Background: I have a step-down transformer P241-5-36, which is a single mains primary coil to 36V Center Tapped (18-0-18) secondary. I would like to use this transformer to build a linear regulated dual rail (+15V/-15V) DC power supply for DIY synth work. I'd like for both me and the power supply to not be destroyed if I happen to accidentally produce a short circuit on the secondary side. The data sheet says for my particular transformer, "36V C.T. @ 0.35A" under the "Secondary RMS Rating" column.
Question:
Do I place a fuse on the primary or secondary coil, or both and should I use a 0.35A value fuse or something else? I think the fuse needs to be on the primary coil (though I'm not sure the value), but when the transformer spec identifies 0.35A at secondary, it is confusing me.
Research: I researched linear regulated power supply circuits and found the following schematics:
While these schematics all to very different things, they all share a commonality in that they show a fuse on the primary side of the transformer. I read through this post asking general safety questions about a PSU schematic. That individual's schematic showed fuses on both the primary coil AND secondary sides.
Proposed Schematic: Here is the schematic I plan to use for my power supply (up to the linear voltage regulators). It is dependent upon the correct location and value of a fuse – I took a guess here:
simulate this circuit – Schematic created using CircuitLab
Please note: I included a switch to float ground because I may want to probe with my oscilloscope and connect the probe's ground clip to something other than ground on my circuits.
Thanks, EE Community.
Best Answer
Your transformer has 0.35A rating for its secondaries (each). You can fuse those with a [say] 0.3A fuse each. (The center tap doesn't nee a fuse.) The fuse in the primary [if you want it] needs to be much smaller. For 115V primary, you have a turns ratio of about 3.20 (to both secondaries in series), so the max current in the primary is about 0.11A. A 100mA fuse should work there.
Beware that if put the secondary side fuses after the rectifier they need to be DC fuses. AC and DC fuses are not the same because the latter need to cut the arc while in the former it gets cut by the AC sinewave itself.