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I'm trying to find the disadvantages of using magnetic couplers versus opto couplers (1). I'm deciding which of the two I should use to create a galvanic isolation between two PCB's that are going to communicate via SPI in a normal environment e.g house living room.

(1) http://www.wisegeek.com/what-is-an-opto-isolator.htm

I have found several websites and documents that show the advantages of magnetic couplers vs opto couplers. Here is one that summarizes most advantages(2)

(2) http://www.nve.com/isopdf/WP1201.pdf (Page 3).

But I'm having a hard time believing that there is not a single disadvantage to using magnetic couplers versus opto couplers for galvanic isolation between two PCB's.

So my question is: Are there any disadvantages using magnetic couplers instead of opto couplers?

Ricardo
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Nick
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3 Answers3

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The magnetic ones (and some other non-optical types) transfer only changes to the state. Although that page you linked says that they work to DC, that's only because there is a latch on the other side that maintains the last known state. Consider the NVE IL710

enter image description here

In some cases, this could be a deal killer.

Secondly, another disadvantage is cost. You'll never see a magnetic type in a cheap SMPS controller because they're more expensive, even if the other quirk didn't exist.

Thirdly, if you're designing things that need to live near high currents or other things that cause large magnetic fields (not uncommon in applications that require isolation), you'd better check out the magnetic field immunity:

enter image description here

Spehro Pefhany
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  • Thank you for your reply! I completely missed the latch. This could be indeed be a deal killer, I'm going take the points you noted with me while I'm further investigating my project. I can not vote up your answer yet reputation but again your answer is much appreciated. – Nick Mar 31 '14 at 14:14
  • @Nick Welcome to the site. You should have the ability to vote now. – Patrick M Mar 31 '14 at 21:21
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    The third point has real significance. Many magnetic couplers will not work if you place them next to a relay coil - and I'm not talking about big relays either. A PCB-mounted low-profile safety relay and many NVE isolators simply won't work if the isolator chip is next to the coil. If your relay is only occasionally energized, it might take a while to realize what's going on. BTDT, had to flip the relays on the board so that the coil wouldn't be next to the isolator chip. – Kuba hasn't forgotten Monica Mar 31 '14 at 21:22
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    @KubaOber Good tip.. those are the worst kind of problems to diagnose- intermittent and/or pattern dependent. – Spehro Pefhany Mar 31 '14 at 21:41
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    @PatrickM i did vote up Spehro Pefhany's answer now. – Nick Apr 01 '14 at 06:44
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Disadvantages of magnetic signal coupling versus optical signal coupling:

  1. Magnetics are usually bulkier and more expensive.

  2. Magnetics don't pass DC. If you need to pass a low frequency signal, then you have to modulate it onto a carrier somehow.

Olin Lathrop
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Generally speaking, it's a lot easier to provide a very high level of optical shielding to an opto coupler than it is to provide a very high level of magnetic shielding to a magnetic coupler. Optocouplers can be made very small and in huge quantities with modern semiconductor processing.

Magnetic couplers can transfer much more power given specific space and cost constraints.

So for small signals, optically coupled circuits can be smaller, cheaper, and more immune to the environment.

For power transfer, magnetic couplers of a given efficiency are generally less expensive.

Adam Davis
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