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.

  • \$\begingroup\$ On something similar I have 3 fuses: one on the primary of the transformer (never blew it) and two more on the secondary, more specifically after the bridge rectifier on the DC rails. Those I blew once in a while. You need two fuses there as the circuits are usable independently and return to ground. Of course you need to size them accordingly. The secondary-side fuses for mine protect the rectifiers as they are rated for a bit less current than the secondaries of the beasty transformer I have. \$\endgroup\$ – Fizz Nov 22 '15 at 4:39
  • \$\begingroup\$ Also some transformers have built-in thermal fuse and you don't generally want to blow it because it's a pain to change it: have to cut and replace rather expensive kV-level isolation tape. \$\endgroup\$ – Fizz Nov 22 '15 at 4:47
  • \$\begingroup\$ some transformers are unsuitable for use without an earth connection, however yours looks safe. \$\endgroup\$ – Jasen Nov 22 '15 at 5:02
  • \$\begingroup\$ How can I tell my transformer is suitable, would it be a primary coil that only connects to Hot and Neutral? \$\endgroup\$ – Dave Guenther Nov 22 '15 at 5:13

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.

  • \$\begingroup\$ AC fuses are fine after the rectifier, as the current still drops to zero, after the filter capacitors DC fuses should be used. \$\endgroup\$ – Jasen Nov 22 '15 at 5:07

Use a fuse for the primaries rated for 1.5 to twice the current. Reason is, that the inrush current is much higher than the average current.

Fuses on the secondary are optional but if you have the space for them I suggest to add them. It's much easier to drop in a new fuse than to change burned out voltage regulators.

Speaking of voltage regulators, the LM317/LM337 can be used with a overcurrent protection. You just have to add a few resistors and a transistor. There is an example for this in the datasheet.

  • \$\begingroup\$ I'll make space for the secondary fuses. I did see the schematic for overcurrent protection on LM337/317 datasheets but thought maybe I misunderstood it's purpose - I didn't realize they could provide protection against shorts. Where can I read more about this inrush current? When I first simulated this circuit, I noticed the first few periods of the secondary signal to draw very high amperage as caps charged, and then it eventually leveled off to a few milliamps per period. Is this the same thing that you are talking about? \$\endgroup\$ – Dave Guenther Nov 22 '15 at 5:11
  • \$\begingroup\$ With slow-blow T4 fuses I found that over-sizing unnecessary in my case, although it is certainly allowed by the NEC; up to 250% [for the primary] in fact [and 167% for the secondary]. Having nasty memories of having to change the thermal fuse on some transformers I prefer to not push it with the oversized fuses... even if they are allowed. It would depend on the transformer too. A toroidal one would need oversized fuses more (has higher inrush). \$\endgroup\$ – Fizz Nov 22 '15 at 5:25
  • \$\begingroup\$ @DaveG Like capacitors, transformers have inrush currents as well if they get energized. The peak current can be huge for some milliseconds. The effect is more drastic for torod transformers than to the square shaped ones. For more info check the wiki-page: en.wikipedia.org/wiki/Inrush_current#Transformers \$\endgroup\$ – Nils Pipenbrinck Nov 22 '15 at 5:25
  • \$\begingroup\$ @Dave G: yes you can surely make a current limiter (and I have some too) but what if it fails or you don't build it correctly, or if you short your board somewhere before it (say with a probe or screwdriver)? It's always good to have at least one fuse... it's very hard to get a fuse wrong. \$\endgroup\$ – Fizz Nov 22 '15 at 5:31
  • \$\begingroup\$ There's also an issue I forgot about; IEC fuses are differently rated than the US [UL] ones. That's why you need to oversize the US ones for inrush. No oversizing for IEC fuses on transformers. \$\endgroup\$ – Fizz Nov 22 '15 at 5:35

The fuse on the primary is there to prevent a short on the transformer melting wires in your wall and starting a fire in your house. Fuses won't necessarily protect against a brief overcurrent thru silicon (that may be enough to destroy it before the fuse melts).


Fuse always connect primary side of distribution transformer the reason is that when any surge come and also overloading this fuse protect the transformer from damange...... Transformer is costly so we protect transformer from damnage by putting fuse at its primary side.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.