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# Looking for a specific transformer

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#### Electric-T

Jun 4, 2017
212
Hey guys.
Im looking for a specific transformer for a project im dreaming up.

Input: 120vac
Output: 15vac

I cant seem to find it. If my math is right it would need to have a power rating of 2kW. Since V×I=P (120×15=1800). Correct me if im wrong.
If anyone is really interested i can post a picture of a my schematic (so far).

#### Audioguru

Sep 24, 2016
3,656
Do you have a very high power 15V heater that uses 120A? (15V x 120A= 1800W).

#### Minder

Apr 24, 2015
3,479
It is usually very easy to modify the secondary by adding turns or removing if needed, if you have a transformer
That is close,
M.

#### ramussons

Jun 10, 2014
467
A SMPS would be better option.

#### 73's de Edd

Aug 21, 2015
3,636
Sir Electric-T . . . . .

If anyone is really interested i can post a picture of a my schematic (so far).

You had better do that . . . . . .
Because with your numbers being given below you are not giving us the current specification . . . .
But just multiplying primary voltage times secondary voltage . . .to come up with that oddlyfantasticamacal pseudo wattage specification.

Input: 120vac
Output: 15vac

I cant seem to find it. If my math is right it would need to have a power rating of 2kW. Since V×I=P (120×15=1800). Correct me if im wrong.

73's de Edd

Last edited:

#### ChosunOne

Jun 20, 2010
486
15 VAC transformers (mostly using a 115-120VAC primary, since you're in the USA)
https://www.ebay.com/sch/i.html?_from=R40&_sacat=0&_nkw=15VAC+transformer&_sop=15

Electric-T, you seem to have multiplied your primary (input) voltage times your secondary (output) voltage, but I can't imagine why. The product gives no meaningful measure.
V×I=P, yes that's correct, and 120×15=1800 is correct but it's just arithmetic without units to make it meaningful. If you meant 120A x 15V=1800W, then it's a meaningful equation but it begs the question of where you got the 120 Amps current draw you're using in the calculation. An application with that kind of current draw would usually use the 120V without stepping it down; or better yet, use a 240V outlet and less current.

But I suspect you're not using 120 A in your project and you're still unclear about how to calculate current? If that's the case, then say so and it'll help us to advise you.

For future reference, any time you're doing algebraic calculation, use the units of measure. It stops a lot of nonsense equations.

#### Electric-T

Jun 4, 2017
212
Sir Electric-T . . . . .

If anyone is really interested i can post a picture of a my schematic (so far).

You had better do that . . . . . .
Because with your numbers being given below you are not giving us the current specification . . . .
But just multiplying primary voltage times secondary voltage . . .to come up with that oddlyfantasticamacal pseudo wattage specification.

Input: 120vac
Output: 15vac

I cant seem to find it. If my math is right it would need to have a power rating of 2kW. Since V×I=P (120×15=1800). Correct me if im wrong.

73's de Edd
Do you have a very high power 15V heater that uses 120A? (15V x 120A= 1800W).
I was more thinking about being safe by assuming the maximum i would possibly draw out of the wall (15 amps) so 1800 watts is as high as i would possibly need to protect against if there was a surge. Any higher than that and the breaker would flip.

#### Electric-T

Jun 4, 2017
212
You had better do that . . . . . .
Because with your numbers being given below you are not giving us the current specification . . . .
But just multiplying primary voltage times secondary voltage . . .to come up with that oddlyfantasticamacal pseudo wattage specification.

Ok so im not just multiplying voltages. I thought the formula was clear. Voltage×Amperage=Wattage (V×I=P). I figured thats as much power as i can pull from a 15 amp breaker before it flips.
(120v×15A=1800watts)

Last edited by a moderator:

#### Electric-T

Jun 4, 2017
212
15 VAC transformers (mostly using a 115-120VAC primary, since you're in the USA)
https://www.ebay.com/sch/i.html?_from=R40&_sacat=0&_nkw=15VAC+transformer&_sop=15

Electric-T, you seem to have multiplied your primary (input) voltage times your secondary (output) voltage, but I can't imagine why. The product gives no meaningful measure.
V×I=P, yes that's correct, and 120×15=1800 is correct but it's just arithmetic without units to make it meaningful. If you meant 120A x 15V=1800W, then it's a meaningful equation but it begs the question of where you got the 120 Amps current draw you're using in the calculation. An application with that kind of current draw would usually use the 120V without stepping it down; or better yet, use a 240V outlet and less current.

But I suspect you're not using 120 A in your project and you're still unclear about how to calculate current? If that's the case, then say so and it'll help us to advise you.

For future reference, any time you're doing algebraic calculation, use the units of measure. It stops a lot of nonsense equations.

I apologize for the confusion. 120V×15A=1800watts. I dont know exactly what kind of power ill be pulling yet, so i just figured this...
I can pull 15A at 120V before i flip a 15A breaker. So 1800 watts seems like the power rating i would need. I hope im making sense.

#### Electric-T

Jun 4, 2017
212
It is usually very easy to modify the secondary by adding turns or removing if needed, if you have a transformer
That is close,
M.
Ok so this might be the best way. Since 15Vac doesnt seem to be very common.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
If you place a 1A fuse in series with the mains and get a transformer rated at maybe 15V 4A, you should be fine.

If course this assumes you need 3A or less, but we don't know that.

#### kellys_eye

Jun 25, 2010
6,514
Hey guys.
Im looking for a specific transformer for a project im dreaming up.
....and what would that dream be?

If we knew what your 'dream' was we could suggest a suitable item.

#### Electric-T

Jun 4, 2017
212
So just so we all know what im going for...
Im trying to design a variable voltage battery charger. Some that ive seen pull as much as 30 amps. Im not sure if i need all that current. Maybe ill look into making it so i can make amperage variable as well. If this is throwing up any red flags let me know.

#### kellys_eye

Jun 25, 2010
6,514
What kind of battery?

Either way, let's take a 12V battery as an example. 12V @30A is only 360 watts...... add losses and maybe 500W.

There are a gazillion battery chargers on the market already. Many are cheap and sophisticated. Unless you have a notion of a new way to charge batteries - faster is the usual requirement - then you're fighting an uphill battle to create something 'new'.

If all you want is to make your own then there are masses of examples of DIY versions out on the internet. Or you could do it from basics.

Find a particular battery type, look at the manufacturers datasheet for it and discover the charging requirements. Go from there.

Jun 4, 2017
212

#### Electric-T

Jun 4, 2017
212
What kind of battery?

Either way, let's take a 12V battery as an example. 12V @30A is only 360 watts...... add losses and maybe 500W.

There are a gazillion battery chargers on the market already. Many are cheap and sophisticated. Unless you have a notion of a new way to charge batteries - faster is the usual requirement - then you're fighting an uphill battle to create something 'new'.

If all you want is to make your own then there are masses of examples of DIY versions out on the internet. Or you could do it from basics.

Find a particular battery type, look at the manufacturers datasheet for it and discover the charging requirements. Go from there.

I guess i was thinking on the wrong end of the transformer. I was thinking of the maximum power that can be pulled from the wall. 15A at 120V gave me 1800 watts. Thats why i thought that

#### Electric-T

Jun 4, 2017
212
....and what would that dream be?

If we knew what your 'dream' was we could suggest a suitable item.

A battery charger that involving a potentiometer that could adjust the voltage.
I guess a "variable output" battery charger is what i would call it

#### Electric-T

Jun 4, 2017
212
So when i look for power rating on a transformer. I should focus on the output power? Thats what im gathering so far

#### kellys_eye

Jun 25, 2010
6,514
You have to calculate your NEED first - then work backwards, adding all the 'inefficiencies' on the way. You end up at the power INPUT requirement which is the 'size' of the transformer you require.

Older transformers (the heavy, metal laminated versions) operate at the local supply frequency (usually 50 or 60Hz) but modern designs use much higher frequencies.... from the kHz to the 10's or and 100's of kHz. This allows the transformer core material to be 'different' (most often a ferrite material) and is far, far smaller and lighter in weight.

The disadvantage (as was) was the cost of the control electronics to make the high frequency signals but over the years this cost has fallen dramatically and switched-mode power supplies (SMPS) - as the high frequency versions are known - are now cheaper than their older, heavier, equivalents.

#### Electric-T

Jun 4, 2017
212
You have to calculate your NEED first - then work backwards, adding all the 'inefficiencies' on the way. You end up at the power INPUT requirement which is the 'size' of the transformer you require.

Older transformers (the heavy, metal laminated versions) operate at the local supply frequency (usually 50 or 60Hz) but modern designs use much higher frequencies.... from the kHz to the 10's or and 100's of kHz. This allows the transformer core material to be 'different' (most often a ferrite material) and is far, far smaller and lighter in weight.

The disadvantage (as was) was the cost of the control electronics to make the high frequency signals but over the years this cost has fallen dramatically and switched-mode power supplies (SMPS) - as the high frequency versions are known - are now cheaper than their older, heavier, equivalents.

If i could find one that even knocks the voltage to 30vac i can go from there. I just want the amperage. I cant work with 3A.

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