Variable Voltage Power Supply Schematic?

[ragtime8922]

I need a high voltage variable DC bench supply and I can't find a schematic anywhere. I'm using it for tube projects so I'll need a high voltage section somewhere in the range of 0-350volts or even 0-250volts and a higher amperage low-volatge filament section that is variable from 0-12v. (I can just add another DC supply like I use for my effects projects for this section).

I saw Steve Bench's site but I know there are far less complicated ways to do this.

[Paul Perry (Frostwave)]

Best & simplest: a normal power supply fed by a variac (variable transformer).
There used to be a (VERY heavy) HP boatanchor power supply that did this. Just be careful with the variac, you might need an isolating transformer as well. But if you just have a normal power supply (transformer & rectifier & filter) and put a variac in front of it, then that will work. The surplus sites sometimes have nice little variacs, you prob don't need a huge one.

[ragtime8922]

Best & simplest: a normal power supply fed by a variac (variable transformer).
There used to be a (VERY heavy) HP boatanchor power supply that did this. Just be careful with the variac, you might need an isolating transformer as well. But if you just have a normal power supply (transformer & rectifier & filter) and put a variac in front of it, then that will work. The surplus sites sometimes have nice little variacs, you prob don't need a huge one.

Jameco actually has one pretty cheap and I think it said it had it's own isolation but I'm not sure. Anyway, wouldn't the power supply that you are connecting it to be it's own isolation transformer? variac tranny to power supply tranny.

Anyway Paul, thank you very much! I thought a variac would work just fine but I had doubts when the only high voltage variable supplys were priced at $1200-$1500. I thought that there just must be much more to it but I should have just trusted myself. thanks!!!

[Paul Perry (Frostwave)]

When you pay $1,000 + for a high voltage power supply, it is usually stable to better than 1% and has a very low output impedance etc.
but, if you aren't running photomultipliers in a physics lab or something like that... a variac approach is just fine!

[zachary vex]

i bought a surplus regulated HP high-voltage supply and there are other plusses... i can limit the current with a knob to protect my circuit in case of some small catastrophe.  but a variac feeding a linear supply sounds pretty reasonable.  just keep a chemical fire extinguisher handy.  8^)

[ragtime8922]

I've never been to a "surplus site". I will probably build a variac based supply but these surplus sites sound like they'd be very interesting. Any links or names of the better sites?

Also, thanks for the input.

[Ge_Whiz]

One important thing to bear in mind on a project like this is that valves are reasonably tolerant of variation in HT voltage, but much less tolerant of accidental heater overvoltage - *POP* - so make sure that your heater voltage is stable if you intend to make it variable for different valves types. Better still, make it switchable - e.g. 1.5V, 6.3V, 12V etc.

[ragtime8922]

One important thing to bear in mind on a project like this is that valves are reasonably tolerant of variation in HT voltage, but much less tolerant of accidental heater overvoltage - *POP* - so make sure that your heater voltage is stable if you intend to make it variable for different valves types. Better still, make it switchable - e.g. 1.5V, 6.3V, 12V etc.

That's exactly what I did with my fx supply. There are only two switch positions on that one though. It's got a +9volt position and a variable position that engages the pot. For the filament supply I'll probably use a rotary switch for values of 1.5, 5, 6.3, 12 and variable. The variable will be a "just incase" precaution. As you know, the diyer always seem to come across another application for a power supply.