Post by thread:[EE] What is a Galvanic Isolator?

>I have a vague understanding of what it is but why is it called a > "Galvanic" isolator? In its most general sence "Galvanic isolation" means that there is no direct current path between the input and output*. Galvanic action is usually taken to mean the setting up of an electrochemical action when a cell is formed by two dissilar metals and an electrolyte. Often the context is related to "galvanic corrosion" when a metal object inadvertently becomes part of a cell and is damaged by the electrochemical action. A "galavanometer" is an ammeter - usually a very sensitive one. The name comes from Luigi Galvani who discovered in about 1780 that a dissected frog's legs would twitch when a "cell" was formed by the metal scalpel and dissection plate. His theory about why this happened, which he verified with careful experiments, was subsequently discredited but the basic principle which he discovered forms the basis of a vast range of modern applications. (They told me about him in school long ago but Google provides http://www.newadvent.org/cathen/06371c.htm which gives lots of interesting detail. > What I am looking at is a bunch of DC/DC converters > for an industrial control system and they have them labeled as "Galvanic > Isolators". Why not just call them DC/DC power supplies? Are they more > than just a DC/DC converter? The circuit is a simple oscillator made > from discrete components that fires a 1:1 pulse transformer and the > output is just rectified and unregulated. Nothing fancy about it. The fact that thye call them Galvanic isolators suggests that the main task is electrical isolation of input from output. Whether they are used as DC DC converters per se depends on the application. They may take a moderate power input signal and produce an output whose main job is to indictae that input is present. They may be powerful enough to drive a bulb or relay. In such a case the isolation is the key thing and the power provided is necessary but secondary. Or they may provide power as their main function where isolation matters. * "no direct path" is relative. Isolation may be to a DC potential, and/or to AC or to sudden voltage changes. Many different isolation techniques are used. Many provide a degreee of AC coupling (albeit in the picofarad range) that can cause problem sin some circumstances. Where substantial poweer transfer is required, high isolation against transient signals can be a major design task. RM

> >A "galavanometer" is an ammeter - usually a very sensitive one. The name >comes from Luigi Galvani who discovered in about 1780 that a dissected >frog's legs would twitch when a "cell" was formed by the metal scalpel and >dissection plate. His theory about why this happened, which he verified >with careful experiments, was subsequently discredited but the basic >principle which he discovered forms the basis of a vast range of modern >applications. (They told me about him in school long ago but Google >provides http://www.newadvent.org/cathen/06371c.htm which gives lots of >interesting detail. Galvanic isolation keeps the patient from emulating the frog!

Shahid Sheikh wrote: > What I am looking at is a bunch of DC/DC > converters for an industrial control system and they have them > labeled as "Galvanic Isolators". Why not just call them DC/DC power > supplies? Are they more than just a DC/DC converter? Yes. As the name implies, they provide galvanic isolation between the input and output power connections. Something just called a "DC/DC power supply" does not necessarily have this feature. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com

Firstly not all DC-DC converter are galvanic isolated. Most of the small point-of-load DC-DC converters are non-isolated, including the VRM on the PC motherboards. Secondly galvanic isolation is quite important for safety. My company (Pepperl+Fuchs) has a lot of Ex and Non-Ex interface cards to transmit power and signals (from sensors: current, voltage, digital signal, fieldbus signal, etc) for the process automation industry like chemical plants. Most of the telecom power supply with 48V input are also galvanic isolated for safety reasons. All the PC power supply (silver box) should be galvanic isolated as well for safety reasons. Some of them are using simple push-pull converter. Some of them are using much more complicated power supplies. One thing in common is to use transformer, opto-coupler and relays as isolation components. Sometimes there are some safety caps across the primary and secondary side for EMC reasons (better AC ground). But they are governed by the standards and is normally very small. Under what circumstances this could be a problem? Xiaofan >Date: Sat, 25 Dec 2004 23:50:20 +1300 >From: Russell McMahon <spam_OUTapptechTakeThisOuTparadise.net.nz> >... >In its most general sence "Galvanic isolation" means that there is no direct >current path between the input and output*. Galvanic action is usually taken >to mean the setting up of an electrochemical action when a cell is formed by {Quote hidden}>two dissilar metals and an electrolyte. Often the context is related to >"galvanic corrosion" when a metal object inadvertently becomes part of a >cell and is damaged by the electrochemical action. >... > >> What I am looking at is a bunch of DC/DC converters >> for an industrial control system and they have them labeled as "Galvanic >> Isolators". Why not just call them DC/DC power supplies? Are they more >> than just a DC/DC converter? The circuit is a simple oscillator made >> from discrete components that fires a 1:1 pulse transformer and the >> output is just rectified and unregulated. Nothing fancy about it. > >The fact that thye call them Galvanic isolators suggests that the main task >is electrical isolation of input from output. Whether they are used as DC DC >converters per se depends on the application. They may take a moderate power {Quote hidden}>input signal and produce an output whose main job is to indictae that input >is present. They may be powerful enough to drive a bulb or relay. In such a >case the isolation is the key thing and the power provided is necessary but >secondary. > >Or they may provide power as their main function where isolation matters. > >* "no direct path" is relative. Isolation may be to a DC potential, and/or >to AC or to sudden voltage changes. Many different isolation techniques are >used. Many provide a degreee of AC coupling (albeit in the picofarad range) >that can cause problem sin some circumstances. Where substantial poweer >transfer is required, high isolation against transient signals can be a >major design task.

Russell, As usual thanks for the detailed explanation. Much appreciated. Shahid -----Original Message----- From: Russell McMahon Sent: Saturday, December 25, 2004 3:50 PM Subject: Re: [EE] What is a Galvanic Isolator? >I have a vague understanding of what it is but why is it called a >"Galvanic" isolator? In its most general sence "Galvanic isolation" means that there is no direct current path between the input and output*. Galvanic action is usually taken to mean the setting up of an electrochemical action when a cell is formed by two dissilar metals and an electrolyte. Often the context is related to "galvanic corrosion" when a metal object inadvertently becomes part of a cell and is damaged by the electrochemical action. A "galavanometer" is an ammeter - usually a very sensitive one. The name comes from Luigi Galvani who discovered in about 1780 that a dissected frog's legs would twitch when a "cell" was formed by the metal scalpel and dissection plate. His theory about why this happened, which he verified with careful experiments, was subsequently discredited but the basic principle which he discovered forms the basis of a vast range of modern applications. (They told me about him in school long ago but Google provides http://www.newadvent.org/cathen/06371c.htm which gives lots of interesting detail. > What I am looking at is a bunch of DC/DC converters > for an industrial control system and they have them labeled as > "Galvanic Isolators". Why not just call them DC/DC power supplies? Are > they more than just a DC/DC converter? The circuit is a simple > oscillator made from discrete components that fires a 1:1 pulse > transformer and the output is just rectified and unregulated. Nothing > fancy about it. The fact that thye call them Galvanic isolators suggests that the main task is electrical isolation of input from output. Whether they are used as DC DC converters per se depends on the application. They may take a moderate power input signal and produce an output whose main job is to indictae that input is present. They may be powerful enough to drive a bulb or relay. In such a case the isolation is the key thing and the power provided is necessary but secondary. Or they may provide power as their main function where isolation matters. * "no direct path" is relative. Isolation may be to a DC potential, and/or to AC or to sudden voltage changes. Many different isolation techniques are used. Many provide a degreee of AC coupling (albeit in the picofarad range) that can cause problem sin some circumstances. Where substantial poweer transfer is required, high isolation against transient signals can be a major design task. RM

>>* "no direct path" is relative. Isolation may be to a DC potential, >>and/or >>to AC or to sudden voltage changes. Many different isolation >>techniques are >>used. Many provide a degree of AC coupling (albeit in the picofarad >>range) >>that can cause problem sin some circumstances. Where substantial >>power >>transfer is required, high isolation against transient signals can >>be a >>major design task. > Under what circumstances this could be a problem? Sometimes an isolated device's output or input "sits on top of" a potential that is varying very rapidly and/or over a very wide voltage range. The slew rate of the transition provides a very high frequency component to the voltage presented to the isolation barrier. This is necessarily coupled across the barrier by any "common mode" coupling. This MAY appear as if it was part of the true signal that the isolator was intended to transfer. For example, a high side driver may be used to couple a gate drive signal from a grounded control circuit to the gate of a high side FET. And/or the circuitry may be used to transfer power to the gate drive circuit. IF rapid changes in the voltage and high voltage excursions create a signal that is not designed for the high side FET may turn on or off when it should be off or on. This may cause 'trouble" :-). An isolation barrier may be required for safety purposes. Signal inadvertently couple across the barrier due to unintended coupling may also cause "trouble". eg simple case enough capacitance will make it unpleasant to hold onto something with mains on the other side of the barrier. This is a real world problem. I have met a number of laptop supplies that are fully isolated from main but that cause a very unpleasant 'tingle" when the output ground lead is held. Poor design. Even Toshiba is sometimes guilty. Presumably electrostatic shields cost too much. The same voltages may destroy voltage sensitive components such as FET gates. I once long ago had an isolated printer with tingle problems blow up a PC printer port. The printer was not properly grounded by the manufacturer and floated at high impedance 110 VAC (230 VAC mains). Impedance was low enough to damage the PC port. Some manufacturers produce eg optoisolators that have specifications for isolation performance at rated dV/dT values as well as pure voltage isolation values. Russell McMahon