Power supply transformer question, double primary

Question

I'm trying to make a circuit to look like this one:

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

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!

Answer 1

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.

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

Answer 2

^{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:

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:

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):

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.

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.

Answer 3

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.