Introduction and question about transformers

[grincore]

Hi, since it is my first post i'll first introduce myself. I recently became interested in electronics as a hobby.   I studied economics and work as an accountant in the sunny Spain.  Since this is a hobby for me i am taking it with calm and slowly reading and watching videos to understand the basics.

So please be nice with me if i ask stupid questions at first.

As many people recomended i wanted to build a power supply for my "lab" 

I have a couple of old laptop chargers and at least two old computer powers supplys but i wanted my first bench power supply to be linear.

Where i live temperature can easily get to 40C in the summer so i have to be very carefull with heat.

My supply is going to be (at first) powered by the venerable LM317 so that means 1.5A at most and not more than 15W of power being dissipated as heat.  I will eventually move to a better design with pass transistors but i want to keep it simple at first.

So, asuming LM317 as my regulator i was going to use a 7.5-0-7.5 center tapped transformer (at least 2.6 amps per tap).  If my understanding is correct i could use one of the taps as ground and use a relay to switch between the center and the other tap to get:

7.5v AC x 1.414 - 1,4 (rectifier bridge) = 9.2v DC which i can regulate between 1.25 and around 6.9v and
15 x 1.414 - 1,4 = 19.8v DC which i can regulate between 7 and 16.8 v

So far so good but i have a few doubts where i could use your help.

My first question is, how do i switch between taps, will a simple switch connected to a relay be enough?
Second, i've seen videos of people showing how to make your own transformers.  Being able to make a custom transformer will allow me to add more taps to work more comfortably at bigger voltage ranges. Does it make sense? Is it something that a beginner can do?
Another option is using say two 6-0-6 transformers (primaries in parallel, secondaries in series) to get a 1.25 to 30v supply.  Would it work?
Can I add extra taps to an off-the shelf toroidal transformers (assuming the primary is not being fully used)? For instance i could buy a 5 or 6 amps toroidal and add extra 5v and 12v taps  for the voltmeter and fan, respectively? Will it add more ripple to my supply?

[tautech]

I have a couple of old laptop chargers and at least two old computer powers supplys but i wanted my first bench power supply to be linear.

Welcome to the forum.

Yes, learn to walk before you run. 

My first question is, how do i switch between taps, will a simple switch connected to a relay be enough?

Better to have a transformer with 2 identical but separate windings IMHO.
For series and paralell connection use a DPDT switch.
For paralell, you must have the 2 windings in phase, easily checked with a DMM, one way will show the winding voltage (wrong) and swapped (reversed) will display 0V.

Useful transformer link:
http://ludens.cl/Electron/trafos/trafos.html
You can make up your own mind. 

Guessing, I'd head for 2000 to 4700 uF caps and allow space to add extra if required.

[grincore]

Thanks!

[Zero999]

One thing you could do is design a circuit that switches the relay automatically, when the voltage is adjusted, so you don't need to manually operate the switch.

[Kleinstein]

Using 2 connected windings is not so bad. 2.6 A AC is good for someting like 1.6 A DC current. So this is not a problem. here.

With the voltages you have to add some reserve to take into acount undervoltage in the grid (e.g. 5-10 % ) and also ripple after the rectifier (e.g. 5-15% depending on the size of the caps). So the useful voltages may be a little lower.
In the lower setting the rectifier can work with just one diode drop - but that's a minor detail.

Switching can be done per relais or even directly per switch. More steps for the transformer voltage are possble, but get increasingly complicated, especially if operated manually.

The LM317 is thermally protected inside - so not big problem here. Just keep in mind to have sufficient heat sink, not to reach the thermal limit early. 

Adding extra windings to a torroidal transformer may work. However this hinders heat from the inner windings to be removed. So the power rating may drop a little. It may be OK for something like a low voltage, low power auxiallary winding (e.g. display or floating regulator), but higher voltages tend so be hard manually. As the power rating will not increase (details are complicated) there is also not much use in an additional powerfull winding.

[Fank1]

All you have to do is run the 7.5V windings to a bridge rectifier.
Using a DPST switch for the lower voltage ground the transformer center tap and leave the bridge negative float.
For the higher voltage ground the bridge negative and leave the xmfr center tap float.

[Zero999]

You could use a transistor + zener diode to switch taps. Here's an example. It's designed for a 15V-0-15V transformer and to switch at 12.6V so will need to be modified to suit your requirements.