inexpensive CT rings

Erik Friesen

I have designed project to use 4-20ma current rings like this -www.automationdirect.com/adc/Shopping/Catalog/Sensors_-z-_Encoders/Current_Sensing_Transducers/AC_Current_Transducers

Anyways, why are they so expensive? These - www.greegoo.com/interface-operator-components/current-transformers-msq-30.htm are much less expensive, but they are current transformers. Is there a good way to use these, or does the proper circuitry cost $75 to emulate?

My circuitry now consist of a 24v supply through a 50ohm resistor, to the ring, and back to a 100ohm shunt resistor to ground.

W9GFO

Allegro makes a bunch of inexpensive hall current sensors, search Digi-key.

Rich H

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The Servo Boss, a 12 channel servo tester kit from Gadget Gangster.

Erik Friesen

Yes, but I am talking about an industrial sensor that someone can put in place without special equipment, and can measure 25-100 amps. Digikey has a few 4-20ma like that, but they are $100+

kwinn

The automationdirect units have a lot of flexibility and measurement/communications options built in while the greegoo unit is just the core and current windings. If you only need 1 or 2 rings you are probably better off with the automationdirect units. On the other hand, if you need several rings and only need to measure current with one range it may be less expensive to go with the greegoo units and make your own interface circuitry.

Franklin

How about any of these www.crmagnetics.com/ On a side note if you don't want 4-20 ma sensors then don't ask for them in your post.

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- Stephen

Erik Friesen

I guess I don't follow you franklin. I looked over everything digikey had, and nothing was as good as what automation offers. I can't send a unprotected loop or hall sensor out to be installed, it has to be in a ready to go housing. I need something with 4-20ma output.

QuattroRS4

Erik,
There are a few sources mentioned in this thread ..

http://forums.parallax.com/showthread.php?p=795321



Regards,
John Twomey

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'Necessity is the mother of invention'

'Those who can, do.Those who can’t, teach.'
'Convince a man against his will, he's of the same opinion still.'

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Erik Friesen

Thanks, quattro. My deal is that I designed this thing with $80 CT 4-20ma ct rings in mind, and other folks on the team are looking for cheap CT rings. They found the above listed in the $3 range, and someone is trying to say that they are 4-20ma output. What I am pretty sure they are is only current transformers 50a/5a. I do not know what it will take to use these rings, but it seems like it could add significant complexity to my existing circuit to allow their use. I just have this feeling that a $3 ring is in a different class than a $80 ring, if for nothing else than the ease of field installation, plus the reliability aspects.

Carl Hayes

Ah, I'm relieved.· To me, CT means contraterrene.· I don't know of any place you can get an antimatter ring; but certainly I wouldn't try to wear one.

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· -- Carl, nn5i@arrl.net

stamptrol

Theres alot of difference between a 4-20 mA current TRANSDUCER and a plain 50:5 current TRANSFORMER. The transformer solution can be made to work with precision rectification, a current shunt and a/d - d/a.

The transducer has all the hard work already done, is calibrated and safety certified for use on the bus voltage you need. Starting with a $3 CT, you'll quickly burn up the difference in price before you send the thing for safety certification.

One thing I'd suggest is always using a 5 amp to 4-20 mA transducer. That way, as the requirements change, you can feed it with any cheap current transformer, whether you need 1000:5, 100:5 or 10:5.

Cheers,

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Tom Sisk

http://www.siskconsult.com
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Erik Friesen

stamptrol said...
One thing I'd suggest is always using a 5 amp to 4-20 mA transducer. That way, as the requirements change, you can feed it with any cheap current transformer, whether you need 1000:5, 100:5 or 10:5.

That is an interesting thought.

Thanks though, stamptrol, because I need a feel for what I am up against. I've found that many times you can be better off sticking with the apparently more expensive option.