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| Injection transformer for bode plot - my DIY project |
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| nikifena:
Hi guys. I read a couple of topics here and over the net about how to make a DIY injection transformer that covers a lot of frequencies - usually, it is used to test power supplies for stability. As far as I know, the best is the B-WIT 100 from Omicron Lab. So I found a photo of the internals - Dave did a teardown too, and I started to search for a suitable core. The core is "special" - not a ferrite core like most inductors. As I think the best solution is a core from Nanocrystalline. It is similar to the traditional toroidal transformer winded with sheet of metal, but the material is very thin and fragile. I measured it and it is 0.001mm. A piece of it just fell out. So, I did some research and found this inductor from Wurth: 744839208072. Since I've been ordering a lot of Wurth, I asked for a free sample. I needed to remove the original wire, then to dremel some of the existing plastic material until I got a nice round ring core. I measured the inductance and with 41 turns (filling the entire core) I got about 250mH. The B-WIT 100 contains 40 turns. I used a twisted wire from an old lan cable. There is a screenshot of the bode plot I did with my scope and I believe the final result is pretty good for a transformer for free :) Just wondering why there is a peak at the high frequencies? The output of the transformer is not loaded. Can someone measure the inductance of the B-WIT 100 transformer? What do you think guys about the graphs? PS: I will do another capture but I will zoom the frequency response. Now it looks very flat and probably it isn't. |
| nctnico:
The peak at high frequency is self resonance due to inductance and capacitance between the windings. |
| TopQuark:
https://www.eevblog.com/forum/testgear/diy-transformer-for-use-with-bode-plots/ Some good discussions here. IMO the output should be 50ohm terminated for your measurements. A 1:1 transformer means 1:1 impedance ratio, so if you are using a 50 ohms signal source, the output of the transformer should be terminated with 50 ohms, same idea as how you should connect a 50 ohm source to a 50 ohm termination when using a direct coax cable. Recently played with various cores to see if I can eek out a bit of performance with the bifilar transformer arrangement. The consensus has always been nanocrystalline cores are critical for the lower end of the frequency response, but the higher end of the frequency is usually limited to 10MHz. In my experiment, I used a large-ish cheap Chinese nanocrystalline core, and placed a couple of small Chinese knock-off type-8 micrometals core in the center in an attempt to extend the usable frequency pass 10MHz. Results are not great, not terrible. I'd say the transformer is very usable from 10Hz to 10MHz, with the HF -3dB point around 13MHz. Don't think the micrometals core did much. Note that as you approach 10MHz, the measurement setup has substantial influence to the measurement quality. For scope bode plots at low frequencies, I used a 10x probe connected in parallel to the transformer input with a T-connector as my reference input, and the output is connected to the scope's 50-ohm input. Pass 1MHz, I think a network analyser is a better tool for the job as it maintains a consistent 50 ohm transmission throughout the measurement path. I used a spectrum analyser with TG as a simple scalar network analyser, calibrated with a thru normalisation cal. The measured HF -3dB point is at 13MHz with this setup. |
| nikifena:
Thanks! The low frequencies from the scope generator are distorted due to the low-resistance coil. Your design has even fewer turns. What about that? I will post more screenshots tomorrow. |
| Weston:
The Wurth 744839208072 is $49(!) in digikey in quantity one? It seems like the core is indeed nanocrystalline, but unless you have weird sourcing issues or a massive discount you would probably be better off buying the cores themselves, which are stocked on digikey. VACUUMSCHMELZE makes them. Some people on the forum were using this core, and I have built a few injection transformers using it, but it seems to be out of stock right now https://www.digikey.com/en/products/detail/vacuumschmelze/T60006L2040W424/12530400 This core is pretty similar, but most of them should work. They all have really high Al values https://www.digikey.com/en/products/detail/vacuumschmelze/T60006L2045V102/12531748 Typically you are measuring the injection signal on the secondary side, so the response does not need to be very flat, just not too attenuating. Do you have any bandwidth goals? Driving with a 50 ohm source the -3dB low frequency point in Hz is 50/(2*pi*Lpri). With that second core I linked you would only need 30 turns for a -3dB point of 100Hz. Also, just a heads up if you have not noticed, but with the bode plot functionality on the RTB scope you can not directly measure the low frequency -3dB point, as the reference signal is measured on the output of the function generator (after the internal 50 ohm output impedance), so you need to add an external 50 ohm resistor to emulate the driving impedance which determines the low frequency -3dB point. |
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