Another DIY signal injection transformer

[georgeharker]

I also built a DIY signal injection transformer. Used the instructions
https://www.eevblog.com/forum/projects/diy-injection-transformer-for-power-supply-control-loop-response-measurements/ and https://electronicprojectsforfun.wordpress.com/injection-transformers/ and have a write up here:

https://georgeharker.com/2024/05/10/building-a-diy-signal-injection-transformer/

Final product
Test setup
Results

I managed to get at least -3dB at 28.6Hz rising to flat at approx 110kHz and remaining flat to just under 1MHz before rolling off smoothly to -3dB at 5.6MHz.

Pretty happy with the results for a 1 day side project / money saving over commercial gear.

[mawyatt]

CAUTION!!!

Unless I'm mistaken it appears you have the Primary and Secondary Grounds common thru the Aluminum Case BNC Grounds.

Usually you want those galvanically isolated.

Best,

[georgeharker]

So best to replace one connector (test device side) with an isolated bnc connector ?

[mawyatt]

Like you we wanted to save a few $ so went the DIY route and use some 3D printed boxes for the enclosure which are plastic and galvanically isolated for the BNC connectors.

What cores did you select?

TopQuark had some really good cores that exhibit outstanding performance.

https://www.eevblog.com/forum/testgear/diy-transformer-for-use-with-bode-plots/msg4212625/#msg4212625

We need to get some of these cores!!!

Best

[mawyatt]

So best to replace one connector (test device side) with an isolated bnc connector ?

Yes believe that should work out OK. Use the generator side common as the case ground and let the DUT side float wrt to the case and generator ground. An isolated BNC is probably the best solution for the DUT side.

Noted you have a nice Siglent DSO and AWG, you'll find some really interesting use for the Bode Function, and with the Iso Transformer this opens up all sorts of uses, see some of our posts wrt such!!

Keep us posted on your progress

Best,

[georgeharker]

Great. Thanks for the advice, made that modification.

[Grandchuck]

Oscilloscopes that offer Bode plotting make an injection transformer a desirable accessory.  This thread and others inspired me to try to make one.  I started with some Red cores, 27mm x 11.4mm x 6.3mm.  The BW results were encouraging, but the low frequency performance was lacking. Internet searching turned up: http://simprojects.nl/images/DIY_signal_injection_transformer.pdf

Roland Van Roy describes a DIY transformer based on a Green core choke: Coilcraft CMT4-10-15.  At $16.78 plus shipping it is a tad pricey and it is hard to remove the epoxy, but it is cheap compared to commercial transformers.  The wire is from CAT5 Ethernet cable.  It is worth the effort as the results are as Roland reported.

[Grandchuck]

I made a mounting plate with BNCs and also tried a smaller shunt resistance (two resistors in parallel for equiv. of about 10 ohms).  The original load resistor was 51 ohms. The bandwidth is now less but still OK for some use cases.

[georgeharker]

Neat! As per comment (which I had had also initially missed) you probably want a galvanic isolation on the DUT side, so you might want to switch one of the bnc connectors for an isolated one.