Scope – Power Supply

OK, I’m going to say it again.  This project involves very high voltage power supplies.  The +150 and -1200 Volt supplies and/or the energy stored in their capacitors can take your life in seconds.  Don’t even THINK about duplicating this project unless you are trained, experienced and comfortable with working on high voltage circuitry.

High voltage power transformers of the type that were used for vacuum tube circuits are now expensive and hard to find.  For this project I wanted to find transformers that are still manufactured in large quantities and are low in cost.  Hammond makes such a line and the 229 series units designed for mounting on PC boards are particularly cost effective.  T1, the main power supply transformer, has two 120Volt primaries.  These are normally wired in series for 240Volt operation or in parallel for 120 Volt operation.

Here we use just one of the primaries and use the other one as a secondary for the +150 Volt supply.  A bridge rectifier and two section filter complete the 150V supply.  Another feature of the Hammond transformers is a 1700 Volt insulation rating on each winding.  The filament of the CRT must be close in potential to the cathode so both sides of the filament are about 1000 Volts below ground potential.  The high voltage rating of the Hammond transformers allows us to use one of the 6.3V secondaries to power the CRT filament and the other one to power the low voltage circuits operating near ground.  Ideally the secondary driving the low voltage supplies would be 10V or so, but because the CRT filament requires 6.3V, we are stuck with that value for the other secondary as well.  We use two half wave voltage doublers.  One produces about +14V for the positive supplies and the other -14V for the negative supplies.  78xx and 79xx series three terminal regulators are used to provide the +9V, +5V, -9V and -5V regulated supplies.

High Voltage Supply:
The highest voltage transformer in the Hammond 229 series has two 120V primaries and two 120V secondaries.  The primaries are wired in parallel and the secondaries are wired in series to produce 240VAC.  This output is connected to two half wave doublers just as in the low voltage supplies.  Each produce about 600VDC and the most positive point is connected to ground.  This results in an output of -1200VDC.  All with a $13 transformer!  I used a “stack” of 200V zeners to provide a regulated -1000V output.  1N5388 200V 5W zeners are available from Arrow Electronics for $0.12 each, so this makes for a very low cost regulated HV supply.

Back to Scope

Comments are closed.