Transformer overheat

[Lyrica's Reality]

Hi, i've only just joinged the forum and am working on my first project, a multiple 9v power supply for guitar pedals which I own and plan to construct in the near future. I'm pretty new to this, but have picked up the basics and it works! the only problem is that the transformer is getting really hot really quick.

This is the transformer I am using:

http://www.maplin.co.uk/miniature-25...nsformers-3688

Its 240v to 12v and has two black wires on the primary, two red and one white on the secondary (12-0-12). I have the black wires upto the mains, one live and one neutral, and the reds wired to the circuit (one live, one to ground) and the white to a ground on the casing (metal).

So have I wired it wrong? Do transformers usually get too hot to pick up or touch for a few seconds? Is their any way to cool it?

My main concern is that its going to cause a fire, obviously, but could also damage the circuit components or even the guitar pedals.

Thanks in advance for any advice.

[cork_ie]

No it is not normal that a transformer should get too hot to touch. If it feels slightly warm then that is OK.
The transformer you are using is only rated 250mA which is a pretty small rating. Perhaps you are overloading it. What current are you drawing from it?

[mcasale]

What is the transformer secondary driving? Are you doing any rectification?

I assume you need 9V DC - not AC?

It's usually a good idea to put fuses on the primary side (sometimes, both line and neutral).

[nomurphy]

Look on the bottom or sides of the pedals and add up the required current for each pedal (e.g., 9VDC at 250mA). The transformer secondary current rating should be for at least twice that total amount.

[absf]

"Its 240v to 12v and has two black wires on the primary, two red and one white on the secondary (12-0-12). I have the black wires upto the mains, one live and one neutral, and the reds wired to the circuit (one live, one to ground) and the white to a ground on the casing (metal). "

The casing is actually Ground too. So you are connecting one red wire and one white wire to GND which is same as shorting one set of the secondary coils. That's why the transformer gets hot so fast.

Allen

[Lyrica's Reality]

Great advice so cheers to everyone who's replied. In answer to a few of the questions:

1) It will be powering at the moment 4-5 guitar pedals which draw on average 4-15mamp so for now the current shouldn't be a problem (will be upgraded eventually once I get it functional and less hot
2)Its outputting at 9v DC, using a diode rectifier (not sure if thats the correct term but i use a series of diode to prevent the AC current alternating, so to speak. Think thats how it works lol)

"The casing is actually Ground too. So you are connecting one red wire and one white wire to GND which is same as shorting one set of the secondary coils. That's why the transformer gets hot so fast"

So I should attach one red to the positive input on the circuit board and connect the ground from the circuit to the casing ground along with the white wire? Leave the second red unnattached?

I'm assuming the primary wiring was correct since nobody has commented other wise?

[Spence]

You have wired it incorrectly, you should take the transformer output to a rectifier and then a regulator, with smoothing capacitors.

Search google images for 12v power supply circuit, decide on one you like and post the circuit, we will advise you how to wire the transformer.

[TecknoTone]

You need to attach each of the red wires to the anode of a rectifying diode. The two cathodes are joined. This will give you an unsmoothed DC supply which will probably read somewhat over 12V, typically 18V or so. The white wire is your ground or 0V. Next, you need an elecrolytic capacitor correctly polarized across positive and negative. This gives you unregulated DC with better smoothing.
Then comes your 9V regulator which goes in series with the +V and the (usually) centre pin to 0V as a reference. At this point, you have regulated 9VDC.
You then need another electrolytic to smooth the output and you're done.

[strantor]

Did you test the output of the transformer? You should be getting 24VAC output from it, if you are using both the red wires.

Originally Posted by absf

"Its 240v to 12v and has two black wires on the primary, two red and one white on the secondary (12-0-12). I have the black wires upto the mains, one live and one neutral, and the reds wired to the circuit (one live, one to ground) and the white to a ground on the casing (metal). "

The casing is actually Ground too. So you are connecting one red wire and one white wire to GND which is same as shorting one set of the secondary coils. That's why the transformer gets hot so fast.

Allen

What? He didn't say that he connected one of the red wires to the case or GND.

[Lyrica's Reality]

This isn't the one I initially used, but I'm thinking of changing to this schematic. The previous one I can't find but was basically the same but 12v input (I'm now thinking they meant 6-0-6 on the transformer secondary, I have 12-0-12) and different resistor values on the Vadj. If it looks good to you guys I'll change to this:

http://www.generalguitargadgets.com/...lean_ps_lo.gif

I think the problem was that I didn't know to connect both red's to the rectifier. I had the cathodes joined as you say, and the anodes also joined connecting to one of the reds. The other red was tried in the ground and also left unwired (This I'm now thinking could have been a problem, since the primary was coming in, producing a current in the secondary and not going anywhere. Correct me if I'm wrong).

I have a circuit setup as 12v from secondary attatching to diode bridge (Connect one red to each anode right? For a 12-0-12 this would mean 24v input and therefore better suited to the schematic above i'm guessing?), into a 1000uf cap, into a LM317 voltage regulator with a 240ohm resistor in R1 and a 1k and 1k trim in R2 to adjust the voltage on the output to near as damn 9v. I have a 470uf cap from Vout to ground and a couple of small capacitors here and there to ground.

On a side note the transformer gets hot even when not wired to the circuit (Therefore no current being drawn right?), so does this mean anything?

"What? He didn't say that he connected one of the red wires to the case or GND."

I did at one point actually do this. As in one red to rectifier anode, other red to the ground on the circuit and the white to the case. This is irrelevant however as everything else i've tried has still cooked the transformer.

Cheers for all the advice.