Nikkohm_Capability-Brochure-Power-Resistors-html.html

1. Electric

Resistance

By applying either plus or minus voltage to the resistor
and which causes electric current, the straight line
characteristics of voltage and current such as Fig.1 are
observed.  The resistance of the resistor is 1K OHM if
5mA current is observed under the condition of 5V voltage
applying. Resistor has a resistance value (unit :OHM).
Resistance indicates the resistivity for electric current flow.
Relation of resistance R (OHM) and voltage E (volt) and
current I (Ampere) is shown as below.,

)

(

Ampere

In most of cases, the resistor is made with metal resistive
materials which have resistivity

as peculiar properties of

physical fixed number.
By cutting the board-shaped metal resistive material in
Fig.2, resistance between the both sides of the resistive
material is calculated as below.

)

(

ohm

By the calculation of resistance of the metal which is cut
10mm square with 0.1mm thickness, a resistance
between the both side points of the metal is calculated as
Table1.

The resistance metal plate size 10mm x 10mm x 1mm can
be used for milli-ohm level resistor, on the other hand the
resistance metal less than 0.1um thickness can be used
for thin film resistor.
As shown in Table1, the resistance of square type
resistive material depends on only the thickness of
resistive material, and that resistance is called as Area
Resistance which is utilized for the parts product design.
Area Resistance is utilized as an index value for the film
thickness in both thin film resistor and thick film resistor.

2. Heating of resistor
By applying voltage E to resistor, current I flows through
inside and Joule loss so called Electricity loss P is
generated and becomes to heat.
Electric power P (W) is shown as below.

)

(

)

(

)

(

Heat of resistor is released to the air by air convection
from metal surface or heat radiation in major resistor
cases, however metal flange equipped resistor assists
heat release through radiator in the case of power film
resistor.
The structure and heat conduction of power film resistor is
shown in Fig.3.

Fig.1 Voltage-current characteristic of resistor,

dotted line shows non-linearity

-5 -4 -3 -2 -1 0 +1 +2 +3 +4 +5 (Volts)

+6.0

+4.0

+2.0

0.0

-2.0

-4.0

-6.0

(mA)

R=1K OHM

Fig.2 Resistance of metal resistive material

Width, W

Thickness, T

Length, L

Resistivity,

Resistance meter

Materials

Resistivity

(ohm

・

(mm)

(OHM)

Const. 0.49x10^-6

10 1mm 10 0.00049

Manganin

0.44x10^-6

10 1mm 10 0.00044

Ni-Cr 1.10x10

＾

-6

10 1mm 10 0.0011

Ni-Cr 1.10x10

＾

-6

10 1um 10 1.10

Ni-Cr 1.10x10

＾

-6

10 0.1um 10 11.0

Ni-Cr 1.10x10

＾

-6

10 0.01um 10 110.0

Table 1. 10mm x10mm area resistance

Capability Brochure, Power-Film Resistors, Series RNP, RPM, RPL

Fig.3 Resistance of metal resistive material

Terminal, Cu
Resistive film
Insulator, ceramics
Heat flow
Flange, Cu
Heat sink

As shown in Fig.3, heat P(W) generated in resistor is
partly released from the resistor through terminal,
however major parts of heat is radiated from radiator
(metal plate, aluminum board, aluminum block) through
isolated ceramics (alumina board) and copper flange to
the air. Performance of the power resistor is
characterized by less thermal resistance between
resistive material and flange.    Alumina board and flange
are connected by solder and which realize very small
negligible thermal resistance. Thermal resistance of
copper flange is also so small as to be negligible.
Radiator has a capability of transmitting heat of flange to
the air, and the thermal resistance is specified in data
sheet for commercial resistor products, and which is
inversely proportional to surface area of the radiator.
Let’s set thermal resistance between resistive material
and flange as

rf, thermal resistance of radiator versus air

h, and temperature of the air inside of the electronics

equipment box as Ta, then temperature of resistive
material Tr with electric power consumption P in the
resistor is expressed as below.

)

(deg

deg

155

)

(

≤

Rating power Pr is expressed as below, with considering
Pr to give some safety margin under the condition of
flange temperature at 25 degree, and set maximum
operation temperature of resistive material as 155 degree.

)

(

)

155

(

−

Unit of thermal resistance is deg.C/W.
Table2 shows some specific thermal resistance which is
between resistive material and flange in power resistor.

3. Temperature rising of resistor
Temperature of all portion in resistor are started rising
once voltage is started to apply for power resistor which
equips radiator as shown in Fig.3. Fig.4 shows one
specific measurement result of the temperature rising.
Unlike to general air-cooling type resistor, flange power
resistor which has less ability of heat conduction radiation
from the surface of resistor, there is a linear shaped
relation between applied voltage and temperature rising.
Fig.4(a) shows the relation of temperature rising of flange

and resistive material which is inside of resistor, under the
condition to use power resistor of 140W rating equipped
on the radiator which thermal resistance is 0.2degC/W.
By subtraction the rising temperature of radiator from the
temperature rising of flange, temperature rising of
resistive material are indicated which is based on a flange
temperature.
The slope of the straight line indicates the cooling
performance of the power resistor and thermal resistance
between resistive material and flange which shows
0.9degC/W in Fig4(b).
Maximum rating temperature of resistor is defined by the
materials which consist of resistor, and which is 155 deg C
for these power resistors, and derating curve can be
provided by pulling straight line of the slope of thermal
resistance from the point of maximum temperature rating
as shown in Fig.4(c).

Type Package

Heat

Resistance

RNP-20D/E/F TO263

3.3 (deg C/W)

RNP-10S

TO126

5.9 (deg C/W)

RNP-10

TO220

5.9 (deg C/W)

RNP-20S

TO220

3.3 (deg C/W)

RNP-50U

TO220

2.3 (deg C/W)

RNP-50S

TO247

1.3 (deg C/W)

RNP-80S

TO247

1.3 (deg C/W)

RNP-100S

TO247

0.9 (deg C/W)

RPM-200

SOT227

0.50 (deg C/W)

RPM-300

SOT227

0.32 (deg C/W)

RPL 300/310

---

0.32 (deg C/W)

RPM-600

SOT227

0.11 (deg C/W)

RPK-900

SOT227

0.10 (deg C/W)

RPL1200

---

0.10 (deg C/W)

TABLE 2. Heat Resistance, resistor-flange,

Typical

Fig 4. Temperature rise and power derating

From upper, Fig 4(a)-(b)-(c)

0 20 40 60 80 100

Applied Power (W)

160

140

120

100

0 20 40 60 80 100

Applied Power (W)

160

140

120

100

160

140

120

100

-50 00 50 100 150 200

Flange Temperature (deg C)

Temperature Rise (deg C)

RNP-100S Temperature Rise on 0.2 C/W
heat sink. Heat resistance is 0.9 C/W.

Resistor Temp.

Flange Temp.

Resistor Temperature Rise

Slope=0.9 deg C/W

Slope=0.90 deg C/W

=1.11 W/deg C

Rating Power (W)

Temperature Rise (deg C)

4.    Power derating of resistor
The power derating curve of resistor shown in Fig4(c)
indicates the safety operation area with border line.  In
other words, temperature of resistive material in side of
resistor reaches maximum temperature on the power
derating curve.    It is required to let power resistor operate
under the condition not on the borderline but inside of the
power derating curve which correspond to the use of load
reduction, and which purpose is improvement of the
apparatus operation reliability. Though load reduction
depends on the design standard of each system designer,
it may be common to set load reduction around 25% or
50%.

5. Resistance

value

Measurement position of Resistance is just the point
5.0mm apart from the bottom end of mold area or painting
area of resistor, then measurement is executed by using
Resistance Meter and Kelvin Probe. Resistor values like
1.0 OHM or 10.0 OHM etc. are not used continuation
value but used discrete valid value from the rational
standardization point. Those valid values are prescribed
in JISC5063, IEC60063, etc., and E12 or E24 series are
used for power resistor. Nikkohm has used numerical
values of IEC standard with addition of 2.5 or 5.0 value as
shown in Table 3.

6.  Tolerance of resistor

Resistors have unevenness resistance value because of
the industrial products. Unevenness of tolerance is
expressed by using % like an expression “±1% tolerance
guaranteed for this resistor”.    Tolerance of power resistor
is ±1% or ±5% in general.  Such as RNP-20P which is
used for the standard equipment of constant current

power supply circuit, which resistor tolerance of ±0.1% is
so much useful.    In some case, “B” means ±1% and “F”
means ±5%, and it may be expressed like 0.1%,1%,5%
without “±”.

Temperature coefficient of resistance

Since materials of resistor are metal or alloy in general
and which utilize the metal electrical conduction
characteristics, resistance tends to be affected by the
temperature of resistive material and temperature of
environment. Therefore once the environment
temperature of extra precision type resistor like 1000
OHM ± 0.000 OHM, its resistance will be shifted from
1000 OHM.  If the shift value of resistance in 1 deg C
change is expressed in ratio, temperature dependence is
0.005%/deg C or 50ppm/deg C in power resistor.  This
temperature dependence is called temperature coefficient
of resistance “TCR”.  Both tolerance of resistance and
temperature coefficient of resistance adopt the expression
in ratio, tolerance of resistance is expressed in % and
temperature coefficient of resistance is expressed in
ppm/deg C respectively for the purpose of minimize a digit
targeted to reduce the mistake of expression. In the
case of 2 terminals type power resistor, the same as
another type resistors, the effect of copper lead for TCR
(temperature coefficient of resistance) is not negligible,
therefore TCR tends to be increased following to the low
resistance as shown in Fig7.

E6+ E12+

E24+

1.0

1.0 3.3

1.5

1.2

1.1 3.6

2.2

1.5

1.2 3.9

3.3

1.8

1.3 4.3

4.7

2.2

1.5 4.7

6.8

2.7

1.6 (5.0)

3.3

1.8

5.1

3.9

2.0

5.6

4.7

2.2

6.2

(5.0)

2.4 6.8

5.6

(2.5) 7.5

6.8

2.7

8.2

3.0

9.1

Table 3. E6+, E12+, E24+, Significant Figures

Fig 5 Position of Resistance Measurement

5.0

ｍｍ

5.0

ｍｍ

Fig 6 Kelvin Probe, Resistance Measurement

Fig 7 TCR of Low Resistance Power Resistor,

RNP-20S, typical

0.01 0.1 1.0 10.0

Resistance, RNP-20S (OHM)

500

400

300

200

100

TCR (ppm/deg C)

7. Short time overload and pulse overload for power film
resistor
Rigidness for the short time overload of power film resistor
corresponds to power ratings, and the usage under that
overload condition is strictly limited. Wire-wound resistor
is specified to be used under the condition of short time
overload which accepts 10 times higher than power
ratings within 5 second. The reason of the above is
because of the so large thermal capacity of the resistive
material of wire-wound resistor than that of power film
resistor. The short time overload characteristics within
0.1s to some seconds depends on the thermal time
constant which is defined by both thermal capacity (m

C) and thermal conduction (

) of resistive material.

Let’s calculate to obtain a temperature of resistive material
after t second passed from the heat start period (t=0) with
applied heat power W. Thermal capacity (C) is
expressed as follows assuming specific heat (c) and mass
(m).

)

(

)

(

)

(

⋅

Time constant (

) is expressed as follows by using thermal

resistance (

) between resistive materials and

surroundings.

onds

sec

)...

(

)

(

Temperature rising value per time change

△

Tt during the

resistive material heated by electric power P(W) is
expressed as bellow.

deg

...

)

(

)

(

−

Temperature falling value per time change

△

Tt during

cooling resistive material by releasing electric power is
expressed as bellow,

deg

...

)

(

)

(

−

and which shows exponential function-like change.
To obtain the thermal capacity C of both power film
resistor and metal clad wire wound resistor which power
ratings is 100W and resistance is 10 OHM in each, wire
wound thermal capacity is 65 times higher than that of
power film resistor as shown in Table4.

Therefore it is desirable to adopt wire wound resistor to
brake resistors for large sized motor, and to surge current
control resistors for large power supply unit. In the case
of applying pulse shorter than 100 micro second width to
resistor, power film resistor has a capability to apply surge
peak pulse which is exceeding Rating Power. Regarding
pulse overload characteristics, there is rough expression

which is not related with the above but based on the
examination result. The examination is executed by
setting a criteria of pulse destruction of threshold as ±1%
resistance change which is followed the pulse applying
with specific constant period by using pulse generator
shown in Fig 8. The result of pulse destruction
examination is shown by dotted line and rough pulse
rigidness is shown by solid line in Fig 9. Since the
aspect of resistor destruction are different in each case of
applying electric power either the constant current case or
the constant current case, peak electric power needs to
be limited lower than the rating power, or recommended to
proceed enough reliability examination with confirmation
before the equipment on circuit board.

8. Impedance frequency characteristics of power film

resistor

Since power film resistor has a superior high frequency
characteristics compare to the other resistors, power film
resistor shows excellent characteristics in the pulse
application circuit or high frequency circuit board which
realizes wide frequency band and fast pulse rising time.
In the case of alternating voltage application, there is a
phase shift on sinusoidal wave in the relation of voltage
and current from strict viewpoint, so impedance of resistor
needs to be treated by both real number R and complex
number Z. Impedance of power film resistor can be
expressed by LCR equivalent circuit approximately, and
which can express high frequency characteristics in
specific band from DC to 1GHz. Both the equivalent
parallel capacitance and equivalent series inductance of

Fig 8. Voltage Pulse Power Generator

Type Power

Film

Wire

wound

P/N RNP-50S

IRV100

Rated Power

100W

Resistance (ohm)

Material Ni-Cr

Ni-Cr

Specific Heat(J/kgK)

418

Mass (kg)

0.082x10^(-6) 5.3x10(^-3)

Heat Capacity (J/K)

0.034

2.2

Table 4. Heat capacity of

power film and wire wound resistors

Pulse width

Repetition

Peak power

DUT

Fig 9. Criterion of Pulse Power Operation

-8

-7

-6

-5

-4

-3

-2

-1

-0

100kW

10kW

1kW

100W

10W

RNP-50S C 100 F, 100W rated power

Pulse Peak W

power film resistor are shown in Table5.

(

)

−

(

)

ohm

...

)

(

−

radian

.....

tan

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

The measurement result of TO220 power film resistor
impedance characteristics is shown in Fig11. The
characteristics show flat level from DC to 600MHz in the
case of resistor 50 OHM through 200 OHM, and LC series
resonance frequency can be observed at 700MHz.

9. Implementation method of flange resistor
Implementation method of power film resistor which types
are TO126, TO263, TO220, TO247 and SOT227 is
explained as follows. Since metal flange for heat
radiation of these all resistors which is isolated from
resistive material and lead, there is no requirement for
considering isolation (isolated heat radiation sheet) differs
from transistor which equips exposed collector.

Resistors TO126, TO220 and TO247 are connected with
heat radiation plate or metal plate by clip or screw.
Recommended torque of the screw is shown in Table6.

Here are some implementation method of flange power
film resistor shown in Fig.12.

Fig 10. Equivalent Circuit of

Power Film Resistor

R(resistance)

C(capacitance)

L(inductance)

Type Package

C(pF)

L(nH)

RNP-20D/E/F TO263

1.44

8.38

RNP-10S TO126

1.00 8.22

RNP-10 TO220

1.15 8.38

RNP-20S TO220

1.44 8.38

RNP-50U TO220

1.69 9.65

RNP-50S TO247

2.35 11.72

RNP-80S TO247

2.35 11.72

RNP-100S TO247

3.68 12.52

RPM-200 SOT227 13.10 13.67
RPM-300 SOT227 13.10 13.67
RPL 300/310

---

13.10

13.67

RPM-600 SOT227 13.10 13.67
RPK-900 SOT227 13.10 13.67
RPL1200 ---

---

Table 5. Equivalent Capacitance and Inductance

of Power Film Resistor

Fig 11. Frequency Characteristics,

Impedance of RNP-20S

10k 100k 1M 10M 100M 1G

Frequency (Hz)

Impedance |Z| (ohm)

10k

1k

100

10

RNP-20SC1K0F

RNP-20SC101F

RNP-20SC220F

RNP-20SC10KF

Type Screw

(mm)

Ｓｃｒｅｗ

(inch)

Torque

(Nxm)

RNP-10S

Ｍ

3 No.4 0.6Nm

RNP-10

Ｍ

3 No.4 0.6Nm

RNP-20S

Ｍ

3 No.4 0.6Nm

RNP-50U

Ｍ

3 No.4 0.6Nm

RNP-50S

Ｍ

3 No.5 0.6Nm

RNP-80S

Ｍ

3 No.5 0.6Nm

RNP-100S

Ｍ

3 No.5 0.6Nm

RPM-200 M4

No.8

1.2Nm

RPM-300 M4

No.8

1.2Nm

RPL 300/310

No.8

1.2Nm

RPM-600 M4

No.8

1.2Nm

RPK-900 M4

No.8

1.2Nm

RPL1200 M4

No.8

1.2Nm

Table 5. Equivalent Capacitance and Inductance of

Power Film Resistor

Fig. 12 Installation, Power Film Resistors

Via hole thermal

connection to heat-sink,

RNP-20D/E/F, RNP-50F

Insertion cylinder thermal

connection to heat-sink,

RNP-20D/E/F, RNP-50F