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from the Isolation department...
Isolation
Continuous process industries, DCS questions. topic
Posted by Raja Mishra on 21 August, 2001 - 10:57 am
What is Galvanic Isolation and Optical Isolation?
How are they different and what are their use for?


Posted by T. Connolly on 21 August, 2001 - 1:14 pm
Galvanic isolation is used to prevent corrosion at the electrcial junction beteween conductors made of different metals, for example, copper and aluminum. This is commonly done with a connector made of a third metal alloy that is compatable with both metals that will provide a quaility connection while not allowing the conductors to touch each other.

Optical isolators are a device consisting of a photo emitter and reciever. An electrical current turns on the emitter, the light from the emitter causes the receiver, a photo transistor, to turn on. It is used to keep two electrical circuits completely isolated from each other while still allowing the transmission of data between the two. Optical isolators are manufactured as a single IC.


Posted by Raja Mishra on 22 August, 2001 - 10:10 am
Thanks Connolly.
What is this quality connection that does not allow conductors to touch each other in Galvanic Isolation?
And is there service restriction to use any of these Isolation techniques?



Posted by PeterM on 22 August, 2001 - 10:00 am
I assume you are talking about isolating repeater amplifiers / barriers (for Intrinsic Safety).

Galvanic Isolation means that the input and output circuits have no direct electrical connection (e.g. by the ground / earth wire). This means that either side can float from ground / earth and a fault current is isolated (to one side).

Optical Isolation is one method that is typically used to provide galvanic isolation. The 4-20mA or T/C signal is converted (by a LED) to a light signal and then back to an electrical signal by a photo diode + amplifier.

The other method used for galvanic isolation is to use a transformer, converting the DC signal to AC then back to DC again.



Posted by N.Firouzi on 28 August, 2007 - 12:24 pm
Dear Peter,

Refer to conversation about galvanic isolation, Assume that we have 2 digital signals we want to send to DCS system. As you know, digital systems are transferred via a 2 core separated signal cable and basically this 2 DI signals have no electrical connection. Though, why does galvanic/optical isolation use such a system?

Best Regards,
N.Firouzi


Posted by Wayne Shimanis Vortex Technologies, Inc. on 22 August, 2001 - 10:12 am
Galvanic and optical isolation with reference to control are the two methods of eliminating ground loops in a control system.

Optical isolation is typically achieved via A\D conversion, the output of the A\D is fed into an opto-isolator (usually a stand alone IC package). The digital pulses from the A\D are converted to a light or infrared pulse in the opto and transmitted to a receiver also in the opto. The pulses from the receiver side of the opto are then converted to voltage or current via D\A.
Depending on the bit capabilities of the circuit, non-linearity and inaccuracies are common.

Galvanic Isolation is an A\D to D\A conversion also. The major difference is the isolation is achieved via a 1: thereabouts 1 isolation transformer. For the most part, assuming a properly designed product, galvanic isolation
provides a much more linear and accurate isolation.

Regard,
Wayne Shimanis


Posted by Johan Bengtsson on 22 August, 2001 - 11:00 am
Galvanic isolation is when you isolate one part electrically from another. This means you can not run any current from one of the parts to the other and therefore allowing a (quite high) voltage between those parts.

Example:
A normal transformer is galvanicaly isolating the both windings from each other meaning you can have a great voltage difference between the primary and secondary winding (usually some 2000V
or more). The energy is passed from the pripary winding to the secondary by magnetic means.
When you have a signal you want to pass from one part of a machine to the control system and you have another signal from another part to pass to the same system you can not always just ground them together, the solution is then to galvanically isolate one or both sensors from the control system.

One way of galvanically isolating a signal is to use optical isolation. In this case the signal is transfered as light a small part inside the device. A fieldbus using fiber is also optically isolating the signal (a very good idea by the way).

Devices providing galvanic isolation for a signal can be using optical as well as inductive or capacitive ways of transfering the signal thru the isolating part. (In theory you could of
course do it in even more ways such as purely mecanical, by air pressure, by sound, or whatever that is not conducting electricity, but that is perhaps less practical ways).


/Johan Bengtsson

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P&L, Innovation in training
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: johan.bengtsson@pol.se
Internet: http://www.pol.se/
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Posted by wlmostia on 22 August, 2001 - 3:36 pm
I'm not sure that there is a recognized definition for galvanic isolation in regards to electrical signals and circuits, but one definition might be:

Galvanic Isolation - Electrical isolation of a type that no direct electrical connection exists between the two sides being isolated. Common
forms of galvanic isolation are optical and transformer.

Some times the term "physical barrier" is substituted for "no direct electrical connection."

An alternate definition which I have seen is:

Galvanic Isolation - A means of interconnecting two circuits which prevents an unacceptable current from flowing as a result of potential
difference(both AC and DC) between the circuits.

Optical isolation uses light as the isolating means and converts the input signal to light and then converts the light back to the output signal. It is a form of galvanic isolation.

Bill Mostia
===========================================
William(Bill) L. Mostia, Jr. PE
Independent I &E Consultant
WLM Engineering Co.
P.O. Box 1129
Kemah, TX 77565
wlmostia@msn.com
281-334-3169
These opinions are my own and are offered on the basis of Caveat Emptor.

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