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Posts tagged “High Voltage

Pocket High Voltage Generator Quick Build

Pocket HV Generator

There are times you find yourself looking for a relatively high voltage (100V to 200V often in my case) but low current DC power supply. I have zener diodes that are higher than 30V, which makes the lab supply useless, and filament LEDs with forward voltage over 60V. When I need to test them quickly, I used to hook up a simple rectifier circuit to a variable AC power supply (nothing more than a slidac with isolation transformer). While this gets job done, the setup is capable of supplying much too high current (1A or more), so I was always very nervous and extra careful in handling the circuit. All I need is a little HV generator that gives me around 200V DC and only capable of supplying a milliamp or less. Realizing that I do have such design available – one of the Nixie supply circuit – I just decided to put one together to use.

Pocket HV Generator schematics

Quick & dirty build of the tool.

Quick & dirty build of the tool.

A single AA battery seems to be enough to generate over 200V on the output with no load. But the output quickly lowers when you draw 0.4mA. So it feels pretty safe to handle this casually, and I can only feel a bit of tingling, not electric shock when I touch the output terminals. The tool proved to be quite handy and useful in testing variety of things:

  • Zener diodes (zener voltage)
  • Switching diodes (reverse breakdown voltage)
  • Filament LEDsĀ (forward voltage)
  • Regular LEDs (forward voltage – yes it’s ok to use this tool, since it’ll only give less than 1mA even at 2-3V)
  • BJTs (breakdown voltages)
  • Neon & Nixie tubes (not very bright, but you can tell if one works or not)
Pocket HV Generator testing a zener diode

Pocket HV Generator testing a zener diode

Pocket HV Generator testing a Nixie tube

Pocket HV Generator testing a Nixie tube

This is one of the most useful tools that I’ve made. And it only took a couple of hours to put it together.

Variations on Nixie Power Supply Design

Since I started tinkering with Nixie and other Neon tubes, I found the need for simple (read: inexpensive) high voltage power supply capable of generating over 170V from 5V DC.

After a bit of research I found that most of the high voltage power supply designs use boost converter driven by a PWM controller IC such as MC34063, with a high voltage MOSFET switching an inductor. (Here’s an example of the design.)
Those designs looked a bit overkill to me, so I started designing my own from scratch.

Since I’m familiar with transistor based blocking oscillator circuit to boost voltage (i.e. Joule Thief), I wanted to see if I can use a similar circuit. The switching transistor has to withstand the output voltage of 180V so I picked some high voltage transistors and experimented. Turned out that typical high voltage transistors (C-E breakdown of more than 200V) were too wimpy for the purpose, and the simple two transistor circuit that I was using was not capable of very high duty cycle demanded by high input/output voltage ratio (over 90%).

One way to reduce requirement for the boost converter is to add voltage multiplier at the output. I added a 3 stageĀ voltage multiplier to a circuit using pretty ordinary (inexpensive) transistors. This circuit was able to provide required voltage (about 170V) and up to around 3 to 4mA of driving current to medium sized Nixie like IN-12.

After building a couple of prototype Nixie clocks using this circuit, I found a very nice transistor capable of handling 100V and 1A current.

With this new transistor, I can now reduce the voltage multiplier stage to only one, since the boost circuit itself can produce up to 100V (ok, with safety margin, more like 90V). This circuit outperformed the prior version, producing about 8mA at 170V.

Super simple HVPS using only two transistors. 180V output capable.

Simple two transistor HVPS on a Nixie clock controller PCBA. (Inside yellow rectangle – fits in 12mm x 32mm)

While I was happy with this design – especially the size and cost – and built a couple of Nixie clocks and IN-13 Neon indicator tube projects with it, I still wanted to make it better (mostly wanted more power).

If I can find a transistor capable of withstanding over 200V with a reasonably low loss, I can forgo the voltage multiplier. However the only options that I can find were MOSFETs.

After checking the prices of high voltage MOSFETs such as IRF740, I concluded that it can be more cost effective if I can make it work, since I’ll be removing two diodes and capacitors from the voltage multiplier.

After a bit of experimentation, I got it to work! Here’s the MOSFET based circuit. Note that this design needs at least 9V of input voltage to work (due to the MOSFETs gate voltage). So for the 5V powered projects, I’d still use BJT based design.

Super simple HVPS using only two transistors. 240V output capable.

This MOSFET based design is capable of delivering at least 50mA at 200V.