1. You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
    Dismiss Notice
Dismiss Notice
This forum does not allow a single user to have more than one username. If anyone wants to change their username contact an admin and it will be done. Multiple accounts belonging to the same member will be deleted without warning.

current shunts

Discussion in 'Home Brew' started by bob85, Feb 13, 2008.

  1. bob85

    bob85 Supporting Member

    Joined:
    Mar 30, 2005
    Messages:
    3,473
    Likes Received:
    1,382
    im looking for a cheap current shunt, 500a/50mv would be nice for direct readings but other decimal values 200a or higher would also do the job for now,
    do any of you guys know if the cheap shunts that come with digital amp meters on ebay are real shunts made from Manganin or are they just a lump of brass strap?,
    good shunts are expensive and high accuracy is not a real concern i just dont want to buy something that drifts like a big ole johnson as it warms up when i can make my own elcheapo shunt that will be close enough so long as i keep duty cycle down,


    thanks.


     

  2. W5LZ

    W5LZ Crotchety Old Bastard

    Joined:
    Apr 8, 2005
    Messages:
    6,832
    Likes Received:
    805
    Bob85,
    Not being in need of such a thing, I've mostly forgotten how to do that 'shunting' stuff, really did know at one time though. So take this for what it's worth.

    I would suspect that most shunts used in devices are going to be the most economical thingys possible. So finding one made from brass isn't going to be all that rare. If practical (meaning enough room to put the thing in there) plain old wire will work too (beefed up fuse, sort of). I don't think I'd start looking for more exotic materials till you get to the "go to the bank" stage, you know? If you can make one that will be 'close enough' for your purposes, why not?
    - 'Doc

    [If you feel this advice is just absolutely fantastic, you may pay me in coffee!]
     
  3. Captain Kilowatt

    Captain Kilowatt Professional Amateur
    Staff Member

    Joined:
    Apr 6, 2005
    Messages:
    16,513
    Likes Received:
    10,115
    Bob,you could do what Nautel did with their early AM broadcast transmitters to measure the high current in the PA stage. They used a stainless steel bolt or threaded rod with leads connected via ring terminals and double nuts on each end. Calibration was simple by turning the nuts along the length of the bolt and locking them down at the correct distance apart.Simple,cheap,effective, and good enough for commercial broadcasting. (y)
     
  4. Beetle

    Beetle Sr. Member

    Joined:
    Dec 7, 2005
    Messages:
    3,017
    Likes Received:
    1,024
    I've never seen shunts made of manganin except for laboratory-standard ammeter shunts. All the U.S. Navy's DC switchboard shunts are simple brass sheets, stacked and individually machined to provide precision within a few micro-ohms. They'll carry full rated current forever without getting so warm that they drift.
     
  5. bob85

    bob85 Supporting Member

    Joined:
    Mar 30, 2005
    Messages:
    3,473
    Likes Received:
    1,382
    doc i dont need 500amp either i just dont want something that gets hot and drifts with current upto about 200amp,
    it would not surprise me at all if the made in china shunts were the real deal at a very cheap price compared to the uk, on the other hand they could be brass strap, i tried emailing them, they advertise in english but answer in chinese,

    qrn i like the stainless bolt idea, it would be much easier to calibrate than the copper tube or strap i would have used,
    beleive it or not i found one locally at farnell thats easily good enough for my purpose,
    im that used to prices here been high especially farnell it never crossed my mind to look nearer to home, i found this through a link on a forum,
    http://www.datel.com/data/meters/dpm_shunts.pdf

    im going try the stainless threaded bar see if i can make a fairly accurate 100a shunt,
    i can get stainless bar or bolts and some gold plated ring terminals just up the road, maybe compare stainless vs brass :D,
    1ft of 10awg copper wire sucks imho.
     
  6. bob85

    bob85 Supporting Member

    Joined:
    Mar 30, 2005
    Messages:
    3,473
    Likes Received:
    1,382
    beetle im sure if i had enough surface area to keep it cool stacked/spaced thin brass strap would do the job, i dont know how they fasten the plates into the endblocks but it must be a very good permanent low resistance connection.
     
  7. Captain Kilowatt

    Captain Kilowatt Professional Amateur
    Staff Member

    Joined:
    Apr 6, 2005
    Messages:
    16,513
    Likes Received:
    10,115
    Bob,the idea to use stainless comes from the fact that it has higher resistance than brass and thus could be made shorter. I can't recall exactley how long the shunt was but it was not very long. It was used to measure 70 amps continously and was the sample provided to the front panel meter on a Nautel Ampfet 1,a commercial 1 Kw AM broadcast transmitter.Yeah, 10 ga. wire shunts do suck.
     
  8. bob85

    bob85 Supporting Member

    Joined:
    Mar 30, 2005
    Messages:
    3,473
    Likes Received:
    1,382
    qrn can you remember the size of the threded bar length diameter ect something for 70amp would be very usefull,
    i have just been playing with a 68ohm 1ma meter scaled fs 5amp, i cut 2 pieces of silver plated wire so i could slide one up the other, it now reads 1amp full scale and reads spot on down to below 100ma if i do the 5:1 conversion for the scale, just messing around:D
     
  9. Captain Kilowatt

    Captain Kilowatt Professional Amateur
    Staff Member

    Joined:
    Apr 6, 2005
    Messages:
    16,513
    Likes Received:
    10,115
    Gee Bob,that was a long time ago. I think the rod was 1/4 inch diameter and about 6-8 inches long. I do know that the meter was quite sensitive,something like 100 or even 50 microamps F/S. It may be a little harder to do with a 1 mA meter.
     
  10. bob85

    bob85 Supporting Member

    Joined:
    Mar 30, 2005
    Messages:
    3,473
    Likes Received:
    1,382
    i was just playing around with the 1ma meter the shunt is for my fluke;)
     
  11. W5LZ

    W5LZ Crotchety Old Bastard

    Joined:
    Apr 8, 2005
    Messages:
    6,832
    Likes Received:
    805
    What's the name of the stuff used for some filament lines? 'Ny-something', same stuff used for making the elements for electric stove 'rings'. Should be able to handle the current and since it's fairly high resistance, should be sort of short.
    - 'Doc

    (I blame my memory on advancing age. Really 'ticks' me off sometimes...)
     
  12. bob85

    bob85 Supporting Member

    Joined:
    Mar 30, 2005
    Messages:
    3,473
    Likes Received:
    1,382
    nychrome?
    another interesting metal alloy is constantan
    an alloy with a very low thermal coefficient is what needed in shunts
     
    #12 bob85, Feb 14, 2008
    Last edited: Feb 14, 2008
  13. Beetle

    Beetle Sr. Member

    Joined:
    Dec 7, 2005
    Messages:
    3,017
    Likes Received:
    1,024
    You might be able to find a few alloys that are better suited for thermal stability than nichrome!

    Constantan is excellent - a nickel-copper alloy (60% Ni-40% Cu). I've never used it except for thermocouple leads so I wouldn't know if it's available in a sheet.

    The DC switchboard shunts I've worked with (and those in the standards labs) have the elements welded or brazed into the end pieces (current terminals), which then become integral parts of the bussing. The meter connections (voltage terminals) are calculated for either a 50mv or 100 mv drop (depending on what the Navy specifies for a particular contract). The lead length and cross-sectional area become vital parts of the calibration equation also. If memory serves, the total lead resistance for calibrating these meters was 0.026 ohms, which had to be added in series on the bench.
     

Share This Page

  • About Us

    The WorldwideDX Radio Forum was originally established in 2001. We pride ourselves on welcoming Radio Hobby enthusiasts of all types, while offering unbiased, informative, and friendly discussion among the members. We are working every day to make sure our community is the best Radio Hobbyist's site.
  • Like us on Facebook

  • Premium VIP Member

    The management works very hard to make sure the community is running the best software, best designs, and all the other bells and whistles. Care to buy us a beer? We'd really appreciate it!

    Donate to us!