Quick Transformer Question

[kurtlives]

Basically I have a 117V/12V transformer. I want to use it for a 12V DC pedal. Would this work to get 12V DC???



Thanks

[Mark F]

Yes you'll get 12VDC , actually you'll get more (12 x 1.414) I would use a regulator to get a solid steady 12VDC

[kurtlives]

How would I do that? I am new to transformers

[jakenold]

Just follow the right side of this schematic.

Jake

[brett]

Hi
C2 is large (2200uF per amp), while C3 is small (100uF).
IC1 is an LM7812.
You'll need a heatsink on the LM7812 if you are going to draw more than about 100mA.  A small one is ok.
cheers

[kurtlives]

Ok so I just need to add a 2200mF and 100mF cap and a SCR and heatsink.

[kurtlives]

Just follow the right side of this schematic.

Jake

How is it going from 18V to 12V?

[Mark F]

Ok so I just need to add a 2200mF and 100mF cap and a SCR and heatsink.

IC 1 is a 12 volt regulator not a SCR.

How is it going from 18V to 12V?

The 12 volt regulator regulates the incoming voltage to a nice steady , clean 12 VDC.  I hope that is a bit more clear.

[kurtlives]

My transformer has wires for

120V
0V
120V
0V
12V
0V
12V
0V

I only need the
120V
0V
12V
0V

Could I just cut off the other leads?

[brett]

Hi
That is the safest way, but you'll only be using half of the power rating of the transformer.
If you want to, to make full use of the windings, connect the 0 and 120V primaries together in parallel (0 to 0, and 120 to 120), and the same for the secondaries (0 to 0 and 12 to 12V). Assuming it is a modern, correctly labelled transformer, you'll have no problems.  However, note that with old transformers where the direction of the windings and the labels are uncertain or unclear, connecting windings that were never meant to be connected can be hazardous to the point of fire, explosion, etc.
cheers

[kurtlives]

What do you mean I will only be using half of the transformers power rating? Is that an issue?

[brett]

Hi
your transformer will have a rating in Volts x Amps (VA).  It will typically be 6 to 12 VA.  As the output will be 12V, 6VA  = 0.5 A output, and 12VA = 1 Amp output.

Transformer maths can be complicated, but here's a simplified version of reality that will keep you out of trouble.  Assume the VA rating is for all of the windings.  In other words, if there are 2 x 12V output windings in a transformer rated at 12VA, each gives 0.5A for a total of 1A.  (ie Each winding has its own inpedendent rating, and you should only draw as much current as the individual winding is rated for).  So in your example above, ignoring one of the primaries and one of the secondaries will reduce the VA rating by 50%.
cheers

[kurtlives]

Ok so if I cut off 12V, 0V, 120V and 0V I should still ahve the current to power a Tube Driver? 500mA

[kurtlives]

I think this is right, could someone check it? I am mainly concerned about the 7812s pinout.

[brett]

Hi
the pinout for the 78XX series is really simple.  If only other devices were so sensibe.

Hold it up with the metal tab at the back and the legs pointing down.  The left leg is where the unregulated voltage goes in, and the right pin is where the regulated voltage comes out.  The middle leg goes to ground.
cheers

[kurtlives]

Ok thanks...

It looks good then? Also would there be any sense putting in a fuse?

[kurtlives]

bump

[dxm1]

Ok so if I cut off 12V, 0V, 120V and 0V I should still ahve the current to power a Tube Driver? 500mA

That would depend on the VA rating of the transformer.  I don't believe you posted that...

[kurtlives]

I have no idea what the VA is...how could I find that out?

[Papa_lazerous]

Ok thanks...

It looks good then? Also would there be any sense putting in a fuse?

Yes a very good idea putting a fuse in, put it before the transformer, it may or may not save your components from damage but it could well prevent a fire if something goes wrong, although your transformer will have a thermal cut out inside too which will protect too