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I have calculated the maximum (Worst Case) current consumption of my circuit to be 88mA. Looking at transformers I found one at an specified max output current of 90mA for parallel winding which is my case. The next available component has a max current output of 200mA.

Q1: am I cutting it too short if I select the 90mA transformer? is there any derating that I should take into account such as temperature or saturation current as for inductors?

Q2: On the primary winding connected to AC mains. Are elements such as a fuse and a Y2 capacitor mandatory to pass electrical certifications?

Thank you

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  • \$\begingroup\$ what is the measured current consumption? \$\endgroup\$ – Leon Heller Jul 30 '18 at 10:51
  • \$\begingroup\$ The circuit has not been built. Is the analytical calculation of the WC consumption \$\endgroup\$ – RWeiser Jul 30 '18 at 11:47
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If you have a bridge rectifier and smoothing capacitor that feeds your load then it's likely that your prefered transformer is too feeble: -

enter image description here

This picture came from this pragmatic doument produced by Hammond.

What you have to remember is that the DC output is going to be 1.4142 x the RMS secondary voltage (minus a couple of diode forward drops) and with a 12 volt secondary, the DC voltage will be about 15.6 volts. If you are pulling off a current of 88 mA, that's a load VA of 1.37 and the transformer is only rated for 1.1 VA (12 volt x 0.09 amps).

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  • \$\begingroup\$ Is this calculation only valid for a no-load circuit? The specification says 12V at full load so I would imagine that the output voltage will be 12V-(0.7*2)? \$\endgroup\$ – RWeiser Aug 1 '18 at 8:24
  • \$\begingroup\$ The output DC level from a bridge and capacitor is the RMS AC voltage into the bridge multiplied by \$\sqrt2\$ and then subtracted by 2*0.7 volts. That is at full load and of course there will be ripple to consider. \$\endgroup\$ – Andy aka Aug 1 '18 at 8:58

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