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I have a system that I have recently developed for in house (manufacturing) equipment testing. Transformers rated for 80VA (264V, .303A) are used in series and rectified using a full-wave diode bridge. The signal is leveled out by capacitors to produce a DC bus voltage. A fuse is placed in series with the windings of the transformers and rated for .315mA.

Should the system be rated for .303 mA (the rating of the transformers), .315 mA (the rating of the fuse) or higher (more current possibly pulled from the bus caps)?

here is a schematic of the system:

enter image description here

Voltage ratings were easy to figure out. I turned on the system and turned the input variac as high as it would go.

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Make sure your fuse is rated to break 1.05kVAC. I would suggest fusing the primary as well, since the transformers could fail, in fact it's more important than fusing the secondary.

According to the Hammond Design Guide for Rectifier Use, the maximum DC current output for your configuration is about 0.62 of the AC rating, or about 188mA in your case. This is because the RMS current in the transformer winding (and thus the heating) will be higher than the DC current output.

If you're talking about current available at the terminals as shown, you have to subtract the current through the 400K bleeder resistor, which is about 3.8mA, leaving you with ~184mA.

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    \$\begingroup\$ I forgot to include it in the schematic, but the primary side is fused with a 5A fuse. the true system is powered by AC mains that feed the variac and a logic supply, the fuse was placed before a power switch that connects the whole system and was chosen with the power supply, Variac, and transformers in mind. The fuse on the secondary side is rated for 1000Vac. Thank you for the document link as well. \$\endgroup\$
    – mayfield512
    Commented Dec 17, 2014 at 16:25
  • \$\begingroup\$ Sounds good. It wouldn't hurt to fuse the input to the transformers to protect the expensive variac. It's possible to have excessive current through the wiper while still not exceeding the 5A input current if the variac is turned down to a low ratio. It may have to be a slow-blow type. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Dec 17, 2014 at 16:42

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