The percentage of carbon makes it possible to determine the value of the resistance for the given dimensions. The more carbon there is, the less the resistance to a high resistance value.

A more recent manufacturing technique consists in depositing the resistive carbon and binder mixture in the form of a layer around a small special glass tube in which the two terminals are threaded in contact with the resistive material. The resistance thus formed is coated in a thermosetting insulating plastic resin (Figure 2-b).

The terminals inserted deep into the glass support allow better thermal conduction, hence better heat dissipation.

The normalized values of commercially available agglomerated resistors range from 1 Ω to 22 MΩ with tolerances of ± 5%, ± 10% and ± 20%. One finds the following powers : 1 / 8 W, 1 / 4 W, 1 / 2 W, 1 W, 2 W, 3 W and 4 W whose dimensions vary proportionally (Figure 2-c).

The power and the ohmic value of a resistor must appear in clear next to the electrical symbol of this one.

However, certain manufacturers reserve the right, in the design of the diagrams, to adopt graphic symbols which will be interpreted if necessary, according to the indications provided.

Particularly suitable for everyday use, agglomerated resistors whose cost price is low, are widely used in electronic circuits ; they have good mechanical strength, a low temperature coefficient in normal ambient conditions (below 60 °C) and good behavior at high frequencies (negligible parasitic inductance). On the other hand, due to their heterogeneous resistive element, they may have too high a noise voltage and insufficient stability for certain uses.

The film resistors also called film resistances are produced by various techniques; one of them consists of depositing by cracking (deposition by high temperature chemical process) on a central support made of insulating material (ceramic) a thin layer of carbon or tin oxide or of another material resistive.

The ohmic value depends on the thickness of the deposited layer and the substance used. However, it has been observed that the very thin layers, required by the high values, are fragile and unstable. Thus, a layer of 1 µm gives a temperature coefficient of - 180 ppm / °C, while a layer of 1 / 1000 µm has a temperature coefficient of - 1800 ppm / °C, that is to say ten times higher.

This is why one prefers to choose a layer a little thicker and to increase its length by tracing a spiral on the tube (Figure 3-a).

It is obvious that the finer the pitch of the net, the longer and narrower the resistive strip ; its resistivity also increases.

The spiraling is carried out on a special lathe by a diamond wheel. The lathe stops automatically when the desired ohmic value is obtained thanks to a measuring bridge. The accuracy is between ± 2% and ± 5%.

Another manufacturing method, better suited to obtain low resistance values, consists of depositing on a support in the form of a tube or wafer resistive metallic materials (alloys, noble metals or oxides) by vaporization under vacuum and in the form of layers. successively extremely thin until obtaining the thickness which gives the desired ohmic value (Figure 3-b).

With a metallic film deposit on a ceramic support, resistances with low ohmic values and greater heat dissipation are produced (from 0.07 Ω to 4 Ω for 1 W and from 0.1 Ω to 100 Ω for 14 W ) ; the absence of spiraling and therefore of inductance (self) allows their use in high frequency.

Whichever method is used, the deposited material ends in two terminals made of tinned copper wire, fixed by two metal cups or welded on the two ends of the resistive body. The whole is protected by a plastic varnish, a coating in synthetic resin or a molded coating which makes that externally these resistances resemble the current agglomerated resistances (Figure 3-c).

The multiple varieties of manufacturing methods for resistors and types of film adopted by manufacturers as well as the direct correlation between power and dimensions mean that the resistors have quite different electrical characteristics, while having the same external appearance.

Carbon layer resistors whose nominal value ranges from 1 Ω to 22 MΩ with typical tolerances of ± 5% or ± 10%, have a temperature coefficient between 150 and 800 ppm / °C, and, an admissible power going from 1 / 20 W to 2 W. They are used for general use.

They have high reliability and good behavior at high frequencies. In addition, they have low noise and are of moderate cost. They are in many ways comparable to agglomerated resistors.

The table in Figure 4 shows, by way of example, the main characteristics of the carbon film resistors from Philips and the dimensions relative to the admissible power at ambient temperature Ta = 70 °C.

 

 

Fig. 4. - Electrical characteristics and dimensions of carbon film resistors produced by Philips.

The metallic film resistors are used for professional uses, where are required : precision (tolerances less than or equal to 0.1%), the temperature coefficient close to ± 25 ppm / °C, high stability and low noise.

Naturally, the electrical characteristics can vary from one type of resistance to another depending on the specifications of use.

The table in Figure 5 summarizes the technical data and the dimensions of the metal film resistors from Philips and constitutes a comparison element with the carbon film resistors seen previously.

 

 

* For values ≤ (less than or equal) 49,9 W : 100 ppm/°C

Fig. 5. - Electrical characteristics and dimensions of metal film resistors (general use) from Philips.

Metal oxide film resistors, the resistive element of which consists of tin oxide heated to 800° and sprayed onto the support, are characterized by a tolerance of ± 1%, a very low noise level, high stability and good operation at high frequencies ; their job is particularly professional.

"Metal-glass" film resistors have the resistive element obtained by coating an insulating support with precious metal powder such as silver and palladium, mixed with sprayed glass.

Although these resistors do not have the same precision as those with metallic film, they still exhibit great stability, low noise and above all great immunity to atmospheric agents ; they also withstand high temperatures and heavy overloads without damage. With the same spiraling method adopted for the carbon layer resistors and by moving the adjacent turns further apart, resistors supporting high voltages are produced : the electric arc being less stressed between two turns.

The Figure 6 gives some data on these resistances.

 

 

  Fig. 6. - Electronic characteristics and dimensions of metal-glass resistors from Philips

The coiled resistances are obtained by winding a high resistivity wire (for example : nickel-chromium, constantan, manganine) on an insulating ceramic support impregnated, at high temperature, with special resins. The manufacturing techniques adopted for these resistances are also very variable so that one can generalize their shape and their dimensions.

On the Figure 7-a is represented a type of wound resistance obtained by winding a resistive wire around a raw and insulating support, little sensitive to the high temperatures ; the ends of the wire are soldered to the terminals and the assembly and protected with a cylindrical coating of molded plastic material, withstanding a temperature of 220 °C.

This type of coiled resistor therefore looks similar to that of an ordinary agglomerated resistor, with the difference that the first colored ring is twice the others. The power of these resistors does not generally exceed 2 Watts and the values range from 1 Ω to 10 Ω.

 

The wound resistance illustrated in the Figure 7-b differs from the previous one by the external insulating coating which can support a temperature of 350 °C. The ohmic values of these resistors range from 0.1 Ω to 33 kΩ with a maximum power of 30 Watts for everyday use.

In the Figure 7-c, this type of coiled resistors is particularly adapted to dissipate high powers ; they are made with a square section soapstone envelope and sealed with a special cement.

In Figure 8 are illustrated some examples of these resistors, some of which are provided with a steel spring acting as a cooling fin. In this way, the calories are removed more and the nominal power dissipated by the resistance can be increased. This spring also constitutes a member of fixing to the chassis and sometimes it forms integral part of the resistance (Figure 8-b).

There are also some types of resistors provided with a delayed thermal protection device, constituted by a spring strip welded with tin on one of the terminals (Figure 8-c) ; if the temperature of the resistor exceeds a fixed threshold, due to a fault on the circuit, the tin melts, releasing the end of the strip which expands outwards and removes the electrical connection, thus protecting the circuit subsequent damage. When the fault on the circuit is detected, we can fix the strip to the terminal of the resistor, simply by soldering with tin. (In principle, it should even be replaced, because the latter undergoes heating).

Other types of wirewound resistors are illustrated in Figure 9 ; these are copies provided with one or more intermediate sockets and therefore suitable for use as resistive dividing bridges.

They are manufactured with ohmic values and various powers which can reach for the latter from 8 to 250 Watts.

Isolation is achieved by a special vitreous enamel. The ceramic support with circular section (sometimes elliptical to reduce its size) is porous to promote cooling.

In the measuring instruments, resistors are used whose nominal value is less than the ohm, the very high precision and the maximum power of a few Watts ; the resistant wire is wound on a mandrel that is not very sensitive to heat (Figure 10).

 

The two terminals of the mandrel, to which the ends of the resistive winding are fixed, can be connected to the electronic circuit by soldering with tin.

These wirewound resistors are available over a wide range of values from a few tenths of an ohm to 100 kΩ with tolerances on the order of ± 10%, ± 5% and even ± 0.5% for the most precise. They are used when those of the agglomerated carbon type are not suitable : for a high dissipation, a narrow tolerance and above all for a great stability of the ohmic value.

The noise voltage of these resistors is generally negligible but we avoid using them in circuits where the frequency is higher than the audio band because of their high residual inductance.

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