wierd transformer output voltage

[[email protected]]

Hi All,

I have a problem -- output voltage of 230V -> 9V transformer is not 9V,
but around 14V!

A couple days ago I have bought a small 230V to 9V transformer, it
looks like this:
http://www.inetcat.org/stuff/transformer/transformer.jpg.

The transformer is a part of a power supply module, it is followed by
7805 regulator. After I have assembled the circuit I have noticed that
7805 is running hotter than I expected. Soon I have found the problem
-- the output voltage of the transformed is much higher that 9V,
around 13V.

Today I went to the shop again and bought another one. Similar effect:

http://www.inetcat.org/stuff/transformer/transformer-output.jpg.

A little black box on the top is the transformer. White wires go to a
wall socket, 230V AC. Two multimeters measure AC voltages, left -- on
the primary windings, right -- on the secondary one.

Why a voltage on the secondary winding is 14.4V? The multimeters are
ok, I can swap them and get the same readings. Am I lucky enough to get
two faulty transformers? It is theoretically possible, both are likely
to come from the same batch. I fail to see what else can be wrong...

I am thinking about go back to the shop and complain, but would like to
hear somebody else's opinion first. Would anyone please care to
comment? Am I missing something about AC transformers labeling?

Please help.

Kind regards,
Alex

 

[Don Kelly]

Hi All,

I have a problem -- output voltage of 230V -> 9V transformer is not 9V,
but around 14V!

A couple days ago I have bought a small 230V to 9V transformer, it
looks like this:
http://www.inetcat.org/stuff/transformer/transformer.jpg.

The transformer is a part of a power supply module, it is followed by
7805 regulator. After I have assembled the circuit I have noticed that
7805 is running hotter than I expected. Soon I have found the problem
-- the output voltage of the transformed is much higher that 9V,
around 13V.

Today I went to the shop again and bought another one. Similar effect:

http://www.inetcat.org/stuff/transformer/transformer-output.jpg.

A little black box on the top is the transformer. White wires go to a
wall socket, 230V AC. Two multimeters measure AC voltages, left -- on
the primary windings, right -- on the secondary one.

Why a voltage on the secondary winding is 14.4V? The multimeters are
ok, I can swap them and get the same readings. Am I lucky enough to get
two faulty transformers? It is theoretically possible, both are likely
to come from the same batch. I fail to see what else can be wrong...

I am thinking about go back to the shop and complain, but would like to
hear somebody else's opinion first. Would anyone please care to
comment? Am I missing something about AC transformers labeling?

Please help.

Kind regards,
Alex

What is the voltage at rated load current? It appears that you are looking
at the open circuit voltage. Under load this will drop.

 

[Alexander]

What is the voltage at rated load current? It appears that you are
looking at the open circuit voltage. Under load this will drop.

I guess you have an AC-AC transformer, after the transformer you use a
bridge-rectifier and some caps before the 7805.
The problem is that the 9V is AC, it's an RMS value an effective value. The
top of the sinus of the 9V is about 12.7V.
This is also the peak voltage after the rectifier, the caps will load to
aprox that value. Depending on the size of the caps the voltage after the
caps will be 12.5-12.7 or lower if the caps are way to small.

To check if this is the case, put the multimeter on AC en measure the value
on the secondary coil, if it's 9V I'm right.

best regards,

Alexander

p.s. the voltage will also change depending on the load and the input
voltage.

 

[[email protected]]

Hi Don,

Yes, for the second transformer i measure open circuit voltage. But the
first one is already in the circuit, and I get around 13V under 15mA
load. I did not measure the current directly, the load is PIC18F1320
microcontroller, MAX233 RS232 level converter, DS18S20 digital
termometer, and a LED. Power supply schematics is here:

http://www.inetcat.org/stuff/transformer/power_module.GIF

Are you saying that I will get 9V only if I draw the rated current from
the transformer in full? I will try it, need to find a suitable load...

Thanks.

Regards,
Alex

 

[[email protected]]

Hi Alexander,

I have already measured RMS voltage on the secondary and it is around
14V. If I check it with the oscilloscope, I get Vpeak = 19.5V, Vrms =
13.7V. This is under approx 15mA load.

http://www.inetcat.org/stuff/transformer/secondary-voltage.GIF

In front of 7805 I get 18V with 1.2V ripple:

http://www.inetcat.org/stuff/transformer/rectified-voltage.GIF

Regards,
Alex

 

[[email protected]]

Hi,

Right, I have completely neglected internal resistance of the windings.
As Don has suggested I have loaded the secondary windings. I am
getting 9.1V @ 230mA.

Don and Alexander, thank you for your time. The problem is resolved.

Kind regards,
Alex