Resistor vs transformer

Should work in any case. Bill had probably this in mind - Connecting the secondaries in series and tapping the voltage across a pair of the secondary terminals would give me 115V , just right for the fire alarm. I think its the cheapest solution. The dual 115V Farnell offers cost above 14 Euros /17 dollars.

Hans

I easily/cheaply find dual 230V at the primaries in my area. I guess it would do the job too.

Hans

"Phil Allison" schreef in bericht news: snipped-for-privacy@individual.net...

like

Not what you asked for, but I did find some chocolate that tastes like shit:

Hmm, out of stock until 6 september 2006. Can't believe that Thompson has run out of shit.

--
Thanks, Frank.
(remove \'q\' and \'.invalid\' when replying by email)

Cheap transformers would be better than a resistor for AC (Radio Shack i think has a low power transformer and socket/plug adaptors). If you need DC, a cheap SMPS would be better.

"Weinberger Hans"

** BEWARE:

The same PITA Kraut Fuckwit is back.

** Huh ????????

ROTFLMAO !!!

This "Hans" Nazi sprog ARSEHOLE has gotta be the biggest FUCKWIT out !!

** The secondaries are only 6 volts each - FUCKHEAD !!!!!!

** Nah.

A 9 mm Parabellum to the forehead beats it by a mile.

......... Phil

--
OK, but first we\'ll need your address.

--
Not necessarily.

Sloman\'s trick of using the 120V primaries in series and having your
device connected in parallel across the primaries causes the
transformer to become an autotransformer, which would work.

Using the scheme with a transformer with dual 240V secondaries
likely wouldn\'t work well because of the much higher winding
resistance you\'ll encounter causing, I would expect, _very_ poor
regulation.

"John Fields"

** I have yours - you pile of criminal, f****ng Texas shit.

........ Phil

Some more rubbish for that Allison crap man .

lemonjuice

And so what ? Go play with yr toys you troolllll

lemonjuice

--- Geez, Phil, as large a part of your vocabularity as 'shit' seems to be, you must have a real fondness for it, huh?

Do you prefer it with a red or a white wine?

Or, more likely, with some of that "Foster's" swill?

-- John Fields Professional Circuit Designer

Winding resistence doesn't depend ONLY on input voltage range. You can get dual 115V's with higher resistence then dual 230V's

as far as I know neither does regulation ... unless you have some good reasoning to prove that.

lemonjuice

Oops...

/ one of the

--
John Fields
Professional Circuit Designer

I would first open up one of the units and peek at the power transformer. There's maybe a 45% chance the transformer has 120/240V primaries, they're just not configured that way from the factory. Follow the line input, past any switches and fuses and trace the wires to the power transformer. If there are two sets of two wires each going into the transformer from the line, those are two 120V primaries hooked up in parallel. To wire them for 220V, you just have to put them in series instead of parallel. The phase is important-- you'll have to snip one wire from the line, then the opposite windin'g wire from the other line and hook those two together.

For the same core, to get the same flux density with a 240V input would require twice the number of turns as for a 120V input.

The voltage on the secondary is related to the voltage on the primary by:

Es Ns ---- = ---- (1) Ep Np

Where Es and Ep are the primary and secondary voltages, respectively, and Ns and Np are the number of turns wound on the secondary and on the primary, respectively.

Since the transformer is tranferring power, the current in the secondary has to be related to the current in the primary by:

Is Np ---- = ---- (2) Ip Ns

So, for, say:

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

With 120V out of the secondary and a 120 ohm load, we'll have a load current (in the secondary) of:

Es 120V Is = ---- = ------ = 1 ampere Rl 120R

Now, since the voltage across the primary and the voltage across the primary, and:

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

then there must be the same number of turns on the primary as there are on the secondary.

Rearranging (2) to solve for the primary current will give us:

Ns Ip = Is ---- = 1A * 1 = 1.0 ampere Np

Now, with that behind us, let's take a look at a transformer with dual 120V primaries and connect them in series so we can use it as an autotransformer with a 240V input and a 120V output and see what happens:

240V>------+ | ||P ||R ||1 | +-->120V>--+ | | ||P | ||R [120R] ||2 | | | 240V>------+-->120V>--+

Since we have twice the voltage across the inductance of the primary, we'll need twice the number of turns to keep the flux density in the core the same as it was for the 120V case, and that criterion is satisfied with the two windings in series.

Now, just assume, for the sake of the argument, that our first transformer:

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

Really had two 120V primaries which were connected in parallel:

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

In which case each primary would be rated for 500mA and, in parallel, the combination could carry 1 amp.

Now, though, since we're not using the secondary and taking advantage of the current sharing we'd get with both primaries in parallel feeding the load on the secondary, we're forcing one of the primaries to supply the entire load, which is going to cause twice the voltage drop across it than would occur with with the windings in parallel. That, in and of itself, will cause the regulation to be poorer than it would be with a conventional transformer with the same amount of iron in the core.

Finally, consider another autotransformer wound on the same core, but with two _240V_ primaries wired in series with 240V across the ends. 120V will still appear from the center tap to either end, but because the primaries were wound with smaller diameter (higher resistance) wire, and exhibit a higher resistance than the other autotransformer, the drop across the 240V primary with the 120 ohm load connected across it will be even greater than across the other autotransformer, resulting in even poorer regulation.

--
John Fields
Professional Circuit Designer

oops

Haven't thoroughly examined your analysis as I got a tight work schedule here ... but I'll be back to you tomorrow morning. In the meantime ... the secondary is left unconnected in the application so flux induced there, currents flowing there have nothing to do with the problem. Then you are assuming wire radius and type is constant which isn't necessarily true for different transformer types and that is going to vary all the impedances, flux you have calculated. You also assume A 240V has to have twice the windings of the 120V and that depends on what the transformer is designed to do. It doesn't always have to be like that, especially if you are considering different transformer types.

Even across the same types you could use a 240V dual to whatever voltage you want with a 120V to a lower voltage and you'd have the same resistence of the primaries.

I see you mentioning regulation. It only makes sense to talk about that if you have an output voltage . In this case our secondary is left unconnected.

Regards

lemonjuice

--- I see from the rest of your post that you don't understand what's being discussed, so your "analysis" will hardly be of any consequence.

Save yourself some grief and concentrate on your work, it'll probably serve you better in the long run.

---

--- had you taken a little more time to read my post, instead of firing back some half-baked, irrelevant response, you would have seen (well, maybe...) that that eventuality had already been covered.

---

--- Again, had you read the article with a little more care you might have found that the discussion was based on using Sloman's example of a transformer with a couple of 120V primaries being used as an autotransformer, and that my examples were all based on using that transformer's core in order to keep apples equal to apples.

---

--- Read my post again. And again, and again, until it makes sense to you, if you can, then come back with any question you may have.

---

--- Flux density, voltage, and primary turns is what we're talking about, dear boy.

Besides, the resistance of identical primary windings in parallel is going to be half that of the same primaries in series, you know.

---

--- I suppose, then, that the voltage being fed into the load (which is taken from one of the autotransformers' primaries) doesn't really qualify as an output because there's no secondary?

Go back and read the article again before you embarrass yourself further.

-- John Fields Professional Circuit Designer

--

Cut that out. Its very funny.

Another error in your post is you don't seem to realize that its NOT RESISTANCE that is cutting down the voltage BUT inductance. Write an equation for that .

Another error is the resistance depends also on the geometry of the transformer.

Another error is the OP was talking about a different type of transformer from Slomans .

etc ... want another error listing so we can all laugh.

lemonjuice