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Friday, December 27, 2013

Testing Chokes



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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* Finished amplifiers, DACs, audio gadgets, upgrades and repairs - this is not our core business and we do it out of passion. We do not have fixed models, fixed price and we customize for each individuals. The sky is limit of creativity.

* Our product lines are always improving and increasing. If you do not see what you want, contact us!

* Please email for volume discounts, distributor and OEM pricing.

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