Power supply - location of the fuse - Page 1
Forum section: electronics / projects

author: ivonenand
author_rank: Contributor
timestamp: June 30, 2015, 08:46:36 pm
content: Hi guys,I'm designing a power supply for the 240Vac line and I have a question regarding the positioning of the fuse with respect to the input EMI filter. I've looked through a few (tens actually) of designs and also opened a few ATX power supplies and there seem to be two positions where the fuse is put:1. The fuse is put directly to the input, L or N, (attachment 1)2. The fuse is put behind the emi filter (attachment 2)So my questions is, what is the benefit of using one over the other? Which one is prefered?I'll appreciate any input on the issue as I have no idea why some designers go for the no. 1 others go the no. 2.

author: madires
author_rank: Super Contributor
timestamp: June 30, 2015, 08:54:00 pm
content: A fuse directly at the input would protect also the (faulty) EMI filter.

author: ivonenand
author_rank: Contributor
timestamp: June 30, 2015, 09:00:57 pm
content: That's my thinking too; let's say a capacitor in the EMI filter blows or the common mode choke shorts; if the fuse is directly at the input then you also cover these scenarios. But, I'm still wondering why there are so many designs where the fuse is put behind the EMI filter

author: void_error
author_rank: Frequent Contributor
timestamp: June 30, 2015, 09:07:05 pm
content: Quote from: ivonenand on June 30, 2015, 09:00:57 pmI saw some designs like that but the EMI filter was incorporated into the mains IEC socket.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: June 30, 2015, 11:33:17 pm
content: The fuse will only really protect the downstream wires/pcb from going on fire.The plug is probably 10 amp rated anyway and so would have a fuse or breaker upstream of up to maybe 20A.As long as it is before the Caps I don't think it matters.Though I would put it first unless you are likely to have EMI problems.

author: codeboy2k
author_rank: Super Contributor
timestamp: July 01, 2015, 09:04:05 am
content: Quote from: void_error on June 30, 2015, 09:07:05 pmThis is the 2nd design exactly. That one is showing a hybrid EMI filter, likely built into the IEC mains socket, so you are forced to put the fuse after it.There are some IEC sockets with both EMI filter and a built in fuse holder. In those, it's likely that the fuse is before the filter section.http://www.ebay.com/itm/1pc-NEW-EMI-Filter-Fuse-Socket-Switch-10A-115-250V-AC-/121249859369(I am not making an endorsement of this product, it's just an example from eBay )

author: ivonenand
author_rank: Contributor
timestamp: July 01, 2015, 09:41:25 am
content: I also though of that; if you use a socket with the integrated filter, then you are forced to put the fuse behind it. So I took a look at the sockets with integrated filters, fuses and switches. Same mess here.1. Tyco puts the fuses on the input (attachment 1, Tyco pn PS000DMXE)2. Schurter puts the fuse and the switch behind the filter (attachment 2, Schurter pn CD24.4101.151)I know I might be making an elephant our of this, but this design issue really bothers me

author: ciccio
author_rank: Frequent Contributor
timestamp: July 01, 2015, 12:25:25 pm
content: I recently got the approval documents for CE marking of one product I designed from an independent (and certified, and really expensive) test house.The insisted that the component flow be this :1) input IEC connector2) fuse3) switch (bipolar)4) filter5) internal circuitThe reason are the following:- the fuse must protect from a short inside the switch (very rare, but I've seen two or three in 35 years, due to broken plastic in the switch actuator)- the switch must be before the filter because the filter has an operating life, that is shortened if always connected to mains.I cannot quote which part of the applicable IEC standard is involved, but they showed it to me.The one-piece socket + switch + filter shown by the OP has a fuse too, and in those I've seen the fuse is immediately after the input connector (there is a copper bar connecting the fuse to the live pin).Best regards

author: madires
author_rank: Super Contributor
timestamp: July 01, 2015, 05:36:05 pm
content: Quote from: ciccio on July 01, 2015, 12:25:25 pmThere is another good reason for this. The X and Y filter caps pass a few mA, which would prevent the power-off mode to be really 0W.

author: codeboy2k
author_rank: Super Contributor
timestamp: July 02, 2015, 02:51:29 am
content: Quote from: ciccio on July 01, 2015, 12:25:25 pmYes, that certainly makes sense especially with a DPST switch, where line and neutral can short out. Here in the North America we're used to just switching the HOT line and not the neutral, so the switch is often placed before the fuse.However, I really do like the reasoning behind placing the fuse before the switch. Even in the single pole case, switching only the hot side, there is still a very remote possibility that the switch might come loose from it's mount (due to stresses of use), might even get broken, can wiggle about and then touch the grounded case. In that scenario, you still want the fuse to blow.So I think I will change my ways, and always put the fuse first, before any switch.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 02, 2015, 04:41:08 am
content: In more or less every PC power supply I opened, the switch (if there is one) is always before the fuse and most also have some filtering right behind the mains connector.

author: madires
author_rank: Super Contributor
timestamp: July 02, 2015, 12:35:12 pm
content: Quote from: NiHaoMike on July 02, 2015, 04:41:08 amIt's cheaper that way. If you want to place the fuse right behind the IEC socket you would need to place a fuse holder (expensive) next to the socket or run 4 additional wires to/from the PCB (also expensive), when the fuse is mounted on the PCB. I would pay the extra buck or two for it, but I'm not the common user. If something happens to the PSU and the fuse blows, the PSU is broken anyway. And average Joe simply gets a new one. Why spend extra money on a feature Joe doesn't need? Win-win sItuation

author: madires
author_rank: Super Contributor
timestamp: July 02, 2015, 12:41:37 pm
content: Quote from: codeboy2k on July 02, 2015, 02:51:29 amFrom the design point of view it's the best way. And it's quite simply because you can get tons of IEC sockets with an integrated fuse holder (one or two fuses, or one plus one spare).