Fuse for Power Supply - Page 1
Forum section: electronics / projects

author: rstor22
author_rank: Regular Contributor
timestamp: March 14, 2015, 05:38:54 pm
content: Regarding the attached power supply schematic which is powered by an HP SMPS brick (32V 1.1A). I would like to add afuse at the point indicated. The power supply is current limited depending on the value of resistor selected by the rotaryswitch. The maximum selectable current limit is approximately 1.05A.I am trying to determine the amp rating of the fuse to use. I was originally thinking about the following fast blow fuse:http://www.digikey.ca/product-detail/en/0314001.HXP/F3214-ND/777288Which seems to be able to run indefinitely at around 1A and at 1.5A it would break in 10 seconds (if I am reading it correctly)However I am wondering what would happen during short circuit conditions, as it may take time for the selected current limitto take effect. Looking at the datasheet for the LM138K/LM338K it indicates an internal current limit of 12A 0.5ms peak.Does this mean I would need to select a fuse that would be able to withstand 12A for 0.5ms, worst case scenario(assuming the HP SMPS which is powering the power supply exceeds its rated 1.1A) ?Thank you

author: rstor22
author_rank: Regular Contributor
timestamp: March 14, 2015, 06:00:41 pm
content: Quote from: 3roomlab on March 14, 2015, 05:45:13 pmTo my understanding the LM138/LM338 internal current limit, when reached, would not cause the device to break, rather it would reduce the output voltage.So I am thinking I would want to allow this to happen (12A 0.5ms max to occur during short circuit) without the fuse breaking, however if there is a continuous draw of greater then 2A (for example the LM138K/LM338K has gone bad and shorts) then the fuse should break. Does this seem reasonable? Perhaps in this case I should consider a slow blow 1A fuse such as this:http://www.littelfuse.com/~/media/electronics/datasheets/fuses/littelfuse_fuse_325_326_datasheet.pdf.pdfWhich would break at 15A @ 10 ms and 2A @ 10 seconds

author: rstor22
author_rank: Regular Contributor
timestamp: March 14, 2015, 07:28:26 pm
content: Would the original fuse 1A fast blow fuse I selected be a better choice? Opening at 8A @ 10ms, 2A @ 0.4 seconds ? The graph on the datasheet does not go past 8A however it may be ok for 12A @ 0.5ms

author: calexanian
author_rank: Super Contributor
timestamp: March 15, 2015, 03:35:20 am
content: I would never use any of those 3 term adjustable regulators in the 138 or 317 TO220 series at any more than a watt or two dissipation, and thats with a generous heat sink. They are internally current limited and the amount varies by manufacturer. Rule of thumb is never more than 1.5 amp peak and if you are drawing more than an amp continuously you need to look for a better, most likely switching solution.

author: rstor22
author_rank: Regular Contributor
timestamp: March 15, 2015, 01:29:34 pm
content: Quote from: calexanian on March 15, 2015, 03:35:20 amI am using the 138/338 TO-3 package each mounted on large heat sinks (with fans) drawing a max of 1.05A. I want to put a fuse at the point indicated in the schematic I attached should something fail, however I am unsure what value fuse to use. Looking at the STMicroelectronics datasheet for the 138/338 the (internal) current limit maximum is 12A peak @ 0.5ms, so I am assuming that I would need a fuse that should be able to withstand this during short circuit, however it should break at a much lower continuous current draw such as 1.5-2A (The SMPS powering the power supply is rated at 1.1A@32V). Would this be the correct way to determine the type of fuse to select? Is a fuse even necessary or recommended at the point indicated?

author: rdl
author_rank: Super Contributor
timestamp: March 15, 2015, 03:30:07 pm
content: The HP power brick may have all the protection you need already. Have you checked? You may not need a fuse at all. Your LM338K shouldn't be able to pull 12 amps from a 1.1 amp rated source. If you're that concerned about a short circuit, just add something like a 3 - 5 amp fast blow type. It won't have any problem running at 1 amp, but will pop quickly if there's an actual shortThe LM338 is not the same as an LM317. It can handle up to 5 amps. Yes, you have to watch out for heat from power dissipation, but that's just part of normal design process. More than a watt or two is not a big deal at all.

author: rstor22
author_rank: Regular Contributor
timestamp: March 15, 2015, 05:02:24 pm
content: Quote from: rdl on March 15, 2015, 03:30:07 pmI connected the HP power brick up to a constant current load and was able to draw up to 1.9A @ approximately 31V. That is the maximum amps my constant current load will go up to. Is it normal for an SMPS power brick to allow for current draw to exceed its max rating (in this case 1.1A)? Would you suggest connecting a large power resistor to see if it can handle higher loads?

author: Siwastaja
author_rank: Super Contributor
timestamp: March 15, 2015, 05:30:09 pm
content: If you are building a lab supply with a current-limiting and temperature-limited chip, the only thing you should (and even can) use the fuse against is that your regulator chip somehow fails and stops limiting current. So you won't be protecting the regulator; the only case where the fuse blows is that the regulator already blew, and you are just containing the damage; protecting the wiring etc. from excessive thermal load. Pick a fuse that isn't nearly blowing all the time. Fuses are not precision devices, and you don't want to have a nuisance fuse. You already have active protection, fuse only being the secondary one, limiting amount of damage in that rare case where damage is inevitable anyway. So, for 1.05A maximum continuous current, I would pick something like a 1.5A fuse. It protects the wiring just as well.If your regulator is properly designed, the fuse will never do anything. It's not super important; you can generally trust the regulator current limiting.And, pay attention to cooling. It's not trivial as you have quite a lot of heat coming off.The big question is how your SMPS works. It may have a Constant Current mode current limiting, or a latching over current limit. The latter is naturally much better in your case.

author: nctnico
author_rank: Super Contributor
timestamp: March 15, 2015, 05:58:14 pm
content: Two rules about fuses:- They won't help to save a semiconductor- They are intended to protect against fire by being the weakest link instead of wires catching fireIf you need a current limiter either use a chip which has one or build one from scratch. I very much prefer to use power semiconductors which have internal current and thermal protection.

author: calexanian
author_rank: Super Contributor
timestamp: March 15, 2015, 06:13:48 pm
content: Quote from: nctnico on March 15, 2015, 05:58:14 pmThat pretty much says it all.

author: paulie
author_rank: Frequent Contributor
timestamp: March 15, 2015, 06:20:48 pm
content: I was thinking the same thing since post #1 then figured maybe better for noobs to learn the hard way. Note that an LM317 based limiter for milliamps or amps can be built at less cost than a fuse with holder.

author: rstor22
author_rank: Regular Contributor
timestamp: March 15, 2015, 07:43:24 pm
content: Quote from: paulie on March 15, 2015, 06:20:48 pmTo my understanding the first part of the circuit I posted does this (limits current)Quote from: Siwastaja on March 15, 2015, 05:30:09 pmYes this is my intention. I am trying to determine the rating of the fuse to use under short circuit conditions (e.g. the output of the power supply I am building is shorted). The fuse should not blow when the output of the power supply is shorted.Please correct me if I am wrong, however it is my understanding that before the LM338 (in the circuit I posted) starts limiting current, there may be a brief moment where the current may exceed the max limit I have set of 1.05A (selected by the rotary switch). Is this right?What that amount is, I am unsure, however I do not think it can be greater than 12A @ 0.5ms as this is the internal current limit of the LM338. Correct?Quote from: Siwastaja on March 15, 2015, 05:30:09 pmLooking at the datasheets of some fuses, I think a 1.5A or even a 1A fast blow fuse would work. However whatever fuse is selected, it should also not break during short circuit conditions (at the output of the power supply) where the current could reach up to 12A @ 0.5ms (if I am understanding the data sheet correctly).AsQuote from: rdl on March 15, 2015, 03:30:07 pmI don't think the HP SMPS (which is powering my power supply) would be able to provide 12 amps, however I am unsure how it is limiting current, so I figured I'd plan for the worst case scenario.So I think I will use a 1A-1.5A fast blow fuse, as looking at the datasheet it appears it may be able to handle 12A for more than 0.5ms (though unlikely), and therefore if the output of my power supply is shorted, the fuse should not blow.

author: nctnico
author_rank: Super Contributor
timestamp: March 15, 2015, 08:47:21 pm
content: Even fuses have datasheets. In there you'll find a graph which says how long it will take a fuse to blow when a certain current flows.

author: rstor22
author_rank: Regular Contributor
timestamp: March 15, 2015, 08:53:15 pm
content: Quote from: nctnico on March 15, 2015, 08:47:21 pmThis is the datasheet of the fuse I was looking at earlier:http://www.littelfuse.com/~/media/electronics/datasheets/fuses/littelfuse_fuse_314_324_datasheet.pdf.pdfFor the 1A fuse the curve only goes to 8A. Is it ok to continue the graph further to estimate when it will break at 12A?

author: Kjelt
author_rank: Super Contributor
timestamp: March 15, 2015, 09:00:16 pm
content: Quote from: rstor22 on March 15, 2015, 08:53:15 pmAs you can see it is not linear so your interpolation outcome has not much value.Just buy 10 and blow 5 up with a 12Amp current measuring the time at least you know.

author: rdl
author_rank: Super Contributor
timestamp: March 15, 2015, 09:36:40 pm
content: The LM338 has a time limited capability to work at currents up to 12 amps. This is so it can be used with loads such as motors or devices with a lot of input capacitance. Your circuit is never going to be able to put out 12 amps because of the current limiting. Put a fuse in if it makes you feel better, but I'm pretty sure that function is being taken care of by the HP power brick. By the way, what exactly is the model number of the HP power supply? Is it UL listed?

author: rstor22
author_rank: Regular Contributor
timestamp: March 16, 2015, 02:40:34 am
content: Quote from: rdl on March 15, 2015, 09:36:40 pmModel (?) 0950-4491BPA-8561WWIt is UL listed.

author: rdl
author_rank: Super Contributor
timestamp: March 16, 2015, 04:06:54 am
content: I've only taken apart a few laptop power supplies, but every one I've opened had a 2 amp fuse soldered on the pc board at the mains input. Being HP branded and UL listed, it's probably fused inside and not going to start any fires if short circuited, but you could always pop it open just to be sure. You can still add your own in line fuse if you want, it won't hurt anything.

author: rstor22
author_rank: Regular Contributor
timestamp: March 16, 2015, 06:01:27 pm
content: Quote from: rdl on March 16, 2015, 04:06:54 amFor the ones you've opened, were they poly fuses?While looking at fuses, I also noticed that some fuses have DC ratings and that fuse manufacturers stress that DC rated fuses should be used in DC applications and that AC fuses should not be derated for DC applications. Recently I replaced the 400mA range fuse on my Extech EX430. The manual indicates to replace with a 0.5A/250V fast blow. Contacting the manufacturer with regards to the DC rating, they recommended a fuse by Divine Lighting (I contacted them and they indicated they do not have they DC rating for the fuse in question) or Littlefuse 0217.500 (their website only indicates an AC rating).In part of my email to Extech support I indicated:QuoteTheir reply was:QuoteDoes this sound right?

author: rdl
author_rank: Super Contributor
timestamp: March 16, 2015, 06:44:22 pm
content: Quote from: rstor22 on March 16, 2015, 06:01:27 pmI haven't opened any newer ones, they may be using poly fuses theses days. Here are a couple from my junk box that I could find and photo quickly. Fuses are standard glass types, 5x20mm and 3.6x10mm (?) both are marked on the pc board as 2A, one as 250V the other as 250VAC. The upper adapter is from an old Micron P166 from the late 90s, the lower from a Toshiba from around 2008 I think.

author: rstor22
author_rank: Regular Contributor
timestamp: March 16, 2015, 06:48:22 pm
content: Quote from: rdl on March 16, 2015, 06:44:22 pmThanks for the information and photos! Much appreciated.