Hobby electronics projects-- fire safety?

It's basically up to you to ensure a short doesn't happpen but in the event it did such adaptors typically are protected by an internal thermal fuse.

Graham

Plugpacks are usually protected with an integral thermal fuse. But good practice would be to add your own fuse to your application. You could use an line fuse holder that can be wired in series with your the plugpack lead.

Dave.

"David L. Jones" wrote in news: snipped-for-privacy@m37g2000prh.googlegroups.com:

Interesting...about them having internal fuses. I've cut many, many of them open...to get out the small transformers for various projects...and have never encountered a fuse...thermal or current...on either side of the transformen, on the the little PCB's, or in the transformer windings. I'm talking about the straight transformer/diodes/capacitor versions...not regulated or switching. I thought they depended on crappy, easily saturated transformers for safety limiting.

???

Ken

"Ken Moffett"

** Not all plug pack transformers have a thermal fuse. However, all must pass a safety test where the output is shorted until the unit fails, without any hazard to the user. ** Yep.

** Some transformers are "inherently short circuit proof " so will not heat the windings enough to burn even if shorted indefinitely.

Others simply burn and go open - but because of the construction method, there is no risk of the primary and secondary becoming connected.

........ Phil

A thermal fuse would be 'buried' in the windings. Did you cut the transformer open ? Externally you wouldn't be able to distinguish a blown thermal fuse from a 'blown' transformer winding.

Graham

I don't know where all this talk of "thermal fuses" comes from. After all, a regular fuse has always sufficed to protect things.

The only reason something gets too hot is because too much current is going through it, and a regular fuse can stop that.

I was under the impression that a lot of those low current transformers were wound so that the primary would break if too much current went through them, and that was the "fuse".

On the other hand, I had an old Sony ac adaptor that did get shorted, and nothing happened so it got really hot, enough to scortch the surface it was resting on. It was, though, one dating from about 1971, so one hopes such things don't happen nowadays.

Michael

"Michael Black"

** Seems YOU don't know a whole bunch of things. ** Far from the truth. ** There are many circumstance where a wire fuse is not able to do that reliably or accurately enough.

Happens when the maximum safe operating current and the unsafe or overload current values are similar.

Happens when the overload current is only 50mA.

Happens whenever the fuse is accessible to a user.

** You labour under many silly misconceptions, then. ** That can still happen.

....... Phil

That only illustrates how little you know about product safety.

Graham

They have thermal fuses.

Graham

a thermal fuse blows before the varnish ignites.

--

Bye.
   Jasen

Really? I've seen a lot of the thermal fuses brought out to separate terminals. One lead of the primary was soldered to one of the terminals, and the fuse could be replaced, or jumpered for testing to determine if the unit was worth repairing. Some transformers even have a notch in the nylon or plastic bobbin to make it easy to remove the fuse and replace it.

--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

from

The only transformers I've seen with the leads brought out were ones with resetting thermal switches. These are a bit more upmarket.

I've also come across transformers with buried self-resetting thermal switches too.

Graham

Or melts, causing a potentially dangerous fault condition. It's all about temperature rise. Most of the common insulations are rated for no more than 130C and the transformer winding itself won't fail open at those temps.

Graham

I just opened one that had failed to get the wire from it. The fuse was not apparent, until I dissassembled the laminations to get to the bobbin, and removed the tape so I could unwind the primary. The fuse was between layers of tape.

Ed

Probably a thermal fuse placed to get blown if the windings overheated.

Power supplies / "wall warts" that are UL listed as a "Class II transformer" or "class II power supply" are supposed to be reasonably safe even if the load malfunctions.

However, I would use some common sense with the load. If the load is a prototype or an experimental device, I would add a fuse that is actually tested to reliably blow if the load fails short or encounters its most probable failure modes that result in increased current draw (component failure or shorting within the load). With the fuse reliably blowing even when the Class II wallwart is warmed up and its winding resistances are higher. Otherwise only operate the experimental/prototype device when you are around to unplug it if things start smoking. I know that Class II power supplies are supposed to be reasonably safe from shorts and overloads, but I suspect some barely pass certification and some may cut corners compared to the units that got sent to the certification lab. For example (with a different device), I have known a fire to start from a fluorescent lamp ballast overheating when a recently-failed lamp caused a starter to "get stuck" (fail short).

If you see any fluorescent lamps blinking on and off or steadily glowing only at the ends, remove the lamp ("bulb") or the starter. If there is steady end glow, the starter failed short if there is a starter in the fixture.

- Don Klipstein ( snipped-for-privacy@misty.com)