Analog Circuits Training - Understanding Analog Circuits 0-10VDC 4-20mA

dva.gifWhat is an analog circuit?  

As we have learned in previous lessons, a discrete signal is simply an ON/OFF signal such as toggling a light switch.  An analog signal is infinitely variable in theory between two points.  A simple example of an analog device would be a dimmer switch that is used to raise and lower the brightness of a light.

Types of analog circuits - The two most popular types of analog circuits are voltage and current.  Voltage signals are usually ±5 V, ±10 V, or 0 V to 5 V, and 0 V to 10 V.  Current signals are usually 4 to 20 mA or 0 to 20 mA.

Analog Voltage Circuits 0-10 VDC - Analog voltage circuits are the simplest as they are easiest to connect and troubleshoot.  However they are not suited for most industrial environments where they may run beside motor circuits, solenoids, and other forms of electrical interference.

Analog Current Circuits 4-20 mA - A 4-20mA signal is an analog signal that regulates a current loop to transmit a value from point A to point B.  That is the simplest definition I can give. However, even it needs broken down a little more.  

  • Analog - As we have learned in previous lessons, a discrete signal is simply an ON/OFF signal such as toggling a light switch.  An analog signal is infinitely variable in theory between two points.  A simple example of an analog device would be a dimmer switch that is used to raise and lower the brightness of a light.
  • Current - The amount of electricity passing from point A to point B.  Almost all other signals you will work with on industrial systems are based off of voltage or the potential electrical difference between two points.  Make sure you read our Ohm's Law Power Formulas and Pie Chart to help you understand the terms power, voltage, current, resistance, and how they are associated.  Also read our How to measure a 4-20mA signal without blowing the fuse in your voltmeter to help you understand how to measure and troubleshoot 4-20mA circuits.
  • Range - 4-20
    • Low 4mA - Unlike many analog signals, a 4-20mA signal's low end does not start at 0.  This is called having a "Live Zero" and allows the device receiving the signal to determine the difference between a "0" signal and a faulty signal such as a failed transmitter or a broken wire.
    • High 20mA - There are many reasons the industry may have settled on 20mA but as for a general understanding it is a good compromise maximum transmission length (lower mA = less voltage drop) and having power available for loop powered devices to consume on typical industrial power sources such as 10VDC and 24VDC.  

Examples of 4-20mA analog devices - While the list of analog devices are as infinite as the signals they create, here are a few examples.

  • Flow
  • pH
  • ORP
  • Level
  • Pressure
  • Temperature
  • Decibel
  • Turbidity
  • Vibration
  • Speed
  • Distance or length

Conclusion

Now that you understand what analog circuits are, continue on to the Analog Circuits Training - Wiring a Potentiometer to a Meter - Basic Voltage Circuit lesson.

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