how do these turn out a steady 5 volts???? dc....
do they use a zener to pull the volts down ????

 

One uses a step down transformer to reduce the voltage, a rectifier to make the AC inot DC, a filter capacitor to reduce the ripple, and a regulator like a 7805 to produce stable five volts.

Further reading - http://www.allaboutcircuits.com/vol_2/chpt_9/6.html.

 

http://www.electronics-tutorials.com/basics/power-supp-hi-regulated.htm

There are several types of supplies all with varying characteristics. The one above in Fig 2 is a typical linear regulator, the one in Fig 3 is a very high current linear regulator.

 

are you referring to a direct rectification? that is you dont use a stepdown transformer.

moz

 

Generally a 100 - 230volt down to 5 volt supply rectifies the mains voltage, then uses high frequency Switch Mode techniques to turn this high DC voltage into high frequency AC with a Mark Space ratio that is monitored and varied to ensure that the rectified output stays at 5 volts.

 

Not always a step down transformer and a voltage regulator are used. Some PSUs are pulse modulated.

 

Normally a small High frequency transformer is used in switch mode PSU's to maintain isolation between the secondary and primary. Without this isolation there might be safety issues as well as tripping of Earth Leakage breakers (GFI).

 

Normally a small High frequency transformer is used in switch mode PSU's to maintain isolation between the secondary and primary. Without this isolation there might be safety issues as well as tripping of Earth Leakage breakers (GFI).

Agree, but normally it is a 1:1 ratio transformer to keep the volume of the PSU small (in some cases 2:1 ratio transformer is used).

 

I have never checked ratios, but wattage and core size would be constant no matter what the ratio.
I would have thought a step down ratio would be used otherwise the high frequency rectifier diodes on the secondary would have to be rated at over 350volts to cope with peak voltage, increasing expense..??? .. Also, if the output cap/filter failed would we get 350 volt DC pulses on the output...?

Efficiency increases with frequency and correct core selection, and that has more to do with physical size especially when you consider most SM PSUs operate anywhere up to 300kHz.

 

I have never checked ratios, but wattage and core size would be constant no matter what the ratio.
I would have thought a step down ratio would be used otherwise the high frequency rectifier diodes on the secondary would have to be rated at over 350volts to cope with peak voltage, increasing expense..??? .. Also, if the output cap/filter failed would we get 350 volt DC pulses on the output...?

Efficiency increases with frequency and correct core selection, and that has more to do with physical size especially when you consider most SM PSUs operate anywhere up to 300kHz.

Actually, a 1:1 ratio transformer is smaller than a step down transformer with the same wattage.