When engineers use the EM231 RTD temperature module for the first time, they often encounter the following problem: after the module is powered on, the SF light on it flashes, even though there is no input connection. This raises the question: is the module faulty? Actually, no. First, we need to know what this "SF" light is used to detect. This SF LED is actually used to identify "disconnection" on the input connection. If the module has an input that is not connected, then the flashing SF light indicates "disconnection", and the CPU information will display the module status as "range out of bounds error".
If any channels on the RTD module are not connected, the unconnected channels must be connected in parallel to the used channels or short-circuited with a resistor. Taking the UN 231-7PB22 two-channel RTD as an example, only when channel A is in use:
1. Connect channels A and B in parallel, with the input of channel B connected to channel A in a one-to-one manner.
2. Short-circuit channel B with a resistor. Channel B is connected as a two-wire sensor.
For resistor R, the resistance value is chosen based on the type of sensor set in the DIP switch.
The above question is easy to explain and understand. So, if the engineer has used all the channels and the DIP switch settings are correct, but the SF light is still on, what could be the cause? As a technical support staff member, the first reaction might be to suspect a broken RTD connection! Then we need to determine what type of RTD the customer is using and how it is connected. This requires us to understand how to connect two-wire, three-wire, and four-wire RTDs. Below, we will explain in detail.
1. Four-wire thermal resistor sensor wiring method (highest accuracy)
As shown in the diagram, before wiring, we first need to determine the two common terminals of the RTD. If we use a multimeter to measure the resistance of the four wires of the RTD, the resistance measured by pins 1 and 4 is 100Ω, the resistance measured by pins 2 and 3 is 100Ω, the resistance measured by pins 1 and 2 is very small, and the resistance measured by pins 3 and 4 is also very small. Therefore, we can determine that pins 1 and 2 are the common terminals, and pins 3 and 4 are the common terminals, thus obtaining the wiring diagram above. If the common terminals are not determined and wiring is done directly, the terminals may not correspond, and the resistor may not be connected to the module. This can easily result in the module being unconnected, causing the SF light to flash.
As can be seen from the wiring diagram, the four-wire heating resistor does not require leads, thus eliminating the influence of lead resistance. It is mainly used for high-precision temperature detection.
1. Wiring method for three-wire resistance thermometer (most common)
As shown in the figure, this is a three-wire heating resistor with three wires. Measure the resistance of any two wires. Two sets of wires are identical; one set has very low resistance. Connect the two terminals with the low resistance to the negative terminals of the channels on the module, for example:
The resistance measured by 1 and 3 is 100Ω, the resistance measured by 2 and 3 is 100Ω, and the resistance measured by 1 and 2 is very small. Therefore, connect the wires according to the wiring diagram above. It can be seen that a three-wire heating resistor requires one lead. Compared with a two-wire heating resistor, this method better eliminates the influence of lead resistance and is the most commonly used wiring method.
1. Two-wire heating resistance sensor wiring method (lowest accuracy)
As shown in the figure, a two-wire heating resistor has two wires. The wiring is as shown in the diagram above. It can be seen that a two-wire heating resistor requires two leads. The wiring method is very simple, but the resistance of the leads increases the measured resistance value (the resistance of the entire circuit is the resistance value of the heating resistor plus the resistance value of the two wires). The measurement error is large, and the magnitude is related to the material and length of the wires. Therefore, this wiring method is only suitable for occasions with low measurement accuracy.
For more services, please follow Yiwei's WeChat service account (unimat2014)!
