Resistor vs transformer

--
I apologize. 

I hadn\'t noticed \'til just a few moments ago that you\'re posting
from Google groups...

------>>>>current (in the secondary) of:

Now you are tapping current from the secondary ... wheras the secondary remains unconnected !!!!!!

--------------->>>>>>>then there must be the same number of turns on the primary as there

The transformer Sloman had in consideration was a step down => number of turns on the primary NOT equal to turns on secondary

Irrelevant as your completely off topic

-------------->>>>>>>>>Really had two 120V primaries which were connected in parallel: ??????????????????????????????????????????????????????????????? Parallel primaries would give you 240V on the primaries NOT 120V !!!!!!!

You talk about voltage regulation with a load connected to the secondary . This isn't happen. Man you give a bad name to this newsgroup!!!!!!!!!!!!!!!!!!!

why should they necessarily be wound with smaller wire??????????? Grow up man . You don't know what at Transformer is!

I REPEAT .......Go ask your granny she'll tell you thet : You talk about voltage regulation with a load connected to the secondary . This isn't happening in the case we are considering. LOL ....

Who said the wire size was constant ? You of course ... because you don't have a clue about transformers at which point you should shut up.

So its the resistance of the wire thats cutting down the voltage???!!!! LOL This is the stupidest thing I've heard in years.

No ... You have a bad assumption. Even your high school equations don't suggest that .

You said the OPs suggestion of using a transformer with 2 primaries rated for 240V was wrong . Refresh your aging memory .

Go on vacation it may help you.

lemonjuice

--
I don\'t know what you\'re looking at, but from my point of view what
I drew was:

120V>--+ || +-->120V>--+
       P || S          |
       R || E        [120R]
       I || C          |
120V>--+ || +-->120V>--+

where we have a transformer with a primary connected to 120V mains
and a secondary with a 120V output clearly connected to a 120 ohm
load.

Perhaps you\'re not viewing the ASCII schematic with a fixed-pitch
font, like Courier?

--
Then your education is _sorely_ laking.  Whether or not you\'re so
stupid that you can\'t understand it, it _is_  the resistance of the
wire (among other things)  which cuts down the voltage available to
the load, and if you ask me politely I\'ll explain it to you in a way
even you can understand it.

"lemonjuice" = a sourfaced f****it

** The actual resistance of the primary of a mains power transformer depends on the particular unit's voltage rating, VA rating (size) and regulation percentage.

Eg: a 12VA transformer with a 230 volt rated primary has a primary resistance of about 250 ohms. The same transformer may have a split primary ( 115 +115) with each winding half the above resistance. So, when connected in parallel for 115 volt use, the primary resistance will be 63 ohms (at room temp).

** The voltage regulation spec of a mains transformer is almost *entirely * due to winding resistance.

Load current causes a voltage drop ( IR in each case) to appear across the primary and secondary windings which act to *reduce* the voltage seen at the secondary. At the main frequencies of 50 or 60 Hz there is no significant contribution from *leakage inductance* to the voltage drop under

*resistive* load.

Taking the 12VA, 230 volt example again, primary full load current is 12/230 = 52 mA ( neglecting I mag)

250 ohms x 52 mA = 13 volts drop or 5.6 % of the supply voltage.

The secondary winding contributes another similar IR drop so the overall regulation is about 11%.

The windings get hot in use and copper resistance rises by 0.4% for each degree C.

The final regulation figure for operation at 75C ( 50 degree rise above ambient) is then 13.2%.

....... Phil

"Ancient_Hacker" <

** The Kraut OP is such a pathetic bloody imbecile he refuses to do even that.

Or even just connect an ohm meter to the units AC plug's pins.

Makes you suspect he has NOT got one in his possession and arrogantly will NOT admit that.

Typical Kraut.

......... Phil

Why ARE YOU aren't connecting the primaries in parallel ? OP/Sloman were talking about a series connection. In parallel you are NOT solving anything. Do you know the equivalent impedance of a RL parallel connection?

seen at the

Nice try but ALL totally irrelevant/offtopic to the case the OP/Sloman brought up.

lemonjuice

"lemonjuice"

** You are clearly a schizophrenic psychopath.

Go blow you brains out - anytime.

........ Phil

Reread his post. Whether he denies it or NOT, BOTH were stepdown transformers.

Thats what I've been repeating in all my posts, huh you finally understood that .

The point is you failed to answer my question. ASCII drawings of transformers may be impressive to your clients but as far as I'm concerned ...

The OP uses 240V. 240V applied to a 2 primaries connected in parallel gives you 240V at the Primaries. If you can't understand something so elementary ... I think I will have to stop wasting my time here.

You still didn't answer my criticism above . So I'll paste it here again. ---

Lets get this argument settled then we can talk about personal matters.

The truth is quite obvious

Wow ... I already answered that question zillions of times in my posts . Read carefully man.

lemonjuice

"lemonjuice"

** You are clearly a schizophrenic psychopath.

Go blow you brains out - anytime.

........ Phil

It makes no sense to use a transformer this way. If he can buy a transformer then he should get a 230->115 step down.

"Fred Bloggs"

** Of course.

Sizes start from about 20 VA in wall wart form - eg :

Just beware if the device draws power on one half cycle only - OR its internal transformer fries running on 50 Hz instead of 60Hz.

........ Phil

It's a question of price and availability. Split primary transformers are a commodity product available off the shelf in a vast range of sizes. Auto-transformers are a niche product, and you are likely to have to buy a bigger transformer than you need at a consumer price.

Phil Allison's example cost some $A59 (including tax) equivalne to $43.60 or 36.5 euro which is a couple of times more than you'd have to spend on a regular transformer and a box to put it in.

--
Bill Sloman, Nijmegen

Thanks for the advice. It certainly is the cheapest. I will check when all the units are here . I think they will most likely be SMPS inside, but even then your idea should still work .

Hans

** None of which are intended for use as 2:1 mains auto-transformers.

** Is not an "auto-transformer" - but has full safety isolation.

Check out the symbols including the "double square" symbol.

** From a chain of retail, consumer electronics stores.

Buy 10 from the wholesaler and them get for half price.

Much better, much safer & ready to use with no effort.

Da Slow Man's mad ideas are just as whacko as ever.

........... Phil

Man wake up. You are risking blowing up your companies equipment, plus starting/not preventing a dangerous fire.

Look carefully. If you hook up a series connection at the primaries for

240V you are doubling the turns ratio! Which means you get twice the desired voltage at the secondary.

If you want to follow that way, I'd say check if you can also get a SERIES connection at the secondary . Otherwise don't touch/change anything.

Can't you offer a coffee to a secretary(ies) to do a Google search for you on CHEAP STEP DOWN TRANSFORMER 230 240 120 110 ?

Otherwise for 8-9 dollars (transport excluded) each I could get supply you 50 of those in plastic casing rated at 45 watts ! Email me if you're interested.

lemonjuice

--
Yes, of course that\'s the most sensible way to solve his problem,
but as an exercise in what can be done to come up with alternate
solutions, I thought Sloman\'s scheme was pretty clever.

--- Well, it seems that with every post you write the hole you're digging around yourself gets deeper and you're getting deservedly closer and closer to being admitted into PA's Asylum for the Criminally Insane.

Here is a schematic of a transformer with, say, a 100 turn, 120V primary connected to 120V mains, and a single 120V secondary connected to a 120 ohm load

120V>--+ || +-->120V>--+ P || S | R || E [120R] I || C | 120V>--+ || +-->120V>--+

what is the turns ratio, primary to secondary?

Np Ep 120V ---- = ---- = ------ = 1 Ns Es 120V

So the secondary will also be wound with 100 turns.

Here is a schematic of a transformer with two 120V primaries wired in parallel and connected to 120V mains, and a single 120V secondary connected to a 120 ohm load

120V>--+---+ || | P || | R || | 1 || 120V>--|-+-+ || +-->120V>--+ | | || S | | | || E [120R] | | || C | +-|-+ || +-->120V>--+ | P || | R || | 2 || +-+ ||

Since the primaries are wired in parallel, they will each, and both, have 120V across them and, since the secondary will also have 120 across it, the turns ratio will be:

Np Ep 120V ---- = ---- = ------ = 1 Ns Es 120V

Assuming that the secondary has 100 turns on it, just like the transformer used in the previous example, means that since each primary has 120V on it, the only difference between the transformers is that the single-primary unit will have the entire load current circulating in the primary, while the dual-primary unit will have half of the load current circulating in each primary.

Now, looking at the same dual-primary transformer with the primaries connected in series:

120V>------+ || P || R || 1 || +-+ || +-->120V>--+ | || S | | || E [120R] | || C | +-+ || +-->120V>--+ P || R || 2 || 120V>------+ ||

It should be easy to see that it if there are 100 turns on each primary, then there will be, effectively, a 200 turn primary wound on the same core with a 100 turn secondary.

That means that with a turns ratio of 2:1, primary to secondary, the output voltage from the secondary will be half of the primary voltage.

That being the case, for a 120V output, the input would have to be

240V.

So, your advice to:

"Look carefully. If you hook up a series connection at the primaries for 240V you are doubling the turns ratio! Which means you get twice the desired voltage at the secondary."

Is totally flawed because doubling the turns ratio (primary-to-secondary) halves the secondary voltage.

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--- No. With a series connection on the secondary as well as on the primary, the output voltage will doubleas the input doubles, since the turns _ratio_ between the primary(ies) and secondary(ies) will not have changed

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--- If they're cheap and rated for 45 watts, I'd suspect that their output voltage would be way high with only six watts passing through them because of poor regulation.

-- John Fields Professional Circuit Designer