Fuse rating - Page 1
Forum section: electronics / beginners

author: macross
author_rank: Contributor
timestamp: December 25, 2011, 03:07:08 pm
content: I need a fuse to protect transformer on the primary side of my bench power supply, but i don't know what rating fuse should i use... Is there some kind of formula for calculating the rating?Transformer spec: Primary 230VAC 50hz secondary: 12v at 2 amps max.

author: Zero999
author_rank: Super Contributor
timestamp: December 25, 2011, 04:30:35 pm
content: That's 24VA so assuming no losses the primary current will be just over 100mA but it'll be more with losses and the power factor not quite being quite 100%. Go for 200mA or 250mA slow blow.

author: PetrosA
author_rank: Frequent Contributor
timestamp: December 25, 2011, 06:49:05 pm
content: If it's a magnetic transformer, you might also want to check the starting inrush current. Something tells me a 250mA fuse might get vaporized at start-up, but who knows...?

author: Zero999
author_rank: Super Contributor
timestamp: December 25, 2011, 07:53:03 pm
content: Yes, but toroidal seem to be the worst for that, in which case you could go for 315mA or more but even 200mA should be fine for an E-core transformer.I've used a 500mA slow blow fuse with a 230V 100VA E-core transformer and it was fine. Yes maybe I should've gone for 630mA but I was being paranoid as it was an isolation transformer for experimental use and didn't want to overheat it if I overloaded it.

author: Unknown
author_rank: Unknown
timestamp: December 25, 2011, 10:00:53 pm
content: Transformer manufacturers will usually specify the correct fuse. For some toroidal transformers this may be a time delay fuse because of the high surge current. A primary fuse serves to prevent the mains transformer from catching fire, not to protect the circuit.

author: Simon
author_rank: Unknown
timestamp: December 25, 2011, 10:12:29 pm
content: my 15V 25A power supply blew a 5A fuse on the primary with practically no load, not sure how long it had been in there probably years

author: Unknown
author_rank: Unknown
timestamp: December 25, 2011, 10:24:14 pm
content: Old fuses will sometimes blow due to fatigue, which is why some people will first replace the fuse and see if its replacement blows before starting troubleshooting if its an old piece of equipment.

author: Simon
author_rank: Unknown
timestamp: December 26, 2011, 09:07:30 am
content: yea that is what I figured for my power supply and it was so

author: macross
author_rank: Contributor
timestamp: December 26, 2011, 09:52:46 am
content: Quote from: Hero999 on December 25, 2011, 04:30:35 pmThanks i will go for 250mA will see how it goes.

author: jahonen
author_rank: Super Contributor
timestamp: December 26, 2011, 10:43:10 am
content: It should be noted that most fuses are very slow for small overcurrents, say 1.5-2x times the nominal current, regardless whether they are slow or fast. For example by comparing following fuses, it can be seen that difference between slow and fast fuse become clear only beyond 3x the nominal current:http://www.littelfuse.com/data/en/Data_Sheets/Littelfuse_Fuse_216.pdf(fast)http://www.littelfuse.com/data/en/Data_Sheets/Littelfuse_Fuse_215_Series.pdf(slow)Both take 30-60 min to blow with 1.5-2x nominal current.Regards,Janne

author: PetrosA
author_rank: Frequent Contributor
timestamp: December 26, 2011, 04:28:45 pm
content: Quote from: jahonen on December 26, 2011, 10:43:10 amTrue, but that's only for constant loads. Inrush can be as high as 20x-30x the rated current for some transformers. I had a 2000 VA torroidal transformer at my cafe in Poland that bucked to 120V for some appliances that would instantly trip the 3-pole 25A main (shutting down my entire cafe...) when plugged in with no load. It all depends on where on the sine wave it energizes and how that affects core saturation.

author: jahonen
author_rank: Super Contributor
timestamp: December 26, 2011, 06:04:23 pm
content: Quote from: PetrosA on December 26, 2011, 04:28:45 pmThat indeed tends to be the case for larger transformers. My point was to imply that fuses tend to tolerate considerable overload for considerable amount of time. Another point is that fuses perform very poorly protecting semiconductors, since semiconductors fail in microseconds, fuses only thousand times slower. Thus fuses tend to be useful for preventing fires and other similar collateral damage.Regarding the big toroids, I have built an amplifier in the past which has 1600 VA toroidal transformer and during testing I soon realized that I had to design an inrush current limiter (IIRC 5x75 ohm resistors in series, bypass relay) and startup sequencer to reduce the inrush to sane levels. Sequencer is also part of the protection circuits. At worst case it can be assumed that only primary DC resistance limits the current if the core is completely saturated. Larger transformers also recover slowly from the saturation, due to small primary resistance. Idle current of those transformers is very small, IIRC just few tens of milliamps.Hereis a measurement of the startup transient after limit. Maximum current is now limited to just under 5 amps. That includes the transformer saturation and capacitor charging currents (4x10 mF total).That reminds me that I have couple of 545 VA toroids which also tend to blow their fuses (If I have to switch them off and on again, I usually just get some spare fuses, it usually works after one or two tries), but haven't yet got around building proper inrush limiters for them. Proper inrush current limiting is a bit difficult especially against short breaks.Regards,Janne