adding fuses to secondaries

[mooreamps]

well, I just ran this experiment last night ; again ; since I don't normally do this. I use a 1 amp fast blow on one of my 36 watt chassis. It powers up fine. Then, I toggle the switch which shorts out the high voltage and the line fuse pops. Maybe these fancy dancy guitar amp building books need a re-write for printing secondary fusing.

-g

[R.G.]

well, I just ran this experiment last night ; again ; since I don't normally do this. I use a 1 amp fast blow on one of my 36 watt chassis. It powers up fine. Then, I toggle the switch which shorts out the high voltage and the line fuse pops. Maybe these fancy dancy guitar amp building books need a re-write for printing secondary fusing.

Of COURSE you can clear a primary fuse by shorting a high voltage winding.

I said as much, although I did rip through it pretty fast to get to the issue I thought we were discussing; that issue being whether secondary fuses are a good idea, or whether you can rely on the primary fuse for everything.

Just give me a minute to go back and quote myself here...

...The primary in our now well-worn transformer is carrying 1000 times the rated current (if we used a 1A fuse there.)

Which pretty clearly says that the primary is trying to go to 1000A, yes? 1000 times the rated current of 1A? Right?

Here's another based on a different set of conditions, but still a secondary short:

So if we short the secondary of a transformer with similar specs, the AC line voltage is driving a six ohm resistor into a short circuit...
The current that flows in the primary is about 120V/6 = 20A...

So that pops a 1A fuse, right?

I'm sorry if I have been unclear. I'm just a little amazed that you seem to think I'm saying that you can't pop a primary fuse with a secondary short on a high voltage winding. If I had known that was what you were talking about, I'd have addressed it directly, as I'm trying to do now. Remember me remarking about whether I was being unclear or mistaken?

06-29-2010 01:59 PM
I'm confused. Maybe you could explain that a bit more. I'll kind of ramble through some more analysis and you tell me what I get wrong.
Perhaps I missed something in my analysis. Here are the things I based the math on:

I'm always willing to learn a new trick or two, so please tell me which parts I missed.

06-29-2010 01:59 PM
Please - tell me clearly if I missed something.
- Was my analysis on Isc being important to the primary AC line fuse...
- Does Isc have anything to to with secondary currents in the face of...
...
And as always, if my analysis in this note was incorrect, please tell me plainly and succinctly "This was wrong, here's the right way to calculate it." I really would like to learn the right way if I'm missing something.

Yes. Of COURSE a short on a high voltage winding can make a primary fuse open. That's never been an issue in this discussion.

We started with my showing how a secondary short on a LOW VOLTAGE HEATER winding can leave a primary fuse un-opened for long enough to damage the transformer. And further that a secondary fuse on a heater winding can prevent damage when that happens while a primary fuse MAY NOT be able to stop it. You even tacitly agreed with that by wanting to talk about something else instead of addressing it when I asked.

So you're OK with my analysis for filament windings, right? You're only worried about whether a fuse is a good idea in a high voltage secondary? OK, let's go there.

To which your reply was:

Well, I'm not going to do this any more. If you put a short on the secondary, it's going to reflect on the primary. The primary current is going to shoot up higher than the secondary current.

(Sidebar: Why I quote things like this.
I've been on the internet since before the days when there was a world wide web. I've also been the target of some character assassination attempts which were based on rewriting on-line history. I got in the habit of quoting sources in publicly available places where readers could check what I said. This is NOT personal. It's just my habit, and a way of keeping everything on a factual basis. And it keeps me from making mistakes. Sometimes. Sometimes I get it wrong anyway.

In all of that time, I've made mistakes. I find that it works best for me to flatly admit I was wrong when I've been wrong. So I quote and point to the sources for what I think are the facts. It helps me stay straight with the facts. And helps me embarrass myself fewer times.)

As to a high voltage winding short, I did the math to show how I think one could get a handle on what happens there, and to illustrate that a secondary fuse on the high voltage secondary may - not certainly will, but may - offer better protection than only a primary fuse, because the transient current overload is bigger there than in the primary (the voltage is bigger!) and might pop a fuse rated for just the normal current in the high voltage winding, which as you note is smaller than the normal primary current. I did not say, or mean, that a high voltage secondary short can't pop a primary fuse. Just that there are alternatives that may be less stressful on the transformer than relying only on a primary fuse, along with some math and a rationale for why I thought that.

I really don't understand how you got the idea that a high voltage short popping a primary fuse was even in question, so I'm taken aback that you feel the need to state it as though it was an issue, or go do a test.

Let *me* be excruciatingly clear. I'll ask once more: if you disagree with any of these statements, please clearly respond to one statement at a time, saying why you disagree with it and the facts which support that, along with any calculations that go with the facts so I can understand where I may have made a mistake. So,
1. In a typical tube amp power transformer setup, a short on the heater winding can burn out one or more windings on the power transformer before any reasonably sized primary fuse can stop it.
2. A properly sized secondary fuse in the winding as in 1 above can stop the damage to the power transformer.
3. In typical tube amp power transformer setup, a short on the high voltage winding can and will open a properly sized primary fuse.
4. In a typical tube amp power transformer setup, a secondary fuse in the high voltage winding can interrupt the fault current in the secondary, and under some circumstances can do this closer to the rated current of the winding than can a primary fuse.
5. In the situation of 3 and 4, whether the primary or secondary fuse opens first is complex to evaluate; but there are means for doing so.

Please, if you disagree with any of these, state clearly which one(s) and what the basis of the objection is.