Understanding 420 mA circuits
The most common way to transfer an instrumentation signal from one device to another is by use of a four to twenty milliamp (420 mA) circuit. Most controls on Heatec products incorporate such a circuit. These are series circuits, which means the components are wired in series with each other. While there are variations in the circuits they all work on the same principle.
Three basic electrical terms are used to describe key properties of a 420 mA circuit:
The number of volts indicates how much electromotive force is applied to the circuit. It’s the force that produces electric current or amps. (Note: amps and amperes mean the same thing.) Electromotive force is often compared to pressure applied to a water hose. In that case it’s the force that makes the water flow.
The number of milliamps is a measure of electric current or rate of flow. (One milliamp is one-thousandth of one amp.) Electric current is defined as flow of charge.* Current in a circuit is often compared to how fast water flows from a water hose. Or how much water flows from the water hose over a given period of time.
|*Flow of charge is used in this document instead of the traditional term “flow of current.” Critics argue that “flow of current” is a misnomer and that “flow of charge” should be used instead. Despite evidence that “flow of current” is bad terminology, it may be more familar to you because it has been used extensively in text books and in courses on electricity. You can find an interesting discussion of this topic on the internet at http://amasci.com/miscon/eleca.html#poynt
The number of ohms indicates how much resistance there is to the electrical current in a circuit. Electrical resistance is often compared to a kink in a water hose. The kink restricts or slows the flow of water through the hose.
The interaction of volts, amps and resistance with each other is defined by Ohms law or the equation: amps = volts divided by resistance. But instead of using equations you can still get a basic understanding of how these three properties interact with each other from some simple rules. You need to understand the following rules to understand 420 mA circuits:
Applying one volt to a circuit that has a resistance of one ohm causes a current of one amp.
Increasing the voltage increases the amps in proportion to the voltage increase. Decreasing the voltage decreases the amps in proportion to the voltage decrease. Thus, doubling the voltage causes twice as much electrical current or double the number of amps. And halving the voltage causes half the amps.
Increasing the resistance or number or ohms has the opposite effect. It reduces the current or number of amps. Decreasing the resistance increases the amps. The change in amps is in proportion to the change in resistance.
The total resistance in a series circuit is the sum of the resistances of the individual components, including the wiring.
The amps or current in a series circuit is the same in all parts of the circuit. Thus, the current through one device wired in series with another device is exactly the same.
An ammeter is used to measure the current in a series circuit. It too must be connected in series with the other devices. That means you must disconnect one of the circuit wires from a terminal and connect one lead of the ammeter to that wire. Connect the other lead to the terminal where the wire was connected.
Since the current is the same in all parts of a series circuit you might think that the voltage is also the same in all parts of a series circuit. But it is not. Voltage in a series circuit has some interesting aspects that we need to consider.
VOLTAGE IN A SERIES CIRCUIT
Voltage can be present in a circuit, even when there is no current. It’s like a water hose with the faucet shut off. There is pressure on one side of the faucet, but there is no water flowing with it shut off. Likewise, voltage may be present at certain points in a circuit, but there will be no current if the circuit is open.
As mentioned earlier, voltage is a force similar to pressure in a water hose. Pressure exists when there is a difference in force at two points. Thus, measurements of voltage are always taken across two points in a circuit, without the need to disconnect any circuit wires.
For example, to measure voltage across a resistor in a series circuit, connect the two leads of the voltmeter to the same two terminals where the resistor is connected. The voltage shown on the meter (while the circuit is in operation) is actually the difference in voltage between the two points. It is the same as the amount of voltage applied to that resistor. And it is the same as the voltage dropped across that resistor.
When current is present in a series circuit, the voltage from the output device is divided or apportioned among the components of the circuit. As you would expect, the sum of voltages across all components is equal to the total voltage from the output device.
When a series circuit has two or more components, the voltage across each component depends on two things:
The resistance of that component.
Its percentage of the total circuit resistance.
Consider a series circuit with two components, one with a resistance of 250 ohms and another with a resistance of 750 ohms. The total resistance is 1,000 ohms. The resistance of 250 ohms is 25% of the total resistance. The resistance of 750 ohms is 75% of the total. Thus, 25% of the total voltage is applied across the 250 ohm resistor. And 75% of the total voltage is applied across the 750 ohm resistor.