current transformer input / rect cct ?

Discussion in 'General Electronics Chat' started by bjhbjh, Jan 4, 2007.

  1. bjhbjh

    Thread Starter New Member

    Jan 4, 2007
    3
    0
    Hi,

    I am looking for suggestions for a circuit to accept input from a current transformer and convert it to a dc voltage or digital value. I find surprisingly little on Google.

    Thanks,

    Brian H.
     
  2. wireaddict

    Senior Member

    Nov 1, 2006
    133
    0
    I've done this with no problem although I've seen warnings about high open circuit voltage from CTs. It's been over 20 years ago & I don't have a drawing for it anymore but I think I connected the CT to a 120V:24VAC potential transformer & the secondary to a MOV to protect the low-voltage components from overvoltage then I connected this to a diode bridge or voltage doubler & filter cap. Then I connected all this to an analog input for a programmable controller & it worked great.
     
  3. rootboy

    Member

    Jan 4, 2007
    13
    0
    First off, you will want to be sure that the wires coming off of the CT (which is the secondary of course) always have a completed circuit. The voltage on a CT with an open secondary will try to reach infinity.

    That's usually not what you're after. :)

    As for picking voltage off of a CT, I would go with a string of diodes for each direction. You could of course use a zener, but the startup currents for a motor on say a 200:5 CT can go as high a 8 times the normal 5 amp current (this is based on the assumption that the motor draws 200 amps of current at normal load).

    So you can see the problem with using a zener, trying to find a zener that can handle 40amps might be tricky.

    And I like wireaddict's idea of placing a MOV in parallel with the diodes.
     
  4. bjhbjh

    Thread Starter New Member

    Jan 4, 2007
    3
    0
    Well perhaps I have used the term current transformer erroneously.

    What I mean is a current pickup device intended for use with a multi meter that clamps around an AC conductor and reads the current as a mv in some proportion to the actual current. I have several of these.

    See ebay auction #140053983956 for the device in question.(not my auction btw, just a convenient pic of exactly what is at hand)

    I have an electric furnace with four 5kW elements. I'd like to monitor the current flow patterns of these on an html page. The furnace switches these elements on & off sequentially each time it runs, but elements sometimes burn out with no indication. I want to be able to keep an eye on these 4 currents via the web and send myself email if currnet isn't flowing in one of the elements when it should. (As a side benefit, I intend to collect data for archival purposes and energy consumption calcs too.)

    I have all I need, except an analog circuit to allow my computer to read the mv's off the pickup devices. I have the A/D convertion in hand already.

    As I see it, what I need is a precision rectifier and sufficient gain to spread the 0 - 25A AC readings across 0 - 5V dc. I have looked around at op amp web pages from manufacturers and enthusiasts like this but any suggestions they make seem to involve op amp devices that are rather dated and in many cases discontinued. I'd like to use current technology devices.

    Suggestions ?

    Thanks,

    Brian H.
     
  5. wireaddict

    Senior Member

    Nov 1, 2006
    133
    0
    The clamp-on current sensor is indeed a CT of sorts. I did something similar only I measured the main motor current on an automatic press. The controller compared the current when the ram raised with that when the ram lowered and made a decision to add air to the counterbalance or vent it until the two currents were about equal. This required building a current-to-voltage converter.

    I probably also used a 0 to +5V analog input from the current-voltage converter [though it may have been 0 to 10V]. Since I didn't get quite enough voltage I used a voltage doubler instead of a diode bridge. I don't have the schematic for this anymore but I described the gist of in my first reply. You may need to experiment with different transformers [i.e., ratios]. I recall connecting the low voltage winding to the CT and the high voltage winding to the voltage doubler. Also, connect a 6V zener across the analog input to protect it from overvoltage. If you still need assistance I can probably sketch most of it for you.
     
  6. wireaddict

    Senior Member

    Nov 1, 2006
    133
    0
    To my surprise, I found the sketch for the current-voltage converter I built in '86. Unfortunately, the file size exceeds the limit so I can't attach it here. If you need a sketch send me a private mail or e-mail so I get you e-mail address and I'll send it that way.
     
  7. rootboy

    Member

    Jan 4, 2007
    13
    0
    Okay, that makes it simpler. Why not use a DI-158UP and set it to x512 gain? (giving you a range of 125 millivolts AC):

    http://www.dataq.com/support/documentation/pdf/datasheets/158ds.pdf

    By using the differential model you don't have to worry about any signal conditioning. Let the data acquisition unit do that for you. And you don't care about the AC aspect since all you are concerned about is amplitude.

    Though trying to read 0 - 25 amps using a 0 - 400 amp clamp-on is going to really hurt your resolution...
     
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