Protection from overheating of a switching transistor that delivers

Abstract

A protection circuit protects from overheating a switching transistor that delivers current from a power source to a daytime running light on a vehicle. The protection circuit of the present invention open-circuits the switching transistor from the power source when the temperature at the switching transistor is sensed to be above a predetermined temperature. In this manner, excessive current is prevented from flowing through the switching transistor. In contrast, prior art protection circuits control only the gate of a MOSFET or base of a BJT switching transistor. In that case, if damage to the transistor results in an effective resistance, constant flow of current through such an effective resistance results in excessive overheating or even a fire. The protection circuit of the present invention open-circuits the conductive path from the power source through the effective resistance such that current does not flow through the effective resistance. Furthermore, a latching circuit in the protection circuit of the present invention maintains the switching transistor to be open-circuited from the power source once the temperature at the switching transistor reaches the predetermined temperature. Thus, the traffic hazard or degradation of the switching transistor, which may result from the daytime running light flashing on and off, are also avoided with the protection circuit of the present invention.