The basic principle of the Marx generator is the best way of charging a number of capacitors in parallel and discharge them in series. Generally, these generators consist of a N number of modules where each module consists of N number of sub modules. Then each module consists of a set of two registers and capacitors. These Marx generators are mainly works as a high voltage impulse for testing when the voltage levels required higher than the required supply voltage.
Marx generators are originally described by E. Marx in the year of 1924. Normally Marx generators are used in the high energy physical equipment and also in the power line gear and aviation equipment’s.This article discusses about the Marx generator design, principles and developments.
Marx Generator Design Principles & Latest Developments
The circuit generates the high voltage pulse by connecting the capacitors in parallel and connecting in the series of capacitors in the circuit. Normally ‘N’ number of capacitors are charged in parallel to the voltage (v) connected in series with the resistance Rc. In this operation the circuit acts as an open circuit when all the capacitors are charged. When the sparks gap uses a switch which has the voltage (v) across them, but these sparks gaps are having breakdown voltages greater than the normal voltages. To generate the output pulses at the first spark gap causes breakdown effectively to short the gap when placing the first two series applying a 2 volt placing the first two capacitors there in the series then applying voltage of the second spark gap consequently. The second gap is consequently add the third capacitor to stack and the process continues to break down all of the gaps. The last gap is connected to the output of the series stack of capacitors to the load circuit.
Ideally the output voltages will be nV, The number of the capacitor times can be connected for charging voltages, But in practice, the value is less. So none of these charging resisters RC is subjected to more than the charging voltages, But in the practice the value is less. None of the charging capacitors even when the capacitors have been erected. The charge available is limited to the charge of the capacitors. So the output is a brief pulse as the capacitor discharge though the load. If spark gap is not conducting then the output voltages will be increasing.
The total circuit depends on the connected capacitor ratings and the timing will be discharged. The switching pulses can be depends on the doping times of the electrons.
Design Enhancements and Considerations
Constant Current Source
Marx generator is charged from the constant voltage source, The energy is dissipated to the charging resisters and that will be equal to that stored in the max capacitors. If the bank is charged with constant current source, this energy loss can be substantially reduced by using the constant current source.
Integrating the Wave Shaping into the Generator
In this mode of operation of the capacitor discharges from the impulse generator, The shape of the pulse is controlled by external impedances at the output of the pulse generator. When the voltage gets higher its difficult to get practical resistors with low parasitic inductance that will also stand the full impulse voltage. The usual remedy for this is to include the wave shaping resistors in the Marx generator.Integrating the wave shaping into the generator illustrated in the following fig.
Reducing the jitter
If the gap in the Marx generator doesn’t fire exactly at the same time, The leading edge of the impulse will have steps and glitches as the gaps fire. These delays also result in a longer rise time of the impulse. If the jitter in the gaps is reduced, then the overall performance is improved.
The traditional Marx generator operates in the air. It has all the gaps in a line with the electrodes operating horizontally opposed to the electron beam. These systems allow the UV from bottom gap to irradiate from upper gaps. This type of operation is used for reducing the normal jitter. Reported in Crags and Meek showed that obstructing the UV led to greatly increased to the jitter, Which they attribute to the lack of UV irradiation to the upper gaps.
When the Marx generator is immersed in oil to enclosed spark gaps. Resistors or capacitors to the networks can be used to propagate the trigger pulse to all the gaps, rather than relying on the over voltage of the upper gaps to the fire. For these purposes a design from Maxwell labs uses a series of resistors to apply the trigger impulse to all the gaps.
Laser Irradiation and Triggering the Goods
These Crags and Meek is also report and use of radioactive sources included within the gap of electrodes to reduce the jitter.
Other Switching Devices
Marx generator has been used to generate impulses of several kilovolts from a relatively low charging source using avalanche transistors as a switching devices instead of a spark gap of devices. In this application, the resistors need to be chosen to keep the transistor is turned on.
Alternate Charging Schemes
Particularly for a lower output voltage of the Marx generators, capacitors can be charged in parallel from a common source through a series resistor or an inductor. The charging impedance has to withstand the full output voltage for this top stage. This is a solid state Marx generator running at a few KV. This isn’t as much of a problem as it would be for a megavolt range lightning impulse simulator.
Applications of the Marx Generator
Box Switching Operation
Box switching is one of the application of Marx generator and it is also called as a boxcar switching of a pockels cell. Four Marx generators are used in this application, two electrodes of the Pockels cells being connected to a positive pulse generator and another one is a negative pulse Marx generator. Two generators are having opposite polarity, in that one of the electrode is first fired to charge the Pockels cell into one charging polarity. These techniques are used to charge the two generators, but not trigger them to the pulses of each generator, Because they needed only partly charged and leak through the Marx resistors which needs to be compensated by a small bias current through the generator. At the trailing edge of the boxcar, the two other generators are fired to “reverse” the cell.
Marx generators are used to provide high voltage pulses for the testing of insulation of an electrical apparatus such as large power transformers or Insulators used for supporting power transmission lines. The voltages applied may exceed two million volts for high-voltage apparatus of the Marx generators.
Operation of Low Jitter for Pulse Synchronization
The low temporal jitter of the impulse Marx generator may be required for multi source applications or timing applications. Marx generator is a variable candidate for phased array systems, low jitter must be reduced to a small fraction of pulse width of 200 ns to the 1ns impulse signal. The main function of this low jitter to be achieved for multiple pulse addition circuits such as Gatling systems and bi static radar systems applications.
Based on the closure of the trigger switch, the reflected pulse of -1/2 the spark gap-charge voltage and a length is twice of the charged transmission line propagates toward the spark gap, as shown in Figure. Arriving at the end of the trigger pin, the pulse doubles in magnitude, resulting in a potential of minus one-half the charge voltage. This results in a highly distorted field between the electrodes due to the presence of the sharp pin at the negative potential.
The low jitter performance is measured using the three Marx generator system is described. Each Marx generator is connected to a common trigger circuit, This is a krytron-based circuit designed to hold off 15KW. These Marx generator is normally connected to a single output transmission line. We connect temporally because we have to separate the individual pulses, The trigger lines are connected to each Marx generator to the trigger circuit that are unique in the same length.
Generation of UWB Signals
The impulse Marx generator was tested for its ability to directly generate microwave energy in the form of an UWB signal. The Marx generator directly drives a rudimentary TEM horn antenna signal. Generally it measures 100meters from the source, For example an EMCO 3106 antenna was used for radiate measurements. Additional measurements were made with a crystal detector.
Single Pulse System with BWO
In this application we are using a narrow band Marx generator, used to drive the cathode of a Russian made BWO. It was measured in Texas tech university. The max generator directly groove the cathode and was temporarily aligned with the magnetic field pulse. An Uncalibrated, b-dot probe was used to monitor the generated signal as well as the Uncalibrated integrated B-dot probe is used to monitor the generated signal, then as well as a florescent witness plate.
Now the BWO was designed to deliver a 35 Hz signal within a 3-4ns. The system is delivered to appear a 20ns window of microwave energy that was approximated to be 30 MW in peak power. This power delivers at TM0 mode of the BWO. The BWO delivered a TM01 mode.
The three Marx generator is orthogonally connected to the common transition line, And for the purpose of demonstrating the generator have a common trigger system. A capacitive voltage probe is placed at the trigger input of Marx generator, and a current viewing resistor is placed at the output of a common transmission line system. So each of the generator will be designed to deliver a 140 KW pulse with sub Nano sec rise time.
The output of the current viewing resistor demonstrates the system’s ability to generate three district high voltage pulse, each having an amplitude excess of 125kv range.
The Gatling Marx generator system was connected to the TEM horn used to earlier in this Marx generated system. Now we have to take precautions were taken to ensure the antenna did not break down during the first pulse, so later pulse would be radiated.
High Voltage Based On Marx Generator Principle Using MOSFETs
This project is designed to develop a circuit to deliver the o/p around triple to that of the i/p voltage based on the principle of Marx generator. This project generates high-voltage pulses using multiple capacitors in parallel to charge during the on time, and then connected in series to develop a high voltage during the off time. When the i/p voltage is applied around 12v volts DC, then the o/p will be around 36 volts DC.
This project uses a 555 timer in the mode of astable, which sends the clock pulses to charge the capacitors which are connected in parallel during ON time & the capacitors are brought in a series during the OFF time via MOSFET switches; and hence, develops a voltage around triple to the input voltage but little less, instead of actual voltage(36V) due to the drop of the voltage in the circuit. The o/p voltage can be calculated by using a multimeter.
Thus, this is all about Marx Generator Design Principles and Developments, applications of Marx generator.Furthermore, any queries regarding this topic or electrical and electronic projects please give your feedback by commenting in the comment section below.