沧灿萧5GN30S东方马达
If, on the other hand, the switch should close spontaneously some time after the charging cycle was completed, but before the next normal pulse, then a normal pulse will be formed. A normal charging cycle will then be started. But while this cycle is in progress, the normal trigger occurs, the switch closes, and then the process for over-voltage goes into operation since now a charging cycle is started with finite charging current already flowing.

Transformer Over-voltage Prevention

A simple device that will help prevent over-voltaging the transformer (but not necessarily other components) is a fast over-voltage sensing circuit that will automatically prevent the next and all succeeding triggers from being applied to the switch if the charging voltage rises above a predetermined value. A voltage divider giving faithful waveform division is necessary here. A bleeder resistance for draining off the PFN charge should be part of the circuit too. It is prudent also to turn off
the power supply automatically at the same time (see section on continuous conduction).

Other protective measures are possible. One is a spark-gap and low resistance in series across the primary, with the gap set to fire for any amount of over-voltage. Another is thyrite across the primary. Both of these are inherently imperfect but are better than nothing.

Pulse Switches

Obviously a switch with adequate voltage hold-off capability is called for and every effort should be made during design to assure this. Series operation of switches is a possibility, but one that should usually be avoided. One of the problems that has been encountered with series switches is that of making certain that the charging voltage is equalized between the series tubes. This means that the capacitances as well as resistances must be equal since the charging voltage has ac components as well as dc. The capacitances should be measured in an actual circuit to be certain that stray capacitances are not upsetting the balance. Individual triggering of all series switches is recommended for positive closing of the individual series switches. This is relatively simple to do with an appropriate multisecondary trigger transformer or separate, paralleled-primary trigger transformers.

Continuous Conduction of the Switch

Another difficulty that may occur is that the switch may conduct continuously. An over-voltage is not created initially. However, the charging inductance and the filter capacitor go through a half cycle of oscillation. At the end of the half cycle the current is stopped by the charging diodes. Now the voltage is reversed on the filter capacitor. Current now flows from the power
supply to recharge the filter capacitor. But this is a situation completely analogous to resonant charging a PFN having a reverse charge, except that the capacitive element is now the filter capacitor and the inductive element is the inductance. The result is a tendency to charge the filter capacitor to more than double the normal power supply value. Of course, all the succeeding pulse components are then correspondingly over-voltaged. Obviously, the power supply circuit breakers and current sensing circuits should be fast-acting for the case of continuous conduction of the switch.

Load Resistance Too High
Proper instrumentation and calibration is expensive in money and time. Sometimes the temptation is to make assumptions regarding the load resistance. Voltage dividers and pulse-current transformers should be used on the load to be certain that the load resistance is correct at full operating voltage. Dummy loads whose resistances vary with temperature should be watched. A mismatch on the high side for the load can allow the transformer voltage to be too high even though the charging voltage is an appropriate value.

Combination of Problems

A common failing on the part of an engineer or technician trying to locate trouble in a malfunctioning pulse system, is the tendency to assume there is only one system malfunction. Actually, it is more common that there are several problems co-existing in the equipment. In testing to see if a particular malfunction exists, as much of the circuit that can be eliminated or replaced by simpler components should be. An example would be that of first operating a pulse modulator into a resistive load at full peak and average power. Then add the transformer operating into a? resistive load, again at full power. Then the diode type load can replace the resistive load. This process can partially avoid assigning of the trouble to the pulse transformer or the diode load or the reaction of these on the circuit when the fault may have been somewhere else.