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LOPT/FBT Tester for SMPS transformers

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Junior Member level 1
Apr 4, 2005
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Hi All
Have Any body Made this Tester ( Not from Kit , But made it all from scratch )
**broken link removed**
Here is the schematic
**broken link removed**
Is this tester Good ?
It said in the PDF that there is a test coil witch come with the kit , Any body know how to make one and did any on test it (The test Coil) with Inductometer



Can i use this tester to test SMPS transformers ?


It should work if your SMPS transformer's operating frequency is around 16-17kHz. For higher frequencies the width of the pulse applied to excite the transformer should be short compared to the natural oscillation period of the transformer under test.

Note that you can do the same test with a function/pulse generator and a scope. I have been using this technique for many years. You should know something about the transformer, like the operating frequency. Then apply a short pulse ≈5-10Vp-p through a low value resistor ≈100Ω to the primary winding while looking at the ringing on the scope (connected on winding side of resistor). If you see just a short spike with very little ringing, you may have shorted windings somewhere. If you see healthy ringing with a nice exponential decay over a couple of cycles, chances are you are OK.

Another test I do sometimes with the suspect transformer out of circuit: Apply a sine wave of fixed amplitude through a resistor from the function gen. Start well below the operating frequency (at least one decade lower), and slowly turn/sweep the frequency up while looking at the scope connected on coil side. A healthy transformer will tend to peak near the correct frequency. A faulty transformer with shorted turns will attenuate the signal over the sweep range.
lopt/fbt tester

Hi E-design
Thanks for your fast reply, it is very useful but i don't have an oscilloscope so i can't do the tests you metioned befor
i know that this tester is for testing FBT witch operates at 15625Hz
but are not there any modifications to make it test SMPS that operates up to 100KHz ?


fbt testr schematic diagram

If you make R5 variable (smaller ) you will be able to get shorter pulses. R4 should be OK as is. They already use a differentiator to provide a short spike, so I don't think the setting will be critical as the period is long at around 100ms. You may need to switch in lower values of C3 to achieve resonance at higher frequencies.

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