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Offline kolias

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Re: Power Supply Unit
« Reply #100 on: January 18, 2011, 01:57:35 PM »
Here is a selection of bridge rectifiers
Nicolas

Offline Hood

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Re: Power Supply Unit
« Reply #101 on: January 18, 2011, 02:13:40 PM »
Any of them would be fine as far as I can see. All are at least 3 times the current and 2.5 times the voltage ratings you are looking at so there should be no problems.
Hood

Offline kolias

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Re: Power Supply Unit
« Reply #102 on: January 18, 2011, 03:08:44 PM »
Alright I will order the toroidal and rectifier tonight, thanks for your help
Nicolas

Offline kolias

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Re: Power Supply Unit
« Reply #103 on: January 18, 2011, 08:12:03 PM »
The last item to purchase for this power supply unit will be the metal enclosure.

Approx. what size box we are talking about?
Noticed in the local stores that they come in various size and they dont have any holes for ventilation. Do I punch some holes on the side?

The cable for the 110VAC will be 14AWG two conductors + the copper ground wire.
What size wiring from the toroidal to the 110VAC terminal strip?
And what size wiring from the toroidal to the rectifier and capacitors?

Thanks
Nicolas

Offline alenz

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Re: Power Supply Unit
« Reply #104 on: January 19, 2011, 03:03:23 AM »
Nicolas,
Re. your power supply enclosure. I built up a power supply that used the identical 24V MPJ transformer that you first selected. It all fit rather nicely into an old scrap desktop PC power supply case. I scrapped the electronics and only kept the metal case, cooling fan and power cord. A piece of Radio Shack printed circuit board mounted on standoffs was used to mount the components.  Milled some straight line trace isolation cuts in the copper to end up with an equivalent printed circuit board.  The newer PC power supply cases are a bit smaller but then I think the toridal transformer that you intend to use will be considerably smaller than the rather huge MPJ one that I used. And old PC’s are essentially free.
I couldn’t believe the prices quoted for metal enclosures.  So I made a separate one for the drives out of inexpensive Ace hardware acrylic sheet. Just couldn’t see spending more for an empty metal box than the electronic components that went inside. Of course a proper metal enclosure even tho a bit more expensive would result in a more porofessional job.
Al
P.S.
Disclaimer: I’m not qualified to make any recommendations where electricity is involved but note that the plastic case only has low-voltage components and the potentially lethal mains voltages are in a grounded metal case. I am sure that there are technical reasons why this is not good practice, but the bottom line is that it has been 100% trouble free for going on ten years now.
al

Offline stirling

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Re: Power Supply Unit
« Reply #105 on: January 19, 2011, 04:48:22 AM »
The last item to purchase for this power supply unit will be the metal enclosure.

Approx. what size box we are talking about?
Noticed in the local stores that they come in various size and they dont have any holes for ventilation. Do I punch some holes on the side?

The cable for the 110VAC will be 14AWG two conductors + the copper ground wire.
What size wiring from the toroidal to the 110VAC terminal strip?
And what size wiring from the toroidal to the rectifier and capacitors?

Thanks

your toroidal already has the wires. Just wire the primary side to your strip and secondary to your rectifier - job done.

Box size: I'm just going to throw in another heads up just in case. Your toroidal will (should) come with fixing bolt and a large "washer". You bolt through the washer and on through the toroidal to your mounting plate. DO NOT allow any part of the metal case to come into contact (or even that close) with the top of the bolt or the washer. (It will short the transformer and that fuse will come in handy again).

Offline kolias

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Re: Power Supply Unit
« Reply #106 on: January 19, 2011, 08:17:45 AM »
It is true that the metal enclosures are quite expensive and I must balance my cost vs a good looking job. For my needs, a PC case although a very good alternative will be too bulky. I know a place that they sell among others old metal switch boxes of various sizes and I will pick one after I get the toroidal and my other parts to figure out how much space I need. You did a nice looking job with yours Al.

stirling that is one thing that scares me about this project – creating a short circuit somewhere. What you said it puzzles me though;

“You bolt through the washer and on through the toroidal to your mounting plate. DO NOT allow any part of the metal case to come into contact (or even that close) with the top of the bolt or the washer.”


Why it matters if any part of the case may come close with the top of the bolt? This bolt will be mounted to the bottom of the case which is metal anyway!!! Am I missing something?

Still I would like to know: Do I need any ventilation holes on the metal enclosures?

What size wiring from the rectifier to the capacitors?
Nicolas
Re: Power Supply Unit
« Reply #107 on: January 19, 2011, 09:45:45 AM »
Look on E-Bay for enclosures.  Mine was a brand-new Hoffman 24"x24"x12" heavy gauge steel NEMA enclosure I got on E-Bay for $99!

You should allow for airflow, but for such a small supply, I very much doubt forced ventilation is required.

If the bolt mounting the power transformer is in any way connected to anything electrically, then it was horribly mis-applied.  The transformer should have a rubber insulator top and bottom completely isolating it from the bolt and "washer".

Regards,
Ray L.
Regards,
Ray L.

Offline kolias

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Re: Power Supply Unit
« Reply #108 on: January 19, 2011, 10:18:48 AM »

You should allow for airflow, but for such a small supply, I very much doubt forced ventilation is required.

If the bolt mounting the power transformer is in any way connected to anything electrically, then it was horribly mis-applied.  The transformer should have a rubber insulator top and bottom completely isolating it from the bolt and "washer".

Regards,
Ray L.

Sorry not clear to me; the metal enclosures here are just a box with cover with no openings whatever on sides. When you say I should allow for ventilation do you mean That I should drill some holes on the sides of the box?

Now I get it with the toroidal but I will post some pics to see how we mount it, it seems critical to me

And no one to tell me the wire size from the rectifier to the capacitors ?
Nicolas

Offline stirling

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Re: Power Supply Unit
« Reply #109 on: January 19, 2011, 12:01:14 PM »
If the bolt mounting the power transformer is in any way connected to anything electrically, then it was horribly mis-applied.  The transformer should have a rubber insulator top and bottom completely isolating it from the bolt and "washer".
Yes of course there are rubber washers and of course they should be used, but this is not the point I was trying to make. Incidentally they (the rubber washers) don't electrically isolate anything that isn't already isolated - they are there as a belt and braces thing to prevent the coil insulation from waring through and causing a short. But as I say this is not the point I was attempting to make.

I don't care how much the transformer is electrically iscolated from the bolt and washer - the case must STILL NOT touch the top of the bolt. Why? because then the case, plate and bolt effectively create a ring of metal through the transformer torus. This effectively creates another secondary and worse  a secondary of ONE turn (think about the current) and worse still a short circuited secondary.
« Last Edit: January 19, 2011, 12:09:24 PM by stirling »