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UNDERSTANDING PULSE & OVER LOAD CAPABILITY
OF WIRE WOUND RESISTORS

Due to existing needs, modern Electronic Circuits & Devices are very sensitive to transients.

For resistors upto 3 watts, the STOL voltage is calculated as :-

STOL Voltage = 5 Times x Power Rating x Resistance value for period of 5 sec.

This in turn has led to the need to “

” transient protection.

design in

HTR

Designers often face the dilemma of the level of protection required and the ability of the individual
component to withstand given transients or pulses.

It is also a fact that whilst there is a plethora of information available on this subject for active devices like
semi-conductors, passive devices such as resistors are often neglected and their role in withstanding pulses /
transients is not fully grasped by the designer.

In actuality resistors are often located in areas likely to experience transients, for eg. Power Supplies &
Lighting Circuits and therefore the full understanding of the ability / capability of the resistor to handle
transients / pulse stresses is indeed a critical issue.

The most common and cost effective type of resistor that can be used for transient protection are wire wound
resistors and this fact is often neglected by the designer as they do not have ready access to the pulse handling
/ over load characteristics of wire wound resistors.

In order to over come this obstacle

has worked closely with designers to produce custom built resistors for

specific protection applications and the data obtained is now given below in the form of tables and data which
has been gleaned through years of research & study :-

As excessive heat both environmental & self produced are the single most common factor contributing to the
failure of a resistor, wire wound resistors have maximum continuous power ratings to limit their temperature
rise.

In practical terms this is in the form of limiting element voltage(L.E.V.).

The limiting element voltage is the maximum continuous voltage that can be applied to a resistor.

For lower values the power rating is exceeded before L.E.V. is reached. However, in the case of higher resistance
values the L.E.V. imposes limitations on the applied power.

Though L.E.V. cannot be exceeded for continuous working, wire wound resistors are the only resistors which
are capable of withstanding 3 times their L.E.V. for pulse applications.

In addition to the above, wire wound resistors also have a well defined overload rating and this is referred to as
short time over load (STOL) on published datasheets.

The short time overload rating of HTR's wire wound resistors are calculated as given below:-

OVERLOADING OF THE WIRE WOUND RESISTORS

For resistors above 3 watts, the STOL voltage is calculated as :-

STOL Voltage = 10Times x Power Rating x Resistance value for period of 5 sec

STOL Voltage =

x 4000 = 223.6 volts

Similarly, HTR's HTA series H9 Type 27K777 5% resistor has a STOL Voltage / rating which is calculated as :-

STOL Voltage = 10 x 9 x 27777 = 1581.12 volts

E = V /R x t = 10000 x 10000/4000 x 0.0015 sec. = 37.5 Joules

1. For very short pulses which are < 1Milli Sec. (1.2/50Micro Sec. as defined by IEC 61000-4-5 & ANSI

C62.41), please refer to Table-1.

2. For longer pulses from > 1Milli Sec. to 100Milli Sec., please refer to Table-2.

3. For pulses > 100Milli Sec. upto 5 Sec., the short time over load capability (STOL) calculation as shown

above should be referred to.

For eg. HTR's HTA series H2B Type 4K0

5% resistor has a STOL Voltage / rating which is calculated as :-

It can be seen that in the above case the H2B type which is rated as 2.5W is capable of dissipating 12.5W for
5 sec.

In terms of energy, this corresponds to 62.5 Joules.

The danger over here is that it might be thought that this device is capable of handling 62.5 Joules

of the pulse width.

At this point it must be remembered that it takes a finite time for the heat produced to be properly distributed
throughout the resistor body and therefore it is necessary to impose limits on the applied pulse energy so as to
prevent excessive stresses due to thermal shock damaging the component.

Now suppose the same 2.5W 4K0 5% resistor is subjected to 10000 volts for 1.5 milli sec., the energy is 37.5
Joules as per the formula given below :-

Now, 37.5 Joules is within the 62.5 Joules shown above but it must be remembered that it would take longer
than 1.5 Milli Sec. for the heat produced in the wire to flow into the surrounding materials and therefore the
temperature of the wire would rise far beyond what it was intended to handle and if the wire is subjected to
temperatures which are beyond its operating limits the resistance value of the resistor will change excessively,
the coating can be damaged and in extreme cases the resistance wire itself can melt which will render the
component useless.

Therefore since resistors can be subjected to different types of pulses, it is very necessary to take into
consideration the pulse width duration and the applied pulse voltage / power.

A general point to be noted is that the average power of the pulse applied should not exceed the rated power of
the resistor.

Also the term pulse implies a single pulse applied to a resistor, which is not already dissipating power and is in

an ambient temperature of 70°C or less.

HTR has collected data for 3 different pulse ratings for their most popular standard wire wound resistors which
should be referred to as given below for the safe operation of the resistors.

5 2.5

irrespective

PULSE HANDLING CAPABILITY DATA

Table - 1 is for very short pulses which are < 1 Milli sec. (1.2/50Micro Sec. As defined by
IEC 61000-4-5 & ANSI C62.41)
Table - 2 is for longer pulses from > 1Milli Sec. to 100Milli secretary.

Sr. No.

Type

Table–1

Table-2

H1B

600 V

525 V

H2B

1500 V

1000 V

3000 V

2000 V

7500 V

5000 V

H12

11250 V

7500 V

HTA SERIES

Sr. No.

Type

Table–1

Table-2

1200 V

750 V

1800 V

1200 V

H3A

2000 V

1300 V

3500 V

2250 V

4000 V

2500 V

H5A

5000 V

3250 V

6500 V

4250 V

H7A

8500 V

6250 V

10500 V

7000 V

10.

H10 / H10A

12000 V

9500 V

11.

H15

15000 V

12250 V

HIA SERIES

Sr. No.

Type

Table–1

Table-2

1200 V

750 V

1800 V

1200 V

2500 V

1750 V

4250 V

3000 V

6250 V

4000 V

7750 V

5250 V

F10

8500 V

5750 V

HFA SERIES

Sr. No.

Type

Table–1

Table-2

H3P

2000 V

1300 V

H5P

6500 V

4000 V

H7P

10500 V

7000 V

H10 P

12000 V

8500 V

HIP SERIES

Sr. No.

Type

Table–1

Table-2

F2P 0/1

1800 V

1250 V

F4P 0/1

3250 V

2000 V

F5P 0/1

4750 V

3250 V

F7P 0/1

6750 V

4500 V

F8P 0/1

8750 V

6000 V

HFP SERIES

Sr. No.

Type

Table–1

Table-2

SR8

10000 V

6750 V

SR11

13000 V

8750 V

HSR SERIES

Sr. No.

Type

Table–1

Table-2

C2A

1500 V

1000 V

2750 V

2000 V

C5B

3500 V

2500 V

4500 V

3000 V

C7B

4800 V

3250 V

C7A

5750 V

3750 V

C9/C10A

6500 V

4500 V

C11

9000 V

6000 V

C17

13250 V

8750 V

HEA SERIES

Sr. No.

Type

Table–1

Table-2

SV4 / SV4A / SV4AU

2750 V

2000 V

SV5 / SV5A / SV5AU

3500 V

2500 V

SV7 / SV7A / SV7AU

4800 V

3250 V

SV7B / SV7BA / SV7BAU

5750 V

3750 V

SV9 / SV9A / SV9AU

6500 V

4500 V

SV11 / SV11A / SV11AU

9000 V

6000 V

SV17 / SV17A / SV17AU

13250 V

8750 V

HSV / HSVA / HSVAU SERIES

Sr. No.

Type

Table–1

Table-2

CP3A

3000 V

2000 V

CP5 / CP5A / CP5Z

4250 V

3000 V

CP7 / CP7A / CP7Z

6250 V

4000 V

CP10 / CP10A / CP10Z

9000 V

6000 V

CP15 / CP15A / CP15B /

CP15C / CP15Z

10500 V

7000 V

CP20 / CP20A / CP20B /

CP20C / CP20Z

14500 V

9500 V

HCP SERIES