Between a mains step down transformer and a SMPS, which would be safer for a bench power supply? I cannot look inside the transformer in either case to judge the insulation quality. But, if the insulation does fail there would be 230VAC at the secondary for the mains transformer. And, maybe 325VDC at the output of the SMPS (is that possible)? I can visually inspect the SMPS PCB to check if there is proper clearance, presence of MOVs, thermistors, fuses etc, but of course cannot see inside the transformer.

So the question is, which is easier to make safer (or screw up)? Which is easier to figure out when new, if failure will be catastrophic in future? Can SMPS transformers fail due to failing mosfets etc? I'm guessing that if a mains transformer works without issues when new (including the problem of interwinding capacitance), I can expect it to work for at least a decade without issues, right? Assuming everything will be fused properly and fan cooled.

It's one thing to get a shock from inadvertently touching bad wiring, and quite another to be exposed while clutching the wires in my hands. So I am concerned somewhat about safety. Of course, an RCD/RCCB should probably be put in, but I don't have that yet.

[PS: I'm planning to put a buck converter and then a linear regulator to drop and adjust the final output voltage, be it the SMPS, or the mains transformer.]


2 Answers 2


If you are making a bench supply for conducting EE design work then you need to have "extra" protection because you may well be touching 0 volts or the "hot" DC output for significant periods of time. Fusing each leg of the DC output and using a zener diode to ground is something I would consider to be safe. When I say ground I mean a proper safety ground. Normally the zeners would not be called on to do anything but, if a transformer fault occured that generated a potential over-voltage situation, the fuses should blow. Zeners and fuses need to be rated accordingly.

For higher power protection, consider using a zener with a crow bar circuit like this: -

enter image description here

Here, the zener passes current to the SCR's gate when the output voltage of the regulator is exceeded. It turns on the SCR and it conducts current until the fuse blows. This is just one example of a crow bar protection circuit and please bear in mind that a lot of SMPS modules may have this feature already.

Other ideas along the same line

  • \$\begingroup\$ I added a diagram for the fuse. Is that what you mean? Zeners rated for 51V. I'm guessing they will need to conduct 8-10A for the 6A fuse to break. So they should survive the surge...? \$\endgroup\$
    – Indraneel
    Apr 12, 2018 at 13:34
  • \$\begingroup\$ Does such a zener exist? The highest I can see is 2.2A and it already costs a bunch! \$\endgroup\$
    – Indraneel
    Apr 12, 2018 at 13:48
  • \$\begingroup\$ In such a high power application, you can use the zeners in a crow-bar protection scheme. They don't need to be powerful but they do need to be rated at the right voltage. I'll add a small circuit. \$\endgroup\$
    – Andy aka
    Apr 12, 2018 at 13:54
  • \$\begingroup\$ Now I understand. But the -ve is not floating anymore. What did you mean by fusing each leg of the DC output? Is it possible to have a floating supply with crowbar to earth, from each leg of the DC? \$\endgroup\$
    – Indraneel
    Apr 12, 2018 at 14:18
  • \$\begingroup\$ Yes, a crow bar to earth from each leg will do the trick. They will sit there passively until called into action. There will be some leakage impedance to ground under normal running conditions, possibly around 1 mA but if that is too much then a lower power sense circuit and other means can be used. \$\endgroup\$
    – Andy aka
    Apr 12, 2018 at 14:22

Look for a CE marking, or its equivalent in your country. That indicates the transformer or supply has been designed to an appropriate safety standard. If it's marked, it should be OK, if not, then it won't, regardless of whether it's a transformer or SMPS.

What a mains-connected piece of equipment must do to gain such a marking is to have adequate isolation between input and output. This means the transformer must use insulation between the primary and secondary, and must have adequate clearance and creepage distances maintained for the terminals.

  • \$\begingroup\$ The equivalent in India is the ISI Mark en.wikipedia.org/wiki/ISI_mark . But it may be difficult to get hold of such a transformer, as they are usually marked only for manufacturers buying in bulk. It is easier to get a custom wound transformer. But, I have no idea what to ask for regarding insulation. As for SMPS, that are all imported from..... and who knows about the authenticity of the CE mark on it. \$\endgroup\$
    – Indraneel
    Apr 12, 2018 at 12:16
  • \$\begingroup\$ Creepage between transformer terminals is not an issue. They are either on opposite sides or wires extending out. But as for the insulation, is there anything I can specifically ask for, from the person winding the transformer? Or check with a multimeter? or check if it burns in a flame? \$\endgroup\$
    – Indraneel
    Apr 12, 2018 at 12:19
  • \$\begingroup\$ Put 1500V AC between a primary wire and a secondary wire for a few seconds. If it survives, it's OK to use it on mains. \$\endgroup\$
    – Neil_UK
    Apr 12, 2018 at 12:55
  • \$\begingroup\$ CE mark is self-certification and not a strict requirement, still no warranties properly speaking \$\endgroup\$
    – Andrés
    Apr 12, 2018 at 13:41
  • \$\begingroup\$ Hmm, I can see a potential use for a taser! Apply it on the insulation, and then the guy doing the winding will definitely do a great job! \$\endgroup\$
    – Indraneel
    Apr 12, 2018 at 13:51

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