Electronics questions

[ahmedragia21]

1.In a requlated power supply why we dont use
1a. Resistors for bridge diodes to limit the current for them while we always do that with normal diodes
1b. Why we always put the rectifier first then put the step down transformer why we cant do the opposite ?

2. What is the use of the current mirror , i know its theroy , but the use ?

 

[Hero999]

1) I don't understand the question. Regulated power supplies are better because the voltage is constant and ripple free.
1a) No a rectifier converts AC to DC.
1b) Because transformers require AC not DC and the diodes would convert the AC to DC which would blow up the transformer and diodes.

2) See datasheest for op-amps like the 741 for examples of current mirrors.

 

[ahmedragia21]

thanks
1a. So at AC we dont put resistors in series with diodes to limit the current ?

 

[Nigel Goodwin]

thanks
1a. So at AC we dont put resistors in series with diodes to limit the current ?

You don't at DC either - there are possible situations where you might with either, but it would be fairly unusual.

 

[ahmedragia21]

how we dont do at dc ???
we dont put a series resistor to LEDS ? to zener diodes ??

 

[audioguru]

A rectifier diode is in series with a load. The current is very high for the moment that the filter capacitor is charging but then the load limits the continuous current. Rectifier diodes are made to pass very high momentary currents.

An LED or a zener diode is the load so of course they need to have a current-limiting resistor in series.

 

[Nigel Goodwin]

how we dont do at dc ???
we dont put a series resistor to LEDS ? to zener diodes ??

Those aren't rectifiers, and are used in a totally different way, for a completely different purpose.

 

[ahmedragia21]

Your just saying their application which is a "rectifier diodes" , which are really PN junctions , and they are diodes , wont they be burnt out when they draw more current which they cant stand like any regural diode ??

AudioGuru , would you explain more please how LED , ZENER is a load ?

 

[Hero999]

Any diode needs something to limit the current flowing or it will blow up.

With a rectifier the load is used to limit the current as Nigel explained.

The same is true with an LED, in this case a resistor is normally used to limit the current but it could just as easilly be an electronics circuit like a switched mode power supply.

A zener needs something to limit the current too, normally a resistor is used but a constant current source can be used instead. Zener diodes are used as low current voltage regulators normally to provide a voltage referance for something else like a larger voltage regulator.

 

[audioguru]

a 1N400x rectifier diode is rated for 1A rectified current. Its max allowed peak current for one cycle of the mains is 30A. when it is in a power supply and rectifies AC then it passes a very high current (up to 30A) for a moment each cycle or each half cycle of the AC to quickly charge the main filter capacitor. The resistance of the transformer limits the current to 30A or less. The rectifier diode heats when it passes 30A then it cools before the next pulse.

An ordinary LED operates at 25mA. It can pass 100mA for a moment if you use a circuit to pulse it. The time between pulses allows it to cool.
A small zener diode can pass 25mA continuously or 100mA for a moment. It also has a max heat dissipation limit.