An Economical
Impulse Generator

This circuit is connected to and uses lethal mains voltages and must not be used by any non-qualified persons.

There are often times when a superimposed impulse is required on a supply to either test the effectiveness of a suppressor circuit, or the capturing ability of a power quality recorder etc. The following circuit provides an economical means to generate a 5/30S impulse which is superimposed on a normal 230VAC waveform. Two impulse voltage levels are available, 50V and 500V.



CIRCUIT DESCRIPTION

impulse generator

The principle is a cascaded gas discharge circuit that discharges into a fairly low-ohms resistor which is placed in series with the main 230VAC signal. The diodes (D1, D2) select whether the impulse will be positive (using S1) or negative (using S2). C1 and C2 are normal smoothing capacitors. When either S1 or S2 is pressed, C4 charges to full voltage within a short period while C3 charges more slowly. C4, however, never charges beyond the firing voltage of G2.

When C3 charges sufficiently and the voltage across G1 reaches firing voltage (230V) the low voltage end of C4 is "pulled" toward the high voltage end of C3. This in turn significantly raises the high voltage end of C4 and effectively places C3 and C4 in series making the total voltage approximately 500V. As this is beyond the firing voltage of G2, it now conducts placing the full charge of C3 and C4 across R9 and R10. R11 and L1 serve to indicate when a strike has happened. R12 is an inrush resistor.

For those concerned about the energy levels dissipated in this circuit, we use the formula J=CV2. The capacitance is taken as 0.235F (2 x 0.47F in series). For the sake of argument we assume it possible that the voltage on G2 could be at 350V without it firing, and then G1 reaches 230V, this makes the total voltage 580V. The total energy dissipated = 40mJ. This can give you a serious whollop but the danger lies in the fact the output is always delivering mains voltage, not the energy during an impulse.



OPERATION

The circuit has a limitation with operating voltage being approximately 185-240VAC. If lower than 185V the rectified voltage (185 x 1.414 = 260) will be to low for G1 to fire. Above 240V the rectified voltage (240 x 1.414 = 340) will approach or be above the firing voltage of G2 thus reducing the effectiveness of the impulse. As this was designed for testing at 230VAC we felt it unnecessary to do much modification. If it is desired to operate the circuit at higher than the specified voltages then R1 and R2 can be decreased to act as a voltage divider with R12 (R12 can also be increased to a maximum of 22k without making the supply to the circuit 'soft' and unreliable).

If operating the impulse generator at 115VAC (used extensively in the mining industry around the world as well as being the mains voltage in the USA) then use a simple auto transformer arrangement as shown alongside. This could comprise of the two input windings on a small mains transformer (as shown).



EXAMPLE WAVEFORMS

The waveform of the pulse can be seen in the following screen shot as recorded on a RPM Power Recorder type 1656 and displayed via their Power Analysis Software.

Spike waveform


The following screen shot shows the pulse superimposed on a normal 50Hz waveform.

superimposed on 50Hz input


This screen shot is a zoom of the above waveform for clarity.

close-up of spike on cycle



PARTS LIST

D1, D2 - 1N4008 diode


R1-R4  - 1Meg
R5-R8  - 240k
R9     - 30 ohms
R10    - 270 ohms, 2 watts, 1kV working
R11    - 100k
R12    - 10k

all resistors 1 watt, 350 volts working unless otherwise stated


C1, C2 - 1F
C3, C4 - 470nF

all capacitors 400 volts working, polycarbonate.


G1     - 230 volts, 20A/20kA gas discharge tube
G2     - 350 volts, 20A/20kA gas discharge tube

L1     - Neon indicator lamp
S1, S2 - suitable high working voltage push buttons



INCREASING THE IMPULSE VOLTAGE

Increasing the impulse voltage is simply achieved by adding more cascades.

First, for higher impulse voltages, it is advised that the circuit be fed from an isolation transformer and that the 230VAC and 0VAC be inverted (the 230VAC now becomes 0V and is also earthed).

As an isolation transformer is being employed, a higher working voltage can be used. Change the gas tubes for 350V on the first stage and 600V versions on the cascades. Ensure all resistors are able to withstand the final impulse voltage (depending on which stage collapses first, the impulse voltage could be found anywhere in the circuit).

The timing is controlled by the final resistors (R9/10). Increasing the resistors increases the decay. The attack is not controllable (depends on the speed of the tubes).

A final word of warning: Anything beyond 1kV can jump anywhere. Please exercise extreme caution.



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© 30.03.04