I'm trying to make a circuit to look like this one: Power supply from Aplication Note

This is from ATM90E26 Application note, this IC needs to be connected to mains in order to measure voltage (220VAC) and current. I tried to look for information about the transformer "TD35-18A-220V"(turns ratio, inductance, etc) but with no luck, all I found was a link to buy bulks of 1000 pieces and no specifications.

So I started to look for a similar one and found this one from wurth elektronik: 750342104

transformer symbol transformer table

According to datasheet for 78L33 maximum input voltage is 18V, so the position of terminal 2 of the transformer (I mean the ratio of (1-2):(2-4)) should be specific to get that output level from terminals 2 and 4, as in the circuit on the first image (terminals 3-1).

From isolated side (terminals 6-10) should be power for digital control electronics, microcontroller and the rest of the circuit (250mA top current). And an isolator (SI8651BB-B-IS1R )to connect the microcontroller to the non-isolated from mains ATM90E26 IC.

My question is if I'm looking at this the correct way?

Is the turns ratio for (1-2):(2-4) a percentage style ratio?, meaning if 220V is the voltage between 1-4, then, if I looking to obtain 15v between 2-4, terminal 2 will be at the 6.81% of the turns between 1-4?

Does this means that the transformer needs to be specific and not "off the shelf"?

Thank you!

  • \$\begingroup\$ No, it's a SMPS transformer, not a mains 50/60Hz. Ask ATMEL for the specifications, if they are serious company they should tell you. \$\endgroup\$ Mar 26, 2021 at 22:06
  • \$\begingroup\$ Anyway, it seems weird at first glance, like using L as ground and then depicted turns ratio 1-3 VS. 3-5 is huge that goes to supply 3.3V LDO. I mean it's like powering the 3.3V LDO with 300V and connecting phase line (L) to frame ground?? You'd better go away from this design if you don't want to hurt yourself, IMO this is not the correct schematics. \$\endgroup\$ Mar 26, 2021 at 22:14
  • \$\begingroup\$ Hello @MarkoBuršič , the things is that in order for the metering IC to measure voltage it needs to be connected to L and N. Through a connection GND-L and a voltage divider circuit for N (880KΩ-1KΩ). \$\endgroup\$ Mar 27, 2021 at 13:54
  • \$\begingroup\$ Responding to your first comment @MarkoBuršič, if it is a SMPS (switch mode power supply) then I should add a switching IC right? Or should I look for another transformer? \$\endgroup\$ Mar 27, 2021 at 14:08
  • \$\begingroup\$ I understand that, but it could be made more safe. For example using an isolated DC/DC small power converter from MCU side 5V/5V and LDO 3.3V, then using digital signal isolator IC for communication between Atmel IC and MCU, you can connect and swap L or N to the Atmel side. Designing a SMPS would be even tougher than this metering app. so I don't advise you. \$\endgroup\$ Mar 27, 2021 at 18:58

3 Answers 3


So, you need about 8..12 V DC on the input of 7833 (to have some room for line voltage variation).

When rectified, AC voltage is multiplied by a factor of 1.4. A single rectification diode will give about 1 V voltage drop. So, the required AC voltage would be (8…12)/1.4+1 (V) = 7…10 (V), not 15 V AC.

Since line isolation is not required (at this point), AC voltage may be obtained both by autotransformer and by regular transformer, The turns ratio have to be 220/(7…10). Note, that in case of autotransformer, primary turns would be w1+w2.

It might be easier to find appropriate separate transformer to power ATM90E26 alone than the combined autotransformer shown in the appnote.

enter image description here

And, for safety reasons, I have to emphasize, that secondary (MCU) part must be fully isolated from the line.



simulate this circuit – Schematic created using CircuitLab

Instead looking for the transformer you can use components already pre-made, like isolated RS485 transceiver module, MCU module, lastly your custom metering module with isolated DC/DC converter and signal isolator IC, like ADUMxxxx

ISO RS485 examples:

enter image description here

enter image description here

enter image description here

The black bricks on photos are isolated DC/DC converters, while the ICs are digital signals isolators TX,RX,RTS.

The MCU can be any kind of it, for example Arduino, Raspberry PI, ...etc. The SMPS you can buy in any electronic store, like Meanwell, Recom, TDK Lambda, Traco, ...

Meanwell EPS15-05:

enter image description here

Then you have to choose a small DC/DC converter 5V/3.3V let's say 2W, example Murata MEJ2S0503SC (instead of custom AC transformer):

enter image description here

Then you need a digital isolator like ADUMxxx or Sixxx for SPI or UART commuinication between your custom board and MCU.

Similar metering application (3 phase) SmarPi.

enter image description here

It has an input SMPS, which supplies RPi or Arduino. Through a digital signal isolator and DC/DC converter (all in one) it supplies and communicates with metering IC.

  • \$\begingroup\$ I think the OP specifically needs some line-referenced circuitry to monitor AC voltage and current, and will then (let us hope) isolate this from the remainder of the low-voltage circuitry. \$\endgroup\$
    – Frog
    Mar 27, 2021 at 20:17
  • \$\begingroup\$ Hello @Marko Buršič , I think i understand what you propose. Metering IC needs to be referenced to 220VAC from L, but this will be no problem for DC/DC isolated converter? \$\endgroup\$ Mar 27, 2021 at 21:13
  • 1
    \$\begingroup\$ @IvánGerber Correct. The app. note lacks of SPI or UART isolator, which is a must if you have to connect to the board. You could even put a small transformer 230/6VAC and then reference the frame ground to whatever potential: L or N, but the best way is to go with a custom board as described. \$\endgroup\$ Mar 28, 2021 at 7:45
  • \$\begingroup\$ @MarkoBuršič thank you for answering, I will do like you say, using a SMPS and an isolator IC with dc/dc converter to power and comunicate with the non isolated circuit wich will be referenced to L by tying it to it's GND. \$\endgroup\$ Mar 28, 2021 at 14:51
  • 1
    \$\begingroup\$ @IvánGerber You can see it in the app. note that L which connects on shunt is tied to circuit ground, so if you connect any isolated supply to Vdd and GND would get L voltage reference on GND, but the dc/dc converter has to have a breakdown voltage larger than line voltage, let's say >=1.5kV or 500VRMS. \$\endgroup\$ Mar 28, 2021 at 20:24

That's an 80 kHz transformer so it must be for a switched-mode power supply and typicall will be fed by a high-voltage DC and controlled by a high frequency switching transistor. It won't work at 50/60 Hz.

Try to avoid any connection between mains and your measurement circuit. Full isolation is the way to go. It will make the device much safer and allow debugging, plugging in USB programmer, grounded oscilloscope leads, etc.

You might find some useful design ideas at the Open Energy Monitor project. They should be good on safety.

  • \$\begingroup\$ Hello @Transistor , so what you are saying is that I need to add 80kHz switching IC like in this example ? Note: metering IC needs to be connected to N and L in order to measure voltage \$\endgroup\$ Mar 27, 2021 at 14:03
  • \$\begingroup\$ What part of the system are you trying to build? The power supply or the mains voltage monitor? You might be approaching this the wrong way in that you seem to be saying "I've chosen to use bass guitar strings, now show me how to make a violin using them". \$\endgroup\$
    – Transistor
    Mar 27, 2021 at 14:06
  • \$\begingroup\$ I'm trying to build the power supply that will power the metering IC and another isolated circuit, this circuits are done on design already. I'm trying to understand how to design the power supply transformer, and how to find a trnasformer that meets the requirements. I see now that the transformer can't be the one that i posted because is 80kHz rated and I need 50Hz rated \$\endgroup\$ Mar 27, 2021 at 14:18
  • \$\begingroup\$ Just buy one ready made. You can get plug-top USB supplies very cheaply - if you don't have any lying around, take the PCB out and mount it securely in your project. It seems that you are a little over your head on this project so I am very concerned that you will try to measure the mains voltage without an isolating transformer. Understand that if you don't then you must assume that the whole circuit is live and you can't connect it to a USB port or attach your 'scope's ground clips. \$\endgroup\$
    – Transistor
    Mar 27, 2021 at 14:27
  • \$\begingroup\$ The thing is that already made power supplies don't have the topology that the circuit needs. So I am trying to understad how the turns ratio on this special transformers work in order to know how to better look for them on supplier pages. Thanks to you now I see that I was not looking well because of frecuency rate. \$\endgroup\$ Mar 27, 2021 at 14:56

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