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LAN_403

HV Enthusiast

Mon Mar 03 2008, 09:14PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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The only limit to the current in this circuit is the impedance of the secondary. If the secondary impedance was say 1 ohm and made with 2 AWG wire, you can draw 253A current at the output (assuming a short circuit)

Marko

Mon Mar 03 2008, 09:18PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

kell wrote ...

The output is lower than the input, so isn't it buck? Boost would be the other way around.

Woops; yes; even after looking at the pic several times didn't prevent me from swapping input and output. I apparently badly need more sleep...

Circuit should work for these voltages no matter from which side it is powered, as long as voltage is right.

So, 11 amps.

Dr. Slack

Mon Mar 03 2008, 10:31PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

But what assures "sharing" of current between the windings

The fact that they are wound on the same core. If we assume the core has infinite permeability and so inductnce (which is only very slightly wrong), then the AmpereTurns product from the primary = -AmpereTurns product of the secondary. As the turns ratio is 10:1, the currents are forced to be in a 10:1 ratio.

What "forces" this sharing? If the AT products are unbalanced, this creates a net magnetising force which creates a very large voltage on both windings, changing the current draw until the AT products balance.

In a real transformer, the inductance is only finite, so the AT products of the two windings is only almost equal, expect 99% to 99.9% equality for good transformers.

Finn Hammer

Tue Mar 04 2008, 06:41AM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

First of all, the ratings are there to humbly suggest a current draw from the transformer, which will keep it from burning into a crust.
You can always draw more current than the rated, but then the transformer will start to get hotter. The maximum current that you can get out of the transformer is determined by the winding resistance, and occours when the winding is shorted. In that case, the winding resistance is the load, and the voltage generated appears across it. This will lead to crusty wire in short order.

No matter which side you connect to mains in the configuration that you showed, the output current

has to flow in the primary winding, which is rated at 1 amp.
Force it to carry 10 amps, and you end up with a crust.

Cheers, Finn Hammer

Dr. Slack

Tue Mar 04 2008, 08:30AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Don't let the fact that the diagram is drawn rather oddly confuse you as to which way the current is flowing. It does appear to show the primary in series between the input and output, so a casual glance at the diagram might suggest that the output current flows through the primary, but that's not so. Having the common terminal at the top doesn't help either (in fact that may have been the major confusion). Here is the digram slightly redrawn as the auto transformer that it is, putting the common terminal at the bottom where it belongs. The arrows show the direction of currents at some moment in time.

The AT product in the primary of 1A x lots of turns is equal and opposite to the AT product in the secondary of 10A x just a few turns, thus there is no flux in the core due to load currents.

The 11A output current is made up of 1A in the primary added to 10A flowing through the secondary, each within their ratings. Agreed Finn?

The 230v output voltage is made up of 253v supply less 23v on the secondary, check the positions of the winding start dots for polarity.

The wire common to the input and output carries 10A or 11A depending on whether it is part of the primary or secondary circuit, the difference being due to the small primary current.

Of course the meaning of input and output can be freely interchanged, it doesn't matter whether you boost 230 to 253 (more likely) or buck 253 down to 230, energy conservation, current addition and voltage adition all still apply.

Marko

Tue Mar 04 2008, 12:35PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

has to flow in the primary winding, which is rated at 1 amp.
Force it to carry 10 amps, and you end up with a crust.

Finn, didn't you check the pic I posted, or you just don't trust workbench simulations? :p
I thought it was pretty self explanationary

As neil says it's probably easiest to visualize it as autotransformer.

Finn Hammer

Tue Mar 04 2008, 06:40PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

So what you are saying is, that for

L1 23V*10A = 230VA and
L2 230V*1A = 230VA

Totalling 460VA input

you get

230V*11A = 2530VA output?

Forgive me if I`m thick, but to me this smells not only like crust, but like free energy.

Cheers, Finn Hammer

ramses

Tue Mar 04 2008, 08:29PM

Registered Member #1208 Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154

agreed, but i would like to take another guess at 10A. keep in mind that your basically putting a 10A transformer in series with the mains, which are NOT current limited. i would guess that you could pull around 25A from the secondary, albeit not safely, and i am SURE you could pull 25A from the mains, but you may blow breakers/fuses/wires/set your house on fire. so i would set it at 10A as rated, but expect up to like 30A

have fun,
ramses

Arcstarter

Tue Mar 04 2008, 09:31PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Yea it would be 1 amp.

Sulaiman

Tue Mar 04 2008, 09:41PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

To stay within the ratings of the transformer the answer IS 11 amps - see NeilThomas's post above for clarity.
If you don't care about staying within the transformer ratings then EVR has given a clear answer above.
If you get a different answer then please think again before posting!

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