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Here is a calculator for transistor base resistor values. Its IMPORTANT that you read the following.
Calculate the current you need to pass through the transistor when its on, that is your collector current.
The current gain, beta, Hfe, is a value you can find in the datasheet for the specific transistor. To calculate for the worst case use the minimum given Hfe value or the correct value for the collector current you need.
Vce voltage is the voltage over collector and emitter. A design guide not included in calculations for now.
Base voltage is the voltage that you use to drive the base of the transistor with.
Voltage drop is the Vbe(sat) value, you will get this from the datasheet looking at the graph for Vbe and Vce(sat) vs. collector current.
To insure to turn the transistor fully on, you can double the value for collector current, which will result in a base resistor value half of what this calculator gives you.
The examples are given for the highest collector current and worst case amplification factor, Hfe. Values for your circuit might vary, be sure to enter values for your own needs.
To calculate for a PNP transistor, enter negative numbers in collector current, base voltage and voltage drop.
Examples are given for NPN transistors 2N2222, 2N3055, 2N3904, BC547, TIP31, TIP31A, TIP31C, TIP41, TIP41A, TIP41C and PNP transistors 2N3906.
This driver is among the simplest, with just six components it will be able of delivering high voltage with a strong enough current for various experiments.
There is also a push-pull version of this driver, more on that can be found following this link: http://wiki.4hv.org/index.php/Flyback_transformer
WARNING!: Working with electricity is dangerous, all information found on my site is for educational purpose and I accept no responsibility for others actions using the information found on this site.
Read this document about safety! http://www.pupman.com/safety.htm
The transistor used in this circuit should not have a voltage rating higher than 250V, due to the higher base current needed of higher voltage rated transistors, this might in some cases cause problems where the self-resonating circuit can no longer drive the transistor.
At voltages higher than 12 VDC input, the components will dissipate a lot of heat and the 2n3055 transistor is most likely to burn as it is not rated for more than 15 A and it has to be derated even further for case temperatures above 25 oC.
MOSFETs are not suitable in this driver.
The frequency of operation in this circuit is determined by the capacitor across the transistors emitter and collector, experiment with the value of this capacitor to find the best performance, this capacitor have to be a good film or foil type (MKP/MKT)
The driver is supplied with 12 to 17 VDC from a computer ATX PSU.
3rd may 2008
The driver is built on a piece of vero board with multiply resistors to obtain the needed wattage rating, this is far from optimal as the load sharing between them is horrible, to ensure better sharing.
The 2n3055 transistor is mounted on a 30 x 10 cm aluminium profile, this is just about enough to keep it alive doing long runs.
4th may 2008
I experimented with voltages between 12 to 17 VDC, there is no incredible performance with higher voltage compared to the amount of heating the higher voltage generates.
Different capacitors were tried, ranging from 10 nF to 300 nF, 220 nF was found to work the best for my flyback transformer.
8 primary windings and 4 feedback windings gave the best results.
7th may 2008
18 to 20 mm sparks were achieved
Keeping the electrodes just far enough apart for no spark to jump, a beautiful corona breakout is visible.
With the output coupled through a home made salt water capacitor it was possible to have loud and very bright sparks.
This is a very simple and cheap circuit to achieve around 20 kV high voltage, but where it excels in simplicity it lacks a lot in stability and efficiency.
A rough estimate is that 25-50% of the input energy is wasted as heat in the transistor.
It is cheap and simple, but inefficient and unstable.