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Analog Voltage To Current Conversion

08/10/2016 8:06 AM

Typically in the industry we require analog voltage 0 - 333 mv to be converted to analog current 0-1 amps . The frequency is 50 / 60 Hz. Is there a circuit which is stable in industrial environment?

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#1

Re: ANALOG VOLTAGE TO CURRENT CONVERSION

08/10/2016 8:21 AM

A 333mΩ resistor?

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#2
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Re: ANALOG VOLTAGE TO CURRENT CONVERSION

08/10/2016 8:30 AM

It will draw current from the source, which is not acceptable.

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#6
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Re: ANALOG VOLTAGE TO CURRENT CONVERSION

08/10/2016 12:07 PM

To measure a voltage one must always draw some current. The first unanswered question then is how small must the measurement current be?

The second unanswered question then is what load must be driven with ±1 ampere of current?

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#3

Re: ANALOG VOLTAGE TO CURRENT CONVERSION

08/10/2016 8:31 AM

Maybe something like this:

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#4
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Re: ANALOG VOLTAGE TO CURRENT CONVERSION

08/10/2016 8:54 AM

It should take 0 - 333 mv AC voltage input.

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#5
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Re: ANALOG VOLTAGE TO CURRENT CONVERSION

08/10/2016 9:53 AM

http://www.analog.com/media/en/technical-documentation/application-notes/AN-968.pdf

You will have to adjust input and feedback resistors around U1 to get the gain you want.

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#7

Re: Analog Voltage To Current Conversion

08/10/2016 1:10 PM

You mention 50/60Hz. Are you using a DC voltage to control an AC current, or what? And (as Redfred says) the load must be specified. You must be more specific if you want a usable answer.

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#8

Re: Analog Voltage To Current Conversion

08/10/2016 2:37 PM

I always hated open ended test questions!

What is the OP's definition of mv? Is it "millivolt" (mV) or "megaVolts" (MV) or ....? "to be converted to analog current 0-1 amps"? Are you talking about milliamperes (mA) 0 to 1,000 milliamps?

Once the OP's is more specific about what they're talking about ..... Maybe we can help them

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#9
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Re: Analog Voltage To Current Conversion

08/10/2016 4:27 PM

I was asking myself this question: what sort of probe generates a 0-333 mV signal (and that seems very specific for some reason). Could it be a thermocouple? Possibly.

Could it be a pH meter, possibly. It could be a potential divider using a HT probe and a snake belly on the ground leg. Why such a high output current, most transmitters I am familiar with go 4-20 mA or possibly 0-20mA, even on controlling some heavy prime movers.

0-1 ampere seems oddball, like this current is powering a small servo?

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#10
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Re: Analog Voltage To Current Conversion

08/10/2016 5:21 PM

I don't know, but this seems to me to be a homework test question. Their leading statement; "Typically in the industry we require ...." And then the actual question; "Is there a circuit which is stable in industrial environment?"

Sounds to me like the student is suppose to do their own research to solve this question. That's unless hdiddee is aka "Engineer Mohamed" and still totally lost.

What good is the internet or a library if you don't know how to do a proper search of either one on your own?

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#11

Re: Analog Voltage To Current Conversion

08/10/2016 8:29 PM

333 mv is generated by split core current transformer. This has to be converted proportionally to 0 get 1 amps, which is required by an energy meter input. The current drawn by the Energy meter is negligible. The split core CT can maximum have a load of 2 VA.

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#12
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Re: Analog Voltage To Current Conversion

08/10/2016 10:27 PM

Sounds like #1 Tornado is #1 again, then. Assuming the impedance of the energy meter is negligible and using E=IR, a 333 mOhm resistor in series with the transformer and the meter input will give you 0 - 1 A corresponding to 0 - 333 mV voltage. And 1 A x 0.333V = 0.333 VA, so the load is within the range you specified.

Of course, this is just the thoughts from a dumb ole mechanical engineer. What do I know?

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#16
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Re: Analog Voltage To Current Conversion

08/11/2016 10:03 AM

1 ampere for a meter drive is not negligible. The scaling of the meter displays 1 ampere when the input voltage is 333 millivolts ac 50/60Hz perhaps is what you mean?

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#17
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Re: Analog Voltage To Current Conversion

08/11/2016 10:29 AM

No. Energy meters normally take either 0-1 amps or 0-5 amps AC since the CT has a ratio of either XXX/5 or XXX/ 1, where XXX is the primary amps.

However, the split core CT does not give the output in Amps, but in voltage of 0 - 333 mv. We need to convert this voltage to 0-1 amps AC so that it can be fed to the energy meter.

Hence this problem.

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#22
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Re: Analog Voltage To Current Conversion

08/11/2016 12:52 PM

I understand now why you couldn't find anything already made for this application.

This will likely require a power rated inverter of some sort (very small power). I find it difficult to believe that the complexity & conversion efficiency of such a circuit, so that you can reuse a particular type of meter will meet your accuracy & repair/diagnostic complexity requirements. In USD$, the equipment will be several hundred dollars if cobbled together from inexpensive available components (PWM inverter), especially after filtering out the harmonics and non-linear components so that your power meter reads properly.

The earlier suggestions assume a conversion of the mV reading to a dc analog value easily read by standard automation components, and converted to numeric data, bypassing the need for the mechanical? energy meter.

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#18
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Re: Analog Voltage To Current Conversion

08/11/2016 10:42 AM

You contradict yourself!

First you say the energy meter input requires 0-1 Amp, Then you say the current drawn by the energy meter is negligible. It can't be both! Which is it?

If the current drawn by the energy meter is negligible, then it does NOT require 0-1 Amp! Rather its input senses voltage!

It sounds like Tornado and Bigg have it right!

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#21
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Re: Analog Voltage To Current Conversion

08/11/2016 10:59 AM

WHAT i MEANT WHEN i SAID THAT THE ENERGY METER REQUIRES 0-1 AMPS, is that the energy meter will measure values only between 0-1 amps AC. Regret the misunderstanding.

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#24
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Re: Analog Voltage To Current Conversion

08/11/2016 3:00 PM

OK, that's better. As others have said, we need to know more about your energy meter.

I've been measuring similar energies (≈500A @ 480V), and the Current Transformers (Rogowski Coils in our case) have internal shunts, and connect directly to the energy meter.

It sounds like your Current Transformer requires an external shunt, and again, a 0.333Ω resistor is the correct shunt value to produce a voltage of 0.333V with a CT secondary current of 1.00A

When I go back and re-read your original post, I get the impression that you intended to say that you needed to convert 0-1 Amp into 0-333mV, rather than the opposite, which you stated.

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#25
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Re: Analog Voltage To Current Conversion

08/11/2016 3:21 PM

This often arises when English is employed as second language due to reverse syntax.

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#26
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Re: Analog Voltage To Current Conversion

08/11/2016 3:38 PM

Indeed! I was going to say something equivalent in that last post, but didn't...

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#27
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Re: Analog Voltage To Current Conversion

08/11/2016 4:10 PM

I am taking that as a left-handed chiral compliment.

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#33
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Re: Analog Voltage To Current Conversion

08/14/2016 8:56 PM

You can certainly take it as a compliment. In the culture with which I'm most familiar, serving with the left hand is considered an insult, and this certainly was not!

You may also know that I'm old enough (or low enough on some scale) to be pretty much locked in to classical physics. The quantum stuff makes my head spin, and I have no idea whether that is clockwise or counter-clockwise...

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#13

Re: Analog Voltage To Current Conversion

08/11/2016 2:39 AM

If this is "typical in the industry" then see where it is used and how they have done it otherwise more information is required

It is difficult (but not impossible) to imagine that the 1A will be sourced from within the circuit. But if it is the case then a 333 milliOhm resistor will give the 0 - 1 A.

The first "need to know" is where the 0 - 1 Amp comes from. Is there an outside power source available? Does it need to be, or is it allowed to be, isolated?

Probably the easiest way is an audio amplifier (externally powered) with a feedback loop fed by the voltage across a 10 milliOhm (or perhaps a 333 milliOhm) series resistor in the output circuit. If you need isolation then an opto-coupler with associated circuitry is available.

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#14
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Re: Analog Voltage To Current Conversion

08/11/2016 4:24 AM

We need external power to give 1 amps. The input is 0 to 333 millivolts AC, and the output should be 0-1 amps AC, proportional to input. We can use external power source.

Care to be taken that input power source should not be burdened.

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#15
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Re: Analog Voltage To Current Conversion

08/11/2016 7:44 AM

1 ampere of current into a 1k ohm load will require a 1000 V supply. That 1k ohm load will dissipate 1 kilowatt of power.

It seems that you are missing many critical details in this puzzle. Hire a qualified electrical engineer to design what you need. I suspect they will be able to buy COTS components to meet your yet to be properly specified needs.

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#29
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Re: Analog Voltage To Current Conversion

08/11/2016 6:57 PM

I believe that you are in need of a properly designed 600/1A current transformer(CT). It appears that what you have in your system is not suitable.

Is this installation for measurement or for protection. The CT must be designed to meet your system requirement. Measurement and protection CT's are totally different in design and application.

A Measurement or Protection Engineer will be able to specify what you need. You will need to know the cable resistances between the CT and the meter/relay as well as the meter/relay specification or at least make and model number. The CT will give you the output to drive the meter/relay.

A non-split CT is preferred for accuracy but you will have to shut the system down to "thread" it over the cable. A split CT is usually satisfactory but must be correctly specified.

An external power source will not be required.

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#19

Re: Analog Voltage To Current Conversion

08/11/2016 10:47 AM

What is the voltage of and current range through the line being monitored?

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#20
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Re: Analog Voltage To Current Conversion

08/11/2016 10:55 AM

It does not matter since there is a CT fixed on the cable. Normally the Voltage is 415 V AC, and the current will be in the value of 500 Amps AC.

We will put a 600/5 CT normally.

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#23
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Re: Analog Voltage To Current Conversion

08/11/2016 1:31 PM

What brand of energy meter is this, what type, and what are its units of measurement? This seems more than a bit odd to me.

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#28
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Re: Analog Voltage To Current Conversion

08/11/2016 6:32 PM

I does matter in that your post #2 stated "It will draw current from the source, which is not acceptable". Just considering 1A against the 500A, you're only looking at a loss of 0.2%, and that's before we've looked at the voltages (and so powers) involved. In any case, using such a CT will result in an absolutely negligible loss.

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#30

Re: Analog Voltage To Current Conversion

08/13/2016 10:26 AM

The OP faces a mismatch between a newer style CT with a 333mVac output and existing metering equipment designed for 1Aac input.

I suspect that his options are limited; replace either the CT or the meter. If there is a signal conditioner that converts from 333mVac to 1Aac, I'm not aware of it. Good luck with Google. The trade off for a lower cost, safe output, 333mV CT is the necessity for a meter designed to read that particular CT output.

Elkor Technologies' application note on CTs is a very well written explanation of the evolution of CTs that covers this situation.
Historically, 5A CT outputs were designed to drive mechanical meters or protection relays.

With the advent of digital metering, mA or mV outputs have come onto the scene, because of construction, size and weight reductions and with the added attractive mV output feature of an internal burden resistor that makes it safe to disconnect the secondary without a shorting bar or knife switch.

The 333mV output CTs appear to be lower cost units. What's the point of buying a lower cost unit that needs an added cost signal conditioner to make it compatible? Plus a signal adds distortion and the idea is to get the best waveform into the meter. A direct connect CT makes the most sense.

Of course, the mA or mV CT outputs are intended for use with digital meters designed for those inputs.

If interested, you can read the full text here (start on page 2):
http://www.elkor.net/pdfs/AN0305-Current_Transformers.pdf

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#34
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Re: Analog Voltage To Current Conversion

08/14/2016 9:39 PM

This makes sense, but what does OP mean in post 11 that max. burden is 2VA?

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#35
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Re: Analog Voltage To Current Conversion

08/15/2016 12:34 AM

I've now come to the same conclusion as several others, that he got a new CT, whose output is 0-0.333mV, and he is trying to connect it to an older energy meter that was designed to directly measure 0-1Amp of input current.

The 2VA tells me the CT can put out up to 6A of current. Since 1A is well within that limit, and 0.333V will cause 1.00A to flow through 0.333Ω, we are back to Tornado's Post 1!

Now back to post 11: If the energy meter measures 0-1A of current, then the meter must draw a maximum of 1A of current. That 1A is negligible compared to the 500A in the main line, but it is not what I'd consider a negligible input for a measuring device.

Perhaps what he meant to indicate was that the energy meter has a very low impedance, and so will have a negligible effect on the current flowing. Another way of saying the same thing is that there is negligible insertion loss of current in the energy meter.

Tornado's solution is clearly the cheapest one. If the energy meter is very old (I'll let you and/or the OP decide how old is very old), the best solution would be to get a new energy meter, but of course then we're talking about a major investment.

On the other hand, our energy meter saved well over the $2k+ that we spent on it in a single month, by eliminating wasted energy! That's a pretty quick Return On Investment!

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#36
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Re: Analog Voltage To Current Conversion

08/15/2016 7:25 PM

I have read the article linked by Carl_E. I think OP is just wrong about the 2VA, but is correct that 'minimal' loading is needed.

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#31

Re: Analog Voltage To Current Conversion

08/14/2016 12:51 PM

The source of the confusion is the CT manufacturer's product sheet. Here's an excerpt:

So what's wrong with this picture? This particular unit has a built in burden resistor which precludes its use in a traditional metering circuit that expects 0-5Amps, not 0-333mV. Buying the right type of CT is the only/easiest answer.

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#32
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Re: Analog Voltage To Current Conversion

08/14/2016 7:18 PM

Manufacturer needs to find a new tech author (just for datasheets - heaven knows what else they screwed up!).

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