Why are the transformers in the DC power supplies from China so small, in a 12v 10 amp supply I purchased it is about 30mm square compared with a 12V 1Amp transformer I have that is about 80 mm square? Have I missed something?

 

Maybe the Chinese psu is a switched mode type does not have a "mains" transformer. Transformers working at 50-60 Hz 10 amps require a lot of iron, and even 1amp makes it fairly large compared with an inductor working at several hundred kHz.

 

Thanks for that reckless, I,m going to learn a bit about switched power supplies.!!

 

Another thing to consider is that there is generally a trade-off between cost and characteristics such as temperature rise, regulation, and of course efficiency. The less suff used the less it costs to make, and some manufacturers cut cost to the bone by eliminating safety and performance margins.

 

I have purchased a switched supply and now understand just a little bit more about them. I do have one question should I earth the secondary side -ve for safety just in case there is no isolation transformer in the unit?

 

No don't connect any part of the secondary side to earth, as you can introduce fault currents, as these chips use Live mains supplies.

Study different smps chips like TEA1733, UC3842 /5 series.

 

That's a worry Dodgy, If there is a fault in these chips I guess mains gets on to the 12v side?

 

In the majority of SMPS there is a small high-frequency transformer which provides mains isolation. It seems yours has one. Before using your supply you can confirm this by checking with a multi-meter for continuity between the mains input side and the 12V output side. There should be none; in which case you can safely ground the secondary -ve if you wish.

 

The only exception of connecting the Negative output to Earth is on metal frame psus, like atx to prevent shock, otherwise any plastic cases that are marked with a double square are not Earthed., they will probably have a suppression capacitor on the Negative output to mains negative input .

 

The 12V/10A power supply, if you were not bought the industrial type, then the current could be not enough as its claimed, probably the current just around 6A or less.

 

I see that in retail markets in Thailand all the time. Unless it is a brand you know and are cofident it isn't counterfit, be very skeptical about specifications.

 

I see that in retail markets in Thailand all the time. Unless it is a brand you know and are cofident it isn't counterfit, be very skeptical about specifications.

From the out side appearance, same design perforated ali. case, same terminal arrangement all available on ebay and amazon they all look to be same. I did measure ohms from L and N to +/- its all open circuit. I think I will risk earthing the -ve side for safety.

 

Maybe the Chinese mm is smaller, or the Chinese volt or current is smaller?