Electronic – Safe 0–300 VDC variable supply

power supplytransformervariable

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?

Best Answer

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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