@ A protection circuit (140, 142, 138, 154) monitors each of one or more charge electrodes (CE1-CEX) used in an ink jet printer. The voltage level on each electrode or the current flowing to each electrode is continuously monitored. A fault condition is indicated if the voltage is below a defined level or the current flow is above a defined level. Since fault conditions can be indicated during normal printer operation, the monitored status of the electrodes is keyed or sampled by gated flip-flop circuits (154) during normally stable periods of the operating cycle of the electrodes so that only true faults are detected. If guard ink drops are inserted between successive printing ink drops to form a guard zone defined by electrical signals (172B, 172D in Figure 4), the preferred sampling points are the end portions of the guard zone signals. Once a defective charge electrode is detected, the associated flip-flop is locked into a fault condition. An output signal of the flip-flop clamps the charge electrode supply voltage and causes a shut down of the ink jet printer so that no damage will be done to the charge electrodes.
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