Continue to Site

Welcome to

Welcome to our site! is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Two current sense transformers to sense "unknown" SMPS current?

Not open for further replies.


Advanced Member level 5
Jun 13, 2021
Reaction score
Trophy points
Activity points
Customer has sent us 150W , 37vout offline SMPS (100-265VAC). They are told it is very special. They gave it to us to check it.

We have no BOM or schem and no gerbers or layout. We only have the PCB.(i think 4 layer). It uses a PQ26/25 core.

There is no output inductor. It kind of looks like a One transistor forward with high voltage reset. One pri and one sec coil. Sec coil is capacitively coupled to the output rectifiers. Neither end of the sec coil has a direct connection to output ground. (the mentioned series capacitors are “In the way”)

We want to scope the power currents. However, we don’t know if the current , in whichever branch, is unidirectional, balanced bidirectional, or unbalanced bidirectional. As such, do you agree, we will get best depiction of the power currents by using two current sense transformers as in the following attached? (LTspice and jpeg)

In the attached , the current is unbalanced bidirectional… using the two CSTs as shown , we are able to piece together what the actual current looks like. Would you agree? This is the cheapest way?..we cant afford a pricey high BW current clamp.


  • Two tran forward.jpg
    Two tran forward.jpg
    139.6 KB · Views: 86
  • Two tran
    3 KB · Views: 82
Last edited:

Now added LTspice and schem in jpeg as attached.
Best to add the series cap..then no DC average component can flow in the secondary, and the CST can thus never saturate


  • Two tran forward with CST.jpg
    Two tran forward with CST.jpg
    136.8 KB · Views: 79
  • Two tran
    3 KB · Views: 77

The thread title is referring to current measurement, related to the actual question the circuit reduces to


Now you are suggesting this modification


As given, I'd comment
1. Second CT seems useless
2. No means for flux reset, CT saturation could be only avoided if either duty cycle is small or CT core is designed for full DC flux.
3. As far as I see, things don't get better with capacitive coupling.
Thanks, but your schem above is not showing the pulse currents at the f(sw) of the just have flat DC current in the primarys.......sorry but that isnt what the question was about. We appreciate DC current cant be measured with a currnt transformer
--- Updated --- fact the attached should also be workable....3 coil CST.
2 unidirectional CSTs on the same core.
--- Updated ---

As you know, the "garanteed success" way of scoping the power current in any smps, is to simply use a 100:1 CST, with a simple 10R resistor across its sec coil....and then put the pri in series with the output and then the input capacitor capacitor bank.....then you have a truly bidirectional current....and from the scope shot, you can glean what you will about the actual power switching currents elsewhere in the SMPS.........its balanced current in the you never saturate...even with just a 10R burden across the sec coil.


  • 3COIL CST.jpg
    3COIL CST.jpg
    135.3 KB · Views: 69
  • Two tran
    3.1 KB · Views: 79
Last edited:

The attched worked fine.....100:1 current sense transformer.
10R Burden.
No secondary diodes or anything...just the 10R burden.
(i admit the primary side is a littel different than shown....(i cant show what is actually there malheauresement)


  • one tran fwd_cap rect.jpg
    one tran fwd_cap rect.jpg
    123.5 KB · Views: 71
  • One tran fwd _cap
    2.2 KB · Views: 79
  • 240vac 150w 100t1 10rburd vacin and isecwht _1.jpg
    240vac 150w 100t1 10rburd vacin and isecwht _1.jpg
    198.9 KB · Views: 86

Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to