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Need a two wire ac voltage rectifier

  1. Aug 25, 2010 #1
    I have a company that makes electrical resistance heaters. We have an application where we are trying to achieve 60 or less watts in a 5' 120VAC heater. The problem we have is that the heating element wires need to be on the order of 24ohms/ft and that ends up being a small gauge wire (ie 40 ga alloy 800). We have half/wave rectified the input voltage with standard diodes that end up delivering 67v RMS and that brings the gauge to a thicker wire. What we would like to achieve is to rectify down to 50 or 40 or 30 Vrms with a two lead device. This reduced voltage would allow us to use thicker wires that are easier to handel. I know nothing about later technology diodes that might clamp or reverse biased the ac wave down to lower levels. This is not homework. I am old and have been out of engineering school for some time.
     
    Last edited: Aug 25, 2010
  2. jcsd
  3. Aug 25, 2010 #2
    Using half wave rectifier you will get 54 V dc for 120 V ac input.
    If you want till less you can use controlled rectifier using thyristors/triacs, which are very common nowadays. By controlling the firing angle the output voltage can be varied smoothly from 108V to 0 V.
    Brief introduction about controlled rectifier can be seen here.
    http://en.wikipedia.org/wiki/Rectifier
     
  4. Aug 25, 2010 #3

    sophiecentaur

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    Presumably you have had to reject the idea of just using a longer length of resistance wire, coiling or zig-zagging or a different alloy? My tables suggest that 40 gauge NiChrome is about 30 ohms per foot and NiChrome is commonly used in heating elements.
    Sorry if this is old ground for you but 'any port in a storm'.
     
  5. Aug 25, 2010 #4
    The length of the wire is not an option for this application. I have looked into thyristors and that could be a good solution. I am interested in a fixed voltage lower than 60VRMS with a 120VAC input. Do they make a two wire version that has a fixed voltage pass? I am also looking at schottky & zener diodes. I can accept most any voltage below 60RMS and have thought about putting two diodes in series with polarities reversed any thoughts?
     
  6. Aug 25, 2010 #5

    sophiecentaur

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    Two back to back diodes will pass no current (only microamps).
    Any form of 'dropper' will get hot, dissipating lots of Power. A Zener / Avalanche diode will dissipate Power equal to the current times the voltage drop. You may as well use a series resistor, which would be reliable and cheap.
    What about a transformer? A 2:1 autotransformer would be a smaller, cheaper option but you might have to have it made up specially. Or I guess an appropriate inductor (choke) would also achieve what you need without getting hot and wasting power. No problem as long as you always want a fixed power from your load.


    I haven't understood why this device has to be "two wire". Surely the 120V turn up somewhere on two adjacent wires next to each other. Have you a circuit diagram that would make it clear?
     
  7. Aug 25, 2010 #6
    I was trying a back bias pass through with the two diodes in series but it sounds like current caring capacity is not there. The reason I would like a two wire solution is for ease of assembly and installation. These products are assembled by my customer and they like the simplicity of a non directional diode like they have now. Doesn’t matter which leg or which polarity. If a thyristor was fixed to pass a given voltage ie the gate had a fixed reference then it could be installed on either input leg W/O polarity. A transformer is bulky, might need a housing and have to be UL approved. The application is for various industrial heat blankets applied in all sorts of harsh environments.
     
  8. Aug 25, 2010 #7

    sophiecentaur

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    I see you mean back to back Avalanche diodes - lots of dissipation there.
    I see the need for simplicity but, as far as I can see, the truly simple and 'lossless' solution would be to have a single series diode. But it can only work for the one Voltage reduction ratio.
    It would be possible to design an active two terminal circuit which would drop a fixed number of Volts in a fairly lossless way but it would take expertise to develop, I fear, and that would cost you.
    What actual resistance of wire are you planning to use, what actual heating power and what is the frequency of your supply? (50 or 60 Hz). That info is all that's necessary to specify the Inductance of a suitable passive choke.
     
  9. Aug 25, 2010 #8
    Can I get the facts of this straight?

    Your 'customer' (and yourself?) are proposing to make money out of the free answer to this question.

    Why are you not prepared to pay a competent engineer to design the appropriate solution?

    Are you prepared to offer Physics Forum some support in return?

    As a matter of interest the phase control thyristor you are proposing would create an horrendous amount of RFI (and be illegal in many countries).

    If this is truly an industrial application get the job done properly before someone is hurt or worse.
     
    Last edited: Aug 26, 2010
  10. Aug 26, 2010 #9
    Mr Studiot
    I am a competent engineer and I have employed other engineers for over 30 years. The product in discussion is a UL approved product (very safe). I don't expect any free engineering although I have provided same to many struggling engineering students over the years. I am exploring design concepts here Mr. Please step aside and do not interfere with the free thinking spirit of this forum.
     
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