Calculating Burden Resistance of CT

Discussion in 'General Electronics Chat' started by Muhammad Khurram Ahmed, Oct 11, 2015.

  1. Muhammad Khurram Ahmed

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    Oct 11, 2015
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    I am using a CT 60/5 A. I would like to know how would I select a burden resistance to convert 5A current to 400mV signal? I understand that V=IR will give the required resistance, however, for 5A even 1ohm resistance will develop 5V and hence the power dissipation of resistance has to be atleast 25W(P = V*V/R) which is huge. How should I calculate the resistance so that it remains in reasonable power dissipation level? What if the burden is rated at 1VA?
     
  2. #12

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  3. GopherT

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    Ohm says, V/A = ohms

    so, 0.400V / 5A = 0.08 ohms.

    0.4 V x 5A = 2 watts.


    I would put four, 1W resistors at 0.33 ohms in parallel. You will have 0.0825 ohms that can handle 4 watts.
     
    Last edited: Oct 11, 2015
  4. strantor

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  5. #12

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    Yes, 0.4V/5A = 0.08 ohms @2 watts actual and 4 watts rated on the resistor.

    OP also said 1 watt for the burden.
    P = I^2 R
    1W = 25 R
    0.04 = R

    If he meant the burden resistor is rated at 1 watt and you want to put in the usual safety factor,
    0.5W = 25R
    R = 0.02 ohms

    A little bit of vagueness goes a long way. ;)
     
  6. Muhammad Khurram Ahmed

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    The lowest resistance I can get is 2.2ohms. connecting 5 in parallel with 3W rating will do the job but doesn't it make an amateur design? Moreover, I have come across some generator control modules which can be connected to CTs to read per phase current. I have not seen resistances in such numbers in those modules. Is there any alternate approach? Is connecting resistors in parallel the only way?
     
  7. Alec_t

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    Another way is to use a CT with a higher turns ratio, e.g. 100:1.
     
  8. GopherT

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    It does not make an armature design. This tactic is used to spread heat across multiple resistors. You do not have to connect them in parallel, you can buy low resistance current-sensing resistors as shown in Post 2 by @#12.

    If you want a completely different solution, you can look at a Hall effect sensor.
    http://www.allegromicro.com/en/Desi...Sensing-Techniques-for-Power-Electronics.aspx
    Or

    http://www.ti.com/lit/ug/tidu522a/tidu522a.pdf
     
  9. strantor

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    Paralleling resistors is not amateur, IMO.
    You don't need (5) 3W resistors in parallel; that's 15W total capacity.
    Your resistor(s) will have 5A through, and 400mV across them. [5A * 0.4V = 2W]
    Your design should be at least capable of dissipating 2W, best safe practice is to design for 3 or 4W as Gopher said.
    If you use an SMD resistor (like most of those in #12's list) instead of through-hole, you can dissipate more heat (into the PCB) with a smaller package(s).

    If you really are opposed to having multiple resistors in parallel, you can use a piece(s) of precision resistance wire cut to the proper length to give .08ohms, or even just use a PCB trace trimmed to give the right value.
     
  10. MrAl

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    Hi,

    I have to ask since nobody else did yet:
    What do you need the 400mv for? That doesnt sound high enough, but of course it depends so i am asking.

    As others have pointed out, if you have 5 amps and 400mv then the power is 5*0.4=2 watts and there's no getting around that unless you change something else. It is often easy to modify a current transformer when you want more output, but to get less output is hard to do, so your other alternative is to get a different transformer or other means to measure the current. Hall effect sensors have become popular, or perhaps a 100 amp 50mv current shunt with a 13.3x amplifier.
     
  11. GopherT

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    Al,

    400mV was selected to limit the amount of total power dissipated by the current sensing resistor (s).
     
  12. strantor

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    Glad you asked! Actually, it doesn't sound low enough. Typical burden on a 5A CT gives a 50mV or 100mV output at full-scale. I assumed OP had a special application. I have seen less common CT configurations rated for 333mV full-scale output (400mV max) .
     
  13. MrAl

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    Hi,

    That's not really the way to do it though. The right current transformer is the way to go.
     
  14. MrAl

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    Hi,

    Well that is interesting that you should say that, because when i worked in the industry we always designed with 2.5v or higher output, in fact 10v would not be uncommon. This provided feedback to the current sensing circuit in order to limit the output current to a safe level for the sine converter. There were rectifiers too and higher voltages seemed to work better because of their voltage drops.
    Ours were wound with a center tap so only two diodes were needed to rectify into DC.
    Hall effect sensors were just starting to enter the market at that time so we did not have any reasonably priced units to work with yet.
     
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  15. strantor

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    Thanks for sharing; That's why I love this forum. I learn something new every day. My knowledge base gained across many experiences leads me to believe something is "universal." Then I realize that my "big picture"/"across the board" view is just a window, maybe even a niche. Sometimes I'm confounded by people with equal or greater knowledge than me, in the same field, might have never seen something that I see on a regular basis (and vise versa).
     
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  16. #12

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    I have found that you can imagine a spectrum of results (like a bell curve) for almost anything about human behavior or beliefs. Very convenient for a teaching tool, and very smart to be aware of this method on a daily basis!

    People like me and strantor, who are very good in our specialized fields, rarely learn any big, new concepts, but we learn little things almost every day.
     
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  17. Alec_t

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    The more we learn, the more we realise that there's a heck of a lot more to learn. And we don't know what it is that we don't know!
     
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  18. MrAl

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    Hi,

    Yeah it's amazing how this works isnt it? It seems that when we start out in school that we dont know much of anything and we gradually learn more and more, so we get the illusion that we are little by little exhausting the bulk of the unknown by learning more of "it". But it turns out that it just expands because the limits of this 'bulk' were just an illusion enclosed in another 'bulk' of the unknown. It seems at some points almost like a belief rather than a science because what we dont know we can not yet even begin to imagine. The "imagination horizon", if you will.
     
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  19. strantor

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    I slept on it, and remembered that I too have encountered these lower current/higher voltage CTs. I used one of these NuWave current-limiting phase angle boards to control a transformer in a hot wire cutter. I had forgotten; I took advantage of the higher voltage and ordered a 2nd CT from them, and used its output thru a bridge rectifier to a DC panel meter.
    Here's the type of (CT + Shunt) I see far more often though:
     
  20. GopherT

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    I completely agree, I've always said that education (or experience) teaches us how much we don't know.
     
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