Electrical Engineering Challenge

the regulator doesn't require it but else the regulator may as well be

Ummm... could you add my last name to your spelling chequer?

This seems to be their first contest. No results yet, but check here near the end of the month:

Incidentally, the original contest page mumbles:

error in the schematic below." which literally translates that they took a perfectly good working schematic and literally "inserted" something. My guess(tm) is that they "inserted" the fuse and switch, in the wrong order. Wrong part values and reference designators don't constitute an "inserted" error.

So what does UL say about having a possible short across the power line "before" the fuse?

That has to be it. In a normal plug-in-the-wall circuit, if you have to change the fuse, you could get shocked, even with the switch off.

This circuit is shown as floating so technically, maybe that's OK here?

A nit pick maybe but the non-polarized caps are shown with the curved side which would usually be two parallel plates. Probably not that.

boB K7IQ

There are plenty of symbols. Most dual flat plate symbols represent a true non polarized cap, from a non-dual internal structure.

A proper representation would be TWO EL caps, back to back, with the center connecting node being unavailable internal connection. That IS physically how they are constructed and why they are twice as large as other EL caps of the same value..

The actual, in use symbol, however, is a simple, curved plate symbol with NO polarization sign.

Folks who over the years got lazy and left that polarization sign out of polarized cap schematic drawings have become part of any problem with identification we see these days. They should ALL carry said sign EXCEPT for any non-polarized use of the symbol.

I'm dubious. First, sometimes the fuse (fuse link) is inside the transformer. Second, I've used AC entry modules that interlock (the AC cord must be removed before the fuse door can be opened). There may or may not be a safety requirement in the fuse/switch order.

That's easy to deal with. Make it a user requirement to not turn off power when the AC line is at the peak of a cycle, either polarity. Add it to the user manual and you are done.

No, good documentation.

The common twist off cap fuse is protected because the AC line hot goes to the far end of the fuse holder. When you unscrew the fuse holder cap, the fuse disconnects from the far end long before the cap is removed from the holder. Of course if someone wired the AC line hot to the other end of the fuse holder, you would get fried if you touch the fuse when extracted.

Open cartridge fuse holders like this are another story. Those are usually hidden behind some kind of protective cover, possibly with an interlock. I wouldn't use them for unsafe voltages (anything over about 50 volts).

The fuses built into the panel mount AC power jack disconnects from the circuit during removal: No shock hazard there.

It would be a rather badly designed product that exposes the line voltage on a fuse during removal.

There have been Engineering Changes to various power supplies in years past because the designer did not understand this principle. :-?

I wasn't that sloppy, but I was involved with a different fuse related hazard. The fuse holder end pin, with the AC hot line, was not insulated inside the radio: The mechanical designer located the fuse near the top of the box, where someone reaching into the box could easily get fried if they touched the exposed terminal. Nobody noticed until I got shocked working on the first shipments. It was unlikely to affect users as it required removing about two dozen screws to remove the covers, but I didn't want to kill anyone servicing the radio. I couldn't believe how much paperwork was required to add some shrink tube around the terminal.

"By accepting a Prize, Participants consent to the use of their name, location, and Entry, in perpetuity, in any and all media now known or hereafter developed (including, without limitation, print, broadcast, and Internet), for all legitimate business purposes including advertising and promotional activities without additional compensation,"

So, if you win a prize, and later get mega-rich, they'll use your name for advertising purposes, and you'll have no say in it.

Sylvia.

all for a network card lol

NT

It's interesting how many people (including me) missed that one.

That's what we have design reviews for, to get 2N eyeballs onto every sheet.

The power entry module we commonly use has the switch before the fuses. (double pole switch and two fuses (one on each leg) if that makes a difference.)

George H.

I think the issue is, does the switch have a 15A/20A rating (i.e. is the presumed branch circuit breaker sufficient fire protection), or is it only good for the lower current that the transformer can handle. If the fuse has only to protect the transformer, it can be on either side of the switch. Because transformers saturate when there's DC on the input, the transformer likely does need protection.

If the whole thing is one integrated module it really isn't so important, is it?

I guess I don't understand why the order of the fuse / switch is important. Is the worry that the switch fails, but doesn't draw enough current to blow the 15/20A circuit breaker and so starts a fire?

George H.

I agree. Why build a regulator that is limited to 35mA? I did not account for the difference between the 05 and the L05 regulators. In other words, I assumed there was no quiescent current used by the regulator. So, the estimate is a best-case analysis.

The input capacitor is the key. The circuit will not function properly without much greater input capacitance. Probably 1000uF is appropriate. I think that was mentioned elsewhere in the thread.

JS