High Voltage Power Supply
Modern power supplies often employ switch mode topographies. Of course, this reduces the size and cost of many components and allows for good voltage and current control. On the other hand, this project is a laser project, not a power supply project, so getting a 20kVDC, 2kW power supply working as cheaply and quickly as possible is the goal.
The internet is full of high voltage adventurers and their spark-throwing creations. From exploding watermelons to railguns, tesla coils to coin crushers, it seems that there are a few people out there that need high voltage, high current, or both -- and lots of it.
This ready supply of components and simplicity of design led me to a big iron linear power supply.
Linear power supplies are pretty basic. (1) Transform mains voltage to the desired voltage; (2) Rectify the output; and (3) Filter the output.
The internet is full of high voltage adventurers and their spark-throwing creations. From exploding watermelons to railguns, tesla coils to coin crushers, it seems that there are a few people out there that need high voltage, high current, or both -- and lots of it.
This ready supply of components and simplicity of design led me to a big iron linear power supply.
Linear power supplies are pretty basic. (1) Transform mains voltage to the desired voltage; (2) Rectify the output; and (3) Filter the output.
Variac (or Variable Auto-transformer)
I tip my hat to the inventor, long ago, of this handy device.
Variac was a brand name for a variable autotransformer produced by General Radio until 2002, when the name Variac was purchased by another company with a less auspicious sounding name. The name Variac sticks to these like Kleenex to facial tissue.
Variac was a brand name for a variable autotransformer produced by General Radio until 2002, when the name Variac was purchased by another company with a less auspicious sounding name. The name Variac sticks to these like Kleenex to facial tissue.
I purchased a new (old stock) panel mount Staco 2510. This model can handle 25A continuous current and weighs more than 20 pounds. It came with the original knob and dial for that professional look. I intend to mount it on a rack mount panel, along with the HV PSU controls and meters.
The input and output share the same winding, thus the Variac is not an isolation transformer. It merely allows the user to select a voltage between 0 and 100%, or a little more, of the input voltage.
The input and output share the same winding, thus the Variac is not an isolation transformer. It merely allows the user to select a voltage between 0 and 100%, or a little more, of the input voltage.
High Voltage Transformer
Thanks to an avid Tesla coiler, I was able to buy this 120:1 PTW-5 potential transformer. It’s capable of continuous operation at 1.5kVA and 15.5kVrms. Since the HV connections are on the top and the whole transformer is encased in epoxy, it’s fairly easy to handle and mount, except that it weighs over 80 pounds. The 4 rectifier blocks fit neatly right on top.
Bridge Rectifier
A full-wave bridge rectifier makes an AC input into DC, typically by inverting the negative component into a positive one (or vv).
I once obsessed about maximum average forward current and peak reverse voltage. I should have been a little more concerned about mechanical construction. Ultimately, I ended up with 4 of those decent quality Chinese rectifier blocks (40kV, 1A) that can be found on eBay.
They are pretty small and fully insulated in epoxy/potting stuff.
I once obsessed about maximum average forward current and peak reverse voltage. I should have been a little more concerned about mechanical construction. Ultimately, I ended up with 4 of those decent quality Chinese rectifier blocks (40kV, 1A) that can be found on eBay.
They are pretty small and fully insulated in epoxy/potting stuff.
Filter
Ripply DC is almost as bad as AC for operating a laser. Assuming the DC negative terminal is earth-grounded (it is in my design), the rectified output would vary between 0V and 20,360V (14.4kV x √2) 120 (2 x 60Hz) times per second. Assuming the laser would start at all, it would constantly be extinguished and restarted every cycle. In order to raise the continuous DC component, a filter is necessary.
Passive filters come in a few popular orientations: Capacitor, pi (or CLC), inductor-input (or LC), and others. I modeled each of these in LTSpice. What ultimately made up my mind? A good price on a big laser capacitor on eBay. Just one.
So, now I’ve got a 25kV, 2μF poly-something capacitor. This particular one might be called “Big Red.”
It had an unusual connection, so I retrofitted it with a brass bolt.
Passive filters come in a few popular orientations: Capacitor, pi (or CLC), inductor-input (or LC), and others. I modeled each of these in LTSpice. What ultimately made up my mind? A good price on a big laser capacitor on eBay. Just one.
So, now I’ve got a 25kV, 2μF poly-something capacitor. This particular one might be called “Big Red.”
It had an unusual connection, so I retrofitted it with a brass bolt.
Coming Together
This is the module that will be mounted in the base of the 21U rack that I'm using for the current regulator and power supply. The base is 3/4" plywood with a zinc-plated steel ground plane.
The transformer is bolted down with countersunk 3/8" bolts. (I love these bolts -- any time you need a crescent wrench on an electronics project is a good time.) The capacitor is similarly mounted with 6 countersunk bolts. The bridge rectifier is wired up with 12AWG solid copper wire.
The transformer is bolted down with countersunk 3/8" bolts. (I love these bolts -- any time you need a crescent wrench on an electronics project is a good time.) The capacitor is similarly mounted with 6 countersunk bolts. The bridge rectifier is wired up with 12AWG solid copper wire.
These are three 100MOhm resistors is series to discharge the capacitor in about an hour. They were salvaged from the regulator's old HV connectors, placed in an oversized straw from Bobby's Burger Palace (delicious!), and filled with Dow Corning 734 silicone.
This is the current power supply schematic. The receptacles allow the current regulator, vacuum pump, etc. to share a single line. The first relay (SPST) allows the current regulator HV switch to turn on the power supply. The switch (S1) allows the power supply to be turned on independently. Conveniently, it's integrated into the power entry module. The second relay (SPDT) momentarily provides the maximum voltage for starting the laser.
Notes:
- If you buy a Staco 2510 without a knob and/or dial, you can buy them from ISE, Inc. Unfortunately, ISE appears to
robcharge ~$70 for a knob and ~$85 for a dial. - Be cautious with eBay purchases. Some sellers don’t know how to package heavy/sensitive equipment that they have come across. I’ve lost a transformer and capacitor to poor packaging/rough handling.
- Dow Corning 734 flows fairly freely until it sets up. That plus its tendency to not set up when placed in an airtight enclosure caused no end of headaches. Eventually it turned out satisfactorily, but only after cursing my ineptitude in fluid dynamics and wiping left over silicone from just about everything on my lab bench