Electrical Engineering Challenge

LED is backwards. Fuse is weird.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Looks like they are harvesting email addresses.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

At least the LED won't kill someone. The fuse is dangerous but so were the "All American Fives", so they may not think that's a mistake.

I'd guess it's a con to get you on a mailing list. ...Jim Thompson

-- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

| 1962 | The touchstone of liberalism is intolerance

OK. You win. The weird fuse kept me from seeing the backward LED.

Getting smarter every day. :o) The weirdness of putting a fuse on the secondary's center tap is actually OK. Apparently it's old school to put a fuse on the primary line. This brings to mind that aphorism about how the most expensive part on a board is there to protect the fuse. :o)

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Don Kuenz KB7RPU

Pin 1 on the switching regulator is duplicated. The one on the bottom should be power ground= 4

Hi,

Please let me know how this could be. It is clearly described that the input nodes comes from a transformer with 24 V. I don't know what VCT means, but if the transformer is isolated I don't see any problems with the fuse as long as the max. current is below 1 A.

Marte

Typical journalist nonsense. No respectable PCB CAD program would allow a part to have two pin 1s.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

"CT" means "Center Tapped", a common and "standardized" transformer nomenclature since at least the 1940s.

What's the point of fusing the secondary? What happens when the diode(s) short? What keeps "the max. current below 1 A."?

Oh, a diode short... the fuse looked different, but I couldn't identify what was wrong.

Thanks, George H.

the fuse in series with the transformer primary.

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  \_(?)_

The diode(s) hopefully will explode and open.

Hopefully. The UL doesn't like hopefully.

If one of the rectifiers shorts, that thing could sit there and burn and bu rn. Just shorted on half the cycle, hell, whatever it is powering might kee p running. Nobody knows nothing. That's the kinda thing that starts fires.

What's more, the secondary need not be center tapped at all because it is a single ended supply. Is putting a center tap on the winding actually cheap er than two 1N400X diodes ? They would still have 120 Hz ripple, everything would be the same, it's almost like they tried to screw u...

Oh.

Yeah, think here. If the LED is backwards, so what. If the IC pins are mark ed in error or whatever, so what. If it catches on fire, not so much. So I am pretty sure it is the deal with the fuse here.

I think it takes an infinite amount of current to explode a short. Irresistible force, immoveable object sort of thing, you know.

If one diode shorts, there will be no voltage available for the load. Draw the schematic to see for yourself.

The transformer will see a half-wave short. That may be enough to ruin the transformer, but consider that the core of the transformer could saturate (depends on primary resistance) which further aggravates the failure.

Yes, a non-center-tapped transformer could be used with a bridge rectifier. That is not the challenge. Work with the problem presented.

From a disaster standpoint, the backwards LED is a mistake but not a problem. It just won't glow. The major problem is the fusing.

Well I suppose the fuse will open if the regulator fails short circuit.

If the diode(s) short then the transformer will get hot and the thermal cutout embedded in the primary winding will operate. There is a thermal cutout embedded in the primary winding, isn't there? :>

piglet

Look at the OP schematic.

On Tue, 5 Apr 2016 03:12:24 -0500, John S Gave us:

snip

A diode can fail open or shorted.

Hell, a current shunt can fail open. There are other parts of the circuit that should be totally fried if it does, but it can still happen.

A circuit "breaker" can fail shorted too. It isn't very likely but it can fuse shut and pass more current than it was designed to open at.