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Are AC adapters (e.g. laptop chargers) electrically isolated from ground because they have a transformer?

I'm asking this because I read that isolated transformers have a 1:1 ratio whereas AC adapters are commonly a step-down.

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You mix up "isolated transformer" with "isolation transformer". The latter is a transformer that has only one single goal and that is galvanic isolation from the mains supply, without changing the voltage. These transformers are often used for testing, so it is somewhat safer to accidentally touching one of the conductors.

All normal transformers, when properly connected, offer galvanic isolation, but beware that there do exist types (like autotransformers) that are internally wired in such a way that they don't. When unsure measure resistance between primary and secondary windings, this should be very high.

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  • \$\begingroup\$ It's also important to note that there's a distinction between transformers which will offer galvanic isolation for the voltage levels which are likely to appear on an AC line, versus those which will offer a "safety isolation" barrier. For example, a transformer might offer two 120V primary windings which can be wired in series or parallel (for 110/220V operation) but one could not safely connect one winding to 110V mains and expect the other to supply an isolated ~100V output. \$\endgroup\$
    – supercat
    Mar 19, 2013 at 16:11
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First off for AC there is really no such thing as perfect isolation. Capacitance is everywhere. That is why non-contact voltage detectors can work. However there is such a thing as sufficient isolation that it is safe to touch the output.

Broadly speaking mains power supplies can be divided into four categories.

  1. Not isolated at all. Such power supplies should only be used in situations where there is no chance of user contact with the output.
  2. "Class 2" isolated. There is no earth connection and isolation is provided between input and output. Common on very small power supplies, but the leakage through the RFI filter capacitors starts to become a problem as the power supply gets bigger. There are ways to reduce leakage while keeping acceptable EMC such as using split-bobbin transformers, but they come at a cost in efficiency.
  3. "Class 1" isolated. The output ground is DC isolated from all the input terminals, including ground but the mains ground is used as a barrier between input and output to redirect leakage away from the user. There is likely to be substantial capacitance between output and ground.
  4. "Class 1" ground-referenced. The input is isolated from the output (though the isolation may be quite leaky) and the output is tied to mains ground.

In my experience the first category while quite commonly seen inside equipment is very rare for separate power supplies, though I have seen reports of non-isolated power bricks being supplied with some dodgy "direct from China" stuff.

Phone chargers and similar low power wall-warts nearly always fall into the second category.

Laptop power bricks and similar-sized generic power bricks are normally in the second or third category, In my experience older laptops and generic replacement power bricks are more likely to be in the second category while the power bricks supplied with recent laptops (at least in the EU) seem to nearly always be in the third category.

Desktop PC power supplies are pretty much always in the fourth category.

Generic open-frame or semi-enclosed power supplies designed for use inside equipment are usually in the third category.

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Actually if you look at many of the switch mode ac adapters today, they have a capacitor from the secondary to the primary. This raises concerns. If you take a voltmeter and measure a voltage from the secondary to the mains you will get a 110 volt reading or so. Obviously if the capacitor fails, you have a much bigger problem.

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Normally they are however it depends on the circuitry inside the adapter.

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  • \$\begingroup\$ Welcome to EE.SE. Thank you for taking the step to answer a question. However, please take care to keep the quality up. In this case: Be more verbous and add examples. \$\endgroup\$
    – Ariser
    Apr 19, 2020 at 11:47
  • \$\begingroup\$ A warm welcome to the site. I think you're using 'Answer' on this site like 'Reply' is used on other discussion sites. Although this is a well-intended comment, it's not an answer and so will most likely be deleted. You can delete it yourself if you like. Thanks. \$\endgroup\$
    – TonyM
    Apr 19, 2020 at 13:08
  • \$\begingroup\$ Sorry that’s all I’m going to give you \$\endgroup\$
    – MR.TECH
    Apr 28, 2020 at 17:46
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Yes, they are isolated from the mains L and N, using a transformer (not necessarily Protective Earth though - PE, see below). The transformer can be any ratio and it will isolate as long as the windings are separated (i.e. it's not an autotransformer). You are probably thinking of specially purposed 1:1 isolation transformers which an EE may use when e.g. designing power supplies or working on high voltage equipment.

All modern consumer equipment should be isolated from the primary mains side (however as Chris notes, some old consumer equipment was not isolated and one should not assume anything until tested and confirmed), though a charger may be referenced to earth ground at it's output (my laptop adaptor is) An easy way to test this is to use a multimeter and see if there is any continuity between the adapter output and the plug earth pin (if it has one - if it doesn't it's completely isolated, or "floating")

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  • \$\begingroup\$ how can an adapter output be referenced to earth if its isolated ? \$\endgroup\$
    – subz
    Mar 19, 2013 at 10:51
  • \$\begingroup\$ It would be isolated from the input line and neutral but connected to the input earth. BEWARE THAT HISTORICALLY, SOME CONSUMER ELECTRONICS WERE NOT ISOLATED AND COULD HAVE A LIVE CHASSIS AT MAINS VOLTAGE though such practice is mostly identified with cost cutting in the tube era, one should never assume where mains connections are concerned. \$\endgroup\$ Mar 19, 2013 at 12:09
  • \$\begingroup\$ @Chris - Good point, I'll add this in. I was thinking modern equipment only but you're right, even then you should never assume anything before testing. \$\endgroup\$
    – Oli Glaser
    Mar 19, 2013 at 15:35
  • \$\begingroup\$ I was thinking of the old boat anchors when I wrote that, but here's an example of a 5v 50 ma un-isolated supply, published in 2012 supertex.com/pdf/app_notes/AN-D30.pdf \$\endgroup\$ Mar 19, 2013 at 15:41
  • \$\begingroup\$ It is relatively common for consumer electronic devices to forgo electrical isolation if there are no exposed connections to anything inside. Nowadays things like television sets have audio/video inputs that are referenced to their internal ground and they thus have to be isolated, but historically televisions would just have a capacitively-coupled or transformer-coupled antenna input and possibly a transformer-coupled headphone/speaker output. Most consumer electronics have some form of exposed connector, but in those that don't, hot chassis are common. \$\endgroup\$
    – supercat
    Mar 19, 2013 at 16:08
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All Transformers are essentially the electrically isolated devices. Since a transformer uses the magnetic field in its working. In AC adapter we are required to step down the value of voltages. Check this link from the Electrical glossary. Because the input is at higher voltages and we require smaller dc voltages, a transformer is added in the adapter.

Whereas in other cases sometimes we don't require a transformer for stepping down, we just need electrical isolation to prevent shocks, so Isolation transformers are built and designed for this purpose.

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  • \$\begingroup\$ No, all transformers are not isolated devices. Google autotransformer. Primary and secondary ground may be joined too. Moreover, the question is four years old. \$\endgroup\$
    – winny
    May 31, 2017 at 7:45

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