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Agilent
Effective Transformer/
LF Coil Testing
Application Note 1305-3
Introduction Transformers and LF coils are used in power supplies, digital networks (for
example, ADSL) and various communication instruments to step up (or down)
an AC voltage or for impedance conversion or filtering purposes. Though
production of transformers is increasing year after year, there are problems that
QA test efficiency and production test throughput cannot be easily improved
because several different measurement instruments and setups need to be used
for testing various transformer parameters. This application note introduces the
cost-effective solutions to the transformer parameter measurements by using
the Agilent 4263B LCR Meter.
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Current Problems on The primary parameters that need to be known for transformer/LF coils are
Transformer/LF Coil self-inductance, dc resistance, turns ratio and inter-winding capacitance.
Conventional low-cost LCR meters have the following shortcomings when used
Evaluation for transformer/LF coil evaluation.
1. DC resistance of primary and secondary windings cannot be measured with
LCR meters. (The DC resistance measurement requires using a separate test
instrument such as a multimeter.)
2. The turns ratio, a key transformer parameter, cannot be measured with LCR
meters.
3. The transformer parameters cannot be measured at 100 kHz because many
low-cost LCR meters do not cover high frequencies up to 100 kHz.
4. The test signal level is automatically selected according to the measurement
range, the test signal level cannot be user-defined for a specified level.
5. Total test throughput on production lines cannot be maximized because of
slow measurement speed.
6. The connections of a transformer to the instrument (test fixture) must be
changed to measure parameters for the primary and secondary windings.
The required connection changes make it difficult to enhance the
measurement efficiency.
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Agilent 4263B LCR 1. DC resistance measurement capability
Meter Solution The Agilent 4263B with option 001 (N/M/DCR measurement capability) can
sequentially measure self-inductance and dc resistance (Rdc) by alternately
applying AC signal and DC signal to the DUT, as shown in Figure 1. DC resis-
tance measurement function covers a wide range from 1 m to 100 M
and enables a low resistance of transformer winding to be measured with a
0.01 m minimum resolution. The N/M/DCR measurement function dispenses
with a separate multimeter and additional measurement step, thus improving
test efficiency.
2. Turns ratio and mutual inductance measurement capabilities
The Agilent 4263B with option 001 also adds turns ratio and mutual inductance
measurement capabilities. A turns ratio (N) measurement value of 0.9000 to
200.00 can be obtained from a voltage ratio measurement for the primary and
secondary. In the N measurement mode, an internal voltmeter and a selector
switch (a unique function of the 4263B) are configured as shown in Figure
2. Mutual inductance (M) of 1