Tubular Vitreous Enamelled
Tapped Resistors See Figure 10
Fixed resistors can be supplied with taps to special order.
Because of the reduced winding length, the total resistance of
a tapped resistor will be less than can be offered on a fixed
resistor of the same size. The reduction is proportional to the
number of taps and Table 5 is intended as a guide.
The minimum resistance per section on all sizes is 1Ω and the
standard selection tolerance for any section is ±10%
Enquiries for tapped resistors must state the following details:
Resistance per section
Maximum dissipation per section
Maximum operating ambient temperature
Maximum permissible dimensions if important
Type of terminations required. (See ‘Terminations’, page 3)
Resistor style or proposed method of mounting.
*Maximum total dissipation assumes that this will be
evenly spread over the total element length.
Low Inductance Resistors
Ayrton-Perry wound elements are supplied for low inductance
applications. This winding style has a maximum permissible
hot spot temperature of 300°C. The maximum dissipation is
defined in Table 1 under the heading Operating hot spot
temperature of 300°C, and resistance ranges are defined in
the three right-hand columns of this table.
When cold, vitreous enamel has excellent insulation resistance.
In common with all insulants the specific resistance of the
enamel decreases with increased temperature; therefore, if
operated at any temperature approaching the maximum, the
resistor cannot be classed as an insulated type and should not
be used in contact with any conducting materials.
The recommended dissipations for each of the resistor hot
spot temperatures applies to resistors mounted horizontally. If
the bore is completely blocked a 15% derating is
recommended. However, wherever possible, resistors should
be mounted vertically with unobstructed bore.
This makes best use of the chimney effect of the heated tube
and will encourage a cooling stream of air through the bore.
Allowances must be made, when tubular resistors are mounted
in banks, for the effects produced by radiation between tubes.
Appreciable reduction of hot spot temperature can be
achieved by arranging that resistors are subjected to some
measure of forced draught. In general, it is most efficient to
extract air from the resistor enclosure and arrange that an air
inlet is adjacent to the bottom of the tubes.
If soft soldered connections are used the resistors should be derated
where applicable to limit the hot spot temperature to 300°C.
© Welwyn Components Limited Bedlington, Northumberland NE22 7AA, UK
Telephone: +44 (0) 1670 822181 · Facsimile: +44 (0) 1670 829465 · Email: email@example.com · Website: www.welwyn-tt.com
Issue E · 10.05