Bridge Rectifier

Discussion in 'General Electronics Chat' started by geek12, Sep 9, 2010.

  1. geek12

    Thread Starter New Member

    Sep 9, 2010
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    Guys,

    I am using a bridge rectifer in half wave mode and is in parallel with Solid state relay with a resitive and inductive load. The product is in market for about 7 months and then I am seeing burning out of the bridge rectifer. Rectifier is operating at about 70-80C. What can cause it to blow? any help would be great.

    Cheers
     
    Last edited: Sep 9, 2010
  2. Wendy

    Moderator

    Mar 24, 2008
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    A schematic is worth a thousand words. Really.
     
  3. geek12

    Thread Starter New Member

    Sep 9, 2010
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    Please see attachment that's the quickest I can make. Please note the TClose switch u1 is used to represent a Solid state relay. And heater element is taking 13a. The operation is as simple as this when heater is operating bridge rectifer is out of it SSR us operational and when Uwave is operational bridge rectifer is operational and we are just putting half wave across rectifer.
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    If what you have labeled as filter is an AC source you have effectively shorted half the AC signal. That is not really a bridge you are showing, just 4 diodes all facing the same way. AC would have to be injected as shown for it to be a bridge, and the filter would be in parallel across the load.
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    What is drawn is not a bridge but 4 diodes in series/parallel. If you wish to do half wave rectification to reduce power in the heater, you only need a single diode rated for the voltage and current. It will have to mount on a heat sink.
     
  6. geek12

    Thread Starter New Member

    Sep 9, 2010
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    Well I just drawn the circuit. It is very old board and I drew it from board. Also the AC supply is input to the filter and output of filter section is shown in drawn circuit. Also the four diodes which are connected is a rectifer GD-25. which is a bridge rectifer rated for 25 A and also if you connect it in the way I have shown you wil get half wave rectification. I don't see any problems with it. What you are doing is just have two diodes in series and then in parallel with similar arrangement. I have used scope to get waveforms and appears to be great. But why can it blow up after 7 months?
     
  7. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Unless you need to put the diodes in series for voltage reasons yo're dissipating twice the power due to the voltage drop across the diode junctions.

    And yes, while that bridge may be rated for 25A that doesn't mean just in free air, it's rated for 25A but at a certain junction temperature which you'll need to keep in line through some method of keping the junction temperature down as in with a properly chosen heat sink with the proper amount of air flow. If it's that hot on the outer case it's far hotter internally.

    http://en.wikipedia.org/wiki/Heat_sink
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    Thing is, schematics matter. If we don't have the schematic we are spitting up wind aboard ship. I assume the filter shown is a capacitor, for example. There simply isn't enough information to begin a guess. I also suspect the schematic is extremely flawed, with major holes and errors.

    Generally components should be run using the 50% of capacity principle. If you are running these diodes (which are 100 volt PIV 1A parts as I recall) right at their maximum specs, plus running them hot (which ages them much faster), then they will have a short lifespan. If you are running them at ½A or less then go with a 1N4004 or better, they have double the PIV rating. If you are near an amp you will need a totally different family.

    Whenever a part is subject to a lifespan test they are run under power in an oven, which speeds aging about 15 to 50 times normal.
     
  9. geek12

    Thread Starter New Member

    Sep 9, 2010
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    Thanks guys for your Help. I will replace the bridge witha single diode and see the results.
     
  10. marshallf3

    Well-Known Member

    Jul 26, 2010
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    1N4002 diodes aren't going to work for a 16A circuit, and it's drawn different now.

    Now you've got the full AC being rectified into DC then to the heater.

    Why put any diodes in there at all if you're just powering a heating element?
     
  11. geek12

    Thread Starter New Member

    Sep 9, 2010
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    Many reasons it can be done in this but my project is to reduce energy.

    And protecting heater getting to acertain temperture very quickly.
     
  12. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Half wave of the AC to effectively cut the power going into the heater in half (as well as its output)

    Perfectly fine, but all you need is one single rectifier that's capable of handling more than 16A

    In your case I'd recommend a 25A or 35A stud mount on a large heat sink.
    The rectifier &/or heat sink will of course need to be isolated.
     
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