Application Notes for
Power Wire Wound Resistors
A. Choosing Power Wire Wound
Resistors and parameters determination :
Resistor power is
calculated by Power W=I2R
W = Resistor Power I = maximum loading current
R = rated resistance value or maximum resistance of Power Resistor
2. Never overload a power resistor
beyond the specified Voltage, rated Power and Current.
3. We recommend choosing a resistor
with a rated power of at least 1.3 to
4 times higher than the actual loading power if your
application requires the resistor to run continuously at full Power.
Extra safe margin Power/Current can extend the resistor’s service
life and lower its surface temperature.
4. If the maximum or surge power larger
than the rated resistor Power,
please tell the actual working condition like peak/surge voltage,
resistance value, duty cycle, loading time and repetition rate.
5. If the surge/peak voltage is larger
than the rated resistor Voltage SQR(P*R), please tell us the Peak to
Peak Voltage range, Duty Cycle, Repetition Rate per unit time or
Frequency and loading time.
6. Our resistor can withstand 5-10
times the rated power for 5 seconds, depending on the current pulse
width, resistor series, installation and cooling system.
7. There is no standard resistance
value for Power Resistor. It is better to tell the working Voltage,
Loading Time and Duty Cycle of your applications for the Low Ohmic
Power Resistors. As different Voltage can induce a very different
resistor current. Different raw materials and production processes
might need to withstand the high Current and Temperature.
For example, load Current for 1 ohm and 5 ohm 10kW Power resistor is
100A and 44A, respectively.
8. The resistor’s maximum working
voltage must obey Ohm’s Law SQR(P*R)
9. We recommend choosing Low-Inductive
Resistors for high-frequency applications.
10. Most of our Power Resistors can
manufacture according to customers’ applications like Resistance,
Rated Power, Resistor Size, Mounting Fixture and Inductive / Low
Inductive, Pulse Voltage condition etc…
11. Do not touch the Resistor when
connected to a power source due to the high resistor surface
temperature and the chance of ELECTRIC SHOCK.
12. The salty, dusty and corrosive
environment can affect the wire wound resistors performance.
B. Other Application notes:
1. The surface temperature of
resistors can reach as high as 100°C to 500°C while at full
load, depending on resistor
series, resistor rated power,
resistance value, working conditions, ambient temperature
and cooling system design. Keeping the temperature below
150°C to 250°C will extend the resistor service life.
2. Use guards and warning labels where
necessary for the hot resistors.
3. Adding a cooling system such as
external forced cooling fans can lower resistors’ surface
temperature and do not cover the resistors.
4. We recommend keeping all
temperature-sensitive components away from the resistor.
5. Do not touch the Resistor after
connecting to a power source due to high resistor surface
temperature and the chance of ELECTRIC SHOCK.
6. Below is one of the Derating Curves
for Power Resistors in general. Please contact us for an individual
resistor’s derating curve.
Always clean the Resistor Tab Terminals before use. Do not
clean Resistor surface with organic solvents.
8. Do not scratch the resistor surface with
any hard or pointed object.
9. DDR-F and
DQR-F series Power Resistors coat
with UL 94V-0 silicone coating. The Resistors should be installed
away from any flammable materials.
10. Silicon coated resistors might emit smoke
during initial power loading. It is a normal phenomenon. After
loading it 100% for 1-2 hours, the smoke emitting will stop.
11. The ASZ, AHR and HER Resistor
external metal enclosure can be a source of interference for most
sensitive circuits. Grounding the resistor metal housing can solve
All our Load banks RB3A, RLB3A, RB, DB, RBA, DSR-WB, DSR3-WB, FVRB
and RBC series should be Ground connected below connecting the Load
Source and loading the load bank.
C. Adjustable Wire Wound
DSR-F / Rheostats
FVR / Rheostat Boxes
DSR-WB series application notes :
Rheostat and Adjustable Wire Wound Resistor is a kind of
wire wound resistor.
Rheostats’ performance is bounded by both Ohm’s Law
and resistance wire Current carrying capacity.
The function of a rheostat is to adjust the circuit current
between the maximum current at the minimum resistance and
the minimum current at the rated resistance.
Rheostat parameters determination
Rheostat rated Power =
(Rheostat maximum load Current)2 x rated Resistance
The maximum load current is determined by the current of an
existing application before the adjustable Power Resistor or
Rheostat is inserted. This consideration is for Circuit
Current adjustment – like a Rheostat in series with a fixed
resistor ( the equivalent circuit).
From a material point of view, the allowable Current is
limited by the Ohm’s Law and the resistance wire’s Current
carrying capacity – whenever the lower one.
Loading Current beyond this can damage the adjustable
resistor / rheostat.
The maximum current for two Rheostats with the same rated
Power can be very different.
For example, load Current for 1 ohm and 5 ohms 10kW Power
Rheostat is 100A and 44A,
There is no standard resistance value for Power Resistor.
can be calculated with the maximum current and voltage.
can be calculated with the minimum acceptable current and
The workable Power will decrease as the resistance is
adjusted to a smaller value.
The workable power at the adjusted resistance is about the
ratio of (adjusted resistance) to (the rated maximum
resistance of that rheostat) x ( rated rheostat power) or
i.e. in material point of view : Power per Unit Resistance
Rheostat Application Notes :
Load Current at the adjusted resistance value
Load Power at the adjusted resistance value =< Rheostat
resistance value is not the same as an adjusted resistance
voltage across Rheostat might need to decrease to avoid over
loading the rheostat when reducing the resistance value
in order to keep the load current below the rated Current.
A fixed power resistor can be connected in series with the
rheostat to protect it from overcurrent damage.
The Rheostat rated resistance = Rheostat Power / (maximum
The Resistor power = (maximum load Current)2
x rated Resistance.
The main role of Adjustable
Power Wire Wound Resistor DSR-F, Rheostat FVR, Rheostat Box
FVRB and DSR-WB is
to decrease, not increase, the electrical current in the
The above is for Continuous
For some situations, we will suggest RBA series Adjustable
Load Power / Current adjustment by preseting steps –
discrete resistance values.
Current is achieved via circuit breakers or power switches
at the control panel.
Each switch can
control a preset resistance value ON/OFF.
ON/OFF combinations, a different load current can be
For example : 250Vdc
with max. current 150A.
The setting can be :
10A, 20A x 2, 50A x 2
= 150A in 5 steps with resolution 10A or
1A, 2A x 2, 5A, 10A x
2, 20A, 50A x 2 = 150A in 9 steps with resolution 1A or
Our Load Bank
supports precision resolutions like 0.5A, 0.2A and 0.1A.
The load Current is
achieved by load Step adjustment.
e.g. 97A is needed,
switch : 2A, 5A, 10A, 10A, 50A is ON
This load bank rated
power is 250V x 150A = 37.5kW only.
When compare with
rheostat, RBA series load banks, this can be a cost saving
Other Rheostat Application
The resistance adjustment is achieved by sliding the metal
brusher across the metal resistance material.
There is a chance of flashover between two metal parts when
the resistance is being adjusted, especially at high
voltage, current and/or power conditions.
It is better to power OFF the load source across the
Rheostat before adjusting the resistance values.
2. Do not
touch the Adjustable Resistor / Rheostat surface when it is
connected to the power source due to
High Surface Temperature and avoid
We recommend choosing a rheostat with a rated power of at
least 1.2-4 times higher than the circuit maximum power if
any application requires the rheostat to run continuously at
the full Power. Extra safe margin Power/Current can extend
the rheostat’s service life and lower its surface
Due to high power application and Rheostat consists of metal
movable parts within the Rheostat.
We suggest installing the rheostat on a fixed and level
bench to avoid vibration.
dusty, humid, high temperature, vibration
and corrosive environment can affect the Rheostat
6. Both above
Section A and B
application notes are also true for adjustable wire wound
Rheostat Bank FVRB /
Adjustable Load Bank DSR-WB
: Ammeter, Voltmeter, Wattmeter,
Ohm meter and Temperature meter