Please, now you make your comments on this schemetics.

Not only do you not need C2 and C3; having them will prevent the circuit from working.

I would connect the 15VAC secondaries in parallel and use a larger filter cap to reduce ripple voltage on the rectified output. That would waste less power in the LM317.

LM317 are spec'ed for a minimum load current of 10mA. If the regulator you use doesn't regulate without a load, increase the minimum load until it does. I had one LM317 that required a 20mA load before it would regulate...

You might be able to use a 78L05 for the 5V for the voltmeter and power it from the 12V regulator.

You never specified the current rating of your transformer. You guessed it might be around 2A. If it's closer to 2A, it might be marginal for the load you plan.

 

I almost give up my project idea, because it seems that it is far beyond my range to build an adjustable voltage regulator.
I plan to go electronics shops to buy a ready device next days, because I nead one.

If you decided to buy, maybe you can buy a new one or second hand as 0~30V/3A.

 

I've not worked out the current flow, but it seems to me that having 2 bridge rectifiers from different taps on the one secondary and each going to the same gnd may cause problems. It is morning here and I'm pre-coffee

 

If you decided to buy, maybe you can buy a new one or second hand as 0~30V/3A.

How much $ you think?

 

I've not worked out the current flow, but it seems to me that having 2 bridge rectifiers from different taps on the one secondary and each going to the same gnd may cause problems. It is morning here and I'm pre-coffee

I have tried to make separate grounds, GRN1 and GRN2. I am sometimes confused when common ground when not.

 

Not only do you not need C2 and C3; having them will prevent the circuit from working.

I would connect the 15VAC secondaries in parallel and use a larger filter cap to reduce ripple voltage on the rectified output. That would waste less power in the LM317.
LM317 are spec'ed for a minimum load current of 10mA. If the regulator you use doesn't regulate without a load, increase the minimum load until it does. I had one LM317 that required a 20mA load before it would regulate...
You never specified the current rating of your transformer. You guessed it might be around 2A. If it's closer to 2A, it might be marginal for the load you plan.

Thanks, a lot. You gave me very good hints.
1) I am using an old PC power supply box and even same pcb but of course, most compononts removed. I kept them from original pcb.
So, first hand I remove them.
2) OK. I will use 15VAC input for LM317 with 2 parallelly connectged rectifiers. I was confused from LM317 datasheet where it says input voltage shall be => 28V
3) I think 2 leds in parallel will consume more than 10mA. Is this OK?
4) My transformer has no label on it. As far as I remember it was 2x15W. But anyway, I will limit my output current as max. 1A.

 

How much $ you think?

Here are some kinds of type that for your reference, if you buy the dual power supply adjustable lab 0-30v, 3a then you can do the op amp experiments for dual power supply, or just buy the single power supply, it depends on your budget and what kind of circuit that you trying to do.

 

Hello,

If you want a fixed 12 Volts an 7812 regulator will do.
The 7812 is good for 1 Amp.
If you want 2 Amp, you at least need a 3 Amp regulator like the 78T12.

Bertus

Hi, Bertus.
I need a variable voltage regulator from 1,2V to 15V with max 1A supply. I will use 7812 for cooling fan power supply.
I have built the circuit shown on the attachment. My problem and question are as follows:
1) Digital voltmeter installed on the circuit box is not showing output voltage correctly (digits are always cahnging) . When I check output voltage with portable avometers which I have 3 kinds, the output voltage seems to be adjusted continuously as I wanted. What is rthe reason? What you think?
2) As you see on the schematics, I have used 2 DC rectifiers, one for aux. power supply ant the other for variable voltage output. Can I make a common ground for both rectifiers? Maybe, this the reason for voltmeter not showing the voltage properly.

 

As you are using just 15VAC for each rectifier, you have a number of options.

1. Parallel the windings for more current and use one bridge and filter cap to drive both regulators.

2. Separate the windings and give each one its own bridge and cap and reg.
Both 0V lines can now be connected together. I forgot to draw that in.

3. Series the windings and use the center tap to ground and one diode in each leg only and have a common cap for both regs.


The third option allows a -ve reg to be used too giving a -12V out.

As it is, you do need to remove the connection between the windings as you are basically using #2.

I think you could do with bigger electros than 330uF too.
And a good idea is to connect the bottom of the pot to 0volts as well as the wiper so if the pot goes noisy it does not go fully open .

 

Here are some kinds of type that for your reference, if you buy the dual power supply adjustable lab 0-30v, 3a then you can do the op amp experiments for dual power supply, or just buy the single power supply, it depends on your budget and what kind of circuit that you trying to do.

Hi, I checked local market. The simplest one costs about 40$ (TTT_Echni_C brand and 1520D model) and it goes upto 100$ with additional features. I live in a big city. Tomorrow, I will go to other shops and I will decide on 40$ one probably. Thanks for your guidance.

 

Hello,

If you want to make an adjustable powersupply for 2 Amps, you might want to have a look at the LM338, the big brother of the LM317.
If you only can get the LM317, you can use a PNP bypass transistor for the higher current:



Bertus