Mind those chassis screws!

[Potato Cakes]

Hello, everyone,

I had an interesting experience today with a toroidal power transformer heating up to the point of letting out the magic smoke. I had recently completed this unit a couple of weeks ago and had done an extensive amount of tests and tweaks to it, having it powered on for several hours with no issues. I put the lid back on and it sat idle until today when I wanted to use it to test some other gear. After being on for a short while the power in the unit sagged and the case got super hot. I turned it off and opened it up and something let out some smoke. Nothing seemed burned or melted, but I noticed that the top plate holding the transformer in place was too hot to touch. I put a fan on it and powered it back on and everything was fine. All the voltages coming off of the PSU were correct and nothing seemed out of the ordinary. I put the lid back on and was almost done when it occurred to me that the screw on the side of the transformer might be in contact with it, so I opened it up and saw where the screw was indeed making contact with the windings. Fortunately nothing was damaged and I didn't get shocked, but it prompted me to come here inform everyone here who is working on a project cramming a bunch of stuff in a tight space to mind the placement and the length of the screws being used to hold the chassis in place.

Thanks!

Paul

 

[scott2000]

For sure! Thanks!

I had a burr give me a workout once......

 

[JohnRoberts]

if the screw pierced the winding insulation it could cause a shorted turn (no bueno).

JR

 

[PRR]

> screw was indeed making contact with the windings.

Teeny toy mike preamp with 120V supply inside. Boxed it up, got a shock. Opened up, no short, no shock, Boxed it, shock. Took far too long to spot the box-screw which was biting the PT.

 

[Audio1Man]

One thing to watch out for is for a SHORTED TURN on a TOROIDAL TRANSFORMER.  The mounting bolt can't connects the bottom and top cover of the chassis and the low voltage and big currents can't flow. In big power amplifiers the bolt can weld to the top cover when it touches. The top side of the bolt/screw must be below the top cover so it can NEVER TOUCH THE TOP COVER. I developed a mounting in the early 70's @ BGW and the next year many of the other mfg were using this new mounting method.

The core hole was filled with epoxy resin, then counter bored so the flat washer, split lock washer and lock nut were below the top surface of the transformer. Now the shorted turn can never happen.
Duke

 

[Potato Cakes]

I don't think it shorted a turn as the AC coming out is still fine. I think it was just enough to pierce the insulation and make contact with one winding. I had it powered on for a while with it opened after the incident and everything seemed fine, so I mended the exposure and used a shorter screw. Still too close for my comfort.

All the specialty transformers that I get from Aaron at Don Audio uses a similar epoxy core you mentioned, Duke, with a hole drilled out so there is no need for a top cover. I haven't found a supplier in the US that does the same thing. If they did I would use them for all power transformer needs.

Thanks!

Paul

 

[pucho812]

layout is everything.