How to test the common-mode rejection ratio (CMRR) of a hot runner system

In order to test a hot runner system's CMRR, a controllable common-mode signal must be injected, and the output deviation must be measured. Next, a formula is used to determine the CMRR. To make that the temperature management system can function steadily even in the presence of electromagnetic interference, this test is usually carried out in a laboratory or high-precision maintenance setting.

1. On-site Testing Method (Needs Expert Equipment)

Procedures

Operating Guidelines

Set up the measuring instruments and signal source.

Utilize a typical thermocouple simulator, a high-precision digital multimeter or oscilloscope, and an adjustable AC power supply to simulate common-mode interference.

Create a Test Loop

Attach the temperature controller to the thermocouple input. As common-mode interference, provide a 50Hz/60Hz AC voltage (such as 10Vrms) to the signal line's neutral point.

Common-Mode Signal Injection

Simultaneously apply an AC voltage of the same amplitude to the thermocouple's positive and negative terminals to simulate ground loop noise.

Calculate the Variation in Output

Take note of the temperature controller's greatest temperature variation and translate it into an equivalent millivolt error (ΔmV).

Determine the CMRR

Determine the CMRR value using the output error and input common-mode voltage.

For instance, when a 10V common-mode voltage is applied, the temperature controller displays a temperature drift that corresponds to a 1.4mV error (≈40℃), followed by CMRR ≈ 80dB.

3. Other On-Site Evaluation Techniques (No Specialized Equipment Needed)

The following techniques can be used to indirectly evaluate CMRR effectiveness in the absence of laboratory facilities:

Observe temperature stability: Look for jumps or spikes in the hot runner temperature control when a high-power motor is starting and stopping.

To test the signal, connect an oscilloscope probe in parallel with the thermocouple wire and look for high-frequency noise superposition.

Compare the efficiency of shielding by turning on or off the shielding wire's grounding and seeing if temperature variations are noticeably reduced.

For testing, swap it out for a high CMRR module. Temporarily attach a temperature control module with a CMRR of at least 100dB, then assess how much the system stability has improved.

Recommendation: A third-party body should periodically commission formal CMRR testing and certification for high-requirement industries including the automotive and medical sectors.

4. Supplementary Steps to Boost Testing Efficiency

Prior to testing, calibrate sensors and equipment to guaranty fundamental accuracy;

To more accurately replicate actual interference settings, test under typical operating conditions, such as full load operation;

Keep track of multi-channel data and evaluate each hot runner's response consistency;

Use TUS testing to confirm overall temperature control performance.

Safety principle: "Professional testing + on-site verification + regular evaluation" guaranties the hot runner system's dependable anti-interference capabilities.