Thursday, September 20, 2012

AC To DC Circuit 1 - Hobby Electronics

Welcome everyone to my blog. In this post I am going to write something about AC to DC converters. How it works, components that is needed to build and a few more words on this.

For hobby electronic projects DC sources are very important, anyone can buy standard dry or any different cells available in the market but that will cost some money. Say you want to build a small amplifier maybe an 1W variant with LM386, you will need to power this circuit, you might want to hook up a 9V battery with it but after certain period of time the battery will die. You can buy an adapter that will convert the AC main line to low voltage DC and use it or you can make such an adapter yourself. It is very easy to make a AC to DC converter.  These circuits are really cheap, versatile and easy to make.

Components needed for a AC to DC converter : 

1. Transformer ( We can build AC to DC converters without transformer see here )

2. Diodes ( It will convert the AC to DC )
3. Capacitors ( These are for smoothing or filtering )
4. LED and Resistors ( Just for pilot lamp, it is not mandatory )

A few words about AC DC & other components:

AC means Alternating current, so the direction of the flow of electrons alters or changes periodically.
DC means direct current & its direction doesn't  change with time. So if we want to convert AC to DC we will need such a circuit that will convert the alternating flow to a direct flow. Diodes are such devices. Diodes are P-N junctions that allows one way flow of electron so they can convert AC to DC.
You can think of diode as a one way gate. It permits only one way flow, but in AC as you can see electron flows in two different directions, diodes won't allow them to do so thus making it DC. 

The 0.7 Volt drop is for silicon diode.

We Will Also Need a Transformer(step-down) to lower the 110V/220V ac Main line to our required voltage like 6v/12v.

Transformers have two coils one the primary & the other is secondary. AC is applied on its primary side & we get AC (lower voltage if its step down & higher if its step up) on its secondary & this could be vice-verse, if we apply low voltage(AC) on a secondary coil of a step down transformer we will get higher voltage on its primary(warning this could be a reason of electric shock). One more thing transformers only works on AC.

The next thing is capacitor. Capacitors charge when they are connected with supply & discharge when the supply is cut-off. So in this circuit the capacitor will fill up the gaps.

For Center Tapped Transformer, this diagram can be used to get DC. A center tapped transformer is shown below.

IN4007 That's how they look like.

A 220V to 12V-0V-12V center tapped transformer that could be used here.Red cables are for 220V AC input & Blue - White - Blue stands for 12V-0V-12V. I'm using this transformer to run 3 of my desktop cooling fans.

IN5408, these are high current diodes. They are larger than IN4007

Just take a look at the diagram u can see that its actually a really simple circuit. Here we are using one step down transformer for getting our required voltage & 4 diodes for full wave rectification & one capacitor for purifying.

Two capacitors that could be used. On the top a 470uF 50V capacitor & on the bottom a .01uF capacitor.
(How to read? here the reading is 103K which stands for 10*10^3picoF that means .01uF & voltage rating is 2KV/2000V K stands for 10% tolerance of its capacitance)

Power Dissipation & Heating:

Guess you might have noticed that during operation transformer and diodes get really hot(in some cases). To determine how much heat it will generate we have to know how much power it has dissipated.

The reasons why transformer gets hot is kinda complex so I will not write much about that now and transformer usually doesn't get much hot either as long as you don't overload. 

Amount of power diodes need to dissipate in this procedure will be

P(diode) = VF * Imax
P(diode) = VF * Imax * 2 [For Bridge Rectification]

Here "P(diode)" means the power dissipated by the diode or diodes, "VF" is the forward voltage drop of the diode, "Imax" is the maximum current through them or you can just use the load current in there.

More power diodes have to dissipate more hot they will get, so before connecting you must check the diode's specification sheet to confirm that everything will be fine.

As in bridge rectification two diodes work simultaneously in any given time, we have to multiply the voltage drop by 2.

Some important notes here.

1. General purpose silicon diodes that we usually use drops about 0.7V. So each diode will drop 0.7V of input voltage.For a full wave rectification as two of the diodes act simultaneously it will drop about 1.4V in total. So if Input RMS is 12V we will get an output of (12-1.4)V=10.6V.
For Schottky diode  the forward voltage drop is 0.4V. In reality general purpose diodes drop about 0.3V and Schottky diodes drop about 0.1V.

2. IN4007 diodes that I have used here can support up-to 700V RMS & 1.0A maximum average forward rectified current. *u can take a look at the datasheet.

You can get datasheets & more stuffs here

Read My Other posts here.