Switch mode power supply output across capacitor increases in volts.why?

[[email protected]]

Hi all,

The subject describes pretty much my question. I have been spending
several days trying to debug a circuit, only to find that it wasn't my
fault:

I have a normal (pretty cheap) switch-mode plug-pack power supply
(240V, 50Hz in; 12V, 200mA out), and nominal voltage appears to be
somewhere around 12V (so far so good). With an electrolytic cap across
the output (correctly polarised), the voltage rises to somewhere
around 17V - 18V. I had no idea this would occur, and was thus trying
to find the problem in my circuit.

Does anyone know why this happens? Presumerably a poor regulating
circuit in the plug pack, but does anyone have some more thorough
answers?

Thanks very much,

-Gus

 

[Phil Allison]

The subject describes pretty much my question. I have been spending
several days trying to debug a circuit, only to find that it wasn't my
fault:

I have a normal (pretty cheap) switch-mode plug-pack power supply
(240V, 50Hz in; 12V, 200mA out), and nominal voltage appears to be
somewhere around 12V (so far so good). With an electrolytic cap across
the output (correctly polarised), the voltage rises to somewhere
around 17V - 18V. I had no idea this would occur, and was thus trying
to find the problem in my circuit.

Does anyone know why this happens? Presumerably a poor regulating
circuit in the plug pack, but does anyone have some more thorough
answers?

** A small SMPS plug pack I have here (12 volt, 780mA ) produces a strong
100 Hz ripple ( ie hum) at the output if you add even 100uF of external
capacitance - but it was built to power a particular ( telecoms ) device
rather than general purpose use.

I suppose something in the output voltage control loop goes out of wack when
you add extra C.

But you one you have sounds like a menace to equipment.



...... Phil

 

[Jamie]

Hi all,

The subject describes pretty much my question. I have been spending
several days trying to debug a circuit, only to find that it wasn't my
fault:

I have a normal (pretty cheap) switch-mode plug-pack power supply
(240V, 50Hz in; 12V, 200mA out), and nominal voltage appears to be
somewhere around 12V (so far so good). With an electrolytic cap across
the output (correctly polarised), the voltage rises to somewhere
around 17V - 18V. I had no idea this would occur, and was thus trying
to find the problem in my circuit.

Does anyone know why this happens? Presumerably a poor regulating
circuit in the plug pack, but does anyone have some more thorough
answers?

Thanks very much,

-Gus

its simple, You don't have a switch mode power supply..

Its your basic all wart type supply that is unfiltered or
you have something serious wrong with the supply.

http://webpages.charter.net/jamie_5"

 

[neon]

switching power supplies as a general rule allways keep a nominal load on it. the magnetics needs current to work with.

 

[[email protected]]

thanks both of you - i guess I buy more expensive, properly regulated
supplies in the future!

Cheers...