Control transistors with
the ohmmeter |
Electric assembly |
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Created it, 06/10/19
Update it, 06/10/22
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In this practice, you will finish the assembly of the indicating whole of state with LED, then, after having carried out the test of it, you will begin the first experiments with the logical integrated circuits.
1. - PREPARATION OF MATERIALS
The materials which you will use in this practice are as follows :
8 resistances of 15 kW
1 / 4 W tolerances ±
5 % (chestnut - green - orange - but)
8 transistors BC 238 B
1 braid of insulated rigid wire (length 10 cm each one approximately)
1 integrated circuit MM 74C04
1 integrated circuit MM 74C08
1 integrated circuit MM 74C00
IMPORTANT NOTE :
Integrated circuits CMOS are characterized by an impedance of extremely high entry (about the million megohm) and an accumulation of electrostatic loads on their terminals can easily give rise to a potential difference electric sufficient to cause, under certain conditions, a destroying electric discharge.
To prevent that that does not occur, it is advisable to use adapted devices, either during the storage of these components, or at the time of their use.
Many nonconducting materials, especially those out of plastic can, by simple prolonged friction, being the object of a static accumulation of electricity.
For this reason, in packing and during transport, integrated circuits CMOS are protected by cases or blocks from spongy substance made conducting. By maintaining the legs of the integrated circuits in short-circuit, one prevents that a possible potential difference does not appear between them. A human being can also take care of static electricity, generally by friction, especially if it wears clothing of wool or fibers synthetic or if it carries shoes to rubber soles, if it goes on the fitted carpet or if it is in contact with seats covered with plastic. To avoid damaging the integrated circuits CMOS which you bought near the electronic trade, you must observe the following precautions:
- do not remove the integrated circuits of the graphited foam which is used to them as support only before their immediate use,
- before taking an integrated circuit with the hands, it is wise to discharge with the ground the possible static electricity from your own body by touching a metal mass connected to the ground, for example the piping of drinking water or central heating,
- as far as possible avoid posing the simultaneous interrupters with several pins of an integrated circuit,
- before introducing an integrated circuit into a support, you always ensure that the food is not connected,
- after employment, always replace the integrated circuit on its mark-sensed foam, which will make it possible to preserve it in good condition.
With regard to the transistors, it is wise to be ensured of their state before using them; let us see how one can carry out their control while being useful oneself of the ohmmeter.
1.1. - CONTROL
OF TRANSISTORS A THE OHMMETER
To check in a complete way the correct operation of a transistor, the surest system is to be useful itself of a transistormeter. However in absence of such an apparatus, it is always possible to check the good state of a transistor by taking measurements of resistance between its three terminals transmitter, bases and collecting by means of an ohmmeter.
In the figure 1-a, one can see how to identify the three terminals of the transistor of the type NPN which you bought, (you also have the possibility of buying documentation, “repertory world of the transistors”), while the figure 1-b represents the graphic symbol corresponding.
To carry out the control of this component, proceed according to following instructions' by holding account that, like that was seen at the time of the test of the LED, the controller to be used must be of the traditional type. An electronic controller or of numerical type does not show the suitable characteristics to carry out this type of control.
Lay out the controller on the gauge W
x 1 000 and place the test probe “+”
of the ohmmeter (generally corresponding to the black casing) in
contact with the terminal of the base, and it test
probe “-” (generally
corresponding to the red casing) initially in contact with the
terminal of the transmitter then with that of the collector as represented on
the figure 2-a. The needle of the apparatus must indicate in both cases a low
value of resistance (direct resistance) ranging between a few kW
and 15 kW.
Now reverse the polarities of the ohmmeter,
as indicated below figure 2-b, while placing the point “-”
i.e. it red on the basic terminal and the other point initially in contact with
the terminal of transmitter then with that of the collector ; the needle
of the galvanometer must in both cases indicate an infinite resistance (opposite
resistance).
Control finally insulation between the
transmitter and the collector by putting the test probes of the ohmmeter in
contact with its two terminals, initially in a direction, then in the other ;
in both cases measurements will have to provide a value of rather high
resistance, practically infinite.
The method that you put into practice can naturally be used to control transistors PNP. It is simply enough to remember that in this case the values of direct and opposite resistance measured between the base and the transmitter and the base and the collector, are obtained while placing the test probes of the ohmmeter in the contrary direction of that indicated for transistor NPN.
It is quite obvious that knowing the polarities of the ohmmeter and the method indicated, it is possible to determine if a transistor is of type NPN or type PNP. Indeed, if one finds a low value of resistance (direct resistance base-transmitter, base-collector) while placing on the basis the test probe “+” (black), it is about a transistor of type NPN. If same direct resistance is obtained while placing on the basis the test probe “-” (red), the transistor is of type PNP.
2. - ELECTRIC ASSEMBLY
a) - Take R8
resistance of 15
kW
1/4 W,
tolerance ± 5 %
(chestnut - green
- orange - but) and after of having suitably
folded the legs, place it on the printed circuit at the site located by the
corresponding initials. Make so that they are well in contact with the circuit.
Carry out the weldings of the legs while proceeding as you already did for other
resistances.
b) - Same manner, weld with the suitable sites of
the printed circuit, R9 resistances,
R10, R11, R12, R13, R14 and R15
(figure 3) very of 15 kW
1/4 W tolerance ±
5 %
(chestnut - green - orange - but).
c) - Lay out the T0
transistor, standard BC 238 B or its
equivalent, in the position indicated by serigraphy (figure 4-b). Take care to
identify its three legs well. Place this component in manner that it is elevated
approximately 3 mm circuit as shown on the photo figure 4-c, then
slightly draw aside the terminals of the transmitter and the collector so that
while turning over the circuit, the transistor preserves the good position;
weld finally the three legs and cut then the surplus length.
d) - Carry out the assembly and the welding of
transistors T1, T2, T3, T4, T5, T6 and T7
(figure 4-a) while following the indications provided for the assembly of the
first T0 transistor.
We will continue this lesson of practice by checking the remainder of the assembly on another page in order not to encumber this one.
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