i am trying to make a power supply for electrophoresis, which requires 100V DC. The project is a funded non-profit for some schools, where there are strict requirements when it comes to the individual price.
One way to make the power supply is as posted in this instructable, where they use a boost converter from a 15v DC supply: http://www.instructables.com/id/Gel-electrophoresis-power-supply/. I have found that this in components will cost about 10$ plus the power supply.
Another way, would be to use the mains directly with a bridge rectifier trough a series resistor to a zener diode with breakdown of 100 volt. I know that it should supply between 200mA and 350mA of current when it is connected.
The top circuit is the one i have tried and the only problem is that the resistor has to dissipate quite a lot of heat. But is has been tested to work.
The bottom circuit is one i found somewhere, which has next to no power dissipation, except for in the load.
The question stands, is there a better way to do it or is the proposed circuit safe. And will the alternative circuit work when you put 230V RMS on it? The circuit was found for lower voltages around 12V, so can i find a transistor that can handle my specs? The benefit of the zener versions is that they can be connected directly to the mains and doesn't need a separate dc power supply, which saves money.