AC Current:DC Current Ratios

[Wilder Amplification]

For a given amount of required DC current, how does one go about calculating the required AC current on an HT winding? Does it depend on rectifier type, be it a FWCT or FWB? Or for a given custom wind, does one go about just telling the winder how much DC current you need and what type of rectifier you're using and let them calculate it? Is there a conversion factor (such as Idc x 1.6 or x 1.8) or is it based on a percentage (i.e. the AC current must be rated X% higher than the required DC current)?

[raiken]

http://www.hammondmfg.com/pdf/5c007.pdf

[loudthud]

The info in the Hammond article is pretty close but you'll find the same info from say, Signal Transformer, will be a little different. The biggest influence on RMS secondary current is the value of the filter caps. Duncan's PSU Designer will give accurate numbers if you use it properly.

Follow these tips:

1) Run the simulation at the correct frequency and for an integral number of cycles. 100mS is good because it's 5 cycles at 50Hz and 6 cycles at 60Hz. Give a nice big delay to get rid of the cap charging effects.

2) You need good numbers for the transformer. You need to measure the resistance of the primary and secondary and the no load secondary voltage. Right click on the transformer, click on the boxes with ... and fill in the data.

3) Cap ESR is important in higher power supplies. Try to find a reasonable number from parts similar to what you will be using if you can't find actual data sheet.

4) You thought a solid state diode drop was 0.6V? In the simulation below it's over 1 Volt mostly because the peak current through the diodes is over 3.5 Amps.

In the simulation the yellow trace is the secondary voltage just to show the cycles, the red trace is the secondary current pulses (right scale) and the green trace is the output voltage.

The box at the lower left is where you'll find the important numbers. The box does not display properly on my laptop, I pasted two screen captures together to show all the numbers. The RMS secondary current is underlined in red as is the Mean output voltage. Note that the RMS secondary current is more than twice the load current and the mean voltage is barely over 6.3V. The min voltage is 6.0055V, not enough to run a regulator.

When you are specifying a custom transformer, if you have accurate numbers, you usually get a reasonable result because the builder will bump up the wire size above what is required to the next bigger available size, or higher.