dunb-noob transfomer question

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I've been working on small gainclone amp, I've noticed something that confuses me. US household voltage is 120vac. However transformers with 120vac primaries seem very rare. Is it common to use a resistor to drop the voltage? I have one that meets all my specifications, I'm just wondering for future reference. Thanks in advance!
 
What voltages are you finding?

120v and 240v are common standards. If they make a transformer with two 120v primaries, they can then sell them worldwide. Wire the two in parallel for 120v operation or in series for 240v operation.

Long ago, they called the mains a nominal 110vAC, many old timers still call the 120v mains "110." Later we saw 115, and for a while 117vac was common. But I haven't seen a production transformer called 110,115,or117 in decades.

No one would not normally drop voltage with a resistor in a transformer primary. If nothing else, the voltage drop would be totally dependent upon the amount of current drawn from the thing.
 
Interesting. I must not be searching correctly, on mouser I get only 18 120v transformers total (of various secondary voltages) and hundreds of 115v. I got a Hammond 183k28 which has specs of 115-115=230 primary going to 14-14=28 secondary. They look nice but I figured I could not use them, they would probably exceed 30v at 120v (if they didn't blow up). Thanks for the info
 
:p To add to the confusion...the 110 or 220 Volts were just multiples of the 55 it takes to trigger a street lighting arc lamp. Pushing the voltage up means you can get more Amps through the mains without getting more losses. Ohm´s Law!
And the frequency - the efficiency of a transformer is partly linear with that.
Some cheaply built "one fits all countries" trannies will hum and get warm on European 50Hz mains and be quite OK with your 60. Aircraft use 400Hz.

Pit
 
I think your miss reading the (may be center taped rather than two phyical seperate windings)tranformer specs. If it says something like 120-120 that means it got two primary winding each rated at 120vs wire in seris for 120v or in parrel for 240.
on the secoundaries your looking for either 0-12 0-12 means its got two secoundary coils each rated at 12 not 24 in total. Or it could be center taped which would be 12-0-12 which if you were to ignore the center tap would then give you 24 or if you use the center tap you'l get two 12 volt outputs.

The choice of secoundary voltages will vary depening on what your amp is and what load your trying to drive.
 
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Hi,
when you see a transformer specified for 115+115 50/60Hz, this indicates it is designed as a universal transformer that can operate on both 50Hz and 60Hz supply systems.

It can be connected to 110Vac to 120Vac supplies by wiring the two primaries in parallel.
It can be connected to 220Vac to 240Vac supplies by wiring the two primaries in series.

115Vac and 230Vac are the two most common transformer standard voltages for domestic use.
 
So, your saying that even though the transformer says 115-230 primary (connected serially) gives 28Von the secondaries (connected serially) would also be able to handle 120V mains? Wouldn't the excess voltage translate in excess voltage on the secondaries also (making somewhat more than 28V)? I'm making lm1875 amps for my brothers for xmas these have a max rating of 30V for power (if I'm understanding the data sheet correctly) that's getting a little too close, plus I do have 120v to 25.2V. So in reality, I'm just trying to learn something at this point. Thanks again
 
Hi,
the transformer has a turns ratio.
That turns ratio determines the ratio between input voltage and output voltage.

If the mains supply voltage is less than the rated input voltage then the secondary output voltage will be less than the rated output voltage.

If the mains supply voltage is more than the rated input voltage then the secondary output voltage will be more than the rated output voltage.

There is another effect. Almost every transformer manufacturer rates the output voltage when the input supply is exactly at the rated input voltage and when the current drawn from the secondary is exactly the rated output current.

If the output current is near zero the output voltage will rise to rated output voltage * regulation.

Lets take a 115:28Vac 7% regulation 280VA mains transformer.

If you feed in 115Vac and take out 10Aac then the output voltage will be 28Vac +-manufacturing tolerance.

Now feed 120Vac to that same transformer with no current drawn from the secondary.

The output voltage will be 120/115 * 28 * [1+0.07] ~ 31.3Vac

The voltage across the smoothing capacitors will be ~43.7 if the diode Vdrop is ~ 500mV at near zero current.

This tells you that a 115:28Vac 7% regulation transformer fed from 120Vac will take your chipamp in quiescent conditions to about 43 to 44Vdc.
 
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