Power Transformers With Internal Thermal Protection Fuse

[TomCarlos]

Friends...
As a novice tech, I had no idea that Power Transformers have an internal protection fuse. I thought this only existed in hair dryers!!

I recently encounted a blown fuse inside the transformer on a Fender amp. It seems cumbersome to go in and replace the fuse, then reassemble the transformer and hope it will not short out internally.

So my questions are:

(1) How common is this?
(2) What is the point of having an internal thermal fuse when the amp already has an external fuse?
(3) When buying a replacement transformer, how do you know if that one also has an internal fuse?

Again, it seems redundant to have the thermal fuse- especially INSIDE the transformer!

Thanks for your comments... Tom

[Chuck H]

Sometimes you can replace the internal fuse. But generally you would just replace the whole transformer since it's often more trouble than it's worth or the fuse is not easy to get to. This from what I've read here. I haven't had the pleasure.

The internal fuse should be rated higher than the external fuse. It's there as a fire prevention measure and probably a legal issue for the transformer MFG. Unfortunately there seem to be some transformers in current amps that are prone to popping their internal fuse. This is an expensive glitch for the amp owner.

[TomCarlos]

Ok Chuck....

That makes sense. I appreciate the reply.

Something I learned long ago- LOOK at the fuse to verify it's value. It could be that the guy who serviced the amp before you decided to over-fuse!

So I now understand that in cases like that, the internal fuse acts as a backup.

In my case, the amp did have the correct fuse. But again, who knows, someone might have over-fuse, burned up the transformer, then stuck the original fuse back in.

Tom

[trem]

How on Earth did you find that.?
I also was not aware of this fuse.
Anybody know what model amps/transformers had this.?
I have a SF Deluxe Reverb with a bad PT.....
And as you say, it is probably advisable (for a few reasons) to just get a new PT.
Thanks

[R.G.]

It's a way to meet the requirements of the safety regulations. There are standards for fire and electrocution safety. An overheated transformer may short out and cause a fire, and the standards say "it can't do that". More importantly, they require it not do that even with any single, worst-case component failure in the box. The standards do not say "put in a thermal fuse"; they say "it can't cause fires or electrocution", and how that's done is up to the maker.

A thermal fuse is a good answer to this in many ways that are important to the makers and safety testers. It's pretty sure to shut down things on overheating before fires get started. The problem is, musicians and manufacturers differ in their opinions of what constitutes an OK overload.

To answer your questions:

How common is this?

Very, very common, and also unknown until you disassemble the transformer.

What is the point of having an internal thermal fuse when the amp already has an external fuse?

They protect against different things. The AC mains fuse protects from electrical shorts that may also be outside the transformer. The thermal fuse protects from internal shorts and fires in some cases.

When buying a replacement transformer, how do you know if that one also has an internal fuse?

You can be pretty sure that if it's made by the original manufacturer, it does. Another replacements source may not. However, pay attention to what follows:

The manufacturer goes to the trouble to specify that inside the transformer at least partially to limit their legal liability in case of a failure. The way that works is that if they have complied with all the safety testing, it's hard for a judge to say they were reckless in endangering customers.

By the same reasoning, if YOU replace it with a transformer without a thermal fuse, you are taking the liability on yourself that the replacement will not overheat, start a fire and injure or kill someone. Remember that.

Next point: if you repair the transformer by putting in a new thermal fuse in the old transformer, do you then subject it to the necessary safety testing? The answer is no- you can't, because the tests are destructive. No non-destructive tests you can do on it will do. And it is unlikely that you'll be good enough at transformer repairs to meet the thermal, electrical insulation, and creepage-and-clearance requirements inside it. If you did, it would involve a complete tear-down and rewind. A rebuild would take enough time to cost more than a replacement in bench charges.

This is just my view of it. In my opinion, you order an exact replacement and put it in. But then I've been trained to be very, very afraid of lawyers. Of course, you may be less concerned with this. Up to you. You have been warned.

[Billy R aka DynaFreak]

let me guess, is that Blues Junior PT? I've replaced a few of those (Fender Tech). Those amps tend to get run very hard, and they always sound awesome (just before they blow). Do not try to repair that. Replace with an exact Fender part, supplied by your local friendly Fender Dealer. Be sure to shop around and spend some cash while you are there!

[J M Fahey]

Short example: your transformer may work regularly, for ages, at, say, 45ºC with an outside temperature of 25ºC. To consider very hot days, many hours of hard rocking and a basement or closed, soundproofed rehearsal studio with no wind at all, they *may* find acceptable that it reaches, say, 65ºC, which is very hot (70ºC is unbearable to the bare hand).
Suppose the wire insulation can stand 120 or 140ºC *but* the nylon bobbin starts to melt or deform at 85ºC.
The manufacturer may decide to split the difference and fit a 75ºC thermal fuse , exactly midway between 65 and 85.
You can't complain to them: "your thermal fuse killed my perfectly good transformer at 75ºC", because if you follow the series of temperatures (under different conditions) I mentioned, you had no business letting it reach that temperature, which in anybody's book *is* a gross overload, well beyond any reasonable working condition.

[Kazooman]

RG and JMF. Would it be possible to have some sort of resettable thermal fuse? It is a crime to have to replace the PT if the internal thermal fuse blows for no apparent reason. Things DO fail.

In some instances, a repair might be the best or only option. I purchased a non-working Fender Dual Showman Reverb head (the much maligned red know model) for close to nothing as a project. It was immediately apparent that the PT was not working although it did not show any signs or smells of abuse. As it turned out, the reason that the head was so cheap is that it is virtually impossible to find a replacement PT and I assume that the seller had already discovered this. I pulled the PT and sent it to Heyboar and they repaired it for much less than a new PT would have cost if I could have found one (about a quarter of what a custom build would have run). They also confirmed that there were no signs of a PT meltdown when they pulled the bells. The head works fine and the PT doesn't seem to run any hotter than other amps I have.

[R.G.]

Would it be possible to have some sort of resettable thermal fuse? It is a crime to have to replace the PT if the internal thermal fuse blows for no apparent reason. Things DO fail.

It's a game of weighing imponderable quantities.
-The manufacturer have to pass safety inspections because of the possibility of huge legal awards in the unlikely event that an amp fails in just the right way to cause a fire or a death.The monetary losses for that are big enough to wipe them out.
- It is unlikely that very many of the transformers will *ever* blow their thermal fuse. Understand that "not very many" is way down in the sub-1% range.
- Of those few, only a tiny fraction will be traced to the thermal fuse.
- Of those, only a few owners will be told the cause, and only a few of those will be angry.
The net result is that sales will be a lot smaller if it's even a few bucks more expensive, and only a very, very few customers will be unhappy, and that will be years later. It's a good reason not to spend the extra money. Manufacturers who spend the extra money have lower sales, stay smaller, and go out of business more often. It's a Darwinian situation - only the manufacturers who pinch pennies survive and become big. The little ones fail more often, and this selects for the big ones, too.
- It is possible to have a resettable thermal fuse. It is just much more expensive than the non resettable kind.
- resettable thermal fuses are more prone to failure themselves because they're more complex than the non-resettable ones; as you note, things DO fail
- Whether you know it or not, you are a very rarified, unusual, informed consumer of amps.

In some instances, a repair might be the best or only option.

You have to look at this from the viewpoint of the government, legal system, and manufacturers. In their view, "repair" means "replace the failed transformer". It does not mean that someone should hack in a fix and compromise the safety features and testing.

There's another side to this. If you own an amp, and it fails with a thermal fuse in the transformer, then YOU insist that the tech repair that transformer instead of replacing it with the approved, tested replacement, and later sell the amp to someone, who is harmed by the amp, the courts can - and have - held that YOU would be partly responsible for the harm and according to the principle of joint and several liability, make YOU responsible for 100% of the eventual damages, even decades later.

I am very much aware how much this chain of events is like trying to stack marbles. Still, very similar things have happened.

... it did not show any signs or smells of abuse... there were no signs of a PT meltdown when they pulled the bells. The head works fine and the PT doesn't seem to run any hotter than other amps I have.

Yep. I believe you. However, from the maker's viewpoint:
- the amp lived out its warranty time; the fulfilled their obligations to the customer
- there were no internal signs of overheating, so the thermal fuse (if that's what it was) did its job and *prevented a potentially bad situation from getting worse*; the whole point of protective devices is to sense the edge of a damaging situation coming, and stop things before it gets irrecoverable; viewed in this way, the thermal protector (if that's what it was) did its job perfectly
- the bad situation may have had nothing to do with the transformer; it could have been a high ambient temperature outside the transformer; note that a high temperature *period* is what softens and degrades insulation, so even if the transformer would have survived the temp, the protector should have opened, and was doing its job; or the protector could be faulty
- a rebuild by Heyboer is probably a solidly built, reliable thing; that's not what the government, safety inspectors, legal system and manufacturers think of when they hear "repair a failed transformer"; they think of some junior tech with no experience or training, wrong materials and no idea about safety issues jury rigging a transformer in a clumsy, sloppy way

In some countries it may well be a crime to repair and refit the same one. In the USA, it's not. But there are big legal penalties for being wrong and unlucky if the marbles do stack up.

[J M Fahey]

*Excellent* analysis.
In my particular case, I live in Argentina where thermal fuses the type we are talking about are not available.
To be more precise, since they needn't be fit by Law, 99% of electronics suppliers don't even know what I'm talking about, and only a couple very tiny shops carry them.
I'm talking, for example, a guy who repairs plane electronics, controller boards for monster newspaper printers and stuff like that.
He has, for example, tiny toggle switches or 1/2" single digit LED displays for which he charges U$50, each.
Why?
Because they are the exact toggle switch or Led digit or whatever needed to repair a Collins HF9070 airplane radio, and they need it *now !*.
Well, this guy does carry thermal fuses of all kinds, along tons of wondrous stuff, but at his prices .... forget it!!
At the same time, I can buy tons of coffee percolator type bimetallic switches, which of course are self-resetting, for peanuts.
Guess what I use.
Before stoning me, remember that:
a) I'm not in the USA
b) Thermal fuses are not mandatory here, so I'm in fact *increasing* reliability or safety compared to the masses.

[Kazooman]

Thanks for the great analysis. I'm in the "fix it for fun" camp and don't plan to resell the head so I guess I needn't call my lawyer just yet.

Having said that, I really should look again to see if a replacement PT has become available. I tried my best at the time and there were none to be had.

[R.G.]

Before stoning me, remember that:
a) I'm not in the USA
b) Thermal fuses are not mandatory here, so I'm in fact *increasing* reliability or safety compared to the masses.

I'd be the last to criticize that. It is a very rational response to your situation.

The funny thing about the USA situation is that thermal fuses are not mandatory here either. But if you guess wrong, and are legally unlucky, the possible losses are very big. Not only that, but if you have a lot of unhappy customers *with* the thermal fuse (for instance) then those customers can sue you for putting them in, on the theory that you made a defective product that fails too much.

What's really going on is that the product liability law "industry" in the USA has become a kind of legal-industry lottery. A very few people get lucky in huge ways and the publicity on that is enough to get nearly everyone sued for something at some time, even if it's completely irrational and unfounded, and the vast majority of these suits are thrown out of court. So there is some customer, somewhere, that will search diligently to find anything you *might* have done wrong, or failed to do right enough. I have had to adapt my views to that upside down state of affairs. Ugh.

[Enzo]

You really don't want resetting breakers. At least not the self-resetting kind. Those will open when a failure arises, but then they sit there clicking back on every so often. Once powr is removed from the failed circuit, we don't need it powering back up for a few seconds once a minute.

Your mains fuse opens when current is excessive. The thermal fuse inside the PT opens when the PT gets too hot. Why might you want both? That mains fuse represents an amout of power - in other words a 4A fuse on a 120v main means 480 watts can flow through that fuse all day. Your transformer can get extremely hot without using all those 480 watts. You don't want your transformer melting down while the fuse continues to handle it. SO a thermal fuse protects against that.

[R.G.]

The circumstances on a transformer are different. Most transformers of any size have a thermal time constant of half hour or more because of their high mass. A resettable thermal fuse inside a good sized power transformer would take at a guess 10-30 minutes to cool enough to click back on, and the same to get hot again if it was not massive overload causing the heat rise. I've done this test before. Things like heat sinks and small circuits with shorter time constants do the on-off cycling. Transformers are thermally ... slow...

My choice would be to have the thermal breaker cause a "thermal overload" indicator come on to provide info so the user could make an informed decision about when to turn it back on. I've seen some guitarists where no amount of information would help, but that's an other issue. We can't sue their parents for that.

Yet at least. 8-)

[J M Fahey]

The couple times (no more) it happened I got the amp back on Monday, with the comment: "don't know what happened Saturday night, the amp turned off, we checked everything but couldn't turn it on again, had to use a backup, yet on Sunday, back at the rehearsal room, it turned on and worked perfectly well, please check it just in case".
Obviously after 5 "endless" minutes it had not yet cooled enough, and the show *had* to go on.
Oh well, being an impatient Teen again !!
My only explanation is that they connected "just" an extra speaker, and the lower load impedance was enough to overheat it but not enough to trip the short protection.
Mine triggers an SCR which disconnects the speakers through a relay, you have to turn the amp off 1 minute for it to self-reset and, of course, remove the short or it will trigger again.
I've had complaints (even spread locally on the Internet) about my amps "turning themselves off for no reason, 10/20/30 times in a row"
Oh well.
EDIT: the "thermal overload" sign would be a good thing, can be backlit with a neon lamp across the thermal switch contacts.
I *used* to put a "short-check speakers" labelled Led across the Relay coil/bobbin (it's energized to *cut* sound) but Musicians didn't like it, they were angry that it cut their sound at all.
Soundmen (which are supposed to be Professionals) were angry that with my amps "they could not add/pull an extra monitor in the middle of a song because that killed that entire monitor net"
The idea that you don't do such things in the middle of a song fell on (literally) deaf ears.
Oh well [2]