Hello friends.

I have a transformer which is 18v-0v-18v with center taped.. The current rating of transformer is 500ma
And I have 8v and 12 voltage regulators which are 7808 and 7812.

I want to arrange this regulator in the transformer to gain 12v and 8v output dc and current 200ma..

I have attached the Transformer image which I have ....so please have a look...

I want to gain 8v and 12v directly from 18v transformer..
I don't want to gain 8v by adding the 8v regulator at the output of 12v...

Thanks in advance...

 

The 18V transformer will give you more like 24V if you use a simple full-wave rectifier, employing the centre tap with two diodes.

If you used a bridge rectifier across the full winding, you would get twice as much voltage (don't do it!)

Even with only 24V input, the regulators would be dropping quite a lot of voltage, particularly the 8V one. Take care that they will be able to handle the power loss, with an adequate heat sink as necessary.

 

Hi Adjuster.. Thanks for your reply...
Can you give me the circuit diagram according to my needs...Because I am new in electronics ... and this is my first project.. So please help friend !

Thanks in advance

 

Here, have a look at the attached.

It should take care of your supply needs for now.

Everything is necessary, or I would not have put it in the schematic.

[eta]
The schematic has changed; R1 is now 30 Ohms, 5W and R2 is 47 Ohms, 5W.

I did not update this schematic; the updated schematic is posted in my 2nd reply on page 2.

 

Thanks for your reply SgtWookie !

Will this circuit give 200ma output if not then How do we get it !

Very very very thank you my friend..... for your help

I have some questions below regarding the circuit above......... so please..... answer...

1)What is the value of R2 & R1, is it in ohms , kilo ohms or mega ohms !

2)What is the use of R2 & R1 !

3)Is is using full wave rectification or half wave rectification !

4)Will the center wire of transformer will be common ground ! (You can see at my images of the transformer above , the secondary winding has two green color wire and one red color wire in middle).

5)How can we reduce or increase the output current !

6)Will the 12v output can drive 12v relay (Relay needs 60ma to work properly on 12v).

7)What is the advantage and disadvantage of this PSU !

8)Can we turn on 8v and 12v supply simultaneously without any problem !

9)If suppose the Load exceeds the 200ma current then what will happen ! Will it work or will we have to do any changes in circuit !

10)If this circuit is used properly at all condition 24hr , then what will be the life expectancy of this circuit !

============================================================

Friend , I have mentioned some question please give the answer...
As I am asking this question because I am new In electronic and need some help to create this PSU..

Thanks

 

On reflection, the comment that I originally made here may have been a little harsh and ill-considered. Nevertheless, I think that knowledge of some basics such as Ohm's Law may be useful for you. This would allow you to understand things like this power supply without needing to have things described in so much detail.

 

Will this circuit give 200ma output if not then How do we get it!

If your transformer is indeed rated for 500mA, then you should be able to draw up to 200mA from either regulator. Note that you should supply heat sinks for both regulators, or the power dissipation will cause them to overheat.

I have some questions below regarding the circuit above......... so please..... answer...

1)What is the value of R2 & R1, is it in ohms , kilo ohms or mega ohms !

Their resistance is specified in Ohms.

2)What is the use of R2 & R1 !

The resistors relieve much of the power dissipation load in the regulators. With these resistors, the regulators still have to dissipate up to around 2.5 Watts of power. Without the resistors, the regulators would overheat and shut down, as you would be asking them to dissipate 7 Watts or more at full load.

At peak loading of 200mA, there will still be a few volts above the output voltage on the input of the regulator.

3)Is is using full wave rectification or half wave rectification !

Full wave rectification.

4)Will the center wire of transformer will be common ground !

Yes, and you should also connect it to earth ground. This protects you in case there is a fault in the transformer.

(You can see at my images of the transformer above , the secondary winding has two green color wire and one red color wire in middle).

Yes, I saw that.

5)How can we reduce or increase the output current !

The regulators will regulate by output voltage. They will supply the current necessary to maintain the output at their ratings, until the load becomes so great that too much voltage is dropped across the resistor on the input. At that point, the output voltage will drop.

If you need more current than that, you will need to decrease the size of the resistor on the input of the regulator. However, that will make the power dissipation in the regulator increase; so you will need a larger heat sink.

6)Will the 12v output can drive 12v relay (Relay needs 60ma to work properly on 12v).

The 12v output is limited to approximately 200mA. If the total output load is <= 200mA, then the relay will function normally.

7) What is the advantage and disadvantage of this PSU !

Advantage: simple and inexpensive.
Disadvantage: Very inefficient.
For the 12v output, 1-(12/33)*100 = 36.4% efficiency.
For the 8v output: 1-(8/33)*100 = 24.2% efficiency.
Most of the power is dissipated in the resistors and the regulators.

8)Can we turn on 8v and 12v supply simultaneously without any problem !

Yes.

9)If suppose the Load exceeds the 200ma current then what will happen ! Will it work or will we have to do any changes in circuit !

The regulator output voltage will decrease.

10)If this circuit is used properly at all condition 24hr , then what will be the life expectancy of this circuit !

There are far too many variables which I don't know about that will affect the life expectancy of this circuit. If the temperature is very hot where you are, the life expectancy will decrease significantly. If the workmanship is poor, the life expectancy will decrease. If the heat sinks are small, lower life expectancy.

 

After further consideration, R1 and R2 should be increased from 5W rating to 10W rating. Otherwise, if there is a heavy load on the output, 5W would not be sufficient.

 

Where is my post?
Oh, maybe it is at the other website.

 

Where is my post?
Oh, maybe it is at the other website.

Apparently: I'm tempted to say it is like a parallel universe, but with different series resistors (somebody may have made a slip with their arithmetic). http://www.electro-tech-online.com/...help-using-transformer-voltage-regulator.html

Edit: More to the point, I would doubt those small reservoir caps with big series resistors are a good idea. It might or might not simulate OK, but a lot is being asked of those caps. SgtWookie's circuit is orthodox, with a decent-sized reservoir so the ripple should not dip too low.

 

It gets confusing when the same thread is on two different websites. Some of the answers are the same and some are different.

 

Well, the worst case ripple (if the load doesn't exceed 400mA) on the 2200uF cap should be under 2v - so I don't see it as a problem.

On the other website, Eric's resistors are too small, and placing large caps next to the regulator kind of defeats the purpose of the resistors, if the load has a lot of transients.

[eta]
After further thought, I realized I made a mistake, so values have changed for R1 & R2.

Schematic & simulation attached.