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Old 11th December 2006, 08:10 AM   #1
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Default Little help understanding Transformer ratings

OK I am slowly getting into electronics and am looking to embark on a dual variable regulated power supply. I have found some basic design ideas. standard bridge rectifier into an lm317 etc. Obviously since I am building a dual supply I need a centered tapped trafo. 48VCT to be exact. that should give me a decent range on the variability now heres the golden question. If I find a 110 primary 48VCT secondary rated at say 3 amps. that is 3 amps total at 48V if I take the CT and do +24 and -24 then I am only going to get 1.5 amps per leg correct? so If I wanted to overbuild a bit I should be looking for a 4-6 amp trafo. ( headroom is good) to allow me to comfortably pull up to 1.5 amps per side ( -24 , +24 ) AM I Understanding this correctly. ( bear with me I am a newb and still doing MUCH research before I start playing with mains)

Here is the basic design idea.

http://orca.st.usm.edu/~jmneal/dual_psu/

Thanks,
Randy
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Old 11th December 2006, 09:35 AM   #2
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Smaller traffos tend to have worse regulation, i.e. the rails drop more when fully loaded, so usually do need to be oversized slightly if you intend pulling the full current from the PSU. To avoid this, I'd be looking at something rated, say, 25% above what you need, so for your supply I'd look at 2A per winding. Any more is overkill, fun, but overkill.
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Old 11th December 2006, 11:17 AM   #3
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So if I need 1.5 per side then I obviously need a 4-5 amp (3A for no overhead) trafo then? just want to make sure I am understanding the amperage relationship properly. 48vct 3A == to +24v 1.5a -0- -24v 1.5a Center tapped correct? I realize this is a basic question. Also trying to fully understand Va ratings. I understand that the rating is similar to Wattage (sorta but slightly different). P=I*V so if I have a trafo with a 100Va lets say at 20VCT then I get 5A ( which if used as to legs + and - I get 2.5A per rail?) Sorry if this question isn't worded as well as it could be ( English is my first language so I have no excuse ) As previously stated just getting started and trying to wrap my little brain around some basic concepts.


Thanks for your help.
Randy
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Old 11th December 2006, 11:39 AM   #4
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Just one misconception remaining as I see it. Traffo outputs are measured as AC RMS. This is a kind of averaging, so the peak voltage put out, (and what you get when you rectify it), is actually 1.41 times the AC RMS, so for your 48v CT, you will actually get 48x1.41=67.7V, or near enough 33V rails. So for 24V rails, you need to be looking at around 18-0-18, or 36V CT. Does that make sense?
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Old 11th December 2006, 12:29 PM   #5
Geoff H is offline Geoff H  Australia
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Hi Randy

"48vct 3A == to +24v 1.5a -0- -24v 1.5a Center tapped correct?"

Not quite. The VA rating is based on the total voltage x the current.

in this case 48 x 3=144 VA.
In a split supply, you have (24x3) x2=144VA


Rail voltage would be in the order of +/-32 volts at no signal.

That transformer should be good for about 30w out in an 8 ohm load and about 50W into a 4 ohm load, using bipolar transistors. Not bad for your first amp. My first s/s amp was.... 3 w/ch. (Please don't tell anyone)

Geoff.

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Old 11th December 2006, 12:32 PM   #6
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Quote:
Originally posted by Geoff H
That transformer should be good for about 30w out in an 8 ohm load and about 50W into a 4 ohm load, using bipolar transistors.
Shame he's building a variable PSU then!
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Old 11th December 2006, 12:33 PM   #7
AndrewT is offline AndrewT  Scotland
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Hi,
taking your example of 20Vct and 100VA =>5Arms.
the 20Vac *5Aac =100VA. simple for a resistive load.
the 5A rating applies to either secondary winding or both.
So you have 10-0-10 and can draw 5A from both ends, or 5A from the whole secondary.

Do you plan to take the secondary output into a capacitor input smoothing circuit.
Then you must down rate the transformer to about 70%. Your 5Aac becomes 3.5Aac.

When you load up the output of the smoothing caps you are drawing power from a higher voltage (141%) so to make the power the same, the DC current should be reduced to match the maximum power the transformer can supply.

This effectively means about 2.5Adc * 141% * Vac=3.5Aac *Vac

Now the answer from all the preceding is the maximum continuous DC current from a capacitor input filter is approximately half the maximum AC current.

Eg. 20Vac @ 5Aac => 28.3Vdc @ 2.5Adc. if this were a dual polarity supply =>+-14.14Vdc @ 2.5Adc.
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Old 11th December 2006, 12:36 PM   #8
Geoff H is offline Geoff H  Australia
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Hey pinkmouse, we'll have to convince him that DC is boring. Variable duty, variable voltage, and variable frequency combined the right way sounds much better.

Geoff
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Old 11th December 2006, 01:13 PM   #9
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Quote:
Originally posted by Geoff H
Variable duty, variable voltage, and variable frequency combined the right way sounds much better.
Indeed.
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Old 11th December 2006, 02:52 PM   #10
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Cool guys I think I understand. ( famous last words ) I am building a bench top dual Variable regulated supply so I can start breadboarding different circuit. What I am looking at should give me a variable supply from about 1.5v to about 30 V . After the power drop from the diodes.

I understood ( in the most simple of terms) the V * 1.41 being peak ( compared to RMS) . I just want to make sure that I overrate the trafo a bit so that I don't have to worry about heat so much ( i believe heavily in over engineering) I would like the ability to pull up to about 1.5A at Voltages between 1.5V to about 30V. (that way I know that I have a PS that should serve MOST of my needs into the future) As stated earlier I am still just doing preliminary research.

To AndrewT. Yes I will be using capacitor filtering after a full bridge rectifier. If I understand correctly I need to overrate the amperage a bit (141% is that correct?) due to the sudden draw from the cap's ( at power up) to reach their full capacitance( might be the wrong word here) ( thinking in terms of a battery they place a heavy load on the trafo to reach the point where they are charged ? Hope I'm not completely butchering EE here ) after that I will be using an lm317 to regulate. I am still very early in the learning process and don't plan on sticking my screwdriver in the socket at least for another hour or so....

All Kidding aside. I plan on taking some EE courses this coming fall. The only part that looks expensive in this project is the tranformers so I want to keep my eyes peeled and make sure I get the right one.

I am an audio engineer and recently built a couple of the seventh circle audio n72's and not only do they sound *almost* as good as the 1272 we have at work , they were way cheaper and I really enjoyed building them. I decided ( after finding a couple of books) that I would build *most* of my workbench tools . Variable power supply, signal generator etc. I have a tektronixs 2235 on the way ( being recalibrated as we speak, got a good deal on ebay). I found a great book or 2 on building your electronics workshop and test equipment ( 2 seperate books) .
Here and Here

Sorry so long winded ( and random, been up all night)
Just seems to me that building the tools of the trade would definitely help me learn alot faster (not to mention save me some $$$ in the long run)


Thanks for the help,
Randy
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