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Current Transformers

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I'm looking at two projects, both of which appear to require current transformers. The first is to detect the current draw of a AC motor, which is expected to be 10-15 amps. I would like to install a 20A AC panel meter, but the ones I have found all state they require a current transformer. How do I determine the specs for the current transformer?

The second is to determine whether a piece of equipment is on or off. When on and idle the equipment draws a low amount of current (~1A), when off it draws nothing, when running it may draw 5-10A AC. Really I'm looking to tell the difference between 0A and 1A and don't care if it is higher than 1A. The remainder of the circuit I have, I'm just looking to determine how to spec the current transformer for this system as well.
 

Reloadron

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For part two I use these little units. They will give a visual indication of any current flow above 0.75 amps (CR2550 version) and don't cost much at all. Hell if you want one just ask, I have several lying around. I have yet to cut off the LED and scope the leads but would also venture a guess if the LED was removed they would easily drive an opto-coupler for more elaborate setups.

As to the CT aspect. The CT will be specified like for example 50/5 or 100/5 meaning that a 50 amp primary would yield a 5 amp secondary. They are easily scaled. I would suggest for your range (10 to 15 amps or so) using a 50/5 CT. I would suggest one like this one from Simpson but many companies make the things. Here is the data sheet on the linked to unit.

Now as to a display if you want a digital panel meter you can get a meter like this one designed for use with an external shunt and then you simply scale the meter for your range. Here is the meter data sheet.

Another common trick using a CT is to for example wrap a few primary turmns through the CT.So if you have a 50/5 CT but never expect to go over 25 amps then wrap 2 primary turns so now uyou have effectively a 25/5 CT.

Beyond the usual digital panel meters there are also cheaper analog meters designed for use with a CT such as this model unit, and here is the data sheet. Look at the AC variations in the data sheet.

I have much more data at home including setup images and such.

<EDIT> I got home from work today and looked at the bucket of CTs I have lying around. Over the years at work I have been eliminating the old CTs for newer current transducers that give me 4 - 20 mA out, units along these lines. However, I lack the guts to toss the CTs in a dumpster. :)

Attached is a 60:5 CT image. I ran 3 turns through the primary as I mentioned earlier so as pictured it becomes a 20:5 CT. If something like this would help in your project just send me a PM and it's yours for the asking. I'll never use most of these things in my life. All I ask is that you use it and let me and the forum know how things went.

</EDIT>

Ron
 

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For part two I use these little units. They will give a visual indication of any current flow above 0.75 amps (CR2550 version) and don't cost much at all. Hell if you want one just ask, I have several lying around. I have yet to cut off the LED and scope the leads but would also venture a guess if the LED was removed they would easily drive an opto-coupler for more elaborate setups.
If I did two turns through the core, would that reduce the turn on point to 0.375A? Or does it not work like that? The issue at hand is to make a remote indicator that a machine was left on many many feet away (50-100), so I think you mentioning the optocoupler will be a good solution to my distance problem.
 

Reloadron

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If I recall correctly I did try with a few loops through the center and it did work. I just am not sure. I have been wanting to get around to cutting the LED off and using a scope to see what the signal looks like. If they are just half waving a AC small signal the LED would be pulsing. This is why I wanted to get around to looking at the output with a scope to see what is actually there. Unfortunately sometimes real work get's in the way of my playtime around here. :) Maybe I'll make time today to confirm my suspicions.

Ron
 
I was looking at this analog panel meter http://www.electronicsurplus.com/It...ment_ 0-10VAC Scale_ 0-30A AC - 3343-0-30ACA/

The supplier website says the FS=0-10V AC and the face reads 0-20A AC. If you enlarge the photo, the face actually says "ES=10V AC Use Ext. Transformer." I'm assuming I would use a current transformer. (yes assume, I guess we'll see if it makes an a$$ out of me). If a current transformer is amps in to amps out, then am I going to hook the two terminals out of the CT to the meter and place an appropriated sized resistor across the terminals as well. (basically the CT, meter, and resistor all in parallel).

Following what I just laid out. Using a 20:5 CT with 20A in would produce 5A out. For 10V AC, that would be a 2 ohm 50 watt resistor (V/I=R 10/5=2 IV=P 10*5=50). though the math sounds right, I hope I'm doing something wrong. Perhaps as simple or silly as selecting a different CT ratio 20:1 would be 10ohm 10watt resistor.
 

Reloadron

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Something is strange. If I follow your link I see where they say:

Part #: 3343-0-30ACA (That part number implies a 0 to 30 Amp meter range) and they also mention Meter, analog. Meter movement: 0-10VAC. Scale reads: 0-30 AC Amps. Needs external transformer.

However the image clearly shows a 0 to 20 Amp. Now a 20 amp part number would be 3343-0-20ACA and if I Google that I end up with this:

http://www.electronicsurplus.com/Item/127744/SIMPSON - 10VAC METER Scale_0-20AMPS AC - 3343-0-20ACA/

Seeing no image available?

Now for that line of meters this is the data sheet:

http://www.electro-tech-online.com/custompdfs/2012/01/ruggedseal_datasheet.pdf

Discounting digital scalable meters all the analog meters I have worked with were scaled for an input of 0 to 5 Amps AC. So if you uses a 50:5 CT the CT secondary went directly to the meter movement and the meter had a 0 to 50 AAC scale on the face.

Also, the forum seems to have lost its link buttons? :)

More Later
Ron
 
Something is strange. If I follow your link I see where they say:

Part #: 3343-0-30ACA (That part number implies a 0 to 30 Amp meter range) and they also mention Meter, analog. Meter movement: 0-10VAC. Scale reads: 0-30 AC Amps. Needs external transformer.

However the image clearly shows a 0 to 20 Amp. Now a 20 amp part number would be 3343-0-20ACA and if I Google that I end up with this:

http://www.electronicsurplus.com/Item/127744/SIMPSON - 10VAC METER Scale_0-20AMPS AC - 3343-0-20ACA/

Seeing no image available?

Now for that line of meters this is the data sheet:

http://www.electro-tech-online.com/custompdfs/2012/01/ruggedseal_datasheet-1.pdf

Discounting digital scalable meters all the analog meters I have worked with were scaled for an input of 0 to 5 Amps AC. So if you uses a 50:5 CT the CT secondary went directly to the meter movement and the meter had a 0 to 50 AAC scale on the face.

Also, the forum seems to have lost its link buttons? :)

More Later
Ron
Well, I think we're drifting in the same direction. This was the first AC amp meter I saw with a voltage input.

The plot thickens. According to info on the Simpson website, this is a rectifier type meter (not sure what that means). and the Simpson 3343 would require on of these (fancy) current transformers (on the left) http://www.electro-tech-online.com/custompdfs/2012/01/currenttransformer_datasheet.pdf
 
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Reloadron

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I remember those now.Been quite some time since I have seen those in use.

I did actually wander into my lab today and tried those little LED current sensors. I first tried a single conductor and at .75 AAC it is lit and much brighter at 1 amp. So it does what the spec sheet says. Then I looped the primary and sure enough it was brighter. Then I cut the LED wires and reconnected them so I could place a scope across them. Yeah, as expected a 60 Hz. square wave with an amplitude of about 3.7 volts. Obviously it would drive an opto coupler, however, it would just give a 60 Hz square wave out.

Anyway, as I posted earlier if you want one of those little LED gizmos to mess with let me know as well as that 60:5 CT I posted the image of let me know.

Ron
 
I remember those now.Been quite some time since I have seen those in use.

I did actually wander into my lab today and tried those little LED current sensors. I first tried a single conductor and at .75 AAC it is lit and much brighter at 1 amp. So it does what the spec sheet says. Then I looped the primary and sure enough it was brighter. Then I cut the LED wires and reconnected them so I could place a scope across them. Yeah, as expected a 60 Hz. square wave with an amplitude of about 3.7 volts. Obviously it would drive an opto coupler, however, it would just give a 60 Hz square wave out.

Anyway, as I posted earlier if you want one of those little LED gizmos to mess with let me know as well as that 60:5 CT I posted the image of let me know.

Ron
but...they make AC input optos (usually decicted as parallel reversed diodes). Would that not produce a steady on output? BTW, I only need to turn on a light as well, but 50-100 feet away.

Did you try cutting the current with the additional loop?
 
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Reloadron

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but...they make AC input optos (usually decicted as parallel reversed diodes). Would that not produce a steady on output? BTW, I only need to turn on a light as well, but 50-100 feet away.

Did you try cutting the current with the additional loop?
Ya know, I forgot totally about optos used for AC, yes, that should work.

What I noticed during my brief playtime was that adding a turn when at minimum just about doubled the LED intensity. While normally rated for .75 amp they do give light as low as .5 amp just real dim. Adding the amp turn doubled what I had. As I mentioned, if you want a few let me know, you can play around and see if they will do what you want.

Ron
 

Diver300

Well-Known Member
Most Helpful Member
I remember those now.Been quite some time since I have seen those in use.

I did actually wander into my lab today and tried those little LED current sensors. I first tried a single conductor and at .75 AAC it is lit and much brighter at 1 amp. So it does what the spec sheet says. Then I looped the primary and sure enough it was brighter. Then I cut the LED wires and reconnected them so I could place a scope across them. Yeah, as expected a 60 Hz. square wave with an amplitude of about 3.7 volts. Obviously it would drive an opto coupler, however, it would just give a 60 Hz square wave out.

Anyway, as I posted earlier if you want one of those little LED gizmos to mess with let me know as well as that 60:5 CT I posted the image of let me know.

Ron
Does the LED contain a rectifier, or are there two LEDs in reverse parallel? I would guess that the current transformer would saturate and get hot if it were open circuit for half of each cycle, and there would be visible flicker.
 
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