Replace stud diode with bridge diode?

Discussion in 'General Electronics Chat' started by sdowney717, Nov 18, 2014.

  1. sdowney717

    Thread Starter Member

    Jul 18, 2012
    385
    13
    My old boat's charger from 1965 has a shorted stud diode in the battery isolator section, part number 6.
    I need to check it, but the symptom is one bank is .7 volts higher than the others.
    There are 3 screwed into an aluminum plate.
    Stud diodes are hard to get, I already have a rectifier bridge 25 amp.

    The charger is the 20 amp version, R2012-3.

    [​IMG]

    [​IMG]
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    if the voltage rating of the bridege is around 200 volts, you could replace both diodes with the bridge, just dont use the - terminal on the bridge.
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,979
    744
    Bridge reccys have four diodes in, they have two anodes together and two cathodes together,it wont work as you can't separate the cathodes,
     
  4. sdowney717

    Thread Starter Member

    Jul 18, 2012
    385
    13
    Here is a bridge rectifier. The red line shows flow as needed.
    I am thinking join the AC sides together as anode, and the positive side as cathode. That put 2 diodes in parallel. Can handle more power, safety factor?
    Would doing that lower the forward voltage drop, meaning that bank would have a higher voltage than the other 2 banks running off the old stud rectifiers?
    If so, maybe I should just wire it with one AC connector as anode.


    [​IMG]
    What do you suggest replace both diodes with bridge? There are 2 diodes to rectify off transformer. And 3 for each bank as battery isolators.
     
  5. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
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    if you look at the drawing, there are two diodes with their cathodes connected together, why would you seperate them? use the bridge to replace both diodes. and the stud mount holes to screw down the bridge to heat sink it.
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,791
    1,103
    The bridge can be used (if of a suitable voltage rating), by ignoring 1 or 2 of its terminals.
     
  7. wayneh

    Expert

    Sep 9, 2010
    12,101
    3,036
    Why? I was able to find the S3720 without much trouble.

    The original stud is rated to 85A. I would NOT replace it with a bridge rated to less than one third the original.
     
  8. sdowney717

    Thread Starter Member

    Jul 18, 2012
    385
    13
    hI, do you show a link?
    s3720 is the 60 amp model, I have 20 amp so is s3120. I know either will work and higher is better.
    I suppose I can order them, but I have some bridges so it is free. Otherwise free turns into $$.
     
  9. wayneh

    Expert

    Sep 9, 2010
    12,101
    3,036
    Oops, my bad. I just looked at line 1 but that is the AC fuse rating, not the charger rating.

    Free is good, but there are many options such as this that are not very expensive. You can shop for others in the D05 package - that's the stud package you need. 1/4-28 threads.
     
  10. sdowney717

    Thread Starter Member

    Jul 18, 2012
    385
    13
    Ok, cheap per unit item, but minimum order is 300?

    Availability:
    0


    • Per the manufacturer, more stock expected week of 12/22/14
    Not Normally Stocked
    Non-Cancelable / Non Returnable


    Price for: Each 1

    Minimum order quantity: 300

    Order multiple quantity: 5
     
  11. sdowney717

    Thread Starter Member

    Jul 18, 2012
    385
    13
    http://www.alltronics.com/cgi-bin/category.cgi?item=1N1184R

    $4.25 not bad, but then shipping adds on quite a bit for these little things.

    Turns out I think two of the three isolator diodes are shorted. I happen to have two bridges, one is 50 amp, one is 30 amp.

    example for that site $10 to ship for cheapest option.
    That adds up quick.
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    Last edited: Nov 18, 2014
  12. wayneh

    Expert

    Sep 9, 2010
    12,101
    3,036
    I wasn't recommending a particular diode or supplier, just letting you know there are options. ;)
     
  13. sdowney717

    Thread Starter Member

    Jul 18, 2012
    385
    13
    Thanks for the info. If my 2 bridges work, I will order 10 of these here to have on hand.
    http://www.ebay.com/itm/10-pcs-50A-...117?pt=LH_DefaultDomain_0&hash=item48734fbded

    Interesting thing is each bridge, if one diode shorted, I could connect to another terminal to fix.
    Tomorrow I will show some pictures of the repair. I verified that 2 are shorted, only one is ok.
    Do you think the bridge is more robust than these old stud diodes in that charger from 1965?

    Have to be careful with the orientation of the stud diodes. I was looking at some blocking diodes and they reverse cathode and anode relative to the screw base compared to what I need.
     
  14. sdowney717

    Thread Starter Member

    Jul 18, 2012
    385
    13
    ok, it is back together.
    Picture of bridge diodes on aluminum plate.
    I enlarged the holes in the bridges to pass the big wire in and solder them on.
    [​IMG]

    Mounted in charger. I added a heat sink on the plate. Bank1 gets the 50 amp bridge.
    [​IMG]

    Heat sink bottom view. Maybe it will help keep them cooler.

    [​IMG]
    Control board with adjustable sliding resistor. These tend to wear-break according to the manual. It is just a potentiometer.
    Can I replace it someday with a better, maybe a round one?
    Rated at 400 ohms, 12 watts rating. If anyone has ideas on that with a link?
    [​IMG]
     
  15. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    783
    AFAIK; most of the stud diodes have been superseded by the tab style package (TO220, TO3P etc) - might be easier to drill & tap 3 new holes and select a whole set of 3 replacement rectifiers.

    Don't forget to select a smaller bolt size than the hole in the tab - you'll need mica insulating washers under the rectifier and an insulating bush under the bolt head.

    Some rectifier ranges include types with an /R suffix, if the original rectifiers are in electrical contact with the heatsink, you should be able to get tab type rectifiers with the correct tab polarity - in which case you won't need the insulating kit, and can use a bolt as big as the hole in the tab.
     
  16. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
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    sdowney717 likes this.
  17. sdowney717

    Thread Starter Member

    Jul 18, 2012
    385
    13
    That is a good idea. Makes better use of the bridges. It is done now they way it is.

    These diodes are simply for isolating 3 battery banks. Charger is not a smart charger, all banks are isolated but not independently charged. This charger pulses the current.
    I suppose I could have 4 banks running off 2 bridges to charge 4 battery banks. Which is sort of interesting. If you divide up current from 3 to 4, will that slow down charging or will it put the charger under a heavier load and still charge ok?

    If a bridge is rated 50 amps, does that mean each diode in the bridge can flow 50 amps?

    If one bridge was charging 2 banks, then will that bridge experience greater heat stress?
    Maybe for reliability, one bridge per battery bank is better?
     
    Last edited: Nov 19, 2014
  18. wayneh

    Expert

    Sep 9, 2010
    12,101
    3,036
    I wouldn't think so. Under normal AC rectification, where the thing is likely rated, all 4 diodes are sharing the current.
     
  19. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
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    If you trace out the current path through a bridge rectifier, you can see that the diodes conduct in alternate pairs. The peak instantaneous current rating of each diode must be equal to the rating specified - however, each alternate pair is only conducting half the time, so average current is the one you need to watch.
     
  20. wayneh

    Expert

    Sep 9, 2010
    12,101
    3,036
    Yes, instead of implying the 4 diodes share the current equally, I should have said two pairs of diodes - two current paths instead of just one. A 50A bridge can contain 25A diodes. Sort of.
     
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