Abstract: A three-stage Darlington transistor circuit having a power transistor (T.sub.3), a driver transistor (T.sub.2) and an initial transistor (T.sub.1) is provided. The collectors of these three transistors are connected with one another, while the emitter of the driver transistor (T.sub.2) is connected to the base of the power transistor (T.sub.3), and the emitter of the initial transistor (T.sub.1) is connected to the base of the driver transistor (T.sub.2). Connected to the base of the driver transistor (T.sub.2) is a series circuit comprising a first resistor (R.sub.1), which is connected directly to this base, and a Zener diode (ZD), the anode of the Zener diode being connected with the resistor (R.sub.1); and a second resistor (R.sub.2); the second resistor (R.sub.2) is connected in parallel to the emitter-base path of the driver transistor (T.sub.2). Upon the attainment of a predetermined voltage at the cathode of the Zener diode (ZD), the driver transistor (T.sub.2) and the power transistor (T.sub.

Abstract: A safety device is included in a circuit comprising a power source and a user device. The power source has one terminal connected to ground, as does the user device, with the safety device connected in a conducting path leading to the ground in the user device. In the event of an incorrect polarity connection between the power source and user device, a current path through the safety device and to ground is established.

Abstract: To prevent excessive current flow through the control transistor of a solid-state ignition system in which the control transistor is serially connected with an ignition coil, if the power supply voltage should exceed rated value, the voltage monitoring circuit, including a Zener diode, is connected across the power supply source. The solid-state ignition system includes a control transistor which controls the conduction state of the final or output transistor. The over-voltage circuit additionally controls the conduction state of this control transistor to conductive condition if over-voltage is sensed, thereby causing changeover of the final or output transistor to blocked state. A timing circuit including a capacitor is preferably included in the monitoring circuit so that application of control voltage to the control transistor is gradual, causing changeover of the main or output transistor likewise to be gradual and preventing generation of an undesired ignition event.