To fuse or not to fuse. I'm confused.

[.Mike]

Sorry for the long setup on this one...

I've been working on a multi-effects off-and-on for a while now. It's basically ten effects in card-type format (think PCI card, but using Molex connectors to make the audio/power connections) in a big aluminum enclosure. I started putting together a power supply schematic and layout, but realized I didn't have enough knowledge to do it right, so I took a break and spent a lot of time researching, breadboarding, and simulating. I'm back at it again with a much better grasp at what I'm doing, and I have a question on the necessity of using a fuse.

I plan on powering the unit using a 12v DC 1A wall-wart, and regulating it down to 9 volts using LM7809s. I want to avoid having a transformer inside the actual effects box to reduce the chance of interference, and because I don't have anyone local to help me make sure everything is wired in a safe manner. I would prefer not to die because of a simple wiring error.

My plan is to take the 12V, filter it, and run it to three separate regulators-- one for the effects, one for the LED indicators, and one for an output to daisychain external pedals. I'm using three separate regulators so I can avoid having any one regulator dissipating too much heat. The cost is negligible, and the space needed isn't that much of an issue.

Do I need to use a fuse in this situation?

My thought is no. We build pedals powered by wall-warts all the time, and don't use fuses. I'm not doing any rectifying within the actual enclosure. The regulator has built-in protection in the event of a short circuit.

I described my plan over the phone to an EE friend of mine, and he thinks I should put in two fuses-- one when the DC enters the unit, and another on the external power output. He said to fuse it based on the total current that the power supply can handle. His main concern is damaging the power supply if for some reason the unit attempts to draw more current than the regulators can handle. I was under the impression that if this occurs, the regular's thermal overload protection will kick in, and shut the power supply down.

So, I'm confused on how to choose if I should fuse or not fuse. Any help or insight is appreciated.

Thanks,

Mike

[tubelectron]

Hi .mike,

A music shop owner friend gave me a Line6 PCB power supply card which was incorporated in a shop display rack to show the Line6 products. It was dismantled it from the unused rack when I had i. Some details :

1/ There is 10 9VDC outputs, and each of them has its own secondary winding on the transformer, it's own rectifier, regulator (TO-220) and capacitors. There is no fuse on these 9VDC outputs.

2/ The unique fuse was for the primary of the transformer, and it was located in the ACmains IEC 3 prong socket of the rack. The rack was grounded to ACmains earth. It was followed by a RF interference filter, a 115/230VAC voltage selector, which are both mounted on the PCB card.

But your case is different as you use a wall-wart adaptor and not an ACmains direct input in your device. And you're right, I never saw fuses on small wall-wart low-voltage adaptors, except maybe for the 120 to 230VAC US to Europe adaptors (I should check). IMHO, I would put a unique fuse, after the wall-wart low-voltage input of your device.

A+!

[maarten]

Depends upon the type of regulator you use; if they are the 1A types, the three of them may in total draw more than your 1A wallwart can supply, if you are using a lot of fx and leds etc. therefor you should protect the wallwart. The wallwart may have protection in its transformer, but that might be difficult to repair (certainly takes more time than changing a fuse during a gig).
Also, the fuse won't change anything to the performance of your equipment.

Maarten

[doc_drop]

So, just to throw a curve ball at you...

Why bother regulationg to 9v? I use a regulated 12v power supply for all my circuits. I have made several 4-in-a-box type thingies. I just use one of my 12v wall warts to power them. I have never had a problem, even powering 10-12 circuits at a time.

None of the circuits (overdrives, fuzzes, distortions, flangers, choruses, sub-mini tubes, phasers, etc.) have ever had a problem with 12v. Most of them have better head room at that voltage too.

Just something to think about. Simple is good in my book!

(Oh, and I don't think you would need a fuse unless you are going with power form the mains, ie 120v. 12v is still pretty tame.)

YMMV

[petemoore]

 For a fuse to be effective it must blow, otherwise a wire does the same job.
  To blow a fuse requires X power, making a fuse small enough...do they make a fuse small enough?...I've never seen one used on small signal devices using 12v [or 36v for that matter] supply...I realize this isn't a sentence but still helps boils it down I guess.

[.Mike]

Thanks guys.

Bruno, your thinking was my thinking-- it's already rectified to DC, and I've never seen a commercial effect that uses a wallwart that also contains a fuse. Then again, I haven't seen all that many commercial effects, and there are probably some factors I haven't considered.

Maarten, I am using 1A regulators, but mainly for their heatsinking abilities, and not for the current output. I shouldn't even come close to 1A on any of the regulators, with the exception of an accidental short. 10 LEDs @ 10mA would be 100mA. Ten effects, including a couple of PT2399-based delays, can't be any more than 250mA. The external effects will vary, but I can't imagine they would exceed either of the other two. I have about a half dozen 12V/1A wall warts, so I'm not really that concerned about blowing one. I am, however, concerned about starting a fire and burning my house down... heh. If a fuse could help prevent this, it's well worth the half dollar.

Doc_drop, I'm regulating for several reasons. The main reason, I suppose, is for consistency. I know that different effects behave differently with different voltages, and I want my setup to sound pretty much the same regardless of the wall voltage or how much current I am drawing from the supply. Another reason is to hopefully limit noise. I'm mostly an at-home player, and the power in my 90 year old house is just terrible. Any little bit of possible noise prevention is probably a good thing. I went with 9V because a 12V wall wart provides enough headroom for the regulator without out burning off too much voltage as heat.

Pete, thanks for the extra confirmation. My friend recommended using a fuse that was 1.2x the maximum possible output of any one regulator, but the regulator would certainly shut down due to heat before the fuse would blow. I'm planning on using some small aluminum heatsinks that I salvaged from an old motherboard.

So the general consensus is that a fuse wouldn't hurt, but is probably not necessary. Thanks guys. Unless someone has a compelling reason why I must use a fuse, or someone points out something that has been overlooked, I will probably just skip it.

Mike

[amptramp]

If you use a power bar as the source where your wall wart plugs in, you are protected by the breaker on the power bar.  Most voltage regulators have current limiting and thermal overload protection (that essentially reduces the current limit threshold), typically at less than 1.5 times the rated output, so there is no need for a fuse after the regulators.  They should be able to handle a shorted power supply bypass capacitor easily or any other type of shorted output.  I had the breaker in the power bar for my computer shut it down just last week due to some unknown fluid that got into it, so protection should be there.

[PRR]

EEs think too much.

Young EEs have not snorted enough smoke to know what happens in a real world.

The wall-wart is designed to fail "gracefully". A dead-short inside or on the output may melt the case a little bit, but is not supposed to start a fire nor throw sparks.

When it dies, you buy/scrounge another wall-wart.

This has been proven "OK" by millions of cheesy answering machines and other small toys.

A small transformer can NOT pass the huge energy available from a wall. A 12V 1A wart may give 6V 5A or 30 Watts worst-case. (A worst-case wall outlet can dump 60,000 watts for a part-second, or 2,400 watts for hours.) Get a small 40W (nearest size) incandescent lamp and _carefully_ try to start a fire with it. It is possible, but it is NOT easy. (The old dorm-fires usually involved rayon scarves draped over enclosed 100W bulbs.)

Basically the wart "is a fuse".

This pre-Approved wart you use as-is.

The regulators are 99.9% sure to shut-down before they melt. If worst comes to worst, they are what? $2 parts?

If three 1A regs feeding a nominal 100mA load ALL get overloaded and pull 3A total, the 1A wall-wart will sag a lot. It will get hot. (A switcher supply will shut-down.) If it gets real hot it may melt and you buy another.

If the wart does get hot, stink, smoke, and die, be careful taking it off the wall. Obviously it could be hot. While they are supposed to fail nicely, once in a while the wall-prongs gets loose and stays in the wall socket. Go down cellar and kill the breaker, then use plastic tools to get the stub out. (Two safety steps: power "off" and tool "insulated", just like you unload a gun AND keep the safety-lock on.)

> one for the effects, one for the LED indicators, and one for an output to daisychain external pedals.

I would argue for segregating the "external" stuff. You know (can measure) your effect drain, and your LED drain, but you can't for-sure predict how-many how-big external loads the user (even you) may plug-in in the heat of performance. There are pre-made many-effect power supplies, tested in the hands/feet of many users. Since I think your total load even with unknown number of externals is probably under 1A, it is probably fine; just saying something to ponder.

> use a power bar ..., you are protected by the breaker on the power bar.

There may be a 15A breaker in the strip. There is a 15A or 20A breaker in the basement. These protect their own wires, and limit the damage when a lamp-cord gets shorted. They probably won't protect against any short on the secondary of a wall-wart. Nominal 120V drain is maybe 0.1A, winding resistance is high, dead-short secondary may pass 5X-10X nominal current, maybe 1A from the strip or wall.

[.Mike]

Excellent, thanks for the great information, Paul. I'm wondering if I explained it wrong to my friend-- maybe he thought I was bringing AC straight from the wall to an on-board transformer, and was just looking out for me. He's definitely at the point of his career where he doesn't have a smoke sucking deficit.

For the external output, I think one thing that is important to me is to simplify things. I'm constantly lugging my stuff upstairs and downstairs, and I figure that daisychaining off the main unit will make that easier. I rarely play anywhere but at home, and I'm not in a band and don't plan to be. I know there are nice, reasonably priced power supplies like the 1Spot, but where's the DIY fun in that?! If it works out later that my design isn't up to snuff, I can buy a separate supply, and just not use the one in my effects unit. Only a couple of bucks wasted.

Oh yeah, and breaker in the basement? I wish. On all accounts... heh! No basements here in Savannah. In fact, my house is on brick stilts to keep us theoretically safe up through a Cat4 hurricane. And breaker box? I have one of those-- on the outside of my house. I also have two separate fuse boxes embedded in the back of my kitchen cabinets. There's nothing I enjoy more than emptying my shelves to check if I have blown a fuse. I have a nice combination of cloth-covered wire, 2-wire Romex, and 3-wire Romex. Oh, the fun of owning an old house.

Fuse: omitted.

Mike

[PRR]

> emptying my shelves to check if I have blown a fuse

That is, as you probably know, "improper". I don't have a residential NEC handy, but at work it was 30 inches free clear space in front of breaker panels. Aside from annoyance, a fusebox can take HUGE short-surge and while the idea is to keep it inside the box. sometimes stuff (sparks, hot moltn metal) gets out. A box of Cheerios makes good tinder to get the cabinet started burning.

I know from bad experience that the cloth wire will probably make trouble before the fusebox.

But maybe the hurricane will douse the flames.

I did a 10% re-wire in my old house, Got rid of half the worst cloth cable and open splices. That was a lot of work, and relatively easy because it was a former dairy-shed (circa 1830) with some access from above. I used a lot of surface-run WireMold. Not pretty but solid.

Here in my "new" (1983) house 95% of the wiring is tolerable, with a major dent in the main box (interesting tale).... but the plumbing is re-re-re-modified and patched. I just bypassed maybe 20% of it (love PEX) and expect to get the rest in a few years.

[.Mike]

That is, as you probably know, "improper".

Yep. My house was built in 1920, and apparently since had electrical work done on it twice that I can tell. The most recent, as far as my neighbors know, was in the 60s or 70s, when an "electrician" supposedly rewired the whole house. Let me tell you, he did a great job. I only have a handful of outlets that are properly grounded, and no GFI anywhere. At least he had the sense to disconnect the knob and tube wiring that runs throughout my attic.

I pretty much don't use the outlets that are fed by the cloth wire, and I am extremely careful with how much current I pull from the rest of the circuits. I have a map that I've drawn, the best I can, that I refer to frequently whenever I plug in something new. My house is one of the original four in my neighborhood, formerly a rice plantation, and all of my neighbors have similar hodge-podge electrical.

I do plan on fixing and upgrading my electrical. Most of it should be pretty easy, since the lower-floor circuits are accessible from under the house, and the upper-floor from the attic. We have no insulation in our walls to cause any obstruction. I'll get to it just as soon as I put on a new roof... and finish remodeling the bathrooms... and fix the awful water pressure... and insulate the attic... and insulate under the house... and replace only 18 more windows... and get rid of the remaining 1970s paneling... and... and... and...  

Mike