from the image impedance method of filter design. Diodes as shunts Where devices are vulnerable to reverse polarity of a signal or power supply, a diode may be used to protect the circuit. If connected in series with the circuit it simply prevents reversed current, but if connected in parallel it can shunt the reversed supply, causing a fuse or other current limiting circuit to open. All semiconductor diodes have a threshold voltage – typically between volt and 1 volt – that must be exceeded before significant current will flow through the diode in the normally allowed direction. Two anti-parallel shunt diodes (one to conduct current in each direction) can be used to limit the signal flowing past them to no more than their threshold voltages, in order to protect later components from overload. Shunts as circuit protection When a circuit must be protected from overvoltage and there are failure modes in the power supply that can produce such overvoltages, the circuit may be protected by a device commonly called a crowbar circuit. When this device detects an overvoltage it causes a short circuit between the power supply and its return. This will cause both an immediate drop in voltage (protecting the device) and an instantaneous high current which is expected to open a current sensitive device (such as a fuse or circuit breaker). This device is called a crowbar as it is likened to dropping an actual crowbar across a set of bus bars (exposed electrical conductors). Battle short On warships, it is common to install battle short shunts across fuses for essential equipment before entering combat. This disables overcurrent protection at a time when removing power to the equipment is not a safe reaction. Shunting an instrument but series connected in circuit As an introduction to the next chapter, this figure shows that the term "shunt resistor" should be understood in the context of what it shunts. In this example the resistor RL would be understood as "the shunt resistor" (to the load L), because this resistor would pass current around the load L. RL is connected in parallel with the load L. However, the series resistors RM1 and RM2 are low Ohmic resistors (like in the photo) meant to pass current around the instruments M1 and M2, and function as shunt resistors to those instruments. RM1 and RM2 are connected in parallel with M1 and M2. If