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I've got a 12V power adaptor (3.8A). And it got two fuses in it (T3.15AL250V, 3AL250V, one of which (3.15A) has blown, along with mosfet, current sense resistor, 450V cap, and IC. I replaced all failed parts, except the fuse (soldered jumper, since I don't have 3.15A fuse for now).

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What is the point of two fuses in series? I've never seen anything like this before.

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  • \$\begingroup\$ For me it looks like the 3 amps fuse is parallel to your cap. So you have a main fuse for the whole circuit (3.15 amps) and a fuse for a part of the circuit (3 amps) \$\endgroup\$ – Eggi Mar 9 '18 at 9:01
  • \$\begingroup\$ It looks like there is a wire link instead of one of the fuses. \$\endgroup\$ – HandyHowie Mar 9 '18 at 9:22
  • \$\begingroup\$ @HandyHowie, that link on right hand I did solder, as I mentioned in the question description. When I have 3.15A fuse I will solder it in \$\endgroup\$ – Qeeet Mar 9 '18 at 10:05
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    \$\begingroup\$ You sure that's a cap? Looks like a MOV \$\endgroup\$ – user28910 Mar 9 '18 at 14:24
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    \$\begingroup\$ Just speculating: Could it be that the circuit is protected by a fast-acting fuse, while the capacitor has a slow fuse so the inrush current doesn't immediately blow it (while a defective capacitor would)? \$\endgroup\$ – Simon Richter Mar 9 '18 at 14:50
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@Qeeet On the first picture, you drew the jumper and the 3.15A fuse as separate objects linking different node (?). Anyway, you have one trace protected by one fuse, another one protected by another fuse. The explanation is that the 3A fuse blows when the MOV (it's definetly a MOV, not a cap) clamps under accidental voltage surge. The 3.15A doesn't blow in this situation whereas it would blow when there is a short circuit but no voltage surge. Probably the 3A is fast acting, since in case of voltage surge you want to react ASAP. And the 3.15A is slow acting as temporary overcurrent may happen in normal situations and become critical only when it lasts more than a few seconds.

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Possible option: The critical current of the fuse when it blows up is difficult to manufacture exactly and there is no way to verify its accuracy. If you will put 2 fuses in series, they will disconnect the circuit on the minimum of the critical currents of these two. In case if the blow up current of a fuse will happen to be bigger than the nominal, the other one will raise the chance that the critical current will not be exceeded.

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