Testing Capacitors: Motor Capacitor Meter
Build a Handyman Capacitor TesterBy rlarios
Description: Motor Capacitor Meter
Skill Level: Intermediate
Assembly Time: 1 hour
This electronics project enables any handyman or homeowner to test motor-run capacitors on appliances even when there is no cap meter at hand. This kit allows you to get a reading that is related to the actual capacitance of a motor-run capacitor under test. Capacitance range goes from 1uF to 100uF which is good enough to troubleshoot capacitors from fans to air conditioners.
Required tools and components:
- Soldering iron and solder
- Crimping tool for quick disconnect terminals
- Wire cutters/strippers
- Long nose pliers
- Knife or box cutter
- Black electric tape
- Scotch tape
Insulate the Discharge ResistorCut approximately 1-1/8" off the heat shrink tubing and grab one 100K 1/2W 5% resistor. Place the resistor through the tubing and apply heat in order to insulate the body of the resistor and part of the leads. However, leave the ends of the leads uninsulated.
Preparing the Power Cord PigtailsUsing your wire stripper, strip enough of the insulation so the pigtails have about 1" of copper exposed.
Adding two pieces of 1" heat shrink tubingCut off two pieces of 1" heat shrink tubing. Add one piece of tubing to each of the pigtails
Installing the Discharge ResistorGrab one end of the resistor (from earlier step) and wrap around one end of the power cord. Through the use of long nose pliers ensure that no part of the resistor is sticking out. Apply flux to this lead and solder. Spread the two pigtails apart to enable you to solder in the other end of the discharge resistor onto the other pigtail. Once again, do not have any of the resistor's lead sticking out.
Insulating the PigtailsCut two 5/8" pieces of the heat shrink tubing. This tubing will be used to insulate the connections you completed in the previous step. Slide the heat shrink tubing onto one pigtail to cover both the insulated resistor lead and the previously insulated cord (leaving about 1/4" of exposed copper at the Power Cord end). Do the same to the other lead of the Power Cord. Apply heat (refer to the image for example).
Installing the Quick Disconnect TerminalsSlide one of the 1/4" pigtails into one of the quick disconnect terminals. When doing this make sure there are no metal parts from the cord-resistor end exposed. Once everything is in place, crimp the terminal. Do the same to the other 1/4" pigtail.
Making the Metering CoilWrap the hook-up wire 22 times around a cylindrical object (about 2" in diameter). To prevent movement, fix one end of the wire to the cylinder with scotch tape, then proceed to wrap the wire. The coil does not need to be tight as long as you get 22 rotations. When done slide the coil off the cylindrical object and keep in place with electrical tape.
What the 22 rotations does is multiply the motor-run capacitor's actual current to allow you to get the correct current reading in Amps, which would correspond to the true capacitance in Mircofarads when the line voltage is 120V. When the line voltage ranges from 115V to 125V, there could be a 4% error in the reading number. If your line voltage is constant at either end, then you can experiment by adding or subtracting one turn of the coil. For example, 21 turns with 125V or 23 turns with 115V.
Identifying the "Hot" Pigtail in the Power CordIn continuity mode, use one test probe on the multimeter to touch the narrower terminal of the plug, aka the hot lead. With the other test probe, test the quick disconnect terminals for continuity. Once you identify the "hot" conductor, mark it to help with the next step.
Cutting the Conductor Previously IdentifiedUsing the knife or box cutter, separate the conductors of the power cord about 1/2 or 1/3 of its length from the quick disconnect terminals. For at least 6" separate the two conductors from each other. With the wire cutter, cut the conductor that corresponds to the hot leg wire.
Installing the In-Line Fuse Holder and Current SensorUsing the wire stripper, strip 1" of insulation from the ends of the in-line fuse holder and the current sensing coil. Cut one 1" piece off of the heat shrink tubing and slide in one end of the in-line fuse holder leads. Twist one end of current sensing coil leads around the end lead of the in-line fuse. Make sure that the twisted wires do not exceed the thickness of the in-line fuse holder lead insulation. Carefully apply solder to this joint to prevent any heat from reaching the heat shrink tubing. Slide the heat shrink tubing to cover the joint and apply heat (with the lighter) to shrink the tubing in place.
Using the in-line fuse holder and current sensing coil you just soldered as a reference, cut off a section of the hot leg conductor in the power cord. On the plug side of the power cord, strip 1" off the end of this hot leg conductor insulation, cut off one 1.5" piece of heat shrink tubing, and from the copper end slide this conductor through it as far as possible. Twist the free end of the in-line fuse holder along with the power cord end making sure the thickness of both twisted wires do not exceed the power cord's conductor insulation. Apply solder to the joint. When cold, slide the piece of heat shrink tubing over the joint. Apply heat to the shrink tubing to keep in place. Do the same for the remaining end of the current sensing coil and quick disconnect terminal's end of the power cord. Make sure all joints are properly insulated for safety.
Taping the Fuse Holder and Coil to the Power CordTake the 10A Standard fuse and put in into the in-line fuse holder. When twisted, the fuse holder opens and when twisted in the opposite direction the fuse holder locks into place. Do a continuity test between each quick disconnect terminal and its corresponding plug's lead. Make sure that there are no short circuits between the quick disconnect terminals. This fuse is meant to blow in the case the quick disconnect terminals accidently get shorted together or a shorted capacitor gets unknowingly tested.
Once complete, put the in-line fuse holder and current sensing coil pair next to the other conductor of the power cord. For a nicer appearance, tie them together with electrical tape (or plastic ties if available).
Completing the Final TestingDO NOT PLUG IN THE HANDYMAN CAP METER YET!
Insert your handyman cap meter quick disconnect terminals on the capacitor-under-test terminal tabs. Make sure not to touch them in case the capacitor still holds a charge. If the capacitor is charged, it will discharge through the handyman cap meter's discharge resistor.
Clamp the Amp meter around the handyman cap meter's current sensing coil. Using the capacitor label microfarad rating as a reference, adjust the scale (if not auto-range). Example: if 50 microfarads, adjust the scale so the meter reads 50A. Once you are positive everything is okay plug in the handyman cap meter to a well-known wall outlet. If the capacitor is okay, the Amp reading should be approximately the same as the nameplate microfarad. Do not leave your handyman cap meter unattended for long. Once you have taken the reading, unplug the handyman cap meter from the wall outlet and let the capacitor-under-test discharge for one minute. Remove the capacitor and if you need to test another one, repeat the above procedure.
Due to the fact that the handyman cap meter's current sensing coil's wire ampacity is 7amps, to stay on the safe side, you must NOT test motor-run capacitors bigger than 100 microfarads with this handyman cap meter. A 100 microfarad motor-run capacitor draws approx. 4.5A at 120V. Leaving a 100 microfarad capacitor energized at 120V for too long will cause sensing coil to get warm to the touch.
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