Transformer overheat

Discussion in 'The Projects Forum' started by Lyrica's Reality, May 22, 2012.

  1. Lyrica's Reality

    Lyrica's Reality Thread Starter New Member

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    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-250ma-transformers-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.
  2. cork_ie

    cork_ie Member

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    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?
  3. mcasale

    mcasale Member

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    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).
  4. nomurphy

    nomurphy Distinguished Member

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    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.
  5. absf

    absf Active Member

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    "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
  6. Lyrica's Reality

    Lyrica's Reality Thread Starter New Member

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    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?
  7. Spence

    Spence Member

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    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.
  8. TecknoTone

    TecknoTone New Member

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    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.
  9. strantor

    strantor Well-Known Member

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    Did you test the output of the transformer? You should be getting 24VAC output from it, if you are using both the red wires.
    What? He didn't say that he connected one of the red wires to the case or GND.
  10. Lyrica's Reality

    Lyrica's Reality Thread Starter New Member

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    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/diagrams/ultra_clean_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.
  11. Spence

    Spence Member

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    I take it you want a 9v power supply for guitar pedals no?

    this would appear to be the circuit diagram

    http://www.generalguitargadgets.com/diagrams/ultra_clean_ps_sc.gif

    there are simpler 9v circuits that will give you a nice clean 9v.

    Do you want the circuit adjustable, or would you like to trim it to exactly 9v?

    With a 9v regulator lm7809 instead of 7812, you will get 9v from this circuit.

    http://www.reconnsworld.com/power/12vpowersupply.gif

    Instead of the 4 diodes, buy a 4 in 1 bridge rectifier. The rectifier will have + and - output marked so you can't go wrong. Use just 1 tapping of the transformer, the one you haven't fried (0 and 12).

    Decide which circuit you prefer, you will be able to run 3 or more pedal devices from either circuit.

    edit

    inspect the transformer for any malformation or signs of damage
    Last edited: May 24, 2012
  12. Lyrica's Reality

    Lyrica's Reality Thread Starter New Member

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    Just an update to say that the power supply is now working. A combination of correct transformer wiring and changing a few of the components worked. The transformer still feels warm, but not uncomfortably warm where I cannot stand to touch it for long.

    Just one closing question if anybody has the time to briefly clarify this.

    Is the maximum number of devices I could use this power supply to power limited only to the maximum current draw of them combined? Supposing 10 devices use only a total of 150mamp would this be safe to use (250mamp maximum output on the transformer)? Similarly if I bought a higher amp transformer could this mean I connect many more.

    I'm only asking this because I don't wanna be hooking loads of stuff upto it thinking that it'll be safe because the current is bellow the transformers limit only to find there's other factors which determine the limits of what can be powered.

    Thanks to everyone who helped me built my first project, it's been a great learning experience and I can't wait to try out some new, more complex pieces of kit.
  13. Audioguru

    Audioguru New Member

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    The DC current from the power supply is far less than the 250mA AC rating of the transformer.

    A 24VAC transformer has a voltage that is far too high to produce 9VDC.
    The unregulated DC voltage will be 24V x 1.414= 34V minus 2V for the diode bridge= 32VDC. You should use it as a 12V-0-12V transformer instead (with the center-tap at ground and only two rectifier diodes) then its DC output will be 16VDC which is better.

    The transformer passes a max power of 24V x 250mA= 6W. The 6W heats the rectifier bridge and the voltage regulator. Remaining power goes to your load.
  14. #12

    #12 AAC Fanatic!

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    Watch out for stomp boxes that have the wrong polarity on the case ground. They will short out your power supply.
  15. Lyrica's Reality

    Lyrica's Reality Thread Starter New Member

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    Audioguru. What are you saying I should do differently? Have got the outputs to 9v dc which is what I want. But you think this is too powerful for the circuit right? So you think this could overheat?

    I'm using this circuit, only difference being its UK mains 230v on primary:

    http://www.generalguitargadgets.com/diagrams/ultra_clean_ps_sc.gif

    What I really want to know is is it safe, and if not why and how to correct it. I've spent a few hours a day for last 2 weeks getting it to an operational state, so I don't really want to change anything unless its unsafe to leave it as it is. If it's safe, i'm happy to leave it as it is to be honest.
  16. Audioguru

    Audioguru New Member

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    You do not need a rectifier bridge with 4 diodes that makes 32VDC to get 9VDC out.
    You use the center-tap of the transformer with only two diodes and get 16V out.

    The original schematic in a PDF file on the internet also had one diode shown backwards.
    I don't know why the author used four odd-value resistors to set the output voltage when only two common-value resistors are fine.

    Attached Files:

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