INDUCTOR FAQ’s
FAQ – “What impact does frequency have on the measured inductance value?”
FAQ “Why don’t the inductance values measured on our LCR equipment match your readings?”
FAQ “Are the current rating of RCD’s inductors AC or DC?”
FAQ “How do I specify an Inductor if I don’t know RCD p/n?”
FAQ – “What impact does frequency have on the measured inductance value?”
Inductance increases as the frequency approaches the SRF (self-resonant frequency) level. Therefore
the inductance level measured at low frequency is typically lower than that at higher frequencies. Along
the same lines, the measured reading on most commercial-grade LCR test equipment is lower than on
RCD’s military-grade test equipment since parts are generally tested at lower frequencies when using
LCR meters. The difference is typically 2% - 15% but can be greater. Therefore it is recommended that
customers correlate sample readings with RCD or specify the required test frequency.
Another reason for correlation is due to the fact that there can be a fairly wide variation in the values
measured between different set-ups resulting from variations in lead wire length and shape, as well as
the intrinsic accuracy, capacitance & inductance of the test equipment and fixtures. Low values, such as
1uH or below are especially sensitive to the parasitics in test fixturing as well as the otherwise minor
differences in inductance attributable to lead length, etc.
FAQ “Why don’t the inductance values measured on our LCR equipment match your readings?”
When testing inductors, there can be a fairly wide variation in the values measured between different setups, primarily due to test frequency, lead length, leadwire shape, test fixture capacitance & inductance,
and test instrument intrinsic accuracy and self-inductance. Low values, such as 1uH or below are
especially sensitive to the parasitics in test fixturing as well as the otherwise minor differences in
inductance attributable to lead length, etc.
More commonly, the differences are attributable to the test frequency. Most of RCD’s inductors are
measured at Mil-specified test frequencies (refer to data sheets). These frequncies (7.9MHz, 2.5MHz,
0.79MHz, 0.25MHz, etc) generally do not match the pre-set frequencies on most LCR meters (1MHz,
100KHz, 1KHz, etc.). Since inductance increases as the frequency approaches the SRF (self-resonant
frequency) level, the measured reading on most LCR test equipment is lower than on RCD’s test
equipment since parts are generally tested at lower frequencies when using LCR meters. The difference
is typically 2% - 15% but can be greater. Therefore it is recommended that customers “correlate” sample
readings with RCD or specify the required test frequency.
FAQ “Are the current rating of RCD’s inductors AC or DC?”
Inductor amperage ratings are based on DC current. The AC current ratings are much lower, especially at
higher frequencies. Designing AC inductors involves more factors. For instance, instead of dealing with
resistance, AC inductor designs focus on impedance because the impedance is dominant over the DC
resistance and varies with frequency. Material losses must also be accounted for. Another issue is the
skin effect. When RF voltage is used the alternating current causes the current to migrate to the surface
of the copper conductor. As a result the center (core) of the conductor is barely utilized for carrying
current. The best method to minimize the uneven heating associated with the skin effect is to utilize
stranded instead of solid wire. For the same reason, high-frequency AC inductors are typically much
larger in size than DC counterparts. RCD’s engineering group would need to know the voltage, current,
frequency, and size constraints in order to determine the most appropriate design.
FAQ “How do I specify an Inductor if I don’t know RCD p/n?”
1. Advise competitor p/n or if unavailable advise whether it is a conventional through hole part (axial or
radial?) or surface mount
2. Advise construction (wirewound or film, etc.). If unknown, go to #3.
3. Advise inductance value (.01uh or micro henries, 10nH or nano-henries, etc.).
4. Advise inductance tolerance (10%,5%, etc.).
5. Advise current rating (.2A, 200mA, etc.).
6. Advise of any special electrical requirements (max DCR, saturation current, etc.).
7. Advise of any special lead-forming (.4" radial, etc.).
8. Advise of any special packaging requirements (bulk, tape/reel, etc.).