Testing chokes and transformers are easy. One just have to
connect them to an LCR meter, that it, right? RIGHT??? Well, it’s not entirely correct. It is only partially correct.
Take power supply choke for example, a certain DC + AC current
will pass through the choke and the inductance will change due. When the core is
saturated, the inductance will drop drastically and therefore the measurement
of the choke with respect to the DC current will be required. On top of that,
the operation frequency is also needed to be considered where some chokes will
be built to work for 50-60Hz, 100-120Hz, 440Hz or even more if it is for plate or
anode & grid loading.
This will ensure it meets the requirement when it is loaded
at the intended operating DC current. The design of the choke / transformer
must ensure that there are enough flux density margins to avoid saturation the
intended DC + AC being applied. If it saturates, it is as good as not having an
inductor on the circuit and affect the results – high power supply ripple /
noise and etc.
Measured inductance is only correct under intended DC load
conditions.
So, it is crucial to test the chokes and transformer at realistic
or intended DC load conditions. Below is a very typical setup for measuring inductor.
If the LCR meter / LCR analyzer is capable of biasing the measured component,
the you can measure the inductor at the desire DC bias.
If your LCR meter is incapable of providing DC bias, then
another method is required – providing external DC bias current to complement
the LCR meter, as follow:
An external DC power supply – connecting it to the current
output, will provide the desired DC bias to the choke / transformer for
accurate inductance measurement. There is a problem with this setup. Conventional
power supply will usually have large output capacitance and this will affect
the inductive impedance of the device under test – choke and cause measurement
error.
To overcome this measurement issue, one can add a relatively
large (compared to DUT) inductor in series with the DC power supply to isolate
the DUT from the DC power supply. The series inductor will have to be very
large in order provide better accuracy to the measured value.
Another way of doing it would be to use a DC power supply
with a constant current output stage that has high impedance to isolate the DC supply
from the DUT. This enables the DUT to be tested easily. There are some readily
available DC supplies in the market but they are not cheap.
One suggestion that we got from our readers is that one can build
the DC supply themselves – using the 3-pin regulator IC - LM317 and the likes
in constant current mode. Voltage mode will not work as there will be
capacitors at the LM317 output.
We’ve NOT tried so since Hameg 8118 has the DC bias function
for up to 200mA of DC current and it is NOT proven to work. Some other LCR
analyzer has external DC bias option where one can connect an external DC bias
directly to the LCR analyzer for such functions.
If you’ve done so, please let us know if it works. Our
readers would be interested to see if this suggestion works.
Note: External current bias protection circuit might be required, depending on the LCR analyzer used.
J&K Audio Design
28/12/2013
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