Transistor bc847 how to test it
Removed transistors OK?
You can also use a digital multimeter to test transistors when they are installed! See below.
After cannibalizing an old device, you often have tons of transistors in front of you, and the question arises:
Which transistor types are they, and are the transistors still usable?
At the back of the photo on the left you can see small power transistors and at the front various low-power transistors.
As a first approximation, let's assume that everything three Has legs and looks something like the parts in the adjacent photo, is a transistor. In order to test a transistor, however, we need to know two things:
To which one Type is it here: NPN or PNP?
What about the Leg occupancy from where are Emitter, Base, and collector (English: E.middle, B.ase, C.ollector)? Incidentally, the "legs" are called solder skewers in technical terms.
The first method can be to search data sheets on the Internet. But this can be quite time-consuming for old types, especially if you don't know the manufacturer.
Sometimes it is also printed on the transistors: CBE are C.ollector, B.ase, and E.middle
The next method and the faster one to the first test are: A multiple measuring instrument and our fingers!.
We make use of the fact that a transistor consists of two diodes polarized against each other: A PNP transistor consists of two diodes with the cathodes (minus pole) in the common center (base), an NPN transistor reversed with the anodes (plus -Pol) of the diodes in the base:
Most often we will come across npn types!
So it is important to find 2 diodes. This works well with a very simple pointer instrument:
Trick 1: As shown in the picture, you change the test clips.
Or: The RED cable is plugged into the (-) socket of the measuring device, the BLACK cable in the (+) socket of the device. This is the opposite of the usual way of plugging in! Reason: When measuring resistance on pointer instruments, 'Plus' flows out of the (-) socket out and then flows into the (+) socket into it, just like normal measurements, where 'plus' always in the (+) jack flows. The RED cable is therefore now (+), the BLACK cable (-)
In any case, the red clip represents "P" and the black "N".
Trick 2: We mark our transistor so that we can find it again later. (Felt pen, scratches, stickers, ...)
And now we start with the type determination:.
We bend the legs and bring them in line. Then we name the legs 1, 2, and 3, e.g. from right to left or vice versa. And then we start measuring with a resistance range of 'x 1k' or 'x 10k' by first placing leg 1 on (-) (black) and then with the (+) cable (red) leg 2 and then leg 3 touch.
There are now three options:
- We find at one of the passages, i.e. the pointer deflects. Then we have already found the first p-n junction: The diode is from 2 => 1 or 3 => 1 (anode => cathode)
- We find at passage in both places! Hurray, then we found a pnp transistor, with the diodes of 2 => 1 and 3 => 1 (or the transistor is broken!)
- We find at both places no passage! Then it is very likely that we have an npn transistor, or that transistor is broken
We swap the measuring tips on the transistor legs until we have reached a situation where a measuring tip remains on the leg and we can connect the other measuring tip to each of the free legs and there is always continuity.
If the stationary measuring tip is red and red is our (+) pole, then we have one npn transistor infront of us. If it is the black measuring tip, then it is pnp transistor!
If we swap plus and minus, then none of the swaps should lead. If we measure continuity anyway, the transistor is defective or it is not a 'normal' transistor.
|OK, now we know whether we have an npn or pnp transistor in front of us, and we know the pin with the base connection. But where are the emitter and collector? We can determine that very quickly, and for this we leave the base leg unconnected and in peace for the time being. The other two legs are now those Not-B.asisB.a little NBB. If we now apply the two NBBs to the measuring instrument, it should lock in both directions! If not, the transistor is not entirely OK. Well, to determine the collector and emitter connection, we make use of the property that a small base current produces a much larger collector current. This should be shown by the yellow arrows in the picture. And we can generate this small base current by connecting the collector and the base leg with a finger via a stretch of skin (do not connect the legs to themselves in a metallic way!). This should be indicated by the resistance shown in dashed lines. We now attach the plus terminal to one of the NBBs, and the minus terminal to the other NBB. If we have an npn transistor in front of us, then we try to short-circuit the NBB, to which we have connected plus, to the base pin with the middle or ring finger. Does the pointer move in the measuring device? No? Then we have to swap the (+) and (-) connections on the NBBs again and try the thing with the finger on the other plus leg. A clear deflection of the measuring instrument should be detectable. Then the leg with the (-) - terminal is the emitter and the leg with the (+) - terminal is the collector. If we have determined a pnp transistor, the search for emitter and collector is reversed, i.e. we have to create a bridge with the finger from the pin with the (-) - terminal to the base pin.|
Checking the continuity with the "x10k" resistance measuring range between the base (red, plus) and collector (black, minus) of an npn transistor:
If we cannot achieve a deflection in the instrument, either the transistor is defective (unlikely if the above-described test for diode direction was successful), or the transistor has a very small current gain. Then you would have to work with a resistor instead of the finger, between 1k and 10k in size. So much for the test with a very cheap and simple one Pointer instrument.
|We have one digital instrument, a different way of working is required due to the high input sensitivity, because the built-in resistance measuring ranges cannot be used.|
Most digital measuring devices have a continuity tester that is combined with a diode test. This is marked with a diode symbol, either as a separate selection field or combined with the smallest resistance measuring range, usually 200 Ω, or 400 or even 600, depending on how many counting positions the instrument has. For some you can still choose between V and Select Ω, then V is to be set here.
If the instrument lacks this diode test, all measurements must be carried out with the voltage source, as described below.
The test leads are to be plugged in normally, the black to the COM- socket, the red to the + socket. The device now impresses a current, usually approx. 1 mA, from the plus connection to the minus connection. If you then start with the procedure of pin averaging by swapping, then either no continuity is displayed (this again varies from device to device with 1. or O.L. (Off Limits)) or a number is displayed that represents the continuity voltage in mV. In the case of silicon transistors, this is in the range from 550 mV to 750 mV, a value that depends more on the applied current and thus on the measuring device than on the transistor. In the case of germanium transistors, this value is significantly smaller, 200 mV and less.
You can very easily determine the type npn or pnp and determine the pin assignment. And that even mostly with transistors that are still soldered in in the currentless circuit (!), As far as the pins can be reached. If one determines that the through voltage from the base to the emitter and collector is largely the same, and vice versa, that there is no through, then, as a first approximation, one can assume that the transistor is still OK.
But in this way you can't find out where the emitter and collector have their legs. This is only possible with the help of a resistor and an auxiliary voltage source, as shown below.
|Some digital instruments have a transistor test socket, which is usually connected to the selection switch via hFE is activated. |
The adjacent block diagram shows the function: The resistors marked ??? k are high-resistance (220k or more) and allow a current flow like the thin yellow arrow in the sketch above to determine the emitter and collector legs. The electricity generated is measured and displayed. Since the value of the resistors is not standardized and the interpretation of the measured current can differ from device to device, the values displayed for different devices cannot be compared.
Once you have determined the type (npn or pnp), you can try to reach a number in the display by plugging it in the appropriate page. The socket also shows the common assignments: E-B-C or B-C-E, i.e. H. the collector can be in the middle.
If the device used does not have this test socket, the emitter and collector legs can be determined with the auxiliary battery:
|To find the emitter and collector legs, we need an external power source, e.g. a 9V block battery, a power supply unit, or whatever else we can find. A resistance of 10k to 22k (selected here: 10k) limits the current, the measuring device is operated in the 20mA range. The cable ends shown on the right are our measuring tips as when working with the pointer instrument. You can find more details on the adjacent photo: Above right the Block battery. |
The red cable goes from the positive pole to the resistance. From there it goes over the black measuring clamp and its cable into the (+) connection of the Measuring device, from (-) or COM of the measuring device it goes to the red measuring clamp, which represents the positive pole for measuring.
The black auxiliary cables go to the bottom left Negative pole the battery and is therefore the minus pole for our measurements.
Under no circumstances should the resistor be bridged or short-circuited when measuring, because otherwise the full battery voltage will be across the diode in the event of a throughput, which will inevitably be destroyed. Too much electricity then flows!
If the pin assignment could already be determined using the diode test, the collector can now be searched for with the finger test.
And what does it all tell us?
Well, although the instructions were a little longer than expected, we can judge a removed transistor very quickly. If we rate it as OK, then it should be good for collector-emitter voltages of up to 20V and a collector current of 20mA.
If you want more, then you have to study data sheets. For most handicrafts, 20V / 20mA is always sufficient.
Tip for searching for datasheets on the net: For European BC types is sometimes only printed with the number, e.g. B. 337-10. American Transistors are often called 2Nxxxx (with xxxx a four-digit number); Only the four-digit number is then printed on the transistor. And japanese Transistors are called Syxxxx, where y stands for a letter, e.g. A. Only A1234, or whatever number is printed on the transistor.
The pin assignment and the type of transistor are best recorded in a small sketch: The pin assignment is usually shown in the data sheets from below, so we look at the pins on them: We note: Name: e.g. BC108, npn, and paint the first assignment for it (such a nose on the transistor with metal housing always marks the emitter). For a transistor with a plastic housing the picture on the right will apply in most cases, sometimes the base and emitter are reversed
If you then want to search for data sheets, it is usually only possible in English. And then experience is also required; so this is more for old hands.
Transistors that fail our test are not necessarily defective. They can be unknown types (FETs, HF transistors), but also A-types with a low current gain. It is advisable to pack these in their own small box or sachet first. If you have a better idea of the whole thing later, you might be able to do something with it.
What can you do with the transistors? One idea: build an accurate electronic thermometer!
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