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Blocking DC voltage on an AC circuit

Discussion in 'Power Electronics' started by Greenlite, Nov 19, 2018.

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  1. Greenlite


    Oct 31, 2016
    I am having some severe problems with a certain pieces of electronic equipment I use for my business,we design and install outdoor lighting systems and some of the equipment uses triac dimming.
    We use AC IP rated toroidial transformers and have found through testing that the output voltage/signal from the dimming unit carries DC which is damaging our transformers.
    We need to built an electronic circuit that filters the DC but allows the AC to flow through,I must stress at this point I know zero about electronics.We use toroidial transformers in all our outdoor installations due to there ability to withstand physical knocks very resistant to temperature changes and work extremely well with our constant voltage led lights

    Certain manufactures of dimming equipment that we sometimes have to use as they are tied into the internal switching systems of the house do put out this 'dirty' AC voltage via their leading edge triac dimming
    We thought if we could built a filter and fit it in between the output of the dimmer and the input of our transformers we could solve this problem.
    We are based in the Uk and the supply voltage is 230volts

    I have found a circuit off the net and have not a clue if it works ..I wonder if I could ask one of you guys for a little help and give me any advice


    Is what im asking possible ?
    Does this circuit work ?
    Would it be easy for a manufacture to make ?

    Best Regards

  2. AnalogKid


    Jun 10, 2015
    Can you share more information about this? Are these power transformers or small signal? In what way is the DC component "damaging" the transformer? Can you share the schematic that includes the transformer so we can see it in some context?

    davenn likes this.
  3. kellys_eye


    Jun 25, 2010
    As above - how are you measuring 'DC'? Is the triac not conducting on one half cycle? This is what you'd measure (i.e. a DC component) if the dimmer used THYRISTORS rather than triacs. Sure you've not got the wrong type?
  4. davenn

    davenn Moderator

    Sep 5, 2009

  5. duke37


    Jan 9, 2011
    A DC component in the transformer secondary could push the core into saturation and allow the primary to pass very high currents. One way to get arround this is to put a capacitor in series with the secondary. If the offset is in the primary, then a selected diac in the control may help.

    Another way is to use a bridge rectifier and reservoir capacitor in the transformer secondary. Use a MOSFET to switch the DC to the lights.
  6. Greenlite


    Oct 31, 2016
    Thanks for your answers unfortunately some of it is like a foreign language to me for that I must apologise.

    Let me give you some history it may help
    5 years ago we started using a product for switching and controlling outdoor lighting .This worked on a RF based system controlled by hand held remotes and a RF timer.You installed the receivers in the garden fed via 230volt the output side of which was connected to our 12volt lighting via our transformers.You could buy the receivers in 1 ,2 and 4 channel versions depending on how many areas you wanted to control.Over the 5 years we seemed to be changing alot of transformers and light fittings were failing.We had other issues that came to light with this product and so discontinued its use which was a big move for us as we were its biggest independent installer in the UK,and had used it on hundreds of installations.

    We moved over to another product a far larger company called RAKO they took all our light fittings and transformers and tested them ,seeing the slippage if the DC voltage through the their triacs they altered their hard ware to eradicate this DC.Since using Rako over the last 3 years we have had almost zero failures.

    We are about to retail the lighting range to the general public but are wary of the many switching units on the market,Crestron,Lutron,Control 4 etc and could see the same happening with end users blaming our product for any failure not realising its their equipment doing the damage.
    We thought if we could build a simple electronic circuit and have our factory build this into the mains lead in the form of an IP rated torpedo shaped module then we could cease to worry about potential failures.

    Rako have told us that the output voltage from our initial supplier of switching equipment was leading edge triac based its built into their boards along with the other brand names I have mentioned.They told us it was the DC part of the signal coming into the transformer that was doing the damage which they went onto rectify in their dimmer modules.

    Having no electronic knowledge albeit qualified electricians I thought a specialist board forum like this could help us in our endeavour

    Best Regards
  7. AnalogKid


    Jun 10, 2015
    You will get a lot of help here, but ...
    I suggest you rethink this business plan. Not trying to be snarky of defeatist, just warning you of a serious potential problem.

    And, still not enough information to determine where the mysterious DC is coming from or how it is damaging a power line transformer.

  8. Frankchie


    Nov 14, 2017
    It sounds like they have a dimmer control feeding a transformer based power supply for led lighting.

    I found this link that may explain a similar problem.

    2.Do not use regular incandescent dimmers for magnetic low-voltage lighting. Regular incandescent dimmers often contain a small amount of DC voltage, which is harmless to a regular incandescent load but may damage magnetic transformers. Use Lutron low-voltage dimmers, which are designed to prevent DC voltage from being applied to the transformer.
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