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Freitsu

Tue Nov 09 2010, 10:35AM

Registered Member #3147 Joined: Sun Aug 29 2010, 10:53AM
Location: Finland
Posts: 56

I decided to build a regulated PSU of the whole thing, including box!
My dad helped me some with the making of that aluminium box.

Here are the pics:

Note the thermostat that you can see bolted onto middle cooling-element, it turns the fan on at +70C and off at +40C. It's usually cold enough under 15A that no fan is needed.

Things to work on is:
- Get bigger smoothing cap! 47000uF 80V isn't enough, voltage REALLY sags while drawing 30A @ 30V.

Current control isn't so accurate but atleast LM723 regulator can keep those 2n3055 transistors from frying!
The schematic for my PSU is taken from this site:

(not my drawings! I stacked more of power transistors)

Adam Munich

Tue Nov 09 2010, 10:56AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Looks pretty good! Check old stereo receivers for smoothing caps, they have huge ones. They're high quality too; can't have hum in your music!

Proud Mary I've never heard that before. A lot of people just say UF though.

Nicko

Tue Nov 09 2010, 11:02AM

Registered Member #1334 Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615

Steve McConner wrote ...

You can always spot a German on electronics forums because they don't write "uF", they use the actual mu symbol. The reason is simply that German keyboards have mu on a key, but everyone else has to remember Alt+016-whatever, so they just type u instead.

I've always liked the way that sensible s/w packages, like LTspice, convert "u" to "Âµ" in the right places, i.e. for component values.

FWIW, when doing my internship at ITT (actually STL Harlow) I remember using "puff" for pF but always "micro" and "milli" for u & m. "muff" was something completely different...

Cheers

Freitsu

Tue Nov 09 2010, 11:17AM

Registered Member #3147 Joined: Sun Aug 29 2010, 10:53AM
Location: Finland
Posts: 56

Thanks Grenadier!
It looks pretty robust, almost like USSR. ;)

Proud Mary

Tue Nov 09 2010, 11:20AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Good work, Freitsu!

Freitsu wrote ...

voltage REALLY sags while drawing 30A @ 30V.

It may not suit all users, in all circumstances, and applications, but it is often convenient to use industrial or automotive lead-acid accumulators to meet occasional or intermittent heavy current demands. When not in use, the batteries can be kept float charged, so they are always ready for use.

I also find lead acid cells, such as the Enersys Cyclon* I've pictured below, very convenient as floating X-ray heater supplies in grounded anode configurations, and which you might find useful in your own X-ray work:

1289301508 543 FT0 Enersys Cyclon 2v 8ah 001

* RoHS exempt

Freitsu

Tue Nov 09 2010, 11:39AM

Registered Member #3147 Joined: Sun Aug 29 2010, 10:53AM
Location: Finland
Posts: 56

I think I will go the capasitor way, but using sealed lead-acid batteries is not a bad idea eighter!

Tesla Fan

Mon Nov 15 2010, 02:23PM

Registered Member #3353 Joined: Sat Oct 23 2010, 11:21PM
Location: Greece
Posts: 90

Hi Freitsu, nice work with the power supply!

I will show you two 24V transormers i have, maybe you can compare the size to find out about the VA rating of yours. The smaller is 700VA and the bigger 1600VA (input is 220V, i assume you have the same in Finland).The small uses 2.5mm wire and the big somewhere about 4.5mm.

As for the voltage sag, it seems kind of normal to me. 30V x 30A = 900VA which is not a small amount of power. I am pretty sure that it is normal to have a little voltage drop when you get near the max current capability of the transformer. I know for sure that when a 24V transformer (AC circuit) is under heavy load (more than its normal rating), voltage can go down to even 17V.

For example, if you connect a 0.1Ohm resistor to your transformers output, it will "try" to draw 240Amps! Since your transformer is not rated at 6KW, he cannot supply that current and the voltage with sag (big time, since this is practically a shortcircuit for the transformer).

I could be wrong, but be sure of your theory before spending money on bigger caps.

-- WARNING - "REDNECK's", UNSAFE WAY TO GET AN ESTIMATE OF A TRANSFORMERS VA RATING --

It just occurred to me, you can use that as a (crude, unsafe, vicious) way to get a rough estimate of your transformer rating: If you have a multimeter, monitor the primary current draw (eg connect it is series with one of 220V input cables) and for only a second shortcircuit the secondary. Then you can meause the exact main voltage and multiply the two numbers to get the VA rating (4,5A at 220V = ~1000VA). Actually you will get a bigger number, as all transformers can be over-loaded to some extent, and ONLY for a very very short period. I know, i know it is cruel, and i assume you have the skills to do that without blowing up you multimeter or killing yourself (mains 220V are involved).

-- USE AT YOUR OWN RISK as a last resort method only --

Steve Conner

Mon Nov 15 2010, 03:14PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The short circuit method will give you an answer about 10 times bigger than the real VA rating. I just go with 100VA per kg weight.

Tesla Fan

Mon Nov 15 2010, 03:54PM

Registered Member #3353 Joined: Sat Oct 23 2010, 11:21PM
Location: Greece
Posts: 90

Thanks for the correction Mr. McConner

, i guess it is a useless method after all...

pauleddy

Tue Nov 16 2010, 02:19AM

Registered Member #2909 Joined: Wed Jun 09 2010, 12:31AM
Location: fort belvoir, Va USA ( south of DC)
Posts: 145

you need one of these caps
140000UF 50vdc

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