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RE: Are electrical fuses not in common use in electronics anymore?

Posted by JURB (A) on June 14, 2016 at 16:16:43

In Reply to: Are electrical fuses not in common use in electronics anymore? posted by Dave Pogue on June 8, 2016 at 10:25:52:

"I can't find any evidence of fuses in my cable box or flat-screen TV. Have they become relics while i wasn't looking? "

Everything should have a fuse at the AC inlet, but the rest of them can be eliminated. The normal technique in good equipment is to have current foldback limiting which means to detect the current drawn and if it exceed a predetermined value the voltage will drop until the current drops to that predetermined level.

There have been TVs designed like that. For example the original Sony XBRs back in the 1980s. You could short out just about anything and nothing happens. I had a PR-36XBR which was a rear projection. Seriously good picture, better than some direct view CRTS. Very deep deep blues.

Anyway, projection TVs have ICs that are pretty much audio ICs but have to work up to like 50 KHZ to converge the three colors on the screen. They are also operated as close to the rails as possible for the best efficiency. However, that made them the most common failure. Well the Sony had a bunch of fuses on the board and there came a service bulletin from Sony saying to just jump out all of them to avoid the nuisance. Fact was, their main power supply had very good foldback current limiting and they had no need for any fuses other than the one off the AC cord.

Another was the Zenith G type chassis. Those were older and direct view, and used what is called a ferroresonant transformer. there was a capacitor across a winding, and the thing saturated the core and it put out square waves. This of course simplified filtering and also provided primary regulation. It was also inherently somewhat current limiting, but not quite as good as a real electronic current detector. But for the late 1970s it was a good solution.

I also had a Radioshack amp that seemed to do that. If a speaker wire was shorted both channel would be clipping at a low level until the short was removed. I never looked into how that worked for several reasons. First of all you had to buy the service manual back then, no downloading it for free. And now of course I do not remember the model number.

That was my coffee table stereo. Speakers were way across the room but the amp, tuner and turntable were all on the shelf right near the couch. Surprisingly for the low power is was I actually got the cops from the adjoining city across the river at my door for loud music. I had VERY efficient (and good) speakers and you would not believe me if I told you what they were.

And also, I have been put in jail for a loud stereo. I am not going into that story right now but it was funny as hell.

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