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“Double Insulated” SMPS!

By on August 5, 2013
double insulated smps









We have seen many reports of people experiencing small electrical shocks while handling peripheral equipment such as WiFi routers, modems, CD players etc… This problem usually comes from unearthed power adapters, principally Switch Mode Power Supplies (SMPS).

SMPS have many advantages compared to the transformer based DC power supplies. They are smaller, lighter, and generate much less heat than conventional power supplies. They can also give very accurate control of output voltages. Also they are cheap! But they do not provide the insulation like a transformer based power supply does!

Many SMPS, like the ones used for desktop computers, laptops and many other appliances have a 3 pins power plug. Inside the SMPS the earth conductor is connected to the –VDC output (cold ground) either directly or through some filtering circuitry. Those ones will not cause any electrical hazard. However, there are also many SMPS out there that are fitted with a two pin power plug (sometimes the third pin (earth) is made of plastic!). It means that the output is supposed to be floating (no or little potential to the earth). Unfortunately this is not true; there is a potential difference between the output and the electrical earth that, if not deadly, can create a number of inconveniences.


For example, this is a two pin power adapter for a Wi-Fi router. The test screwdriver shows that there is enough voltage at the DC plug for the little neon light to glow. Connecting an AC voltmeter between this point and the electrical earth will show a voltage of around 80 VAC. This is above what is considered as a safe voltage in many countries (Usually 50 V) and enough to cause a small electric shock. The current that this unexpected source can produce is low as the internal resistance is high. We can measure it by connecting an AC miliampere meter between those two grounds. The value measured was 0.15 mA. Not enough to kill someone (in normal conditions) but it could damage sensitive electronic components. I tested several different SMPS adapters, all producing similar results.


The question is why is it like that? In a typical SMPS the primary (high voltage) and secondary (low voltage) are insulated through a step down transformer. The feedback from the secondary to the primary is insulated through an opto-coupler. There will always be unwanted capacitive and inductive coupling between primary and secondary, but not of this magnitude. So where is the culprit?


To investigate further, let’s open the enclosure of one of the SMPS I measured. The one I choose is one that is rated double insulated. A Class II or double insulated electrical appliance is one which has been designed in such a way that it does not require a safety connection to electrical earth (ground). The basic requirement is that no single failure can result in dangerous voltage becoming exposed so that it might cause an electric shock and that this is achieved without relying on an earthed metal casing…


Looking at the PCB you can see that the primary and secondary are clearly separated. It has a transformer, an opto-coupler and the gap in the PCB, standard to all the SMPS. However, one extra component is connected between the primary and the secondary circuitry, across the insulation area. This is the famous (blue) Y capacitor. In this particular SMPS the Y capacitor’s value is 3,300 pF… It is connected between the hot ground and the cold ground of the SMPS. The hot ground (primary) voltage usually stands half way between Life and Neutral, i.e 120 V AC.  The reactance (AC resistance) of a 3,300 pF capacitor at 50 Hz is 965 kΩ. So the cold ground of our SMPS is like a voltage source of 120 VAC with an internal resistance of 965 kΩ. The short circuit current would be 0.12 mA. Close enough from what we measured.

The “Y capacitors”, are a special type of capacitor, tested and certified by safety agencies. Usually of ceramic type, they are self-healing and their failure mode is open circuit instead of short circuit as most other types of capacitors would be. Y capacitors are used in SMPS to provide a return path (basically a short circuit) for the high frequency noise that is transferred, mainly by capacitive coupling, between primary and secondary. Without the Y capacitor the SMPS would probably not pass the EMC (Electromagnetic Compatibility) compliance tests.

The two pictures below show the voltage between DC output and electrical earth with the capacitor connected and after disconnecting the capacitor.



With the capacitor connected we have a strong 50 Hz signal (somehow distorted) of over 300 V peak to peak. Disconnect the capacitor and this signal decreases significantly. Further measurements will show that the short circuit current dropped down to a few micro Amperes and the test screwdriver lamp doesn’t glow anymore. This looks much safer! However, we can see a high frequency signal that was not there before. This signal would be seen as conductive EMI (Electromagnetic Interferences) and would fail the SMPS during the EMC compliance tests. That’s what the Y capacitor is here for, to short cut this signal.

So the Y capacitor solves one problem by creating another one. As mentioned at the beginning of this article, the electric shock might not arm you but can damage sensitive components. When you connect a device powered with one of those SMPS adapters into earthed equipment you might see a small spark; really scary if the earthed equipment is your laptop and the plug you connect to is a USB port! Some manufacturers put a warning on their equipment to say it will be damaged or damage other equipment unless it is all wired up before it is plugged into the mains. This is certainly a good advice but it would be a better idea to have all the SMPS adapters earthed through a 3 pin mains connector. Obviously this is not in the design specifications of most of the manufacturers!





Gerald Musy

Penang Malaysia


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One Comment

  1. Alan

    September 23, 2019 at 5:35 pm

    fantastic article. I have just experienced a mild shock from a Switched mode PSU to earth. Like tingling, almost not sure if it's even a shock. It has left me sore though with lots of nerve tingles. I have now replaced all my SMPS with linear transformer based supplies. I measured 80VAC ilk you and was surprised that this is common place.


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