What is use to protect the diodes in a rectifier and capacitor input filter circuit?
Answers
In a generic electronic system there are some inputs that are controlled by the end user. These inputs are read by electronics and acted upon by using outputs. The inputs can come from a myriad of sources: buttons, switches, sensors, relays, and communication devices, to name a few. In certain environments and situations, these input signals can pose a threat to the electronics reading them – especially if those electronics are designed without thought of protection. One such environment is the world of industrial electronics.
An important aspect of designs for this environment is interfacing sensitive electronics with inputs coming from the harsh conditions of a factory floor. Usually, inputs are read by some sort of intelligent processor such as a microcontroller, FPGA, or state machine. In cases like these, it is imperative to protect the processor from the inputs, while still providing a usable signal for the processor to read.
Problem definition
In a typical factory system there may be buttons on a control panel located remotely from the central processing unit. The buttons are connected to central processing via long wires. Unfortunately this can lead to inadvertent electronic failure. Long wires can act as an inductor and when a button is opened or closed, large voltage spikes can show up on the electronic paths. Figure 1 shows a simplified diagram of this situation.
Simplified electronic system
Figure 1: Simplified electronic system.
In order to discuss approaches to overcoming this problem, a more specific example will be used. Typical microcontrollers have input impedance on the order of 20 MΩ. In addition, system voltages range from 1.2 V to 5.0 V. In this case, we will assume a 5 V system. Figure 2 shows Figure 1 reconfigured as a simplified electronic model.
Input model into a simplified electronic model
Figure 2: Input model into a simplified electronic model.
Using this model, it is easy to see the problems with unprotected inputs. Any large voltage that shows up on input pin is presented directly to the interior electronics (microcontroller). Regardless of how this voltage is produced (ESD, induced EMI, switch closure, user error), this can damage the microcontroller, and perhaps cause the entire system to fail. Because of this, different protection strategies must be implemented to create a robust system.
In order to discuss the problem in detail, a simple system will be set up as shown in Figure 3. It is a simple switch that is connected to a microcontroller with a 25 foot wire connection. Note the switch is a 2-pole switch and it switches between open and ground. A pull-up resistor on the microcontroller causes the open position to be read as ‘high’ by the microcontroller.
Simple switch circuit
Figure 3: Simple switch circuit.
When the position of the switch is changed, a large voltage is induced over the 25 feet of wire, and it appears at the microcontroller. This is demonstrated in Figure 4. Note the minimum voltage caused by the inductive ringing is -5.88V. This is more than large enough to cause serious problems within an electronic system.
With this circuit and the simple scope captures, the large voltage problem can be seen. Now it is time to look at approaches to fixing this problem.
Switch from open to ground
Figure 4: Switch from open to ground.
Protection approaches
An important aspect of microcontroller inputs (and the vast majority of any logic ICs) that was left out of the simple model shown in Figure 3 is that they have internal protection diodes that are used to protect the inputs, as shown in Figure 5. These normally forward bias at 0.7 V.
Under ideal circumstances, this can protect the microcontroller. However, if the voltage is large enough or lasts for a long enough time, it can destroy the internal diodes in a shorted position, thereby ‘breaking’ the input pin. Even worse, the input pin is now directly connected to a power rail, so, when the next large voltage shows up on the input pin, it is shunted directly to the power bus, wreaking havoc throughout the microcontroller and most-likely damaging it further.