Fuse on both windings of a center-tapped secondary

[greekie]

Hello everyone,

It's almost 1:30AM here in Denmark, and my brain is therefore not working as well as it should be.

I am contemplating which fuse values to use in a new amp I am building. I have a fuse on both windings of a secondary - should these fuses be of the same rating as a single fuse on, say, the center tap or in series after the diodes?

Thanks in advance.

[R.G.]

They can be smaller, although I'd have to do some research to say how small they could be compared to the single fuse. It may be as simple as half, but I gotta do the math first. The average current in each is half, but that's not what heats the fuse, it's the RMS.

On the good side, it may not matter much. The range of usable fuse is fairly large. This is because the minimum rating is the maximum peak/transient current that needs to pass, and the maximum rating is the value that would burn out the winding. There is a large range of fuse values between the minimum and maximum that will let it operate properly but be protected from burning out.

Sorry, but the exactly perfect value is somewhat arbitrary. Something bigger than half the single fuse value, up to as big as the single fuse value.

[Steve Conner]

I'm going to guess the answer is 0.707, this being the square root of 0.5.

[greekie]

So a 500mA fuse would be replaced by a 350mA fuse... interesting! ;-) I'll see if I can construct a way of testing the practical aspect of it. Because I would like not to have the fuse on the center-tap, and on the AC part of the power supply. Merlin writes about this, but I haven't seen any answer for my question in any if his writings yet.

I'm looking forward to what you'll come up with, RG.

[R.G.]

Well, a 500ma fuse could be replaced by *two* 350ma fuses, anyway. Remember that you have to protect both sides of the winding.

A 500ma fuse in the center tap protects both halves of the winding to the extend that any overload comes through the combined rectifier/filter/load. It doesn't protect at all for a short from end to end, like the wires on the rectifiers, and it may not protect against a short of one winding to ground, because it's sized for a bigger current, the sum of both halves. That requires a kind of specific fault, but could happen.

Fuse sizing is kind of a black art for audio amps. The audio load is so very variable that it is tough to say what the peak demand is, and low long peaks are. On top of that, transformers are TOUGH, being just lumps of iron, copper, and insulation. It is always the insulation that fails first in response to overheating, but the iron and copper on both sides of the insulation are very good at conducting heat away from hot spots and evening things out, as well as soaking up a lot of heat before things rise too much. They can stand huge pulses as long as they get to cool off between them. Overloads are worst on thin-wire windings that have a higher proportion of insulation and less copper to conduct heat away from a hot spot.

The implied binary nature of statements in English get in our way understanding this too. When a manufacturer says their transformer will supply X amperes max, they do not mean that it will fail at 1.01X. It might fail at 1.5X or 3X, but they are not guaranteeing where it will fail, only where it will not. Same with fuses. The makers guarantee that a fuse rated at 1A will NOT blow at 1.0A. This does not mean it will fail at 1.1A. There are elaborate volt-time charts about when a fuse will blow in terms of percent of rating. It ma carry a moderate overload for hours, but blow in seconds (or milliseconds!) on a massive overload.

I think the best advice is to consider fuses to be a blunt instrument, not a scalpel.

[mooreamps]

Merlin writes about this, but I haven't seen any answer for my question in any if his writings yet.

Perhaps the "Grand One" awaits for someone else to derive the answer for him...

I've already posted comments on fusing low current secondary windings... Too steep for this crowd, so I'm just going to leave this one alone....

-g

[J M Fahey]

Too steep for this crowd,

Well, maybe if we promise to take our "Intelligence Pills", would you care to illustrate us?
At least a watered down version, so we don´t choke on it.
Thanks.

[R.G.]

I've already posted comments on fusing low current secondary windings... Too steep for this crowd, so I'm just going to leave this one alone...

I realize that it's sublimely tempting to take a shot a Merlin, and possibly me. But it may not be a great idea.

Yes, you have posted comments about fusing windings. My personal recollection is that those comments were somewhere between confused and flat wrong, and you were not willing to try to back up your one liners with any technical content.

I'll be happy to restart that fuse conversation, or print it out here in toto (I saved it ) if you want. If you think you were, in fact technically correct, but misunderstood, we can thrash out the issue of sizing fuses and how one goes about calculating it.

Shall we start that up again? It's really no trouble if you'd like to.

[Wombaticus]

Perhaps the "Grand One" awaits for someone else to derive the answer for him...

I've already posted comments on fusing low current secondary windings... Too steep for this crowd, so I'm just going to leave this one alone....

-g

If you're so goddamn smart, why don't you tell us yourself, oh great revered rocket scientist? But, oh, wait. Of course, you don't actually understand any of this stuff. So you'll probably need to nick your answer from someplace else. The top of page 42 of Merlin's power supply book might be a good place to start.

[mooreamps]

I don't have his book, nor would I likely want to. Just my attempt to explain the logic of putting fuses on the high current side of the transformer, verses the low current side. Which ever side it happens to ending up being ; primary windings or secondary windings..

-g

[mooreamps]

If you're so goddamn smart, why don't you tell us yourself,

Actually, I kinda am .......

-g

[R.G.]

I don't have his book, nor would I likely want to. Just my attempt to explain the logic of putting fuses on the high current side of the transformer, verses the low current side. Which ever side it happens to ending up being ; primary windings or secondary windings..

We're very eager to hear your logic. Please enlighten us, if you can. I personally would like to see you attempt to explain the logic of putting fuses in transformer windings, period. We did this once before, you remember. Please provide both logic and the calculations to support it.

Here's a hint: I gave you a lot of web references to how to do it last time we talked about fuses. You might want to read them.

If you're so goddamn smart, why don't you tell us yourself,

Actually, I kinda am .......

Once again, a legend in your own mind.

[Wombaticus]

I don't have his book, nor would I likely want to.

Why not? Afraid you might learn something, or are you worried it might be over your head? Ignorance is forgiveable. Deliberate ignorance is just kind of sad.

Just my attempt to explain the logic of putting fuses on the high current side of the transformer, verses the low current....

No, that wasn't what your post was about, at least not the one I was responding to. Rather, your post was all about *you*. In particular, you being a conspicuous dickcheese and taking a gratuitous shot at a respected member of the community who was not even involved with the thread. It's particularly galling, of course, that the member in question is actually knowlegeable and helpful -- in stark contrast to yourself. It's a miracle, really, that the shitty advice you dispense here on a regular basis hasn't yet hurt an unsuspecting novice.

[Steve Conner]

Gentlemen, no fighting in the war room!

Gary, here is an experiment you can try. Take a tube amp that has a bias tap on the PT secondary, or a separate bias winding. Short the bias winding out and tell us if the primary fuse blows before the PT goes up in smoke.

You can also try this with one of those hybrid bass amps, that have a huge transformer with a low current, high voltage winding added to provide B+ for the tube preamp.

You don't actually have to do this (or at least if you do, don't blame me for the damage) but at least treat it as a thought experiment.

[Zozobra]

I never knew fuses could be so controversial!