A lot of batteries lying around but don’t know which one is still alive
One can test common household batteries with a multimeter to check if it is dead. For this, the selector should be set for proper range on the DC scale. Household batteries like AA and AAA are rated at 1.5-volts, so select a 4-volt range on the DC scale for common batteries. If testing a 9-volt battery, then move the selector to the 40-volt range.
Touch each end of the battery with a probe. If the red and black wires are reversed, then a minus (-) sign will appear in front of the numbers. A 1.5-volt battery is “dead” if it registers 1-volt or less. If a device does not work, but the 1.5-volt battery registers (for example) 1.38-volts, then the issue is with the device. A 9-volt battery is “dead” when it fails to read about 7.5 to 8-volts.
When lamps leave you in dark
There are two possibilities if a bulb doesn’t glow. One might be when the light bulb burns out other would be the problem with the lamp. This can be found out by two tests. First, set the multimeter to measure resistance in ohms. If there is more than one resistance setting, choose the most sensitive one. Set the meter to the 400 volt AC selection. Touch one probe to the side of the bulb socket. Touch the other to the tip at the bottom of the socket.
The meter should read about 120 volts. Sometimes, expected voltage in the bulb socket and also the bulb are good, but the lamp still might not light. If the socket is older, the contact tip at the bottom of the socket may have lost its springiness and it may not make dependable contact with the centre tip of the bulb. Use a popsicle stick or screwdriver to lift the tip. Then screw the bulb into the lamp socket and it should work.
NOTE: This test will not work with the compact fluorescent bulbs and with fluorescent tubes because they do not have a continuous conductor running through them, like an incandescent light bulb.
To check the fuse
From household gadgets to automobiles almost all electronic instruments use fuses. If such devices are not working first thing should be to check the fuse before calling for an electrician. First thing here would be to turn off the device, remove the power and then remove the fuse. Set the multimeter to CONT. (continuity) and do the continuity test by putting the positive and negative leads on either side of the fuse and check the reading. If the meter beeps there are continuity and the fuse works, if no sound then it means that the fuse is bad.
When air conditioner quits the job
It might be during the worst weather conditions your AC fails to work. Spending a muggy night is the last thing you would want to do. Some small multimeter tricks would help you find out what actually went wrong, can you fix it yourself or is there a need to call an electrician.
Air conditioning system may have more than one set of fuses protecting it. There may be a set of circuit breakers in your main electrical panel. Check to see that the circuit breaker toggles have not moved to the “off” position due to a sudden overload. If it is still in “on” yet the Ac is not working, then shut off all power to the air conditioner. Turn the air conditioner thermostat to the “off” position also turn off the power at the breaker box too. First check the fuses for continuity, if the fuses are fine, then discharge the capacitor by running the metal tip of a screwdriver between the two terminals to rid the capacitor of any electrical charge. There may be a spark when this happens.
Checking the capacitor
Disconnect the wires of the capacitor using needle nose pliers. Set your multimeter to read Ohms or resistance, on the highest setting your meter has. This is done by turning the dial on the multimeter to the setting labeled “cap” or that has the ohms icon on it. Place the black lead in the hole marked “com,” and the red lead into the hole that says cap or ohms icon by it. Place one lead on each of the capacitor’s terminals and see what reading you get.
This reading should be within 6 percent of your particular capacitor’s rating. If it is not, then your capacitor is faulty and must be replaced with a new one. If that’s not the problem locate the box that contains the fuses and Do the continuity test. Detailed explanation can be found on how to check an air conditioner