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58
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 consideration since they cause an
instantaneous temperature rise in the resistor film. This application note
can guide you through these considerations.
For applications with transients, pulses
or surges the following must be consid-
ered:
1. Do not exceed the normal rated oper-
ating voltage of the device.
2. Using the figure at right, estimate the
energy (E
a) and the pulse duration
(t
a) for a single pulse in your applica-
tion.
3. Calculate the energy ratio in percent
(E
r) between the nominal energy rat-
ing of the model you have chosen
(see table) and the single pulse
energy in your application (E
a from
step 2) using the formula:
E
a
Er = –––––––– x 100
Enominal
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.
V
t
V
t
V
t
V
t
E = –––
V
2
t
R
E = –––
V
2
t
3R
E = –––
V
2
t
3R
E= Energy (joules)
t=Time (seconds)
V= Voltage (volts)
R= Resistance (ohms)
C= Capacitance (farads)
E = –CV
2
1
2
t = RC
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 volt-
age of 3.5kV is acceptable.
1
E
a = –– CV
2
= 6.1J
2
ta = R C = 1 ms
6.1J
Er = ––– 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 chosen,
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
Ohm Value
Example:
100R = 100Ω
2K40 = 2400Ω
Tolerance
F = 1%
G = 2%
J = 5%
K = 10%
Series
TFSA100KJE
Energy Rating
joules
A= 6
B= 9
C= 11
RoHS Compliant
D= 33
E= 44
F=55
Standard Part Numbers for TFS Series
6 Joules 9 Joules 11 Joules 33 Joules 44 Joules 55 Joules
Ohms Tol. 0.5 Watts 0.5 Watts 0.75 Watts 1 Watts 1.5 Watts 2 Watts
100 1% TFSA100RFE
100 5% TFSB100RJE TFSD100RJE TFSF100RJE
220 1% TFSA220RFE
270 5% TFSA270RJE TFSC270RJE TFSD270RJE TFSF270RJE
470 1% TFSA470RFE
470 5% TFSB470RJE TFSE470RJE
680 5% TFSA680RJE TFSC680RJE TFSE680RJE TFSF680RJE
750 5% TFSB750RJE TFSD750RJE TFSF750RJE
1,000 1% TFSA1K00FE
1,000 5% TFSA1K00JE TFSB1K00JE TFSC1K00JE TFSD1K00JE TFSE1K00JE TFSF1K00JE
1,500 5% TFSA1K50JE TFSC1K50JE TFSD1K50JE TFSF1K50JE
2,200 1% TFSA2K20FE
2,700 5% TFSB2K70JE TFSE2K70JE
4,700 1% TFSA4K70FE
4,700 5% TFSA4K70JE TFSC4K70JE TFSD4K70JE
4,990 1% TFSA4K99FE
5,000 5% TFSA75K0JE
6,800 5% TFSB6K80JE TFSE6K80JE
10,000 1% TFSA10K0FE
10,000 5% TFSA10K0JE TFSB10K0JE TFSC10K0JE TFSD10K0JE TFSF10K0JE
16,000 5% TFSF16K0JE
20,000 1% TFSA20K0FE TFSF20K0JE
20,000 5% TFSB20K0JE TFSD20K0JE
22,000 1% TFSA22K0FE
27,000 5% TFSA27K0JE TFSC27K0JE TFSE27K0JE
47,000 1% TFSA47K0FE
50,000 5% TFSA50K0JE TFSF51K0JE
51,000 5% TFSB51K0JE TFSC51K0JE TFSD51K0JE
75,000 5% TFSE75K0JE
100,000 1% TFSA100KFE
100,000 5% TFSB100KJE TFSC100KJE TFSD100KJE TFSF100KJE
TFS Series
Surge Capable
Thick Film Non Inductive
SINGLE PULSE ENERGY RATING
ORDERING INFORMATION
Maximum Individual Pulse Rating
Energy Rating %
Pulse Width
1µs 10µs 100µs 1 ms 10 ms 100 ms
0.1
1000
1
10
100
