Current and voltage amplifier circuit to make Splitcore CT to drive Schneider EM - Page 1
Forum section: electronics / projects

author: getfast_kiran
author_rank: Regular Contributor
timestamp: June 05, 2018, 04:51:33 am
content: I have been trying to connect the CT of following configuration,CT Configuration :-CT output ratio : 50A:333mVACSplit Core Type.The output from CT is being connected to the Schneider electric EM. But the problem is when I connectsplit core to EM I do not get current measurement but normal closed type CT would give the output.The same Energy meter when connected to the Split-core is not giving me a reading in the same meter connected to the same load.What could be the possible reason behind.I suspected that may be the Split core CT which is of Voltage output type might be finding it hard to drive the Energy Meter output.Hence I did a current amplifier and voltage amplifier section to work on the same, still it did not work. But I am not at all good with such amplifiers of whichI have attached the diagram could you tell me what have I done wrong or is there another way to solve it.Please take a look at the attachment.

author: getfast_kiran
author_rank: Regular Contributor
timestamp: June 06, 2018, 05:42:10 am
content: Mention Energy meter is a Schneider EM6433. I just measured the resistance at the terminal of current sensor and schneider input they are 40 ohm and 0.6 ohm. I hope someone could help me with it.

author: capt bullshot
author_rank: Super Contributor
timestamp: June 06, 2018, 05:51:59 am
content: These meters are made for use with a current output CT. The nominal secondary current of these CT is 1A or 5A at rated primary current. So your amplifier has to supply a current (implemented as a current source) of 1Arms or 5Arms at full scale. Your circuit is a voltage source that can supply approx 20mArms - that cannot work.

author: getfast_kiran
author_rank: Regular Contributor
timestamp: June 06, 2018, 06:10:07 am
content: Hi, thank you so much for that diagnosis of the circuit and problem. Can you help me with a solution as reference to start with so that I could get the meter to read this CT I have with now. Is there any circuit reference that you can provide for me or point me too so that I could make it read in Energy Meter.

author: getfast_kiran
author_rank: Regular Contributor
timestamp: June 06, 2018, 06:15:17 am
content: Quote from: capt bullshot on June 06, 2018, 05:51:59 amCan you please give me a reference for making a circuit that could provide the current as current source of the current scale as you told.

author: capt bullshot
author_rank: Super Contributor
timestamp: June 06, 2018, 06:40:21 am
content: You'd have to build a circuit like Fig. 4.15 of The Art of Electronics, Third edition. The figure shows a bipolar voltage controlled current source. You'd have to add a beefy buffer (that can supply +/- 1.5Apk or +/- 7.5Apk in between IC1's output and the sense resistor.LT's app notes AN-4, AN-18, AN-45 may give you some ideas of that kind of circuits.

author: getfast_kiran
author_rank: Regular Contributor
timestamp: June 06, 2018, 07:43:34 am
content: Quote from: capt bullshot on June 06, 2018, 06:40:21 am,Really thankful for so much reference to start with..I will follow the reference you have provided. Is it usual in industries to do the same circuit to connect split core CT that have voltage output type?

author: capt bullshot
author_rank: Super Contributor
timestamp: June 06, 2018, 09:29:12 am
content: Quote from: getfast_kiran on June 06, 2018, 07:43:34 amNo, one in industry would consider doing it that way. It's quite a waste of time and effort, and a really inefficient and expensive solution anyway. One would search for a electrically matching split core CT, or use a "normal" CT and take the effort to open up the circuit to apply the CT, use a clamp-on CT with suitable output (these exist) or use a different meter.IMO that split core CT is intended to be used as a part of a circuit (meter or whatever) that is developed to match its output. An industrial engineer wouldn't even consider to use such a CT with such a meter, as the CT doesn't adhere to the defacto standard for this application.If it was my meter and split core CT (e.g. for a one-off project at home), I wouldn't go that route either. I'd open up the meter, analyze the current sensing circuitry (expecting to find either a small value shunt resistor, or a smallish PCB mounted CT) and modify the meter to accept that CT directly.

author: CopperCone
author_rank: Super Contributor
timestamp: June 06, 2018, 01:53:40 pm
content: I think you might get better phase response using an active circuit like you described but indeed the idea of building a crazy power buffer to power a freaking meter is rather insane.imo its like trying to connect a dremel to a v8 engine lol

author: getfast_kiran
author_rank: Regular Contributor
timestamp: June 07, 2018, 12:37:31 pm
content: Quote from: capt bullshot on June 06, 2018, 09:29:12 amOkay that was a nice explanation.I was trying to read into material that is given by you last time. I was also feeling like I am going in a wrong way. I think as you told it would be better for me to find a suitable split core to work with this meter. If I could not find one I think it would be better for me to open up the meter and see what the current sense circuitry is doing to modify it accordingly.

author: getfast_kiran
author_rank: Regular Contributor
timestamp: June 07, 2018, 12:43:19 pm
content: Quote from: CopperCone on June 06, 2018, 01:53:40 pmThe example you took to explain me the circuit I was trying to build was indeed useful to visualize it. Thanks for that example.

author: CopperCone
author_rank: Super Contributor
timestamp: June 07, 2018, 01:55:27 pm
content: And the other thing is even if you really need to do it getting precision from a 5 amp circuit will be high.What you might be able to do though is switch the resistor inside the meter to a higher value one so your signal power levels are more reasonable. Maybe?

author: getfast_kiran
author_rank: Regular Contributor
timestamp: June 12, 2018, 11:55:51 am
content: Quote from: CopperCone on June 07, 2018, 01:55:27 pmDid you mean to get precision for current higher than 5 ampere will be difficult ?Quote from: CopperCone on June 07, 2018, 01:55:27 pmWhat value might you suggest ?

author: capt bullshot
author_rank: Super Contributor
timestamp: June 12, 2018, 12:22:31 pm
content: Quote from: getfast_kiran on June 12, 2018, 11:55:51 amFirst, you'll have to convert the voltage output CT to a current output CT and determine the ratio and full scale current of the result. If you CT has a magnetic core, this might be as simple as removintg its internal termination resistor. Full scale current and CT ratio can be measured or calculated from this resistors nominal value and the datasheet values for output voltage at input current.Next you'll have to analyze the meter's circuit to see if there are any issues replacing the resistor.Then you can calculate the correct resistor value from the meters nominal full scale current, the orginal resistor value and the desired full scale current (as from your modified CT). Voltage drop at full scale (original) shall match voltage drop at your target full scale current.You'll have to take the meter's abilities to set the external CT ratio into account, in a way that the meter is able to show the correct value. E.g. if you can set the meter to an external ratio of 15A : 5A, your resistor shall have the nominal voltage drop at 15A primary current through your CT.

author: getfast_kiran
author_rank: Regular Contributor
timestamp: June 18, 2018, 08:24:04 am
content: That was indeed helpful. I have never opened up a meter before. I will try to do as you say If I get permission to open up the meter.