Using an AC-to-AC wall wart transformer/adapter in reverse

[Matt]

Suppose I have a plug-in transformer that reduces 120VAC power to some
lower AC voltage (say 12VAC or 24VAC) and suppose I want to do the
reverse---increase a low-voltage source back up to 120VAC.

Would there be a problem in connecting the low-voltage power to the
plug-in prongs of the wall wart (instead of the normal use: plugging the
prongs into the wall outlet) and expecting to get high-voltage power
back out the other end?

I guess my question amounts to asking whether there are any components
other than the transformer inside the wall wart.

My immediate need is to use a 24VCT (center-tapped, 3-wire, 12VAC or
24VAC) to convert 24VAC back up to 120VAC.

 

[Charles]

Suppose I have a plug-in transformer that reduces 120VAC power to some
lower AC voltage (say 12VAC or 24VAC) and suppose I want to do the
reverse---increase a low-voltage source back up to 120VAC.

Would there be a problem in connecting the low-voltage power to the
plug-in prongs of the wall wart (instead of the normal use: plugging the
prongs into the wall outlet) and expecting to get high-voltage power back
out the other end?

I guess my question amounts to asking whether there are any components
other than the transformer inside the wall wart.

My immediate need is to use a 24VCT (center-tapped, 3-wire, 12VAC or
24VAC) to convert 24VAC back up to 120VAC.

If it (the wall-wart) is just a transformer, then it is bilateral
(reversible). Most wall-warts are switchers and are not bilateral.

 

[Matt]

If it (the wall-wart) is just a transformer, then it is bilateral
(reversible). Most wall-warts are switchers and are not bilateral.

I'm sure there is a transformer in it, since it is quite heavy. The
question is whether the transformer is the only thing in it. I can't
tell whether you have helped answer that.

 

[PeterD]

I'm sure there is a transformer in it, since it is quite heavy. The
question is whether the transformer is the only thing in it. I can't
tell whether you have helped answer that.

Use your meter and figure it out. Not rocket science. Or just try it?

 

[Matt]

Suppose I have a plug-in transformer that reduces 120VAC power to some
lower AC voltage (say 12VAC or 24VAC) and suppose I want to do the
reverse---increase a low-voltage source back up to 120VAC.

Would there be a problem in connecting the low-voltage power to the
plug-in prongs of the wall wart (instead of the normal use: plugging the
prongs into the wall outlet) and expecting to get high-voltage power
back out the other end?

I guess my question amounts to asking whether there are any components
other than the transformer inside the wall wart.

My immediate need is to use a 24VCT (center-tapped, 3-wire, 12VAC or
24VAC) to convert 24VAC back up to 120VAC.

Sorry, folks. Obviously I made an error.

I meant to ask what voltage I would get at the prongs if I hook low AC
voltage to what is normally the output.

BTW I am measuring about 25.7 VAC at the output when the input is 117.5
VAC.

The thing is labeled "Plug-in class 2 transformer, Input: AC 120V 60Hz
35W, Output: AC 24VCT 700 mA".

So it is producing well over its nominal voltage. Can I conclude that
it is unregulated?

 

[Phil Allison]

"Matt"

Suppose I have a plug-in transformer that reduces 120VAC power to some
lower AC voltage (say 12VAC or 24VAC) and suppose I want to do the
reverse---increase a low-voltage source back up to 120VAC.

Would there be a problem in connecting the low-voltage power to the
plug-in prongs of the wall wart (instead of the normal use: plugging the
prongs into the wall outlet) and expecting to get high-voltage power back
out the other end?

** Yep - the " other end " will be at a lower voltage.

Think it through.

I guess my question amounts to asking whether there are any components
other than the transformer inside the wall wart.

** No.

My immediate need is to use a 24VCT (center-tapped, 3-wire, 12VAC or
24VAC) to convert 24VAC back up to 120VAC.

** A "wall wart" will reduce 120 volts AC to about 28 volts AC when there
is no load - so the winding ratio is 4.3: 1.

The same ratio will apply in reverse, so 24 volts becomes 103 volts with NO
load.

Under load, expect about 90 volts.

There is your problem.




...... Phil

 

[Matt]

"Matt"


** Yep - the " other end " will be at a lower voltage.

Think it through.

Yes, I goofed when I stated the direction of hookup. I should have
asked what happens at the input when I hook low voltage to what is
normally the output.

** No.

No, there are no other components, or no, the questions are not equivalent?

 

[Matt]

Whatever you do, do not put this in a commercial product. You will be
asking for litigation.

No, just looking for some flexibility in prototyping or in a one-off
design, in case I don't have the ideal parts on hand.

 

[Sylvia Else]

Sorry, folks. Obviously I made an error.

I meant to ask what voltage I would get at the prongs if I hook low AC
voltage to what is normally the output.

BTW I am measuring about 25.7 VAC at the output when the input is 117.5
VAC.

The thing is labeled "Plug-in class 2 transformer, Input: AC 120V 60Hz
35W, Output: AC 24VCT 700 mA".

So it is producing well over its nominal voltage. Can I conclude that
it is unregulated?

Or is just mislabelled. If you assume it's really a 110V transformer,
then the output is exactly what you'd expect.

If it's just a transformer you want expect any sort of regulation.

Sylvia.

 

[Sylvia Else]

"Matt"


** Yep - the " other end " will be at a lower voltage.

Think it through.


** No.



** A "wall wart" will reduce 120 volts AC to about 28 volts AC when there
is no load - so the winding ratio is 4.3: 1.

The same ratio will apply in reverse, so 24 volts becomes 103 volts with NO
load.

Under load, expect about 90 volts.

Since when is the output voltage of a simple transformer a significant
function of load?

Sylvia.

 

[Phil Allison]

Sillier than Anyone Else

Or is just mislabelled. If you assume it's really a 110V transformer, then
the output is exactly what you'd expect.

** Totally absurd CRAP.




....... Phil

 

[Phil Allison]

"Matt"

No, there are no other components, or no, the questions are not
equivalent?

** An AC output wall wart contains ONLY a transformer.

The transformer may have a thermal fuse incorporated in it's primary
winding - which makes it part of the transformer.




...... Phil

 

[Eeyore]

Since when is the output voltage of a simple transformer a significant
function of load?

Since Michael Faraday ? first wound one maybe ?

Graham

 

[Eeyore]

With no load ? Transformers are normally rated at their stated load.

Probable.


Or is just mislabelled. If you assume it's really a 110V transformer,
then the output is exactly what you'd expect.

The US supply voltage is officially 120V.

If it's just a transformer you want expect any sort of regulation.

Uh ?


Graham

 

[Sylvia Else]

Since Michael Faraday ? first wound one maybe ?

Graham

The voltage on the primary is determined by the rate of change of flux,
but also has to equal the supply voltage.

Since that flux is linked to the secondary, the secondary sees the same
rate of change of flux, with the voltage on the secondary being related
to the voltage on the primary just by the number of turns.

In the absence of hysteresis losses on the core, flux leakage, and
resistance, this relationship remains true regardless of load.

There are of course such losses, but the transformer is designed to
minimise them. The 25% loss suggested by Phil seems extreme.

Sylvia.

 

[Phil Allison]

Sillier than Anyone Else on Earth

The voltage on the primary is determined by the rate of change of flux,
but also has to equal the supply voltage.

Since that flux is linked to the secondary, the secondary sees the same
rate of change of flux, with the voltage on the secondary being related to
the voltage on the primary just by the number of turns.

In the absence of hysteresis losses on the core, flux leakage, and
resistance, this relationship remains true regardless of load.

There are of course such losses, but the transformer is designed to
minimise them. The 25% loss suggested by Phil seems extreme.

** What a PILE of ABSOLUTE DRIVEL !!


The figures I suggested were for circa 15% regulation which is typical for a
mains transformer of 20 VA rating.

Transformers with a 5VA rating have about 30% regulation.

The drop of output voltage under load is almost entirely due to the winding
resistances of a transformer.

You PIG IGNORANT DAMN TROLL.



...... Phil

 

[Sylvia Else]

With no load ? Transformers are normally rated at their stated load.



The US supply voltage is officially 120V.



Uh ?


Graham

I meant "won't". The OP appeared to be thinking in terms of DC style
plug packs, of which some are regulated, as in they provide a more or
less constant voltage regardless of load.

Sylvia.

 

[Jasen Betts]

I guess my question amounts to asking whether there are any components
other than the transformer inside the wall wart.

often there's a fuse built into the secondary winding of the
transformer. other than that no.

 

[Jasen Betts]

I'm sure there is a transformer in it, since it is quite heavy. The
question is whether the transformer is the only thing in it. I can't
tell whether you have helped answer that.

the only way to be sure is to do a test, put 1.5v in the 24V end with an AA cell
and see if a spike comes out the 120V end when you disconnect it.
(you could use a piezo transducer, neon indicator, small sparkgap,
or just lay a finger across the pins)

 

[Jasen Betts]

Since when is the output voltage of a simple transformer a significant
function of load?

whenever the windings have significant resistance.