Okay I need to understand this concept because I'm playing with mains and I do not want to fry myself... A little background : I'm quite good with low voltage stuff as well as logic level design but I've a big lack on switch mode power supplies . Due to a project requirement to implement a custom smps I've developed an unisolated one through the help of an engineer that works quite well (5v 0.4 amp).. leaving away the discussion about the unisolated power supply that goes into an abs encapsulated box with no end user interaction so it's perfectly safe and legally fine , I run into a problem ... as soon as I've connected the entire device to a serial converter , both burned out ... I've asked to my engineer what happened and he told me to run the device through an isolated 1:1 transformed to bring down the ground to earth ... we'll I'm actually a little lost , I understand that the neutral or hot wires becames the reference ground in my smps and that connecting them to a different ground that's on a different potential level may cause what I actually experienced , but I don't understand why my usb cable has earthed ground and why an isolation transformed changes the potential level on my input ac... I hope to have explained that in the right way !! Thanks !

  • \$\begingroup\$ Your PC has its low voltage GND tied to the frame. The frame is connected to PE and that one is connected to N at the station transformer. That's it. \$\endgroup\$
    – Janka
    Sep 12, 2017 at 19:16
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    \$\begingroup\$ Stop what you are doing. DC power supplies MUST be isolated from AC mains. Period. You might kill yourself, or, worse, somebody else who might be unaware of your creativity. And destroy equipment. And fire/report that engineer who helped you. \$\endgroup\$ Sep 12, 2017 at 19:21
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    \$\begingroup\$ The whole point of an unisolated power supply is that it must only be used for things that are totally enclosed in an insulated box (for instance an LED night light). The moment you connected your bit of equipment to anything else, you broke that requirement. \$\endgroup\$
    – Simon B
    Sep 12, 2017 at 21:33
  • \$\begingroup\$ If you plan to connect your serial interface in future I would seriously recommend implementing an isolated (optical usually) interface if you are not prepared to make the PSU isolated. If you look at what is in a generic cellphone charger you will see very little but it is enough to be isolated. What you have described is a bootstrap supply and this should never have a connection to the outside world. \$\endgroup\$
    – KalleMP
    Sep 12, 2017 at 21:48

1 Answer 1


If it's a desktop PC then the metal case, all the shields of all the cables, and the internal GND, are all tied to protection earth.

So USB GND is tied to Earth.

The other side of your USB-Serial converter has GND tied to mains.

Thus you shorted Live to Protection Earth and exploded whatever bits of electronics were in the way... I hope the lights were out in the lab after this. If you still had power, this means you have no RCD so you are at risk.

Now, if you had used a laptop running on batteries, then nothing would have blown, since the laptop is floating. However... every bit of metal in and on that laptop would have been at live mains potential.

Also everything that is connected to it via USB.

This includes lots of stuff you're supposed to put your fingers on: mouse, keyboard, that shiny aluminium case... so many ways to kill yourself!...

The insulation transformer turns your uninsulated SMPS into an isolated one, so it could work. I have no idea why you chose to make an uninsulated supply, maybe to save a few cents?...

Note: put the isolation transformer on the DUT, not on the PC! If you supply the PC from an insulation transformer, then you'll be in the "laptop" case, and you're gonna fry.

If you insist on the uninsulated SMPS, a much better solution to talk to your micro is to simply isolate the USB-serial interface with optocouplers. Then, there s no need for an isolation transformer. Serial is easy to isolate. Also this will save your PC USB port from unhealthy voltages.

But of course, no fingers shall go near the live device...

  • \$\begingroup\$ I was in need of a very miniaturized smps and was pointed versus a reference design of unisolated one for space reasons .. the engineer is from a very big world company so actually I fell that he must have misunderstood my knowledges , I can't believe that they suggest those solutions if they are widely used .. I won't make the name of the company but it's a very big one... \$\endgroup\$ Sep 12, 2017 at 19:47
  • \$\begingroup\$ Well if the device is inside a plastic box and no-one can put a finger in the wrong place then an uninsulated supply is OK, but it's really annoying for debugging the circuit... You should opto-isolate the serial connection, but if you want to probe the thing with a scope, you'll need to use an insulation transformer on the device, because the scope is also earthed. Don't blow your scope! \$\endgroup\$
    – bobflux
    Sep 12, 2017 at 19:52
  • \$\begingroup\$ There is no reason you can not design a small enough isolated supply... for that power level, it should be the same size as the unisolated one... \$\endgroup\$
    – MadHatter
    Sep 12, 2017 at 20:07
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    \$\begingroup\$ well honestly i do not know where to start because I do not have found any reference design on design tool to start with an isolated one. Do you have any suggestion ? Also I still do not understand how an 1:1 isolated transformer takes my potential to ground level...does the 1:1 transformer secondary is earth grounded ? \$\endgroup\$ Sep 12, 2017 at 20:12
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    \$\begingroup\$ The secondary is isolated from mains and floating. When you connect the USb cable to an earthed PC, this sets the secondary potential to earth level. The important thing is that the isolation xfmr isolates and prevents current from flowing from live mains into the earthed PC ground... \$\endgroup\$
    – bobflux
    Sep 12, 2017 at 20:15

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