power input for small transformer, good practice - Page 1
Forum section: electronics / beginners

author: IvoS
author_rank: Frequent Contributor
timestamp: June 26, 2018, 10:19:28 pm
content: I will have a small linear power supply with this 10VA or similar transformerhttps://www.mouser.com/ProductDetail/Bel-Signal-Transformer/14A-10R-28?qs=sGAEpiMZZMvwUzoUXIIvyd9mRDTtMBVeVfiyOmBntwwckc0OwNq22Q%3d%3dBridge rectifier, caps etc. on secondary side followed by low noise LT1963 (or similar) regulators.What else besides a fuse should be on the primary side of the transformer? MOVs? spark gaps? ........ Low noise power supply is my priority. Thanks.

author: exe
author_rank: Unknown
timestamp: June 27, 2018, 08:52:41 am
content: I'd put a fuse. Also EMI filter of lowest current rating possible as those have bigger inductance and, presumably, filter better. They also come at least in two flavors: normal and medical. I don't remember the difference, but you can google it.

author: IvoS
author_rank: Frequent Contributor
timestamp: June 27, 2018, 08:02:33 pm
content: Fuse will be there definitely. I want to put MOV on the AC line. How do I size it for 120V?

author: exe
author_rank: Unknown
timestamp: June 27, 2018, 09:53:54 pm
content: Quote from: IvoS on June 27, 2018, 08:02:33 pmI dunno. But for sure it must be rated higher than 120V*1.1*1.41 (nominal voltage * 10% variation * peak sine value).I didn't put a MOV because I decided it's not necessary. Transformer itself is quite robust to transients.

author: Cerebus
author_rank: Super Contributor
timestamp: June 29, 2018, 01:12:41 am
content: Quote from: exe on June 27, 2018, 08:52:41 amMedical filters have low earth leakage currents, which in practice usually means no Y capacitor(s) between the phases and earth.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: June 29, 2018, 01:34:43 am
content: 10W it approx 80ma at full load, so fuse would be 0.15A ?there are some good filter, you can make some similar drop some details like thermistor,I bought common chokes , for low power filters ...https://www.aliexpress.com/item/-/32703970119.html

author: exe
author_rank: Unknown
timestamp: June 29, 2018, 06:14:14 am
content: Quote from: GigaJoe on June 29, 2018, 01:34:43 amUhm, sounds about right for 115V mains. It's never precise anyway as it needs to account for inrush current, etc. I normally install the smallest one possible and increase current rating until it stops blowing. But the rule of thumb, afaik, is to have fuse for 150%-200% of max expected current. They are also come in two favors -- fast blow and small blow.Quote from: GigaJoe on June 29, 2018, 01:34:43 amI just bought one. May be not as good as this one, but saved a lot of time.

author: Karlo_Moharic
author_rank: Regular Contributor
timestamp: June 29, 2018, 11:32:15 pm
content: Just put 600V MOVs (at least a pair) , that is all you need on primary side (plus already mentioned fuse). Given that this is mains transformer , you don't really have to worry about noise. If you do get some noise from outlet , put a ferrite bead on the mains cable.Don't use glass fuses , instead use either resettable fuse or HRC fuse , which ever you can get cheaper , the minimum voltage rating of a fuse needs to be 300V.

author: CopperCone
author_rank: Super Contributor
timestamp: June 30, 2018, 12:32:50 am
content: if you read my threads one of them will be about protecting transformers. I need to revisit it to better formulate my thoughts.The idea was that you can use a reactor in addition to everything mentioned in this thread to limit the short circuit current at the cost of a voltage drop. Most people will stop at what is mentioned in this thread, it covers the classical solution that is widely used and considered by most engineers to be decent.

author: exe
author_rank: Unknown
timestamp: June 30, 2018, 09:14:58 pm
content: Quote from: Karlo_Moharic on June 29, 2018, 11:32:15 pmWhy not?

author: Karlo_Moharic
author_rank: Regular Contributor
timestamp: June 30, 2018, 09:38:13 pm
content: In case of HV surge , easily blows up , sending glass everywhere and damaging nearby components

author: ejeffrey
author_rank: Super Contributor
timestamp: July 01, 2018, 03:25:01 am
content: Quote from: exe on June 27, 2018, 08:52:41 amMedical has less or no capacitance to ground (class Y capacitors). This increases EMI or requires a more complicated filter to reach the same EMI suppression. The tradeoff is that it has less leakage to chassis/signal ground. Low leakage current is good when you are powering something with electrodes that might be attached to someone or even inserted into someone.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 01, 2018, 03:43:52 am
content: Quote from: Karlo_Moharic on June 29, 2018, 11:32:15 pmHRC fuses are total overkill in a plug-in instrument with a 10 W transformer. Standard glass fuses in a holder are totally appropriate here, and almost universally used in such situations.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: July 01, 2018, 04:20:23 am
content: If you want to explore transient suppression, depending on your environment and type of loads, etc., have a look at this article from ON Semi:http://www.industrologic.com/transient.pdf

author: exe
author_rank: Unknown
timestamp: July 01, 2018, 08:57:38 am
content: Quote from: Karlo_Moharic on June 30, 2018, 09:38:13 pmI use EMI filters with built-in fuse holders. Fuse slots are contained, so in case of explosion glass won't be spread (no sure about damage to the filter). There are also fully enclosured fuse holders. Like this one:https://www.tme.eu/en/details/0031.3501/pcb-fuseholders/schurter/. There are also covers available:https://www.tme.eu/en/details/keys4245c/pcb-fuseholders/keystone/4245c/. Cable fuses are also often fully enclosured:https://www.tme.eu/en/details/8601.2001.08/fuseholders-for-cable/schurter/.I wonder if ceramic fuses would be better in this regard. Like this one:https://www.tme.eu/en/details/0001.2501/fuses-5x20mm-time-lag/schurter/. But, of course, the physical size limits maximum voltage it can withstand. So, if a surge in the order of kV it won't help.I mean, HRC would be event better, but they are expensive :/ . On the other side, I read transformers (due to implicit high inductance) are quite robust to surges.Anyway, I don't want to make any suggestions, just offering alternatives to a classic open fuse holder and a glass fuse.

author: Cerebus
author_rank: Super Contributor
timestamp: July 01, 2018, 11:28:48 am
content: What people are forgetting here is that fuses aren't there to protect the equipment, they are there to protect the local supply wiring between your equipment and the upstream fusing. So when choosing a fuse's breaking capacity you don't consider the load, you consider the supply. If you're lucky enough to live in a civilised country that mandates high rupture current fuses in all mains plugs then you can happily stick glass fuses in your equipment. If you don't, or your equipment might be used in countries that don't, then you would be wise to specify high breaking current fuses (i.e. not glass ones) in your equipment.And whoever it was that said HRC fuses are expensive is talking out of their hat: 1A 250V 20mm x 5mm fuses, RS UK pricing - cheapest glass fuse £0.111 each, breaking capacity 35A, cheapest HRC fuse £0.142 each, breaking capacity 1.5kA. One third more expensive, 43 times the breaking capacity.

author: Yansi
author_rank: Super Contributor
timestamp: July 01, 2018, 11:41:02 am
content: Fuses protect not upstream, but downstream wiring.But it comes down to the definition of direction: Downstream means from the supply, to the load. So, your breaker panel protects the downstream wiring to the wall socket. And your small glass fuse does in fact protect against fire of the load - i.e. your small 10VA transformer, that could otherwise catch fire when severely overloaded.The glass fuse in your appliance have nothing in common of protecting the wiring from the socket to it. That is the job of the breaker panel.**Some freaked out countries even put another fuse in their crazy looking plugs, to compensate for the simple and dangerous design of their plugs.

author: Cerebus
author_rank: Super Contributor
timestamp: July 01, 2018, 12:02:59 pm
content: Quote from: Yansi on July 01, 2018, 11:41:02 amIt's a circuit, not a river. If the fuse at the appliance end opens, then current stops flowing without the inconvenience of a branch circuit breaker or fuse opening and taking everything on that branch down with it.Upstream/downstream is generally defined from where you're standing, not from the head end of the river/supply, otherwise everything is downstream. But, arguing about definitions of upstream/downstream is pointless when the issue is that one should consider the supply capacity not the load in determining fuse breaking capacity.Oh, and breaker panel fusing usually 15-30A in 220V countries, typical IEC cable rating 6-10A. So the breaker will open when the cable from the wall is overstretched will it?

author: exe
author_rank: Unknown
timestamp: July 01, 2018, 02:02:02 pm
content: Quote from: Cerebus on July 01, 2018, 11:28:48 amConcerning the price, it seems I was wrong. I took the price for fuses in DMM, but they have higher voltage rating. Indeed, ceramic fuses are not much more expensive than glass ones. I didn't knew they are also HRC. Do they have sand inside?

author: Cerebus
author_rank: Super Contributor
timestamp: July 01, 2018, 02:13:13 pm
content: Quote from: exe on July 01, 2018, 02:02:02 pmAll the ones I've seen cracked open do have silica inside. Yes, good meter fuses are chuffing expensive.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: July 01, 2018, 08:41:52 pm
content: Quote from: Yansi on July 01, 2018, 11:41:02 amI actually think the UK plugs are the safest design out there. They are huge, but they have a ground that makes first and breaks last, and the have insulation on the love and neutral which prevents any thin metal object (e.g. metal ruler) that may have slid down between the plug and the wall from shorting out. The fuse in the pug is a brilliant idea, though I would still put a fuse inside the piece of equipment as well.I have seen tons of blown glass fuses in equipment at 120V here in the US, and none of them have ever ruptured the glass. No argument the HRC are better, but I am just not sure how important they are at 120V, especially when in a proper, covered fuse holder.

author: mariush
author_rank: Super Contributor
timestamp: July 01, 2018, 09:00:56 pm
content: If you want low noise, wouldn't it make more sense to go with a toroidal transformer?At the very least, it would allow you to easily wrap it in some conductive material like copper foil and then ground the foil, and you got yourself a nice shielding. Even without extra shielding, the magnetic field would be inside the toroid and all that..At 10VA the transformer would also be smaller and if my memory is correct, at such low VA rating, the output voltage should not vary so much with the load (compared to standard transformer)You may have higher inrush current

author: C
author_rank: Super Contributor
timestamp: July 01, 2018, 10:20:17 pm
content: Quote from: Cerebus on July 01, 2018, 12:02:59 pmThe main purpose of a breaker or fuse is to prevent fire.So how hot will your typical IEC cable rating 6-10A get with a breaker panel fusing usually 15-30A.Heat of cable = time + power.What is the temperature rating of the insulation?To have possibility to start a fire you have up to when insulation fails.And NO the fuse or breaker is not there to protect the source of power in a properly designed power system. The power source has a fuse or breaker that serves this function.Simple electricity rules apply.

author: Cerebus
author_rank: Super Contributor
timestamp: July 02, 2018, 12:18:28 am
content: Quote from: C on July 01, 2018, 10:20:17 pmFrom what you've said I suspect that you don't understand that we've been talking about the breaking capacity of a fuse, not its load capacity. Theload capacityof the fuse is the current it opens at (very approximately - the fuse's rating is more properly specified as some value of I2t). Thebreaking capacityis the flowing fault current that it can effectively interrupt or, put differently, the largest fault current that it guarantees to interrupt. That's why I say you have to consider the supplyin choosing the breaking capacity of the fuse.The earlier example was of a 1A load capacity fuse with a 1.6kA breaking capacity versus a 1A fuse with a 35A breaking capacity. If the current flowing in a fault exceeds the breaking capacity of a fuse, that fuse will fail to open the circuit and can even induce a higher current fault itself, tripping the next bit of protection upstream. So, an example for the latter case would be that 1A glass fuse with a breaking capacity of only 35A. It's very easy to generate a fault current of >35A with a simple accidental dead short - loose wire, loose screw, that kind of thing. The 1A HRC fuse would break that fault current, the 1A glass fuse wouldn't and might even become a lower impedance than the fault it was trying to isolate.A fuse that fails to isolate a fault in a point downstream of itself becomes a liability to points upstream of it and part of ensuring that does not happen is to choose a correct breaking capacity based on the upstream's potential for supplying a fault current to a downstream fault. Yes, part of the criteria for a fuse is "Does this have the breaking capacity to protect the upstream supply?". So in a "properly designed power system" properly chosen fuses/breakers protectboththe load and the supply.

author: Cerebus
author_rank: Super Contributor
timestamp: July 02, 2018, 12:20:08 am
content: Quote from: FlyingHacker on July 01, 2018, 08:41:52 pmUntil you stand on one while barefoot. Then they become the mostpainfulplug design out there.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: July 02, 2018, 01:57:27 am
content: Quote from: Cerebus on July 02, 2018, 12:20:08 amHahah! Like the Danish foot torture device known as “LEGO.”

author: C
author_rank: Super Contributor
timestamp: July 02, 2018, 02:00:40 am
content: CerebusI understand what you stated in your post.As the fault current becomes a greater % of supply you have a chance to have this kind of need.But looking at total system changes things..To get from 35A to a 1.6kA state you would need a very fast change. You would need a fast change just to get to 35A point.The connecting wire inductance will reduce fast changes a lot. Each stage of breakers or fuses is separated by more inductance.Short of a lighting strike, I see little chance. And here it becomes a race against time for what gets blown.The rare case of the glass exploding just shows a need for an enclosed fuse holder or use of fancy fuse that does not explode. That is selecting one of two options..But there is a point where you are just selling numbers with little purpose.You could make the same clam for breaking voltage. Above a point nothing or very little is gained.This chain of posts started with a power supply that would have a cable that connects to an outlet. This is part of the facts.

author: exe
author_rank: Unknown
timestamp: July 02, 2018, 07:39:27 am
content: Quote from: Cerebus on July 02, 2018, 12:18:28 amOne thing that puzzles me is how this is possible. I shorted glass fuses several times, they blew instantly. Not even a spark was visible. The only way I see fuse don't open is when somehow evaporating wire inside the fuse turns into plasma or something. But I thought it's more danger for DC, not for AC as plasma disappears at zero crossing.So, how can a glass fuse conduct under overcurrent condition?

author: Cerebus
author_rank: Super Contributor
timestamp: July 02, 2018, 11:46:44 am
content: Quote from: C on July 02, 2018, 02:00:40 amReally, you're either just arguing for the sake of it or out of ignorance. If it is hard to create a fault current in excess of 30 odd amps why have all of us experienced fuses/breakers of that kind of size tripping? Why do regulations, in my country at least, require minimum breaking capacities - 6kA for a domestic main breaker (BS EN 60898) and 6kA for a plug fuse (BS1363) if this is all fanciful and of no practical concern?

author: Cerebus
author_rank: Super Contributor
timestamp: July 02, 2018, 12:13:34 pm
content: Quote from: exe on July 02, 2018, 07:39:27 amBy being a neat little glass tube full of metal vapour. If you consider how readily a column of non-conductive gas (Neon discharge tubes, xenon discharge tubes, lightning in air etc) will conduct electricity once you get it hot enough, how much more enthusiastic to carry current is the vapour phase of something that was already a conductor at room temperature going to be?It's a misnomer that plasma discharges necessarily disappear at zero crossings. That only happens if there's enough off time passes that the plasma system cools to below the temperature that it is a plasma. Fluorescent discharge tubes run quite happily on 50/60Hz AC as we all know from experience.Fortunately the 'perfect storm' necessary for a puny 5x20mm glass fuse to turn into a raging torrent of plasma is rare or we'd all have horror stories of equipment bursting into flames because of them. However, it can happen and as the manufacturer's data shows theguaranteedbreaking capacity of 5x20mm glass fuses is low. That doesn't mean that theywillfail to operate correctly at higher fault currents, just that it is a possibility - one that is easily avoided by paying 2p more for an HRC fuse.Also bear in mind that theguaranteedbreaking capacity is more prosaic than preventing the perfect storm. All a fuse has to do to fail dangerously is to leave equipment live that ought to have been isolated, even if that is under circumstances where a failed 'blown' fuse is simply carrying less current after failing than before failing - it ought not to be carrying any current at all. You can start a fire with a smouldering bit of insulation, it doesn't require a can of petrol and a flare or a 6kA plasma arc.