FEATURES
• Ideal for medical surge protec-
tion applications
• Ideal to replace standard carbon
composition resistors
• Custom dimensions, values,
tolerances and characteristics
available
SPECIFICATIONS
Material
Resistive Element:
Thick Film
Encapsulation:
Screen Printed
Glass
Electrical
Resistance Value:
100Ω up to
100KΩ
Temperature Coefficient:
100ppm/°C
Tolerance:
1%, 2%, 5%, 10%
Operating Temperature:
-55°C to
+200°C
Test:
VDE 0750 (Pulse Duration
10 msec)
TFS Series
Surge Capable Resistors
Thick Film Non Inductive
C
A
B
0.50mm
H
E
0.25mm
U
(KV)
3
3.5
4
7
7
11
Energy*
(J)
6
9
11
33
44
55
Power
(W)
0.5
0.5
0.75
1
1.5
2
Dimensions (mm)
B
C
H
5.5
10
0.7
5.5
10
0.7
5.5
10
0.7
8
10
0.9
10.5
10
0.9
10.5
10
0.9
SINGLE PULSE ENERGY RATING
Although Ohmite’s TFS Series resistors have been specially
designed and developed to absorb much more energy than
standard resistors, pulses and transients require special consid-
eration since they cause an instanta-
neous temperature rise in the resistor
V
2
t
film. This application note can guide
V E = –––
R
you through these considerations.
t
For applications with transients,
pulses or surges the following must
E=1
2
–CV
be considered:
V
2
1. Do not exceed the normal rated
t = RC
operating voltage of the device.
t
2. Using the figure at right, estimate
the energy (E
a
) and the pulse dura-
V
2
t
V E = –––
tion (t
a
) for a single pulse in your
3R
application.
t
3. Calculate the energy ratio in per-
cent (E
r
) between the nominal
energy rating of the model you
V
2
t
V E = –––
3R
have chosen (see table) and the
single pulse energy in your appli-
t
cation (E
a
from step 2) using the
(joules)
E = Energy
formula:
t = Time
(seconds)
E
a
V = Voltage
(volts)
E
r
= –––––––– x 100
R = Resistance (ohms)
E
nominal
4. Refer to the Pulse Chart. On this
chart find the point where the energy ratio (E
r
), found at step
3, and time (t
a
) coincide. Qualify that this point falls below the
maximum pulse energy curve. If the point is above the curve a
bigger model should be chosen.
C = Capacitance (farads)
Type
TFSA
TFSB
TFSC
TFSD
TFSE
TFSF
A
9
11
13
21
21
26
E
1.1
1.1
1.1
1.3
1.3
1.3
*Published energy rating is for 10ms pulse. For shorter pulses energy rating has to be
derated according to Max. Individual Pulse Rating chart (below) and Single Pulse Energy
Rating considerations (left).
O R D E R I N G I N F O R M AT I O N
RoHS Compliant
½½½½½½½½½½
Series
Ohm Value
Type
See chart
Example:
100R = 100½
2K40 = 2400½
Tolerance
F = 1%
G = 2%
J = 5%
K = 10%
MAXIMUM INDIVIDUAL PULSE RATING
1000
Energy Rating %
100
10
1
0.1
1
µs
EXAMPLE
A 1µF capacitor is charged
to 3.5kV and model TFSC,
1KOhm has been selected.
Model TFSC is rated for 4kV,
so the peak voltage of 3.5kV is
acceptable.
1
E
a
= –– CV
2
= 6.1J
2
t
a
= R C = 1 ms
6.1J
E
r
= ––– x 100 = 55%
11J
According to the pulse chart,
an energy ratio of 55% for a
pulse width of 1ms falls well
above the energy curve. The
limit is actually located around
25-30%. Model TFSC cannot
be used for this application.
A bigger model should be cho-
sen, for example TFSD. Model
TFSD, 1KOhm, can be used
for this application because
we have an energy ratio E
r
of 18%, which is below the
energy curve.
6.1J
E
r
= ––– x 100 = 18%
33J
10
µs
100
µs
1 ms
Pulse Width
10 ms
100 ms
NOTES
• For pulses over 100ms, refer to the momentary overload
specification on the product data sheet. Always verify designs
with pulse and surge conditions through thorough testing of
the design at maximum operating temperature and maximum
pulse loading (or some margin above maximum pulse load-
ing).
• Damage to the resistor by excessive pulse loading is generally
indicated by an increasing resistance of the resistor.
• Energy ratings are based on single pulses (at least 1 minute
between pulses).
• For multiple pulse applications the energy pulse rating should
be reduced and the average power should not exceed the
nominal power rating of the selected model.
Ohmite Mfg. Co.
1600 Golf Rd., Suite 850, Rolling Meadows, IL 60008 • Tel. 1-866-9-OHMITE • Fax 1-847-574-7522 • www.ohmite.com • info@ohmite.com
51
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