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!
Best Answer
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.