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I have a power supply design that makes use of a half-wave rectifier and a non-isolated buck Swtich Mode Power Supply IC to step down 120V 60Hz AC power to 12V DC.

I then use a Linear Voltage Regulator (78L05) to step the 12V down to 5V. The 5V will be used to control a micro-controller in a prototype, so there is a risk of people touching the pins.

In this design the Neutral line of the AC supply is connected to the 12V DC after the buck converter.

I realize that this could be dangerous since the 5V supply to the micro-controller is not isolated from the AC supply, so my question is:

Will the use of an isolation transformer just after the AC supply isolate the rest of the circuit from the supply. Will this make the design safer and/or less efficient?

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    \$\begingroup\$ Is it a buck regulator, or a Switch Mode Power Supply IC? The two are different, and if you have a SMPS IC then you may already have the isolation you require. I have to ask: why not use a 5V wall-wart power supply? \$\endgroup\$
    – Steve G
    Commented Jul 21, 2016 at 15:05
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    \$\begingroup\$ Why not just use and isolated flyback converter instead of a non-isolated converter? Doing the isolation inside the converter means you can get away with a smaller transformer. \$\endgroup\$
    – alex.forencich
    Commented Jul 21, 2016 at 15:13
  • \$\begingroup\$ @SteveG I believe it is a buck regulator. I cannot use a wall-wart due to size constraints. Also I need to switch on a relay that will allow the AC current to flow through a heater that is part of the system. \$\endgroup\$
    – user41391
    Commented Jul 21, 2016 at 15:19
  • \$\begingroup\$ @alex.forencich Thanks for the suggestion, I will look into it! \$\endgroup\$
    – user41391
    Commented Jul 21, 2016 at 15:19

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Will the use of an isolation transformer just after the AC supply isolate the rest of the circuit from the supply. Will this make the design safer

If you have a GFCI (used in the US) or RCD (used in the UK) upstream of your isolation transformer then you have lost the protection that these devices can offer. These devices work by sensing an imbalance in live and neutral wires with the assumption being that any current flowing in live that isn't flowing in neutral must be flowing to earth (possibly through some bodily contact).

For instance, if you touched "live" it would trip said device and protect you. About 30 mA is the current that might flow into you before the device trips. Still enough to give a big jolt but after a few tens of milli seconds the power would trip. If you touched neutral you probably wouldn't notice any sensation at all and the device wouldn't trip. If you touched both live and neutral together, there would be a big earth current imbalance (flowing mainly from live as before) and you would be safe after a few tens of milli seconds.

The output of the isolation transformer is different. If you touched either wire individually you might feel a slight tingle due to leakage capacitance to earth. Nothing there to worry about except that if you touched both isolated live and isolated neutral you would fry. The GFCI (or RCD) upstream of the transformer has no knowledge that you are frying.

Transformers are less efficient to answer your other question.

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  • \$\begingroup\$ Thanks. So assuming there is a GFCI upstream, then an isolation transformer will actually make the device less safe. Judging from other comments, it seems like an isolated flyback converter is the way to go. \$\endgroup\$
    – user41391
    Commented Jul 21, 2016 at 15:24
  • \$\begingroup\$ I am not sure if follow-up questions are allowed in the comments on this forum, but since only Neutral is connected to the 12V after the buck converter (which is not isolated), would it be safe to touch the DC 5V and ground wires going to the micro-controller? \$\endgroup\$
    – user41391
    Commented Jul 21, 2016 at 15:32
  • \$\begingroup\$ What if live and neutral gave been swapped in the wall socket mistakenly? What if the socket switch is switching N instead of L. What if anywhere from your buikding'a incoming power neutral is lost or swapped? \$\endgroup\$
    – Andy aka
    Commented Jul 21, 2016 at 16:59
  • \$\begingroup\$ @Coffee: See my answer to wiring earth to neutral in a 2-pin plug. \$\endgroup\$
    – Transistor
    Commented Jul 21, 2016 at 17:46
  • \$\begingroup\$ @transistor hey stop hijacking my answers LOL. \$\endgroup\$
    – Andy aka
    Commented Jul 21, 2016 at 17:47

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