Test points for microwave UV power supplies allow maintenance teams to quickly and easily troubleshoot the system’s three basic circuits: the photocell, filament, and magnetron current. Located on the front or back of the power supply, some units may have three sets and others only two but all the systems that utilize these tools have the same basic functionality, helping you check to see if your UV lamp is working properly.
While not all microwave UV systems on the market utilize test points, every Miltec microwave UV system does, and these helpful tools are not only good for troubleshooting but can be used to predict maintenance cycles, and end of life signs for consumable parts before they fail.
Those systems without test points rely on internal controls to report faults. But with up to sixty different fault codes to try to pinpoint the exact cause, the ability to test the three basic circuits or test points is often more helpful and can reduce downtime, scrap, and line down failures.
Test Point 1: Photocell
The photocell or photoresistor’s basic job is to ensure the UV bulb is on and functioning normally on the 12-volt DC circuit. Your photocell test point can also help you to know if the health of your systems optical parts (UV bulb and Reflector). This could be seen in the photocell voltage slowing raising over time.
With the unit in “standby” mode, turn the meter to DC volts and measure the photocell test point. You should see a reading of 12 VDC give or take a half volt either way. This indicates normal function. When you go to “lamp on” mode the voltage will drop within a few seconds to less than 2 VDC depending on your system.
Test Point 2: Voltage
The second common test point is the filament voltage test point. This is typically read in AC volts. With the unit to “standby” mode, you will see a reading between 114- and 121-volts AC and in “lamp on” mode about 82 volts AC plus or minus a few volts.
Test Point 3: Magnetron
The final test point set is the magnetron current test point. While not all units have this tool, if your power supply does, here are a few ways to locate them and various readings you may see when utilizing them.
The test point set for all 10-inch microwave systems uses three points labeled Mag A, Common, and Mag B. For those who use a 6-inch microwave system, there are two test points labeled Magnetron or Mag test point. The reason for this difference is that all 10-inch microwave systems utilize two magnetrons and run at either 400- or 600-watts per inch while the 6-inch microwave system only has one magnetron. Some systems may read in millivolts DC and others may read in DC volts.
On a 10-inch system your test points should be equal or close to equal no matter the power level running. If they go too far from even (one side running properly and the other with failing parts), a power imbalance fault may trigger. If it doesn’t trigger, the lamp output will be low or lower than normal.
Wildly fluctuating readings (half a volt or 50 milliamps high or low that never stabilizes) normally indicates that one or both your magnetrons are moding and are an unstable load for the power supply, hence the voltage swing. This moding is the normal end of life indicator for all magnetrons, signalling that they are ready to fail and should be replaced as soon as possible.
It should also be noted that running in this state the UV output can be 25% to 30% down on normal power causing uncured material and possibly triggering an investigation into coatings and process issues that may not be happening.
Because the location of the test points is so convenient, monitoring test points over time should become a part of any preventive maintenance plan, and the more you test, the better the insight and the system.
With more than 70 years of combined UV experience, Miltec UV’s service team has traveled the world helping maintenance workers repair systems and better understand how they work. In our UV service blog, they will share their knowledge and some of the lessons they have learned over the years. Got a service-related question? Email the team at firstname.lastname@example.org.