Question About Selenium or Copper Oxide Rectifier

Discussion in 'General Electronics Chat' started by PGB1, Apr 23, 2013.

  1. PGB1

    Thread Starter Member

    Jan 15, 2013
    58
    3
    Hi Folks!
    I've inherited an old "Electro Chemical Cleaner". As far as I can determine, it is used to de-rust items by electrolysis, which I've done for years using more modern equipment.

    Inside is a rectifier which looks to be either a selenium or copper oxide type.
    It has 5 plates and measures 2-1/4" long and each plate is 1-1/4" square.

    For testing, the machine was connected to 120vac through a GFCI protection device & an isolation transformer. Powering the rectifier is a step-down transformer, delivering 7.14 vac.

    I've never studied such a rectifier before, so I'm quite unfamiliar with these. My questions are:

    The voltage drops from 7.14 input to 1.9 output across this device. I'm guessing this is normal for such an item. Or, is something terribly amiss?

    The output frequency is zero. How do these devices do such a fine job of rectification?

    This question may seal the unit's fate- Every 20 or 30 seconds, it will make a faint 'Pop" sound and a tiny spark will be seen between a plate. The spark location varies from plate to plate. It also smells like rotten eggs when energized.
    Is this rectifier doomed?

    If it is doomed, is there any way to judge the current requirement for a replacement bridge rectifier circuit? The transformer has no markings on it. My only clues are the size of the transformer matches a modern 60 va unit.
    The wiring is all 16 gauge. But, it is old, so who knows?

    In the photos, the wire on the right end, plus the yellow bus bar near the center are the DC outputs. The red wires are AC input and enter near the center of the rectifier.

    Thanks you for any ideas or thoughts you have on the rectifier & sizing a replacement!
    Paul

    Here's an off-topic side note- This device came with a lead bucket which served as the anode. It would be filled with an electrolyte (Modern times- Water & Sodium Carbonate). Can you imagine how much lead is leached into the water or fumes off with the hydrogen & oxygen? Scary!
     
  2. #12

    Expert

    Nov 30, 2010
    16,295
    6,806
    P=ie
    60 = i (7.14)
    i = 8.4
     
  3. PGB1

    Thread Starter Member

    Jan 15, 2013
    58
    3
    Thanks #12.
    I phrased my post confusingly. I don't know the va of the transformer. It is physically about the dimensions of a modern day 60va.
     
  4. #12

    Expert

    Nov 30, 2010
    16,295
    6,806
    Just get a 25A, 50V, potted FWB and throw it in there and don't quibble about whether it's supposed to be 9 amps or 11 amps.
     
  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
  6. PGB1

    Thread Starter Member

    Jan 15, 2013
    58
    3
    Thanks Guys Very Much!
    I appreciate your advice.
    Paul
     
  7. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
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  8. PGB1

    Thread Starter Member

    Jan 15, 2013
    58
    3
    Thanks for the heads-up about heat.
    This machine looks like it was built for convection cooling, so adding a small muffin fan should be a simple task. The air will flow perfectly across all components.
    Paul
     
  9. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
    463
    You still may need a heatsink for the bridge rectifier.

    I have used a Pentium II heatsink for a bridge rectifier, attached it with thermal grease and "No More Nails" Silicone compound. This is not regular installation sillicone. It has proven to have excellent heat durability, the bridge recitifer was operated for 3 months with the sink turning very hot at times.
     
  10. #12

    Expert

    Nov 30, 2010
    16,295
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    Just try it. I've done this and found that the sheet metal I bolted the rectifier to was enough for 3-4A. Just try it. If it gets hot, add a fan.
     
    shortbus likes this.
  11. PGB1

    Thread Starter Member

    Jan 15, 2013
    58
    3
    Thanks #12!
    I'll give it shot.
    There surely is a lot of metal to suck up heat. The case is made of 280 sq inches of 1/4" thick plated steel (or maybe iron?). I can bolt the FWB right to an existing tapped hole in the case. Almost too simple.

    I test ran it with a small lamp for a load just to see the convection current from the heat of the existing selenium rectifier & transformer. It really does flow nicely in the bottom & out of the slots above the components. Seems like a nice design.
    Paul
     
  12. #12

    Expert

    Nov 30, 2010
    16,295
    6,806
    ps, you can put heat sink compound between the rectifier block and the 1/4" steel, which seems well equipped to carry heat away!
     
  13. PGB1

    Thread Starter Member

    Jan 15, 2013
    58
    3
    Thanks for reminding me. I would have forgotten.

    I use that in my trade & have a big quantity (that I forget to use).
    Step One- Open Jar
    Step Two- Get compound on new, expensive arc flash shirt & pants
    Step Three- Be certain to step in some & track it into the work truck

    Thanks Again!
    Paul
     
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