It is common practice to use digital open-collector outputs for control units in industrial applications. Using these outputs, you can switch loads, such as relays, lamps, solenoids, and heaters. One possible problem inherent to this type of output stage is a short circuit from the output to the supply voltage (often, 24 V). This condition can destroy the output transistor if it lacks protection. The simplest approach to solving this problem is to use a fuse. This method has a disadvantage, however: You have to replace the fuse after it blows. A PTC (resettable) fuse is often too slow to protect the transistor under the short-circuit condition. Another possibility is to use a current source as the switching element. This approach is safe and simple, but it produces heat during the error condition. If the power rating and the cooling of the transistor are inadequate, the transistor fails because of thermal overload. The circuit in Figure 1 shows another simple approach to the fail-safe protection of such switching devices.
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| Figure 1. | This circuit provides fail-safe protection of an open-collector output stage. |
The principal function of the circuit is to switch off the transistor if the voltage on the collector is higher than a predetermined value. Under normal switching conditions, transistor Q1 should saturate when it turns on with a voltage lower than 0.2 V between the collector of Q1 and ground. If a short circuit exists on the output J1 or if the impedance of the load is lower than specified, the voltage on the collector of Q1 rises because too little base-current feed comes from the control logic (via R4) to saturate Q1. If the collector voltage of Q1 reaches the switching voltage on the base of Q2, Q2 turns on, and Q1 switches off. You can adjust this switching point with the R1-R2 voltage divider. Now, the voltage on the collector of Q1 rises to 24 V, and the output stays in the switched-off condition. To reset the circuit, you must switch the steering output from the control logic to low. Now, the Schottky diode, D1, is forward-biased and thus discharges C1 and switches off Q2. If the steering output from the control logic again switches to the high state, Q2 stays in the switched-off condition during the charging of C1. If the output of Q1 is not overloaded, Q1 saturates again and stays switched on. If the output has a short circuit to the supply or it is overloaded, then Q1 switches on only during the charging of C1; after this time, Q2 switches off Q1. The maximum load current depends on the value of R4, the output voltage from the control logic, and the current gain of Q1.
