Fuse on backwards transformer?

[mth5044]

Fuses are used universally on the mains side of equipment, and I've seen at least one example were a fused is used on the center tap of the HT side, but when using a setup like the example on GEOFEX:

http://www.geofex.com/Article_Folders/tube-wah/tube-wah.htm

With a 12VAC wall wart feeding and 12V transformer into the secondaries, is there any purpose of putting a fuse in that system? Are we assuming the 12VAC adapter has a fuse, or some sort of safety?

Thanks!

[R.G.]

Fuses are used universally on the mains side of equipment, and I've seen at least one example were a fused is used on the center tap of the HT side, but when using a setup like the example on GEOFEX:
[...]
With a 12VAC wall wart feeding and 12V transformer into the secondaries, is there any purpose of putting a fuse in that system? Are we assuming the 12VAC adapter has a fuse, or some sort of safety?

It is firmly debatable. 
Fuses are complicated. Just to start with, a 1A fuse (just to take an easy example) is not designed to blow at 1A. It is designed NOT to blow 1A. When fed, for instance, 1.1A, it may take an hour or so to blow. It won't blow quickly with less than 2A in it, and even that depends on whether it's designed to be "slow blowing". or not. So protection done with a fuse is dependent on how clever and psychic you are about what overloads the fuse will see.

Fuses are also intended for different kinds of protection. A fuse on the AC mains side of equipment is not there to protect the equipment from overloads. It may do that, even accidentally, but a primary fuse is to protect the building and the people around the equipment from being burned up or electrocuted when the equipment has failed so badly that it's pulling vastly too much electricity.

It is possible, if you're clever, to protect a transformer with a fuse by putting a fuse in series with every single secondary winding on the thing. The fuse value then needs to be set so the fuse will open before overloading heating can burn out the windings. So in a tube amp (which is where this kind of question usually comes up) you cannot count on protecting the transformer with a primary fuse. You need secondary fuses for that, one per winding. Same if you want to try to protect an OT.

In the setup as shown with a 12Vac secondary feeding a 12V "secondary" as a primary and stepping up to a high voltage on the second transformer, you might want to put a fuse in the 12Vac loop. Or not. Most small transformers unavoidably have so much wiring resistance that their secondary voltages sag badly, and as a consequence, the overload currents may not be sufficient to overheat them. Some small transformers are designed to be "inherently safe" so that they either can stand the heat of a shorted secondary, or have a thermal cutout inside them that opens when they get too hot. This is set up and encouraged by the safety standards people to prevent fires again.

So whether a fuse would be good in the 12V side is firmly debatable. Depends on the transformers and conditions.

[mth5044]

Thanks for the reply, RG! My biggest concern is the safety of the operator and those around them. If something fails and only takes out the PT, I'd consider it a win. I've done some work with mains wiring, not much, but nothing I've read about it has been called 'debatable', so your use of that word puts me a bit at ease.

'Depends on transformers and conditions' - so in the example on your site, if there was a short on the 120V 'secondary' side, what would happen? Would we rely on the wall warts protection? Would it self destruct? What if we up the wall wart to 1A and a  backwards 12V/120V 12W transformer, or a 2A supply and use a backwards 240V/12V 24W transformer for power.

I suppose the biggest confusion is because I can't answer this: are we less concerned about this type of powering schem as we would be if mains were present in the box because compared to wall power, there's much less current, or are we less concerned because the wallwart is doing the heavy lifting and we are assuming that its design will be enough protection?

[PRR]

It is firmly debatable. 

I disagree. 

Wall-warts seem to be designed to burn-up harmlessly. They have so much resistance that large current is unlikely. The wire is very thin. There's not much flammable inside. The case is sturdy enough to reduce flame-throwing. They often fail silently. Worst-case seems to be a bubble in the wart plastic.

On the secondary: that plan uses maybe 10mA? You can't "protect" the operator: a prolonged 5mA is legally dangerous, so you build so the operator can not get *across* the 200V. You can't hardly find a 10mA even 25mA fuse, so you can't protect the circuit. (Which will never suck bigger current unless something fails.) Is protection even needed? The 100K on the tubes ensures the tubes will never get past warm. (100K with 100V across leaves 100V at 1mA or 0.1 Watts in a 1-Watt tube.) If you imagine the the 10nFd shorting, the MOSFET can do whatever it likes with the 200V, limited by the 1K in the PS and probably higher total impedance through the double transformers and rectifier, and the 500mH coil. It may go over 2 Watts. The proposed part can stand 1W naked, and 2W for several minutes. You do want a good cap there.

[R.G.]

My biggest concern is the safety of the operator and those around them. If something fails and only takes out the PT, I'd consider it a win. I've done some work with mains wiring, not much, but nothing I've read about it has been called 'debatable', so your use of that word puts me a bit at ease. [...] are we less concerned about this type of powering schem as we would be if mains were present in the box because compared to wall power, there's much less current, or are we less concerned because the wallwart is doing the heavy lifting and we are assuming that its design will be enough protection?

We're less concerned because the AC mains power safety is all cased up inside the first transformer attached to the AC outlet. The isolated 12V-12V loop floats and does not present a shock hazard if one is touching a grounded microphone, for instance. The isolated-twice 120Vac output is also not going to be a shock hazard to the rest of the world, although it can and may electrocute you if you accidentally get across both sides of the transformer winding. or from + to - on the DC it makes. But the high voltage out of the second transformer doesn't realistically offer the same electrocution ability (it's less sneaky) or fire starting ability as the AC wall power, so fusing it for safety is probably not needed at all.

Transformers best described as "small" generally have high internal resistances, so chances of burning the second one out are small, and it would take some fancy fusing to try to save the transformer. But it might be done.

It is firmly debatable. 

I disagree. 

I vigorously agree with your right to disagree. 
However, I hold some reasoning about my side of things, which I will debate with you, and therefore, my opinion that it is debatable is justified. 

Let's debate.

Wall-warts seem to be designed to burn-up harmlessly. They have so much resistance that large current is unlikely. The wire is very thin. There's not much flammable inside. The case is sturdy enough to reduce flame-throwing. They often fail silently. Worst-case seems to be a bubble in the wart plastic.

Yes. It was not clear to me what the OP's question was, so I answered many factets. It was not til his last post that it was cleared (a little, to me at least) that it was the safety aspects of the 120V high voltage side. That focus changes the answer a bit, in that all the safety you're going to get is in the first transformer plugged into the wall.

The 12V-12V connection may or may not hit currents in the 1-3A range. 12V wall warts commonly get up there. So there may be some advantage in the trying to save the second transformer with a fuse in the 12Vac loop. It's questionable whether this offers any real advantage or not, but one could argue it. It's debatable. 

On the secondary: that plan uses maybe 10mA? You can't "protect" the operator: a prolonged 5mA is legally dangerous, so you build so the operator can not get *across* the 200V. You can't hardly find a 10mA even 25mA fuse, so you can't protect the circuit.

Well, yes, you could fairly easily protect the circuit, if not the operator. I agree that a fuse is probably not the right technology, though.

So - yeah, it's debatable. 

[merlinb]

Only the first transformer matters. A ceramic fuse on the mains primary protects the appliance from gross surges and fire; it is mandatory. A glass fuse on the secondary protects the transformer (but rarely the circuit); it is optional. The second transformer in this circuit is irrelevent and needs no fuse.

[mth5044]

Thanks everybody! Sorry if I was not explicit enough with my question. Glad it got a little debate going though 

I shall leave it fuseless.