Friday, June 28, 2024

Damaged Current Shunt Resistor in Cen-Tech Multimeter

The 20mA range current measurement stopped working on my Cen-Tech multimeter.  The other current measurement ranges still worked.  I suspected a damaged current sensor resistor, which is about 10Ω.  I opened up the meter and quickly identified a 9Ω resistor which was measured open.  And the discoloration indicated damage, which was probably a result of overcurrent.  A nearby 1Ω resistor also showed some discoloration, but was still intact.  The resistor is of size 0805, which probably has a power rating of 1/4W or less.  So the maximum current is 167mA at this rated power.  The fuse is rated 500mA, which would not be able to protect the resistor.  Once the resistor was replaced; it worked again and was checked against a Fluke 87 multimeter, the measurements are within 20uA of each other around 20mA, or 0.1%.  When it is compared with a Fluke 87, the Fluke uses a 0.991Ω 0.1% 2.5W resistor for mA range current measurement.  The maximum current is 1.6A, which is greater than the 1A fuse rating.


No comments:

Post a Comment