The automobile is a tough venue for today’s electronics. Supply voltages can range from 12 to 16 V, depending on battery charge state. On top of that are the spikes and ac noise coming out of the alternator, especially in stop-and-go traffic.
Even more noise comes from the ignition system, which produces high-voltage signals that radiate at frequencies that vary with engine speed. Spark plugs produce broadband noise as they arc. Ground current returns through the chassis can run into corrosion and other irregularities that create ground noise for the electronics.
It’s no wonder that automotive sensors and instrument clusters employ open-collector circuits to buffer the pulse trains and pulse-width modulation (PWM) signals the sensors and instruments use to communicate. Not only will open collector circuits handle the wide variations in battery supply voltage, they also will provide high drive with great immunity to noise.
Open-collector drive circuits are also totally immune to accidental short circuits to ground. Such shorts are an ever-present danger in automotive systems where vibration can cause wires to rub away their insulation over time so the conductor touches the chassis. Automotive safety requirements, in fact, require electronic signals to be immune to continuous short circuits, both to ground and to the positive rail.
Unfortunately the open collector is not immune to short circuits to the positive rail. Shorts in this direction can be deadly to the output transistor, which will quickly burn itself out trying to drain the battery through a direct connection. Some form of current-limiting has to be in place to protect the transistor.
That need to protect automotive signaling was the genesis of this Idea for Design (see “Simple Current Limiter Protects Open-Collector Circuit”). Vishwas Vaidya’s vendors for his automotive designs were providing sensors and instruments that used resistors to provide current limiting. But in some cases the resistance was high enough to violate logic-level constraints during long cable runs. In other cases the resistor dissipated too much heat during short circuits, causing circuit failures. Both cases compromised the automobile design’s reliability.
According to Vaidya, his vendors were planning to solve these problems in one of two ways. The first was to put in some fuse protection for the circuits, which would prevent damage to the electronics but add cost and maintenance concerns. The second solution was to implement costly smart switches that could break the short circuit and later be reset for normal operation.
Vaidya had a better idea. The design he proposed added only two inexpensive components to the standard open-collector output design and avoided all the problems with resistive current-limiting.
He is currently using his design in his own equipment during lab-testing of automotive electronics systems within his company. His vendors are in the process of evaluating the design with an eye toward incorporating it into their next-generation automotive products.
For those not working in automotive electronics, Vaidya has a suggestion. The circuit can be just as easily used for current limitation of opto-isolated transistor switches. It can even be integrated as a transistor pair with connections for an external resistor (R2 in the circuit) so the open-collector drive protection is built in.