Voltage drop across a glass fuse at 20A and 24V - Page 1
Forum section: electronics / beginners

author: anvoice
author_rank: Frequent Contributor
timestamp: April 25, 2015, 12:41:43 am
content: Hello,Trying to power a heating circuit with a 24V power supply, which will create a little under 20A of current. In the interest of not burning my house down, I was planning on using either a glass or a plastic automotive fuse in the circuit as a safety measure. However, I'm having trouble finding info on the voltage drop across any such fuse. The cold resistance appears in some cases but apparently it can increase dramatically as the current increases. So if my fuse will be taking out half my power at 20A it'd be bit overboard...So if I use something like a 20A (250V) glass fuse for this application, would the voltage drop across the fuse be exceptionally large with a little under 20A of current draw? Is there a particular type of fuse I can get that would work better?If anyone has additional thoughts and safety considerations, I'd be glad to hear them. Here's a picture of my circuit (fuse not included yet):

author: retrolefty
author_rank: Super Contributor
timestamp: April 25, 2015, 12:57:14 am
content: A fuse operated at or less then it's maximum current rating will have insignificant voltage drop. I would suspect you will have more drop in the wiring to the heater load, so be sure to use acquitted sized wiring. At 24VDC I would have no problem utilizing automotive type fuses as they are easier to physically work with.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 25, 2015, 01:12:09 am
content: Compared to the voltage drop across the heater resistor and even the FET, the fuse is insignificant as retrolefty said.For a project like that I would be MUCH MORE concerned about overheating.To the point of putting a thermal cut-off switch to interrupt the heater power.

author: anvoice
author_rank: Frequent Contributor
timestamp: April 25, 2015, 01:35:14 am
content: Quote from: retrolefty on April 25, 2015, 12:57:14 amThe heat bed already has 12 gauge wiring with ends encapsulated in silicone, so it'd be difficult to replace with anything heavier. I'll measure the drop just in case once I get it working.Thing is I don't really know the difference between the glass and car fuses except that the glass ones can be rated at around 250v and the car ones at 32V, and typically run at 12V. If there's no advantage to the glass fuses, I could go with the car ones. Just need to get the right holders since soldering to the stainless steel legs doesn't seem to work...Quote from: Richard Crowley on April 25, 2015, 01:12:09 amOverheating at the wire contacts? I'm guessing installing the temperature switch would be best at some of the less reliable connections, as they would likely overheat first due to high resistance. Would a thermostat control switch work? Something like 100C or so?

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 25, 2015, 02:46:07 am
content: Quote from: anvoice on April 25, 2015, 01:35:14 amNo. Overheating of the HEATER!Or are you trying to make some kind of incendiary device to burn down your house?

author: anvoice
author_rank: Frequent Contributor
timestamp: April 25, 2015, 02:55:52 am
content: Quote from: Richard Crowley on April 25, 2015, 02:46:07 amThe heater is basically a big pad with resistance wire enclosed in silicone, and it's designed to go above 200C. It's rated at 450W, and I'll have it hooked to 400W so it can't even go that high... Unless there's a short circuit in the heater itself, which is fairly unlikely (when resistance wire malfunctions, it tends to snap off and cut off the circuit rather than become more conductive). I'd consider the heater to be a fairly reliable part of this circuit. I'm a bit more worried about connections (like soldering to the source and drain electrodes, or the fuse holder) which will need to transport a large current.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 25, 2015, 03:00:30 am
content: Quote from: anvoice on April 25, 2015, 02:55:52 amThat's pretty much exactly what every designer said about something that subsequently overheated.

author: cravenhaven
author_rank: Contributor
timestamp: April 25, 2015, 03:00:45 am
content: In my experience, automotive fuses/holders such as the blade type have terrible contacts that cause high resistance connections. In my camper/truck I have given up using the "normal" type fuse/holder for anything over a couple of amps continuous and now use either the maxi fuse style or circuit breakers. There is probably a very good reason that cars and truck manufacturers use fusible link styles for the higher current circuits rather than blade fuses.I dont know about the glass type fuses, but unless you can guarantee a low contact resistance I would be looking for something far more substantial.

author: Psi
author_rank: Super Contributor
timestamp: April 25, 2015, 03:12:13 am
content: You can get 1206 SMT fuses to protect a 20A circuit.etchttp://www.digikey.com/product-detail/en/3413.0332.22/486-1690-1-ND/2870103

author: anvoice
author_rank: Frequent Contributor
timestamp: April 25, 2015, 03:58:48 am
content: Quote from: Richard Crowley on April 25, 2015, 03:00:30 amIt's a piece of wire, so fairly little can fail. The only potential danger is portions of the wire shorting against itself and causing a higher current, which would then cause the fuse to blow.Quote from: cravenhaven on April 25, 2015, 03:00:45 amI can't, as I've never incorporated fuses before (in fact, this is my first non-textbook circuit). What would you consider substantial enough? Looking at the price for circuit breakers, I'd rather be sure it's necessary before getting one of these. Surely no one would make 20A (and higher) glass fuses if they couldn't be safely and efficiently used at around 20A?Quote from: Psi on April 25, 2015, 03:12:13 amIs there an advantage to those over glass? I don't yet have the means of making PCBs to use SMT devices, or can those be soldered to regular pads/wires?

author: PSR B1257
author_rank: Frequent Contributor
timestamp: April 25, 2015, 06:32:41 am
content: QuoteWhy do you bother finding a 20A fuse? Every switch mode power supply has a build in over current protectionPS. You have forgotten the series resistor to the gate of the MOSFET in your schematic.

author: G7PSK
author_rank: Super Contributor
timestamp: April 25, 2015, 08:13:35 am
content: You cannot operate a 20 amp car fuse at 20 amps for very long before it blows, you will need at least a 30 amp fuse on a 20 amp circuit if not a 40 amp one. Take a look at car fused circuits they usually have a fuse that is twice the expected current in the circuit.

author: SeanB
author_rank: Super Contributor
timestamp: April 25, 2015, 08:41:44 am
content: Stay away from the car type blade fuses, unless you get good quality ( Bosch, not OHL ones) fuses, and good quality fuse holders. The glass fuses are perfect, though you probably will find that you need a 25A fuse, if you are always sitting at 20A current. Your fuse should have a voltage drop of less than 1V across it at rated current.Some info here, though it is mostly aimed at AC use but is also good to know.http://www.cooperindustries.com/content/dam/public/bussmann/Electrical/Resources/solution-center/technical_library/BUS_Ele_Tech_Lib_Interrupting_Rating.pdf

author: anvoice
author_rank: Frequent Contributor
timestamp: April 25, 2015, 09:10:01 am
content: Quote from: PSR B1257 on April 25, 2015, 06:32:41 am(1) I didn't know that, but (2) is there a guarantee the over-current is close to 20A? How would the PSU know how much current is too much for my circuit?Quote from: PSR B1257 on April 25, 2015, 06:32:41 amVery good point, thank you. Will add that.Quote from: G7PSK on April 25, 2015, 08:13:35 amIt's actually closer to 17A given the 400W output of my PSU. I can add a 30A fuse instead (max I have on hand), which is about twice the current. I was just thinking it should be close to the expected current for safety. For example, my control board which is supposed to supply a little under 11A to a heater has an 11A polyfuse. Perhaps those work differently though.Quote from: SeanB on April 25, 2015, 08:41:44 amIs there a good reason the glass ones are so much better? Some of those fuse clips for glass fuses don't look that much more reliable than the auto fuse holders. In fact, if you could suggest a good glass fuse holder I'd be really grateful, I spent something like an hour browsing ebay for different ones and have no clue which would be high quality.Thanks for the link, I'll check it out.

author: SeanB
author_rank: Super Contributor
timestamp: April 25, 2015, 09:29:30 am
content: Have fun...https://www.google.com/search?q=car+fuse+meltdown&safe=off

author: PSR B1257
author_rank: Frequent Contributor
timestamp: April 25, 2015, 09:38:59 am
content: QuoteIt won't. But if you got a 500W 24V PSU (http://www.hed-radio.com/pdf/meanwell-datasheet/usp-500-spec.pdf) with a nominal current of 21A.The OCP will kick in at about 1,05...1,3 times the nominal current.QuoteA 20A fuse will carry 20A just fine, hence it has a nominal current of 20A.Here is a datasheet of a ceramic fusehttps://cdn-reichelt.de/documents/datenblatt/C400/F-120KA.pdfIf you have a 30 or even 40A fuse you have to make sure the power supply is able to blow it. A car batterie (due to its very low internal resistance) certainly is. Otherwise you might end up with a 30A short circuit over quite some time. If we take the fuse (the 20A type) of the datasheet above, it will blow after 2000s at 30A.

author: anvoice
author_rank: Frequent Contributor
timestamp: April 25, 2015, 09:51:50 am
content: Quote from: SeanB on April 25, 2015, 09:29:30 amTo be fair, that's about the only hit on google for a car fuse meltdown, and it looks like the fuse block itself melted rather than an individual car fuse. I was planning to solder 10 gauge wire directly to the fuse blades, which would probably be as low resistance as I can get it. But if there's a good quality glass fuse holder out there that can come close to matching a solder connection to a car fuse, I'd appreciate hearing it.Quote from: PSR B1257 on April 25, 2015, 09:38:59 amI see, my 400W PSU indeed claims to have an output protection of 110-150%. I'm guessing it's safe enough to trust that mechanism and not double it with a fuse? It'd be nice to avoid the extra connections and added resistance drop from the fuse.

author: PSR B1257
author_rank: Frequent Contributor
timestamp: April 25, 2015, 10:16:32 am
content: QuoteYes.And as I sayed, the PSU won't be able to bolw the fuse anyway due to it's internal current limiting.

author: anvoice
author_rank: Frequent Contributor
timestamp: April 25, 2015, 10:20:19 am
content: I see, thanks... I guessabsence of problem was confirmedproblem solved?

author: SeanB
author_rank: Super Contributor
timestamp: April 25, 2015, 11:16:05 am
content: Quote from: anvoice on April 25, 2015, 09:51:50 amClick on images and you get a lot more........ I have had that exact same failure on my car, so simply cut the wiring off at the socket underneath and got some inline fuse holders and used the power terminal to attach them to the battery again. Those 3 fuses supply the engine cooling fan, rear window demister and optionally the glow plugs depending on model. They commonly cook themselves, especially the fan one. In my car those 3 do the fan ( the cooked one), the rear demister ( the lot was disconnected, as the damage affected the other fuse holders as well with the meltdown) and the amplifier. The amplifier just was using that as I wanted a direct battery connection, not because it is high power, it is only a 10A load. I had some spare inserts for the connector underneath so used this as it looked neater.I had a lot of fuses fail on my older Mazda as well. I eventually just used to open the fuse box at each service and pull each fuse out, clean it, light oil coat and reinsert, which did help a lot with the pins corroding and going high resistance. My VW beetle didn't have that problem, as there it only had about 6 fuses in the block anyway. Nothing there to fail, and most were 30A old style clip in fuses, which never gave problems.