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Geigerduino (part 1)

Over the last couple of months, Gav and R3becca have been discussing a new project idea - the Geigerduino. The idea being to be able to build an simple device that can detect cosmic rays or other radiation using a Geiger-Müller tube and an Arduino, and then publish the details so others could do the same. In late August, whilst having an R&D group dinner at Rockdale McDonalds, Gav quizzed me about generating high voltages, and I drew a rough schematic of a DC/DC boost converter (on a napkin of course). He told me the goal was to generate a PWM controlled voltage of up to 1000 V, using USB as a power supply. Having planted the seed, over the next two weeks I did some additional research and realised we should try a DC/DC converter using an isolated transformer with a voltage multiplier to get this kind of output. So, two weeks ago, we began prototyping this at the space. I built up a simple circuit on a breadboard, using a small transformer, some high voltage diodes and capacitors, and a MCP14E5 mosfet H-bridge driver chip. I had acquired a small batch of these transformers from a surplus electronics place a few years ago, and they seemed appropriate with a DC resistance of 11.5 ohms on the primary, and 2,300 ohms on the secondary. I had no other specs, but I think they were intended for generating HV to drive EL or CCFL lighting. The MCP14E5 is actually intended to drive the gate of power MOSFETs, but I had it handy, it is fast and can supply significant current, and it seems to do the job. I also needed a signal source to drive the transformer, and seeing as R&D had recently done a group purchase of Bus Pirates (preorder 2), I decided to use one of those. The Bus Pirate has a PWM generator mode that can generate 1 kHz to 4000 kHz signals with varying duty cycles.

gd1

So, what was the result - first we tried 1 low frequency, possibly 10 kHz, and got almost 500 V DC out.

gd2

A bit of tweaking, and we found that these transformers work best at around 50 kHz. This picture was taken when we were using about 8 V at 200 mA to drive the transformer.

gd3

Yes, we are generating nearly 800 V DC using USB and a simple breadboard circuit! Gavin and I then made up these circuits on some prototype PCB’s, and found we could generate 600 V DC quite nicely from USB power. Gavin even incorporated his Arduino DangerShield for PWM control.

gd4

There is a 10 M ohm load resistor on the output of the circuit. This provides some load (50 uA at 500 V), and helps to discharge the capacitors when the PWM is disabled. Be aware that many cheap multi-meters have an input impedence of only about 1 M ohm, and the load will drag the output voltage down below those shown above. Also, many meters and probes are not rated for use above 600 V (including the scope meter used in the photos!) Warning: Experimenting with high voltage is dangerous - both in terms of electric shock, and damage to your equipment. You have been warned. More to come… - Kean Reference material: http://en.wikipedia.org/wiki/High_voltage http://en.wikipedia.org/wiki/Boost_converter http://en.wikipedia.org/wiki/DC_to_DC_converter http://en.wikipedia.org/wiki/Voltage_multiplier http://en.wikipedia.org/wiki/H-bridge http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en531148 http://dangerousprototypes.com/bus-pirate-manual/