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I want to make a 0–300V variable DC power supply (max current 100 mA). I found this design,

variable supply w/ 1:1 120 VAC transformer

which features a 1:1 transformer across the mains for some sort of safety reasons. However, I couldn't find any 1:1 120 VAC transformers that weren't incredibly expensive. Next I looked at this circuit,

variable supply without a transformer

which uses a light-bulb "in order to provide ultimate safety in case of a short circuit or a component failure."

I'm not myself able to ascertain if the correct design precautions are being observed in either of these circuits. The second claims to be lethal to the touch.

My question is: does this second circuit exhibit any obvious safety-related flaws in its design (besides, perhaps, contact lethality)? If so, what is the cheapest method of achieving a 0–300V variable power supply while observing all relevant safety considerations, and will any DIY implementation be, to some extent, inherently dangerous?

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    \$\begingroup\$ What part of "lethal to the touch" doesn't sound safety-related to you? \$\endgroup\$ – The Photon Apr 23 '17 at 2:03
  • \$\begingroup\$ @ThePhoton how would this be rectified? \$\endgroup\$ – Jack Lynch Apr 23 '17 at 2:04
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    \$\begingroup\$ Use a transformer, like in the first circuit. But there's still a risk if the user touches both terminals at the same time. 300 V is inherently risky. \$\endgroup\$ – The Photon Apr 23 '17 at 2:05
  • \$\begingroup\$ @ThePhoton okay, awesome. Are 1:1 120VAC transformers like the one required by that circuit just unavoidably expensive? I guess store-bought high-voltage variable supplies are expensive for a reason... \$\endgroup\$ – Jack Lynch Apr 23 '17 at 2:07
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    \$\begingroup\$ I think it's a worthwhile question for some future reader. Go ahead and post an answer. \$\endgroup\$ – The Photon Apr 23 '17 at 2:47
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Collected Information from the Comments:

  1. The lack of a transformer in the second circuit is prohibitively unsafe, so I'm going with the first schematic.
  2. 1:1 transformers like the one used in the first circuit are, in fact, relatively cheap—e.g., one candidate for $14 and another for $18.
  3. The input voltage for the first circuit is actually 220 VAC—an oversight on my part. However, I can link the output windings of the above transformers in series to achieve my desired output voltage.
  4. However, the voltage and current maximums achieved by the circuit are, in fact, just "ball-parks". Since the circuit deals above the benchmark for lethal current, a variable maximum is far from ideal.
  5. The lack of a fuse on the primary side would render the first circuit illegal in many places.
  6. A 300V 100mA supply is potentially lethal, so don't touch both terminals at the same time. Circuits with dangerously high output voltages shouldn't be built by people who don't know what they're doing (or have only a loose grasp).
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  • \$\begingroup\$ The transformers in your DigiKey links can have their output windings connected in series for your desired output voltage. \$\endgroup\$ – replete Apr 23 '17 at 2:59
  • \$\begingroup\$ @replete awesome, thanks! I added that to the answer \$\endgroup\$ – Jack Lynch Apr 23 '17 at 3:03
  • \$\begingroup\$ Are you following the first schematic, then? It is of course far from a complete design. \$\endgroup\$ – replete Apr 23 '17 at 3:13
  • \$\begingroup\$ @replete yeah, probably. What would be most important to add to it? \$\endgroup\$ – Jack Lynch Apr 23 '17 at 3:20
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    \$\begingroup\$ That is a significant question in itself and comments won't do. I suggest you ask a separate question based on the first schematic specifically about accurate current limiting. Decide on a margin you find acceptable (10mA either way? 5mA? 20mA?) \$\endgroup\$ – replete Apr 23 '17 at 3:35
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300V power supply can push current thru the skin resistance of human fingers thus not really a safe thing in any sense especially when someone accidentally touched both output terminals of this power supply.

To answer your question about 1:1 isolation transformer, if you cannot get one, you can do it by yourself.

For such low current (100mA) output, the easiest way to do this is by connecting two step down transformers back to back. Example, get two 240V to 12V unit with secondary current ratings of 2Amps. Connect the low voltage sides of the transformers so that the second transformer will essentially works as the step up transformer.

Now you should have 240V at the output of the second transformer which is actually isolated from the mains line voltage by in excess of 1000V breakdown ratings.

If the "safe" thing that you mention in your question is about this isolation, than I think it cannot get any "safer" than this..

Good luck.

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