Maximum current for AC/DC adaptors

[autir]

(Why was the previous topic locked, gentlemen? ???)

From what I have read I have realised that the current listed on most unregulated AC/DC adaptors is the one needed for output voltage to take its nominal value. How can I determine the maximum current rating for an adaptor (without destroying it or starting a fire or something exaggerating)? What is the "saturation" phenomenon observed in an overloaded transformer and how does it affect future function? Why is it non-reversible?

 

[audioguru2]

Hi Autir,
Sorry, I locked the other topic by mistake, it's unlocked now.

I don't think that an unregulated AC/DC adapter should ever be operated with a load current beyond its nominal rating. Its maximum current isn't listed and it usually must dissipate more power when the mains voltage is high. All the adapters I own have an input power rating (nominal?) that is much more than their output power calculation.
All adapters don't have a fuse and may cause a fire if excessive dissipation causes a high temperature to melt insulation or the wiring structure.
When the core saturates it is no longer a linear magnetic coupler, instead all additional input voltage is converted directly to heat in the primary winding since its impedance drops very low. Saturation occurs at a certain power and is instantly reversible by reducing the voltage or current. The voltage and current are reduced on each half-cycle of the mains AC anyway. You must not allow the peak voltage or current of the mains to cause core saturation.

 

[autir]

Are there regulated and unregulated AC/AC transformers? Or are all transformers unregulated and the regulation of the voltage depends upon something else (a circuit, perhaps)?

 

[Alun]

Hi autir, the transformer just does the voltage conversion ie 240VAC to 12VAC the diodes convert the AC to DC and the filter capacitor converts this pulsed DC to steady DC. As the change in the load can dramatically effect the output voltage some power supplies include a regulator circuit

 

[autir]

Alun thanks for the reply. I know about the different types of ac/dc conversion, my question was how the change in load can alter the output voltage. Does it depend on the internal resistances of the AC/AC transformer itself?

 

[Alun]

autir, the internal resistance of the transformer is only partly to blame, the voltage drop is mostly due to the capacitor filter discharging between AC cycles, it doesn't appear as a voltage drop but as a dramatic increase in ripple. The ripple voltage is an AC voltage of double the mains frequency superimposed on the DC output the graph below show the output voltage of a heavily loaded unregulated DC adaptor. A DC voltage meter will show a voltage drop as it measures the average voltage. The ripple can be reduced by using a larger filter capacitor.

A voltage regulator solves this problem by outputting the same voltage as long as the input voltage is higher than the output. In the example below a 6V regulator could be used, if 12V is required a bigger filter capacitor and a higher output transformer would be needed.

 

[audioguru2]

Hi Alun,
I have never seen an AC to DC wall-wart adapter produce such a high amount of ripple.

I just measured two 120VAC/9VDC adapters:

Rating 9V, 200mA 9V, 500mA

Primary DC resistance: 634 146
DC output, no load: 18.9V 13.4V
DC output, 100 ohms load: 12.2V 11.5V
Ripple with 100 ohms load: 0.6V p-p 0.7V p-p

The primary resistance is dropping the no load output voltage by a few volts and the secondary resistance would drop the output voltage some more. Their ripple voltage is fairly low.

 

[Alun]

Well I did exaggerate a little, perhaps your right maybe those crappy transformers have poor regulation. I was guessing that as the specification for regulation on a decent transformer I have is 15% then a 12V adaptor should only drop by 1.8V I assumed that they just didn't use big enough filter capacitor in order to save space and money.

Try a 47ohm load on the 200mA adaptor and a 22ohm load on the 500mA model, then measure the ripple. Its interesting how the 500mA unit has the worst ripple, it could vary form one model to another, some might have undersized filter capacitors and others might have crappy transformers and the really bad units could have the worst of both.

 

[audioguru2]

Hi Alun,
My little adapter showed its ripple voltage charging quickly, then decaying slowly into the load, as I expected.
My big adapter showed nearly the same charge and discharge times, like a triangle wave, maybe because its secondary resistance is too high.

Sorry I can't measure more because I don't have many power resistors.