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| DIY Transformer for use with Bode Plots. |
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| joeqsmith:
--- Quote from: mawyatt on May 29, 2022, 02:40:20 pm ---... Of course the intrinsic transformer doesn't have a basic 10Hz 3dB corner, just look at the primary inductance measurements of ~8mH which implies an impedance of 1/2 ohm at 10Hz!!! This is not to say the core transformer low end isn't important for Close Loop Injection use, the transformer must inject a signal into the DUT and this injection level falls off at the high and low frequency end due to transformer and source characteristics, as well as other effects. ... In the Closed Loop Bode measurement the loop gain of the DUT comes into play, and usually the loop gain has a general low pass type charteristic. ... Please spend some time studying the mentioned video (and other related papers on Bode Plots and Close Loop Measurement Techniques), this is an excellent resource for getting an understanding of Closed Loop Bode measurements, and even illustrates how this method can be utilized to measure the very complex non-linear nature of SMPS even tho Bode is a linear type function. ... --- End quote --- Your point about the impedance at such low frequency is certainly valid. In the video you link, he tests his transformer manually with a 10 ohm load, 50 ohm source and from 300kHz to 4Hz. I suspect he has higher inductance. He drops the drive to 100mVp to avoid saturation at 4Hz then seems he stays with that drive level. While the video was talking about SMPS and it sounds like that is also your area of use. I don't see why it couldn't be used for testing other control systems stability. If I attempt to look at the last core I made using his technique, probes set to 1X, 200mVp-p drive. Shown is the difference from 10kHz to 4Hz. I understand your comment about the DUTs loop response. |
| joeqsmith:
--- Quote from: tautech on May 29, 2022, 10:47:50 pm ---Well really Joe why did you even dig out that old storm in a teacup ? :-// --- End quote --- Because OP asked for the data. |
| joeqsmith:
If I run a similar test with the other transformer (much higher inductance) we can see that the lower frequency coupling is much better. Phase isn't great. Again, what we expect. For testing SMPSs, again I agree that it may not be useful to have such a transformer. Should mention this was taken with 10X probes rather than 1X. Same drive levels. |
| mawyatt:
--- Quote from: joeqsmith on May 29, 2022, 11:08:09 pm --- Your point about the impedance at such low frequency is certainly valid. In the video you link, he tests his transformer manually with a 10 ohm load, 50 ohm source and from 300kHz to 4Hz. I suspect he has higher inductance. He drops the drive to 100mVp to avoid saturation at 4Hz then seems he stays with that drive level. While the video was talking about SMPS and it sounds like that is also your area of use. I don't see why it couldn't be used for testing other control systems stability. If I attempt to look at the last core I made using his technique, probes set to 1X, 200mVp-p drive. Shown is the difference from 10kHz to 4Hz. I understand your comment about the DUTs loop response. --- End quote --- We just used the magnet type wire that came with the CM Filter and twisted the 2 wires together and wound this back onto the core. If we had more wire then we could have increased the primary inductance with more turns, but didn't, so used what we had. Need to find some old Cat 5 cable to strip!! In the video, he uses more wire and turns, thus greater primary inductance. This is why we were asking about the various core types, especially the large core shown by TopQuark but have no idea how to order from Taobao!! Wanted to use a larger core with more windings to create a larger primary inductance. Your core looks to have more windings and should yield a higher primary inductance than the ones we made. Actually our use is more inline with op-amp type circuits as well as SMPS, both are difficult but the op-amp types usually have lots of loop gain which creates problems with accurately sensing small signals and the reason for the low frequency somewhat randomness in the plots, and keeping the signal low so the core doesn't saturate is important for good results. This is the type of area where synchronous sampling pays big dividends, but that's another complex topic best served in a separate thread. Edit: This method is certainly valid for testing all sorts of feedback control systems, from op-amps, SMPS circuits, optically coupled systems, electromechanical systems and so on. Very powerful technique and why we were amazed this is essentially a scope built-in feature, and with the excellent front end with low noise, high dynamic range, and accurate scaling factors, can cover quite a range of uses. By far the best value electronic instrument we have :-+ Anyway, hope this helps. Best, |
| joeqsmith:
--- Quote from: mawyatt on May 29, 2022, 11:43:08 pm ---In the video, he uses more wire and turns, thus greater primary inductance. This is why we were asking about the various core types, especially the large core shown by TopQuark but have no idea how to order from Taobao!! Wanted to use a larger core with more windings to create a larger primary inductance. Your core looks to have more windings and should yield a higher primary inductance than the ones we made. --- End quote --- He talked about 7 meters of wire in the video you linked. He backs that up with his comment about his DC resistance. Yours appeared closer to 25 turns so yes, I wasn't too surprised. Again, I started out with 30 turns on tape as it seemed it had not been considered. --- Quote ---Primary inductance was 109.4mH Leakage inductance was 4.1uH Coupling capacitance was 119.8pF Resistance 4-wire was 0.589 ohms --- End quote --- Maybe "tape" was glossed over or the assumption was my inductance was off by a decimal point (typo). Anyway, just something to keep in mind. https://www.magneticmetals.com/products-materials/tape-wound-toroidal-cores Also, for the wire I am using Teflon insulated single strand silver coated. I used the hand drill to for the tight twisted pairs. Just an FYI. Good luck with your project and thanks for the all the additional information. |
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