I have a guitar amp that works on 110 V and someone accidentally plugged it in a 220 V socket. I want to replace the fuse (160 mA) but my local store's lowest is a 250 mA. Can I actually use this? Thanks!
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15\$\begingroup\$ You really need to address the problem in your wiring that makes it possible for a 120V load to be plugged into a 220V outlet. They make sockets specifically for each voltage, you should not tolerate a situation where the wrong one has been used merely because it's easier to get. \$\endgroup\$– Harper - Reinstate MonicaCommented May 14, 2021 at 15:39
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6\$\begingroup\$ @Harper, if the amplifier uses a C13/C14 connector then leads may be readily available with 120 and 230 V mains plugs. \$\endgroup\$– TransistorCommented May 14, 2021 at 16:10
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6\$\begingroup\$ The amp may already be damaged. So keep that in mind. Ideally you should get the proper fuse. If you absolutely need to use the amp before getting the proper fuse, it would be better to use the 250 mA fuse than to bypass the fuse altogether. Anyway, next time you plug it in (to 110V), if the fuse blows again immediately, that is probably an indication that the amp is damaged. \$\endgroup\$– user57037Commented May 14, 2021 at 18:43
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3\$\begingroup\$ The fuse has already saved your amplifier once, and given your wiring situation it might need to do it again! It's definitely worth getting the right one. \$\endgroup\$– tomnexusCommented May 14, 2021 at 20:14
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1\$\begingroup\$ @Harper-ReinstateMonica I suspect the problem is bringing an amp from another country, which has to be used with a voltage converter. BC Amparado may be in South America, where there is a pretty crazy situation regarding sockets and voltages, with every country doing something different. \$\endgroup\$– KazCommented May 15, 2021 at 16:00
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3 Answers
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Widening of protective envelope for any circuit is NOT good. the fuse is there to protect the circuit in case of overcurrent. Overcurrent does not mean shortcircuit, it means that the circuit is design at certain current level, in your case according to original fuse is 160mA. everything above that is overcurrent event and a fuse must deal with it to protect the rest of the circuit. even tough it seems like not a large current 250mA in respect to 160mA is increase of 56,2%. It is like driving 95 in 60 km/h zone. just put this in perspective and you will see that even though you can use any fuse to make the circuit work only the one specified for this circuit will be proper.
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1\$\begingroup\$ At my workplace i frequently hear: "what is the problem 150A rated motor it can take 180A no sweat." but when you put it in different perspective which people understand easy and can relate to then they start searching solutions on the load side (clogged pump filters, closed or excessive opened discharge valves) etc... \$\endgroup\$ Commented May 14, 2021 at 12:30
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5\$\begingroup\$ Generally fuses protect wiring. Very often, with electronics, the fuse will only blow after some downstream electronics have already failed. In this case, there is a chance that there is some kind of zener or crowbar over-voltage protection that caused the fuse to blow while protecting downstream circuitry. But it is also possible that the electronics are permanently damaged. \$\endgroup\$ Commented May 14, 2021 at 18:47
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Well, its a gamble your taking.
If you choose to use a larger fuse than the device had originally, you run a real risk of it not doing its job. The 250mA fuse will let the guitar amp pull about 15 Watts more from the mains than the 110 mA fuse will. If you are lucky, the circuit of the amp can handle those additional 15W = (110V * (0.25A-0.11A)) in case of a fault, but if the amp is built at a low budget, its unlikely that the componetns inside will survive such a fault event. Also mind that there is a difference between fast-blow and slow-blow fuses! You will want to get the same type as you had.
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6\$\begingroup\$ @alephzero Could you explain your argument in a little more detail? As far as I know, a fuse will protect (if properly sized) the weakest component that is connected in series to it. You are right, the cord probably would take more current, but you dont know about the internal circuitry \$\endgroup\$ Commented May 14, 2021 at 15:39
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5\$\begingroup\$ @alephzero as far as I know, the logic behind fuses and wiring is the same in the USA as it is in the rest of the world. Fuses usually protect the wiring from sustained overcurrent. Sometimes fuses inside a device may provide a safety function for the device in the sense that after a certain type of failure, the fuse will blow before any dangerous conditions can arise. For example, you may put in an over-voltage crowbar circuit which can protect downstream overvoltage, but will blow a fuse. \$\endgroup\$ Commented May 14, 2021 at 18:51
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1\$\begingroup\$ @MeMyselfAndI It's simply a practical issue: transistors and other semiconductors are so small and easily damaged that it's difficult to protect them with a piece of macroscopic melting wire. Fuses only work well when you're trying to protect other macroscopic pieces of wire (transformers, motors, vacuum tubes) from melting themselves. \$\endgroup\$ Commented May 15, 2021 at 6:37
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1\$\begingroup\$ @user71659: Some devices may be engineered to survive long enough to pop fuses, without other damage, in some overvoltage or reverse-polarity scenarios. Generally it's better to use some other form of protection, but sometimes fuses may be able to protect other devices from damage as well as from melting/igniting. \$\endgroup\$– supercatCommented May 15, 2021 at 6:40
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2\$\begingroup\$ @alephzero Nobody would put a 160mA fuse on a mains cord. I assume this fuse is inside the amplifier itself. \$\endgroup\$ Commented May 15, 2021 at 10:47
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If there is a short at the transformer secondary:
Losses in transformer windings due to resistance are \$ R I^2 \$. Current is squared, which means 240mA will cause 2.4 times more heating in the transformer than 160mA. If the transformer is small and/or "cost-optimized" it might not manage to draw enough current to blow the bigger fuse. In that case the thermal fuse on the windings will blow, which is usually impossible to replace, and if there is no thermal fuse, it will start a fire.
