Friday, 23 September 2011

Single-transistor flyback drivers.

keywords: single transistor flyback driver, single MOSFET flyback driver, simple flyback driver, one transistor flyback driver.

So far, i have dealt with several powerful flyback drivers such as ZVS and 555 timer. To be honest i m more interested in a simpler but powerful flyback driver, so i researched about drivers that utilise only one transistor.

The most popular one-transistor flyback driver is one that uses the ubiquitous 2N3055 NPN silicon power transistor. The schematic is as shown below. The number of turns of the primary is not critical but generally the trickler coil ( the one connected to the Base of the transistor) is having lower number of turns than that connected to the transistor's Collector.

My own variant of the circuit. This topology is also known as Armstrong oscillator, due to Edwin Armstrong who invented and applied this topology for his regenerative FM radio receiver. To recognise this topology is very easy; it has two coils, one having more turns than the other, and it has only one transistor ( or traditionally, a vacuum tube).

Surprisingly, when I substituted the transistor with MOSFET, the circuit is still working. I substituted the Collector, Base and Emitter of the transistor with the Drain, Gate and Source of the MOSFET, respectively. IRF840 and IRFP250N worked successfully, but it didnt work with IRFP450 ( dont know why, if u know why then u can teach me in the comment section). Below is the same circuit, but using MOSFET as an active component.

For both circuits, I found out that my laptop power supply that I m always been using ( 12 V, 3.16 A) cannot be used anymore for these circuits, since it baked the transistors and MOSFETs and if not, degraded them significantly, even though i ve used zener diodes at the base or gate of the devices. I inferred that might be due to large current (3.16 A) of the supply. Therefore I resorted to simpler supply which is the common 9V square battery. Both circuits produced less powerful arc than ZVS or 555 driver ( maybe due to lower power when using battery), and the arc was about 0.5 to 1 cm length. The winding direction of the trickler coils of both circuits can be reversed, and it still works successfully. I know that the circuit can be further improved so that it can fit larger power supply, but my superficial electronic knowledge and tight budget ( i am poor student, T_T) limit improvements and haunt me of more transistor burnings (so far I ve burnt between ten to twenty MOSFETs during three months of experimenting, and each of them worth 1pound. So count by yourself how much lost i ve been facing!). I really hope someone can tell me how the circuits above can be improved for larger power supply. :D

Below is the circuit that didnt work EVEN when using the battery power supply ( I think this circuit actually didnt work, it just damages your MOSFETs and BJT). I just posted it here cuz I am perplexed with most designs of Armstrong oscillator that utilises the following configuration. I dont know the reason behind that, but it seems that most of the designs are using higher power supply. (such as 50, 100 to 140 W, while the highest power supply i have which is the laptop power suppy is just 40 W, in fact, always lower than that!) I also hope that someone experts can explain why the following circuit didnt work.

This circuit doesnt work even when using 2N3055 transistor.

Wednesday, 14 September 2011

Marx generator.

Marx generator is a voltage multiplier that multiply the high voltage direct current to a higher voltage level, by charging a group of capacitors in parallel, and discharging them in series.

I built a Marx generator with original intention to increase input power into my Tesla coil, but didnt working. However its still working as a Marx generator itself, so still worth mentioning in this blog. :P

The operation of Marx generator can be best understood by studying the diagram, retrieved from Wikipedia:

My version of Marx generator has no major difference from any other versions. i used three-stage multiplying, using three 2-litre bottle saltwater capacitors wrapped in aluminium foil as plates. Each of the capacitors was measured to be 5nF in capacitance. There are three spark gaps, where two are small spark gaps as switches and a final spark gap. I ve made the final spark gap larger than the other two, and connecting it directly to ground instead to a voltage divider and load ( as depicted in the above diagram).

For the spark gaps, I have two small spark gaps each of 1.5mm gap length ( it varies around that value cuz u need to synchronise it to get best length in order to avoid corona discharge, cuz corona discharge between the gap may spoil the operation). The spark gap was made by screwing two metal screw into a circular former, either by using glue tape roller, or plastic bottle caps. I used screw to enable easy synchronisation of the gap length during operation.

All the resistors are at 1 megOhm each. The final(big) spark gap was also constructed similar to other two spark gaps. Initially i set the final spark gap length to be 0.5cm, and increased it gradually to get the maximum spark length that can be obtained from the overall configuration of the Marx generator. The maximum spark length observed is about 1.5cm. Yes, its too short for most hobbyists, due to small number of stages ive been using. I am intending on buying more high voltage capacitors to reduce size ( saltwater capacitors are too big and heavy) and make the overall design compact and smaller ( and portable as well) as well as increasing the number of stages to maybe 20 to 30 stages. I wonder how long the spark i will get...

Here is the schematic of the marx generator built:

Here are the photos taken:

The overall setup. Ignore everything outside the A4 papers boundary. Just focus the interior. The noticeable three big bottles are my saltwater capacitors of 5nF capacitance each. I m using my ZVS power oscillator (but changed the 1N4007 diode to BYV26E, IRF840 transistor to IRFP250N, and 10nF capacitor to high voltage rating 1uF capacitor for faster switching, cooler mosfet and higher efficiency and power) fed the oscillating power into the flyback transformer. The transformer rectified the oscillating current in it so there is no need to use diode. The high voltage direct current is then fed into my Marx generator.

Closer view.

Power supply ( laptop power supply, 12V,3.16A as always used before) which is the black wire with yellow wire at its end, breadboard populated with components in ZVS oscillator confg, and a flyback transformer ( the only chunky thing close to the breadboard)

Capacitors and small spark gaps bird eye's view.

Closer look on the small spark gaps. The white one was not used, so i closed the gap.

The small spark gap using screws and plastic bottle cap.

Another small spark gap, also using screws and plastic bottle cap.

As mentioned before, this small spark gap is not used, so i closed the gap. This was constructed cuz i though another small spark gap would be necessary before the final big spark gap, but when finding out it wasnt important later on I just leave it there by closing it. This gap was constructed with glue tape former and two screws.

This is the final and the biggest spark gap where the final product of the Marx generator ( the longest spark) will be observed. It was constructed similar to the unused small spark gap. Just ignore the red tape former over it cuz it is just to hold the wires carrying voltage to the final spark gap in place.

Final (big) spark gap.

Here is the video. I have to put on a video this time cuz it was really hard to capture a photo of a flashing spark. Enjoy it!