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This feels like a silly question, but after taking apart some of our 6W 12V DC power supplies I've noticed that none of them are fused.

Out of curiosity more than anything else, I'm wondering why a fuse isn't required for these power supplies - for the record these PSUs are:

  • Input: 100-240V~50/60Hz
  • Output: 12V DC 500 mA
  • CE compliant (tested by a UK test house rather than just relying on manufactures word)

UPDATE:

Thanks for the responses, so on closer inspection (and based off comments and answers) I think I have found the fuse like component on each board.

enter image description here

enter image description here

My confusion came in that I was expecting a removable fuse as found in a standard 3-pin plug socket.

If someone were to ask me what these were fused to though what would I say? Whatever the input current is stated as? 0.18A?

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    \$\begingroup\$ Do they have a resettable polyfuse hidden on the board? \$\endgroup\$
    – pjc50
    Commented Mar 5, 2015 at 9:33
  • \$\begingroup\$ are they from the uk? dont they have a fuse on the ac plug? \$\endgroup\$
    – Wesley Lee
    Commented Mar 5, 2015 at 9:36
  • \$\begingroup\$ Where do you expect the fuse to be? \$\endgroup\$
    – Andy aka
    Commented Mar 5, 2015 at 9:54
  • \$\begingroup\$ Perhaps they're expecting the power leads to be fused, like almost every standard UK plug? \$\endgroup\$
    – Polynomial
    Commented Mar 5, 2015 at 10:35
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    \$\begingroup\$ Maybe they have a fusible resistor hidden somewhere, often covered by heatshrink tube. \$\endgroup\$
    – AndrejaKo
    Commented Mar 5, 2015 at 12:56

5 Answers 5

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Short answer. I'm not a Transformer expert but in the UK the plug is fused and rated to protect the wiring and also the transformer sometimes.

Also transformers can be self limiting/protecting due to the design (internal protection device or even the wire resistance is high enough to limit current).

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  • \$\begingroup\$ Not a brit, but I'd assume the plug is fused for the maximum rating of the receptacle, probably something like 5 or 10 A. That's hardly going to help when a 6 W device fails. \$\endgroup\$
    – The Photon
    Commented Mar 5, 2015 at 16:40
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    \$\begingroup\$ The typical values in the UK are 13A, 5A, 3A and I thought I saw 1A once. In general the fuse is to protect the wire to the device... once inside that is where the designer takes over \$\endgroup\$
    – Spoon
    Commented Mar 5, 2015 at 18:59
  • \$\begingroup\$ The plug is fused primarlly to protect the flexible cable. As you say 3A, 5A and 13A are the normal values but you can also get 1A, 2A, 7A and 10A. \$\endgroup\$
    – Peter Green
    Commented Dec 9, 2015 at 18:10
  • \$\begingroup\$ I have some plugs with 1A now..toothbrush chargers. As for 7A, it is not a typical value from my experience. I have yet to see them for sale, time to go look :-) \$\endgroup\$
    – Spoon
    Commented Dec 9, 2015 at 18:15
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The primary purpose of the fuse in the plug is to provide overcurrent protection for the flexible cable and to a lesser extent the appliance. This was introduced in the UK at the same time as switching from a large number of low current circuits to a smaller number of 30A (later 32A) circuits. In the case of a wall wart there is no flexible cable to protect.

Power supplies (whether wall warts or otherise and whether british or otherwise) will need to have overcurrent protection to protect against faults in the power supply itself. This is generally provided by a suitable fuse soldered to the board. Making the fuse user replacable would just be a liability, if it blows it means there is something seriously wrong.

The component you have identified in your first picture is almost certainly a fuse (the text "f1" is a dead giveaway). I'm not so sure about your second picture. I'm not so sure about your second picture ("L" as a comonent designation would normally indicate an inductor)

The output cable will likely be protected by designing the PSU itself to limit the current.

IF a PAT test report or similar asks for a fuse rating I would just put N/A as there is no user replacable fuse.

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Just to clear up the fusing in UK AC:

The fuses present on the UK plug are part of the 5 pronged approach to safety

1) the sockets are switched

2) the sockets are EARTHED

3) the LIVE-NEUTRAL are blocked until the EARTH is inserted

4) the LIVE-NEUTRAL are partly shrouded so that the exposed brass isn't "accessible" or contacted until fully inserted

5) each plug is fused due to a ring system in use & the fuse rated for the appliance NOT the wiring - why put a 13Amp fuse onto something that only needs 3Amps?

Under a radial system the fusing could be done at the distribution board BUT it would be rated for the maximum that could be connected (+ any spurs...)

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  • \$\begingroup\$ So you have to change the fuse in the receptacle whenever you plug in a new appliance? That seems unlikely to happen in the real world. \$\endgroup\$
    – The Photon
    Commented Mar 5, 2015 at 16:42
  • \$\begingroup\$ Even if that's the theory, OP's 6 W ac adapter is likely to be plugged in to a receptacle that's been fused for a 100 W lamp or a 1000 W vacuum cleaner. Hardly seems safe to rely on the user changing the fuse to charge their cell phone (or whatever it is). \$\endgroup\$
    – The Photon
    Commented Mar 5, 2015 at 16:44
  • \$\begingroup\$ Umm that is exactly what you do as you put the fuse in and only change it when it blows. What is unrealistic is the load changes. If it did of course you change the fuse... \$\endgroup\$
    – user16222
    Commented Mar 5, 2015 at 17:11
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    \$\begingroup\$ Note that while most UK sockets do have switches they are NOT a requirement. Also while shuttering of the live and neutral is required it doesn't have to be activated by the earth pin. \$\endgroup\$
    – Peter Green
    Commented Dec 9, 2015 at 17:50
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    \$\begingroup\$ @ThePhoton: UK terminology is plug and socket. The fuse is installed in the plug and travels with the appliance. The logic is that the wall sockets could be wired in a ring circuit with 32 A protection while the sockets were rated at 13 A max (giving up to 3 kW available at every socket in the house / office subject to circuit limits). \$\endgroup\$
    – Transistor
    Commented Jan 9, 2016 at 14:43
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The input resistor (red circle) limits the input current and work as fuse to. If PS draws to much current, the input resistor blows up like a fuse.

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Most regulated power supplies are current limited. They have circuitry that senses the current taken by the load and shuts the power supply down if it exceeds the design limit. They therefore do not require internal fuses.

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  • \$\begingroup\$ Any decent power supply will have a fuse. If one of the components in the primary side fails then there is a need to limit the damage. \$\endgroup\$
    – Peter Green
    Commented Dec 9, 2015 at 18:06
  • \$\begingroup\$ I think the OP was talking about primary side protection. The internal fuses are mostly fire protection if components fail not for load monitoring. \$\endgroup\$
    – KalleMP
    Commented Jan 9, 2016 at 15:10

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