What type of fuse should I use for this? - Page 1
Forum section: electronics / projects

author: alank2
author_rank: Super Contributor
timestamp: May 19, 2021, 12:21:23 pm
content: I'm working on a vintage computer project and using these power supplies from Mean Well to power it:(1) HRP-75-7.5 to provide up to 10A of 7.5V.(2) RS-15-15 to provide up to 1A of +15V and 1A of -15V.These power supplies may have some protection in them, but I want to fuse the hot line when it first comes into the chassis. It goes through a switch and then to the power supplies.I want to use an inline fuse for a 5mm x 20mm type fuse, but I'm not sure about the current or whether it is a fast or slow.The 7.5V says AC current input typical 0.9A, but it says inrush current typically 35A/115VAC. The 15V ones (two of them) says cold start 65A/230V which could be 35A/115VAC if it follows the 7.5V PS, but they are much smaller. They say 0.35A typical each. So, 0.9+0.35*2 = 1.6A, but 1.6 * 120V would be 192 watts and the power supply wattage is only 75+15+15 or 95 watts.How does inrush current affect fuse selection?What would be a good choice, 2A? fast? slow?Also, what are good gauge choices for chassis wiring? For the AC wiring? For the DC wiring?

author: TimNJ
author_rank: Super Contributor
timestamp: May 19, 2021, 02:20:10 pm
content: I think I've seen it done a bunch of different ways, but the manufacturer's input current rating may be based on an overload condition. For example, overcurrent protection is set to 135% max. So, 1.35*75W = 101W. As the power supply will continue to operate normally up to about ~135% load, I suppose it makes sense that the input supply can support up to 135% too.Secondly, 75W + 15W + 15W = 105W, not 95W.If we assume the input current rating is based on overload condition, then adjust that to 101W + 20W + 20W = 141W.Multiply 1.6A * 115V (not 120V), as that's the actual spec. The equals 184W.Those numbers make a little more sense.These power supplies are definitely already fused. I'm not sure what exactly there is to gain from a larger overall fuse. Plus, now this fuse has to handle all three power supplies' inrush current at the same time...unless you do some sort of relay power sequencing.For a fuse to handle inrush current, it needs a sufficiently high I2T rating. This basically means: Use a time-delay/slow-blow fuse. The 'T' in I2T implies there's a time dependency, so the peak inrush current numbers alone don't necessarily answer the question. Give the Littelfuse Fusology paper a look:https://www.littelfuse.com/~/media/automotive/catalogs/littelfuse_fuseology.pdf.Page 62 they show some waveshapes and associated equations for I2T. You can use the sine shape.Without actually measuring the inrush current, to determine time 't', you can probably take an educated guess that the 't' value will be <25% of a half-period of 50/60Hz. So for 50Hz, 20ms period, 1/2 period = 10ms, 25% of that is 2.5ms. In reality, most times I've measured, it's been less than 1ms I think. Best option is to measure the inrush current with a current probe, but even that can be prone to error, as you need a really low source impedance, which often is not available.If you can't meet the I2T requirement for a given current rating fuse, try a different product line, or use a higher current rating. As current rating goes up, so does I2T rating. The actual fuse rating is not totally critical, doesn't usually need to be that close to the actual current draw. It is supposed to keep everything safe during a gross overload.

author: alank2
author_rank: Super Contributor
timestamp: May 19, 2021, 04:09:05 pm
content: Thanks Tim - that gives me a lot to look at!

author: TimNJ
author_rank: Super Contributor
timestamp: May 19, 2021, 06:05:35 pm
content: You're welcome. Another approach, instead of guessing at the 't' value in Fusology, is to work backwards from the high voltage bulk cap value. It's the biggest cap on the board, maybe 82-150uF / 400-450V for the 75W power supply.I've done this simulation with LTSPICE before. I've attached an example to play around with. The values I put in for Rser, R1, and L1 are just for reference. Rser can probably be <1ohm depending on building. R1 depends on the ESR of the bulk cap. If there's an NTC inrush limiter, you can add that to R1. L1 is a guess, maybe less depending on how saturation resistant the input filter is.Also, note this is just a half-wave rectifier.The LTSPICE voltage controlled switch always gives me an error at run-time but it always works, so not sure what's up with that.

author: Benta
author_rank: Super Contributor
timestamp: May 19, 2021, 07:51:17 pm
content: Fuses on the main side of such a circuit are not for overcurrent protection (they're lousy for that).The main function is protection in case of fire in the circuit, and this is also what the Codes specify.It should have a current rating and time delay that's suitable for the inrush current, that's all. Making detailed calculations is a waste of time.

author: alank2
author_rank: Super Contributor
timestamp: May 19, 2021, 08:08:36 pm
content: What wire gauge should I use for the AC wiring? DC wiring? (both chassis)

author: james_s
author_rank: Super Contributor
timestamp: May 19, 2021, 08:52:31 pm
content: That depends on the load. Wire ampacity charts are widely available, chassis wiring can usually use a size or two smaller than wiring used in a power cord or within walls but there is no problem with using something heavier. In some cases a larger gauge wire offers advantages in mechanical robustness as well. Given what you've said about the application I would suggest that 18AWG is probably a reasonable place to start, on both the AC and the DC sides.If you add a fuse on the power input, size the fuse to protect the wiring, don't worry about the power supplies, they have their own fuse in them already. 5A is probably reasonable and will protect even the thinnest wire you're likely to use, I'd probably start with normal blow and change to a slow blow if there's ever a nuisance blow without an actual fault condition. This is really not critical though as you have a fixed load here, not something somebody can overload by plugging something else into it. All the fuse needs to do is blow before the wiring catches fire and a short circuit even with thin wire is probably going to trip the breaker in the panel before the wiring burns up anyway.

author: trobbins
author_rank: Frequent Contributor
timestamp: May 19, 2021, 11:05:13 pm
content: alank2, you also need to be aware of the AC mains feed local supply - eg. from a nearby switchboard - and what over-current device is being used for protection (to your equipment), and its trip characteristics. If your equipment requires a significantly high inrush current, then the switchboard device may need to be changed to one that can cope with a higher in-rush, whilst still having the same continuous current rating.

author: james_s
author_rank: Super Contributor
timestamp: May 19, 2021, 11:08:45 pm
content: He listed the power supplies he's using. If his setup is tripping the breaker in the panel then something is seriously wrong with it. That is not something he has to worry about or take into consideration. The breakers in the panel will be sized to protect the wiring in the walls, that calculation has already been done.

author: TimNJ
author_rank: Super Contributor
timestamp: May 20, 2021, 02:45:44 am
content: Quote from: Benta on May 19, 2021, 07:51:17 pmNot sure if directed at me, but if you have no baseline idea of what kind of I2T you need, then how should you determine whether a given fuse is acceptable? Youcouldjust pick the fuse with the highest I2T you can find, and most likely, it will be okay. True.But if you are doing "serious" product design and a fuse and inrush current is involved, you better make sure the I2T is acceptable before you get a few thousand dead units in the field.

author: amishasingh
author_rank: Contributor
timestamp: May 20, 2021, 12:23:11 pm
content: What if you for some sort of MCB kind of thing for this 🤔

author: trobbins
author_rank: Frequent Contributor
timestamp: May 20, 2021, 12:31:33 pm
content: james_s, the OP hasn't said he has turned all those supplies on yet - if he hasn't yet completed his equipment preparation then he hasn't yet confirmed the upstream breaker will or won't trip - so imho you can't say it is something not to take in to consideration. There could be C type breaker that could trip at the lower end of its range - who knows!

author: alank2
author_rank: Super Contributor
timestamp: May 20, 2021, 01:06:33 pm
content: They are just Mean Well power supplies, not huge wattage ones at that, and other have done builds with them before as well. I doubt very much that they would trip the upstream breaker.

author: james_s
author_rank: Super Contributor
timestamp: May 20, 2021, 04:38:05 pm
content: Quote from: trobbins on May 20, 2021, 12:31:33 pmNonsense. He listed the supplies he's using and those supplies all have ratings, I'm familiar with the sort of hardware he's working with. I can absolutely say the upstream breaker will not trip, assuming the power supplies are not defective.

author: Monkeh
author_rank: Super Contributor
timestamp: May 20, 2021, 10:14:17 pm
content: Quote from: trobbins on May 20, 2021, 12:31:33 pmWe're talking about under 100W worth of normal switching supplies. Even a B6 won't blink at it.

author: WattsThat
author_rank: Frequent Contributor
timestamp: May 21, 2021, 04:02:43 am
content: In the US, all you need to know is 14 gauge wire gets a 15 amp breaker and 12 gauge 20 amps. That’s the first thing any sparky in US learns. Sometimes it’s the last as well. Hell, I’ve even heard more than one ask for a “plus” or a “minus” screwdriver with a straight face.In the US, we don’t dick around with different curve breakers in consumer units, known here as load centers, panel boards and breaker boxes. Walk into any electrical supply house and ask for a slow or fast curve 15 amp breaker and they’ll look at you like you’re from another planet. They’ll just ask for the brand of load center you’ll be using as they are different mechanical designs. The list of manufacturers, many now out of business, is endless with at least three common brands, all physically incompatible with one another.Yeah, we’ve got more breaker manufacturers than we have states. But they all have the same basic trip curves.

author: james_s
author_rank: Super Contributor
timestamp: May 21, 2021, 06:50:36 pm
content: There are special breakers available for HID lighting and motor loads. It's true that consumers rarely use anything but the standard stuff off the shelf but they are available. A friend of mine had to get a HID lighting breaker to handle the inrush of a large transformer we used for testing something because the standard breaker kept tripping about every 3rd time we powered up the test rig.

author: Vovk_Z
author_rank: Super Contributor
timestamp: May 22, 2021, 02:41:15 am
content: Quote from: alank2 on May 19, 2021, 12:21:23 pmThose PS have fuses inside, you need just take it apart and look inside.

author: alank2
author_rank: Super Contributor
timestamp: May 22, 2021, 02:12:07 pm
content: The hot line runs from the power cord to a pcb mounted switch and then on to the power supplies. I want to add the inline fuse on the incoming line before it goes through the pcb switch just as extra protection. It goes on from there to the power supplies.

author: madires
author_rank: Super Contributor
timestamp: May 22, 2021, 02:55:44 pm
content: T2 sounds reasonable. T1.6 might be a tad low.

author: alank2
author_rank: Super Contributor
timestamp: May 27, 2021, 01:00:21 am
content: I put in a 2A fast blow; it has not blown it yet after many power on/off cycles.

author: james_s
author_rank: Super Contributor
timestamp: May 27, 2021, 01:47:15 am
content: I'm sure it will be fine. This is not rocket science and there is no need to over-think it. As long as the fuse is the weakest link in the chain but not so weak that it blows when there is no fault condition then you are good to go. At the power levels you are dealing with there is quite a range of fuse ampacity that ought to meet this criteria.