Peter emailed me with a question about Geiger Counters. With his permission, I’ll quote his questions in the paragraph below and answer them following.
it´s Peter again – a few weeks ago I purchased a good, sensitive (and fully functional!!) Geigertube for measuring background radiation – the only problem is that it requires 1000 Volts. Of course I looked at Charles Wenzel´s page –> http://www.techlib.com/science/geiger.html
I built the supply with the single 10 mH choke first, cause I hoped being able to modify it appropriately, but I wasn´t able to get 1000 Volts out of it using 5 Volts 🙁 . There are other versions with a transformer capable of higher voltages using only about 3 or 5 Volts – I´d build them at once if I only knew what type of transformer he means and how to get it (or build it). Wenzel describes it as “a 1:1 600 ohm audio isolation transformer” or “phone xformer”. What is this? Can I build this myself? Have you got any tips for me?As always I´d really appreciate your expert advice!
Wenzel’s Techlib has some very interesting projects, especially when it comes to radiation detectors. I would trust his designs and I would not be afraid to build them, knowing that he has experimented with them and has a good grasp of their performance. Having said that, I haven’t done any radiation detector projects and I do not know if his or others work. I do know that from what I’ve read, the typical Geiger tube requires a very small current, microamps if I remember correctly. So the power supply does not have to deliver much power. The tube has a resistor in series, and this has a very high value, might be a megohm or so.
A quick and simple way to build a high voltage power supply is to use the flash circuit from a disposable camera. The large capacitor charges up to 300 volts or more, and the circuit draws a lot of current from the AA cell. But by making some changes, the power can be reduced. The circuit is similar to a Joule Thief, but with a high voltage winding.
I must put a warning here: DANGER! HIGH VOLTAGE! You can get a severe shock from this circuit. Make sure you discharge the capacitor before you work on it.
Instead of a rectifier and big capacitor, the output should be changed to use a smaller capacitor of a fraction of a microfarad, but the working voltage must be higher than 1000 volts. The output should be a voltage doubler, which should give at least 600 volts. The base bias resistor should be increased so that the circuit draws much less than the hundred or more milliamps it draws when it’s in the camera. It should draw only as much as it needs to generate the current for the tube, and that could be just a few tens of milliamps. Also, 600 volts is not enough, so the battery supply is increased from 1.5 volts to 3 volts, and that should increase the output to the needed voltage.
I have very many of the 600 ohm 1:1 transformers used in phone equipment. I have a blog that explains what it is.
There is a Yahoo group called Geiger Counter Enthusiasts. If you have a Yahoo account you can join this group and find out much more than I can tell you. In order to see this link, you may need to be signed on to your Yahoo account. In its first page it gives links to other Yahoo groups which may be more specialized.
Peter, I hope this helps you to solve your questions and get your project on the road.
