110v to 240v transformer help needed

Discussion in 'General Electronics Chat' started by rob1979, Dec 12, 2009.

  1. rob1979

    Thread Starter New Member

    Dec 12, 2009
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    hello.i was wandering if anyone can help me.i have purchased a tel-ray morley power wah boost guitar effects pedal from america which runs on 110v.i live in the uk and want to change the small transformer so it can work with 240v.i asked an electrician to have a look inside the pedal for me and he said the original transformer was single phase? and couldnt do anything with it and that id have to buy a new transformer so i rang a supplier,RS,and i told them the size of the transformer which i believe to be a chassis transformer,and that according to the schematics for the pedal its original transformer took 110v and kicked out 28v at 40ma.so i was advised on the phone by rs that the nearest transformer they had would accept 240v and dish out 15v in dual phase???? making 30v which is meant to be ok??? i have no idea at all how to wire it in place though? i have unsoldered the 2 power cables going to the pcb and fitted the new one in place but have made no attempt to fully disconnect the old transformer and wire in the new one because i dont know how to.can anyone tell me how to do this by my pictures? the pic of the transformer on its own is the new one.[​IMG][​IMG][​IMG]
     
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Firstly UK mains voltages are dangerous.

    Note that the American transformer has no uninsulated terminals, but the RS one does so after connection you will have to provide insulating covering or boxing for the assembly. This is now a legal requirement for UK mains equipemnt.

    The RS transformer has two primary windings and two secondary windings.

    The primary windings are connected in series which means you connect the 115 terminal from one winding directly to the zero of the other with a short piece of wire across the middle of the transformer at the top to form a bridge.

    Then connect the the mains line and neutral to the remaining two primary terminals.

    Similarly form a bridge across the bottom inner two terminals and draw the 30 volts from the remaining outer terminals of the two secondaries.

    Hopefully my sketch makes this clear.
     
  3. rob1979

    Thread Starter New Member

    Dec 12, 2009
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    so from your sketch i take it the live is connected to the first 0 top far left and the neutral is connected to the top far right 115v from the mains cable and the the lower 2 connections being bottom far left and botom far right are the 2 lives to connect to the pcb? is that correct?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Rob1979,

    studiot has you mostly covered - but I must mention that you have to use a fuse on the primary side of the tranformer - the side that you will connect to mains power.

    The fuse will protect you in case anything goes wrong with your wiring or the transformer itself. The fuse should be in the "hot" wire from the mains plug. The fuse should be the first thing the wire connects to. If it is a low rated transformer, a 1A fuse should be sufficient to protect you. You can put a power switch after the fuse, for a nice touch.

    I cannot tell you which of the mains leads is "hot" in the UK, as I'm in the States. I know very little about the UK mains wiring.

    I highly recommend the use of heat shrink tubing to cover your connections. "Electrical tape" or "Electrician's tape" should not be used for electrical insulation; it will fall off after a period of time.
     
  5. rob1979

    Thread Starter New Member

    Dec 12, 2009
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    Many thanks for that.If you look at my pic the transformer is also wired up to an on/off switch which is why im having trouble knowing where to put what where lol Im very good at soldering etc its just knowing what to put where!
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    OK Rob,
    This is your transformer with studiot's wiring job connected to it. The cyan wiring is the mains side, the red is the low voltage AC side.

    [​IMG]

    I didn't show the fuse. With such a low power secondary, you could use almost any low value for the primary (mains) side; 1/4A slow blow would work well.

    You will need the advice of someone more familiar than I am with UK power as to which is hot and which is neutral.
     
  7. studiot

    AAC Fanatic!

    Nov 9, 2007
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    For the mains side wiring

    american black = our brown = line (not live by the way)
    american white = our blue = neutral

    You really should change the connecting cable to UK colours.
    Note the strain relief gland where the american cable enters the enclosure. This is to prevent it being jerked out, which is a real possibility on this type of equipment if someone trips over the cable. A simple alternative is to feed the cable through the hole and tie a loose knot so it pulls back against the side of the enclosure.

    Since your enclosure is metal you must use a three core (earthed) mains cable and connect the earth wire (green/yellow) very firmly to the body of the case. If your transformer is not bolted directly to the case you should run a green/yellow wire from this connection to an solder lug-eye placed under one of the mounting bolts/nuts.

    The outer sheath of the mains cable must enter the enclosure completely. Once inside the it is stripped back to reveal the L, N , and E wires. The E wire should be the longest so that if the cable is jerked out the Earth is the last to separate.

    The neutral (blue) go directly to the transformer primary terminal.
    The line (brown) goes to one switch terminal. A second brown wire should be connected from the other switch terminal to the other primary terminal.
    It does not really matter if the brown goes to the 115 or the 0 ie it does not matter which way round the L/N are connected to the transformer, so long as the brown is the switched one.

    I don't see a fuse in your photos. Wookie is quite right it would be good practice not to rely on the fuse in the mains plug and introduce your own fuse. In this case the you should drill a new hole near the switch for a fuse carrier and connect brown wiring to it before the switch.
    Do not use a fuse greater than 3A in the mains plug - I would suggest 1 amp would be adequate for this, and yes 1A mains fuses are available.

    Using heat shrink tubing to shroud the terminals is not acceptable practice in the UK any more. Note the more substantial yellow american covers in your photo. These are also available in the UK.

    If you do not understand any of this ask your electrician friend to explain.
     
  8. Duane P Wetick

    Active Member

    Apr 23, 2009
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    19
    You bring up some interesting points about US and UK wiring differences. I have noted the UK power cable color differences which we see more often in the US these days with world trade. Putting the line fuse before the power on-off switch is correct and you would be surprised to learn that many reputable manufacturers put it after the switch, which offers no shock hazard protection should the switch go to ground.
    Additionally, the ground wire must have its own screw connection (brass) and must not share a screw terminal with a screw that holds something else together. Many builders (me) violate this simple principle too. I have seen the substantial yellow insulating covers that are used to cover circuit board mounted fuses, but I don't think they are available in the US. The heat-shrink tubing is both UL and CSA listed for insulating terminals in North America.

    Cheers, DPW [ Spent years making heaters out of op-amps.]
     
  9. studiot

    AAC Fanatic!

    Nov 9, 2007
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    It is a requirement in UK practice to employ fused mains plugs at the wall outlet. So if the problem arose the wiring all the way back to the wall outlet would be rendered safe.

    I was talking about introducing an additional equipment mains supply fuse. In that case your comment holds good as the downstream fuse should always be more sensitive (lower current rating) than the upstream one.
     
  10. rob1979

    Thread Starter New Member

    Dec 12, 2009
    7
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    Thanks for all your comments.you have all helped and i now have a fully working guitar pedal running on 240v mains!
     
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