply 40ma max, safer is around 20
you need either a transistor with a larger hfe, like a darlington pair (tip120 or tip122 I forget), or a smaller transistor(s) to progressively switch higher currents
or go for a logic level power mosfet which doesn’t need high current

OK thanks a lot, I'll come back here when I think I understand the difference between these options. If you know of any real-world tutorials on the web using Arduino, high-current motors and some of the components you mentioned it would be great, much of the stuff I find is either very theoretical (which is fine, btw) or dealing with very low-current motors it seems.

I've used the stp40nf12 nchannel mosfet many times and it does the job well, rated for 40 amps, 120vds,20vgs and the gate threshold is low enough that by 3.5v its starting to turn on and @ 4.5v its nearly fully on atleast enough for most loads make sure you have a flyback diode on there to keep the mosfet safe from back emf from the motor it may not even get warm at that load

Some basic driver crcuits

@Erni: Thank you very much, a very precious page.

A couple of factoids. For a basic transistor [non-Darlington], if you look carefully
at the specs, you’ll see that hFE drops off precipitously as collector current
increases. I don’t know about the one you’re using, but for smaller transistors
the gain can go from 100-200 down to < 10 as collector current goes up up

Also, I didn’t follow everything you said, but hFE can only be measured with the
transistor in the regular operating range, ie not in saturation, so you have to
check that before calculating hFE.

Another thing is that hFE tends to vary all over the place with different transistors
of the same kind and with temperature, that’s why the specs are generally
extremely loose on this parameter.

Also, in regards your burned up resistor, the next thing to do is to calculate
power dissipation in every R.

Pd = (VV)/R = (II)/ R, where V = voltage-difference across the R.

Eg, (2.8 * 2.8 )/10 = 0.78 watts, so what wattage was your 10ohm R?

Thank you for pointing this out. I understand now that the burnout is probably unrelated to the transistor itself — it was just a too small resistor with 9V across it (I think the transistor was the basic 1/4 watt kind, so it figures), which got switched on when I applied current to the base. I'll have to find a bigger load to test the Ib/Ic relationships, like Doc suggested above.

The first thing you think about when seeing low-value Rs [eg, 10 ohms]
is “lotsa power dissipation”, meaning the Rs and also the NPN need to
be largish parts.

Full power into the R would mean 9V*9V/10ohms = 8.1 watt. In anything but
a well-designed high power ckt, that’s a lot of heat.

Also, in this case, the NPN can conceivably dissipate a lot of heat too. Worst
case is for 9V/2 = 4.5V on the collector, which would mean Ic = 4.5V/10ohms
= .45A, and PD(npn) = 4.5V * .45A = 2 watts. A little TO-92 device can only
dissipate 0.3 watts or so before overheating. Even a TO-220 case will get pretty
warm with PD = 2 watts, and you need to think about heatsinking even here.

Then, the other thing that happens is, when the transistor gets hot, hFE
changes a great deal too. hFE is so variable, all in all, that good ckts are
designed to take this into account.

To test a transistor, you can do simple resistance tests on the various diode junctions, as illustrated here, Measure each of the pairs of diode junctions. Collector-emitter, collector-base, base-emitter. Read the resistance of one junction and then read the same junction with the polarity probes switched. One side should read very high resistance, over 1 megohms. And the other should read a moderate resistance, a few hundred thousand ohms. If this is the case for all three junctions, the transistor should be a good working one.

I have used this circuit for years to quickly sort PNP/NPN transistors acquired from “recycling” or grab bag purchases. It can also be used “in circuit”, provided your hardware it belongs to is powered off.

Hm, had a look at that and the instructions are “interesting” to say the least? Not sure I like the idea of the LEDs bursting! But it might be worthe building just for the heck of it.