Simply put, these are tailor made resistors having 3 distinct resistance values which translates into controlling the Blower Motor speeds of an Automotive Air-conditioner.
These resistors are mounted in the Air Flow of the Air-conditioner module and the reason behind this is, to keep the resistor as cool as possible.
In the case of a malfunction wherein the blower motor seizes, the resistor also has a dual purpose of being a safety device as it is fitted with a thermal fuse which will intervene and shut down the circuit
We have seen world-wide that these types of resistors are largely in two types: –
- Ceramic Type
- Metal plate Type
Geographically speaking it can be seen that in Europe Ceramic type is pre-dominant and in the Far East Metal plate type is pre-dominant.
Owing to this there is an ongoing debate among automotive manufacturers and HVAC module manufacturers as to which type is superior.
This article has been specifically written to explore the merits & demerits of both sides of the argument.
It would be a good starting point if one studied the internal construction of both types which is illustrated below:-
The metal plate type consists of a monolithic resistance alloy plate which has been punched out to create 3 different resistance values.
The advantage in this case is that the process can be automated as the alloy metal plate can be punched out.
The distinct disadvantage in this type of construction is that the designer has to make a compromise in the selection of the thickness of the metal plate as he has to obtain 3 different resistance values from the same plate and owing to this he is not in a position to select the optimum material and thickness for each individual resistance value. Therefore, there is an inherent issue of heating, which is detrimental to the life expectancy of the resistor.
The ceramic type resistor consists of a ceramic former on which the 3 different resistance values are obtained by winding 3 distinctly different wire gauges to obtain the 3 different resistance values.
The disadvantage in this construction is that it is more difficult to automate as compared to the metal plate type.
The distinct advantage in this type is that the designer can select and wind the optimum resistance alloy wire and gauge of wire hence leading to minimum heating.
Further, the fact that the wires are wound on to a ceramic former is another advantage as the -ceramic former helps dissipate the heat generated.
As far as the metal plate type resistor is concerned heat dissipation & insulation is aided by sandwiching the resistance element between 2 mica plates which is then placed inside an finned aluminium case and therefore these resistors greatly depend on the flow of the air from the blower to keep them operational as they tend to heat up very quickly which is also a problem when the ambient temperature is high especially in tropical countries.
In the ceramic type resistor as far as heat dissipation is concerned in addition to the ceramic former which aids heat dissipation, the resistors are normally coated with a fire retardant coating formulation which is UL approved.
Therefore, the ceramic type resistor due to superior heat dissipation leads to a resistor with a longer life expectancy.
We have tried to illustrate the advantages and disadvantages of both types and it is up to the designer to select a type which most suits his application.