Mounting a transformer higher then intended.

[peps1]

Hey guys,

Working on a project with a mains transformer (like below) with the two mounting holes that I had planned on directly fastening to the enclosure.



Trouble is, due to poor design my end, I don't quite have what I would consider acceptable clearance between the solder lugs and a PCB.

Is it a fundamentally bad idea to raise the transformer up and off the enclosure by say 10mm using brass standoffs?

[ataradov]

If it works for you mechanically, then why not? Just keep in mind that transformers are heavy, and if your case is plastic, then it is highly likely to break away if you drop the device.

[Ian.M]

I wouldn't do that.  There's too much risk of odd mechanical resonances if the bottom of the core isn't clamped against the chassis, and if it experiences mechanical shock, normal to the plane of the core, either the chassis or the mounting lugs may bend and the mounting screws loosen.  Cut a spacer  the full size of the base out of aluminum plate or use a couple of layers of FR4 glued together.

[Phoenix]

Is it a fundamentally bad idea to raise the transformer up and off the enclosure by say 10mm using brass standoffs?

It's very heavy and the mounting tabs on the transformer are only thin plate metal. With thin stand-offs they will likely flex and bend causing the transformer to slowly drop anyway.

Maybe use a single thick metal spacer that fits under the full transformer? Or large washers that can provide some support underneath the main body of the transformer.

[BravoV]

All you need is just shoot a photo or illustration of your design explaining your situation, experienced members might have clue what to suggest "specifically", as all of these above suggestions are generic.

The devil is in the details.

[peps1]

All you need is just shoot a photo or illustration of your design explaining your situation, experienced members might have clue what to suggest "specifically", as all of these above suggestions are generic.

The devil is in the details.

Here is the problem, the measurements given to me by the manufacturer of the transformer omitted the solder tags, so they overlap the PCB



So looking for the best solution to lift the transformer up another 10mm for more clearance, and was first thinking adding some standoffs to the transformer mounting holes would work, now thinking it may be a better idea to get a small steel bracket that would support the transformer on the ends with the standoffs and in the middle of the bracket? 



This is for a very very small production run, so don't want to just bodge it, and also don't want to pay out to get all the base plates made again.

[Ian.M]

So what's in that corner of the PCB?   If its 'dead' space, or you haven't had the boards manufactured yet,  cutting back the outline for clearance would be an option.

Otherwise, as I said earlier, if the baseplate has holes and the transformer is mounted with bolts, I'd go for some aluminum flat bar stock of a similar width to the transformer core.   Cutting a length and drilling two holes is probably going to be cheaper than making a bent steel bracket, and assembly will certainly be easier with only one extra part rather than three for the bent bracket and standoffs.   The only exceptions would be if the standoffs are going to be riveted or swaged into the case, (but you said you didn't want to rework the case baseplate), or if the existing baseplate has studs for the transformer, so internally threaded spacers are the only viable option - you've then got to decide between your spacer bracket, and a strip of aluminum bar slightly thicker than the spacers, drilled with clearance holes to drop over the spacers.   

[Nusa]

If you rotate the transformer 180 degrees, is there room for the wires on the other side? Hard to tell in the picture.

[peps1]

If you rotate the transformer 180 degrees, is there room for the wires on the other side? Hard to tell in the picture.

unfortunately not.

[ataradov]

If it is a small batch, why not just drill the new holes by hand? It does not look like there are any standoffs, so it would probably be the cleanest and the cheapest solution.

[MosherIV]

Have you tried the transformer lying down on the unused side?
You can use cable ties to hold the transforer in place.

[peps1]

If it is a small batch, why not just drill the new holes by hand? It does not look like there are any standoffs, so it would probably be the cleanest and the cheapest solution.

Unfortunately moving it in any other direction other than up results in collisions with other components. 

An aluminium shim is looking like the best option so far.

[Phoenix]

It looks like they are just the LV taps. Do you need significant clearance or do the tags interfere with components on the PCB? If not maybe just an extra insulator sheet between the tabs and PCB would work, or heat shrink over the soldered connections.

[peps1]

Do you need significant clearance or do the tags interfere with components on the PCB?

They interfere with components, in this case, a rather chunky resistor.

[drussell]

+1 for some aluminum bar stock or small piece of plate... 

It would be easy to cut up into spacers the same footprint as the transformer and drill two holes.

[CopperCone]

Why dont you use a piece of wood. Hakf of the suggestions in this thread are completely insane. Why on gods earth would you stack pcb material. Wood is everywhere. You can cut it in one minute. What the hell?

They build boats and skyscrapers out of it. Omfg

[dgtl]

1) cut some squares out of aluminium plate, drill holes in center and use those spacers to raise the transformer. The spacers shall cover the whole contact area between the transformer and case to avoid the transformer to vibrate or bend.
2) Perhaps it is possible to bend the solder lugs upwards? Then solder the wires and heat-shrink them to avoid accidental contact to pcb.
3) The pcb looks through-hole. Perhaps it is possible to move some components with longer legs away from that hole, just using some other holes on the same traces. Then either cut off that corner or try to bend the terminals so that they go to the top of the pcb.

[Ian.M]

Why dont you use a piece of wood. Hakf of the suggestions in this thread are completely insane. Why on gods earth would you stack pcb material. Wood is everywhere. You can cut it in one minute. What the hell?

They build boats and skyscrapers out of it. Omfg

OP: "This is for a very very small production run, so don't want to just bodge it"

Unless wood is impregnated with a high melting point wax or polymerising resin or thoroughly coated with a pant system it isn't dimensionally stable in the long term.   Prolonged exposure to  high humidity conditions will cause it to swell and the fibres will be crushed due to the clamping pressure of the bolts, then when the humidity is low for an extended period, it will shrink, and the crushed fibres will not recover, so the bolts will effectively be loosened.  Most of the impregnants or coatings will increase its flammability. It may also contribute to poultice corrosion of metal it is in contact with.

Its especially problematic if you are ever going to export the product - see https://www.forestry.gov.uk/planthealth and all the local regulations of countries you may be planning to export to.

FR4 is far more dimensionally stable, (apart from a small tenancy to creep under severe compression loads that can be easily managed using Belville washers to maintain clamping pressure), is flame retardant and should be self-extinguishing, and wont contravene the biosecurity regulations of any country you want to export to.   

[james_s]

While that is true, I've seen wood used as spacers in many commercial products, especially older stuff and it doesn't seem to cause problems. Unless this is some kind of military or aerospace equipment I wouldn't worry about it. Various plastics should also be suitable.

[CopperCone]

bro what the hell, its a god damn transformer connected by wires, its not even solder joints.

I have wood that's been outside for like 10 years, in direct sunlight, expanded and contracted hundreds of times, been through floods....... and it would be perfectly dimension ally stable enough for this application.

And why the hell would you use bolts on wood? Use screws lol

based on the pictures I saw that thing is not going to be hanging off the pylon on an f16, seeing space or being sent back in time to be on a leaky submarine in world war 1

[Zero999]

What are the clearances between the tabs and the case? It's difficult to judge the scale from the picture, but it looks fine to me.

Could you be getting minimum creepage distances confused with clearances?

Unless you put an insulated screed around the mains connections, the case should be earthed anyway and a clearance between the tabs and case of >1.5mm is fine.

http://www.pcbtechguide.com/2009/02/creepage-vs-clearance.html#.Ws6IH3XBgWM