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Is there some simple technique to efficiently produce 120 VDC from a mains 240 VAC supply? All I need is < 50 mA. Can something be done with just rectification and capacitors for example? Large dropper resistor? I want to run a few neon bulbs in DC mode.

EDIT:-

I only actually need one neon to run, but can have others if it makes the cicuit simpler vis a vis total voltage drop. I think a single neon bulb draws only about 1mA.

EDIT2:-

Appologies all, but thanks for feedback. I think that this question is very badly put. It's not the power supply per se that I want, but the neon bulb to run at DC current. Should I ask another, better question?

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    \$\begingroup\$ Exactly how many neons? And do you want them all to be on at the same time? \$\endgroup\$ – WhatRoughBeast Sep 20 '15 at 2:59
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    \$\begingroup\$ How much current will each bulb be drawing? \$\endgroup\$ – Bruce Abbott Sep 20 '15 at 3:07
  • \$\begingroup\$ Probably the easiest way is to find a small mains power transformer that has two separate primary windings for 110Vand can easily cope with the intended maximum current. Connect them in series and you can tap a reasonable stable half power supply voltage from it. The advantage is that you may just have one lying around. You really do want to double check voltages and currents to ensure the transformer and your circuit can cope. When connecting multiple bulbs, you can even consider making two symmetric circuits that somewhat balance the load across both windings. \$\endgroup\$ – jippie Sep 20 '15 at 11:56
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    \$\begingroup\$ Neon bulbs look like short circuits once started as plasma is a very good conductor, so they don't have a well defined voltage drop like LEDs. The important thing is to limit the current, assuming you have an external current limiting resistor, double its value to keep the same current with twice the voltage. This is assuming a bare neon bulb with a simple resistor for current limiting. \$\endgroup\$ – John Meacham Sep 21 '15 at 12:40
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Here's a simple way to get regulated 120 volts, DC from 240 volt mains, but DANGER, WILL ROBINSON!!! it's not isolated from the mains, so if you get 240 hot and 240 neutral connected backwards it could seriously fry some equipment or make you dead.

If you want to play with the circuit, here's the LTspice .asc file you can use to run a simulation.

enter image description here

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    \$\begingroup\$ I would also add a fuse \$\endgroup\$ – hassan789 Sep 21 '15 at 0:40
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You can use the standard linear way if you want something simple:

schematic

simulate this circuit – Schematic created using CircuitLab

Or you can build/buy a switcher if efficiency is more important.

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    \$\begingroup\$ To get 120 VDC at low current from this kind of setup, the transformer ratio would have to be more like 2.828:1, giving about 85 Vrms on the secondary, which has a peak value of 120 V. \$\endgroup\$ – Dave Tweed Sep 20 '15 at 11:12
  • \$\begingroup\$ Good catch, I frequently find I need to divide/multiply by a factor of pi, sqrt(2), -1, etc until things work :) \$\endgroup\$ – Houston Fortney Sep 20 '15 at 13:04
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You could do what some mains powered LED's do, use the reactance of a capacitor.

schematic

simulate this circuit – Schematic created using CircuitLab

This is essentially tuned to the load equivalent impedance via the input R's and AC side capacitor. Supply perturbations are mitigated by the DC side smoothing cap. Prolonged changes to the AC (freq, voltage) has the capability to change the output voltage.

Likewise a change in the load equivalent impedance will change the output voltage.

Losses in this topology are small. An additional DC- FET could be added to provide some controlled variability if required

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  • \$\begingroup\$ How does this provide 120 VDC with a varying load? \$\endgroup\$ – Dave Tweed Sep 20 '15 at 11:07
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    \$\begingroup\$ It won't, but that wasn't asked... \$\endgroup\$ – JonRB Sep 20 '15 at 11:26
  • \$\begingroup\$ I don't know what question you think you're answering. THIS one is asking for 120 VDC at 0 - 50 mA. \$\endgroup\$ – Dave Tweed Sep 20 '15 at 11:34
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    \$\begingroup\$ he doesn't actually say 0-50mA... he say "all i need is <50mA". no where does he say a variable load \$\endgroup\$ – JonRB Sep 20 '15 at 11:39
  • \$\begingroup\$ Regardless of what is being asked, I think describing the behavior of the circuit under less/no load would make a better answer. \$\endgroup\$ – Kevin Reid Sep 20 '15 at 13:27
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When you want so small current, and exact voltage is not critical for you, I suggest to avoid Transformers, instead first use a full bridge rectifier as this : enter image description here

Then in the output use a simple resistance voltage divider to achieve your desired voltage.

Note : If you consume small currents this way would be sufficient, Otherwise you should use a 2:1 transformer and a full bridge with a capacitor.

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    \$\begingroup\$ A resistive voltage divider would have to waste at least 11W of power in order to deliver 6W -- more if you wanted to accomodate a varying load. \$\endgroup\$ – Dave Tweed Sep 20 '15 at 11:10
  • \$\begingroup\$ As I mentioned it's only a good method if he want small amount of energy. As you say 6W is not a small Value! Then it's what I've been said in my answer and resistor voltage divider isn't appropriate. \$\endgroup\$ – HOPE Sep 20 '15 at 13:13
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The above two answers give you a simple schematic to obtain DC from AC. Here is a little modification to the above schematic in order to obtain a stable DC voltage at the output. There is just the addition of a voltage regulator and one more capacitor -

schematic

simulate this circuit – Schematic created using CircuitLab

The transformer (you can ignore the name) will be used to step down the AC voltage. The diode bridge is the full rectifier and the capacitor C1 (usually around 100U) gives a smooth DC output. In order to reduce the variations in the output we use a voltage regulator. The voltage regulator will give the exact DC output. For 12V output you should use 7812 Voltage Regulator IC. You can tap the output across C2.

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    \$\begingroup\$ The op asked for 120VDC, not 12VDC. Can you add a part number for an 120V regulator alternative? \$\endgroup\$ – jippie Sep 20 '15 at 7:03
  • \$\begingroup\$ TL783 works up to 125V DC output (actually in-out difference)... but it might be a bit risky given possible input variations. \$\endgroup\$ – Fizz Sep 24 '15 at 19:12
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A neon bulb will run on AC or DC. They often have internal current limiting resistors. If not, you can use a current limiting capacitor.

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