Multimeter Operation: Radio Shack 22-179

Multimeters

A multimeter is an inexpensive piece of test equipment that is capable of measuring AC volts, DC volts, and resistance. By learning how to use this equipment, it is possible to troubleshoot controllers, solenoids, field wiring, and to verify AC and DC voltage levels.

Electrical Terms

AC volts (VAC) - Alternating Current, this is household voltage. Most irrigation solenoids operate on AC voltage. Rain Bird voltage specifications are 117 VAC ± 10% (105 - 129) volts.

DC volts (VDC) - Direct Current, the normal source is a battery. DC voltage is polarized, meaning that there is a positive (+) and a negative (-), sometimes referred to as ground. The meter must be connected properly to prevent meter damage, the RED lead is (+), and the BLACK lead is (-).

Resistance (ohms ) - A measurement of how difficult it is for the current to flow through the electrical system. This is very similar to friction loss through a piece of PVC pipe.

These may be easier to understand if they are compared to irrigation terms that are more familiar.

Irrigation term Electrical term
Pressure (PSI) Voltage (volts - V)
Friction Loss (PSI) Resistance (ohms - )
Flow (GPM) Current (amps - A)

 

Additional Terms Used in Measuring Resistance

Resistance is the most useful function for troubleshooting irrigation systems. For irrigation applications, a solenoid is considered to be good if its resistance is between 20 - 60 ohms. It is best to compare resistance measurements to that of a new solenoid. Example, if a solenoid measures 44 ohms, it would be considered good because it is between 20 - 60 ohms. However, if a solenoid of the same brand measures 24 ohms when new, it would be an indication the solenoid is deteriorating and should be replaced.

Short - when the resistance measured is below 20 ohms for a single solenoid. This allows excess current to flow through the circuit breaker or fuse. If the amount of current exceeds the rating of the devise, it will open, therefore stopping the 24 volts to the valves.

Open - when the resistance is above 60 ohms, the flow of current to the solenoid is reduced. Think of this as having a rock lodged in the mainline of a sprinkler system. The resistance may increase to the point that the solenoid does not get enough voltage to operate.

HINT... To help remember the difference between an open and a short. An open drawbridge stops the flow of traffic. A short is a short cut for the flow of current through the solenoid.

NOTE...When two valves are operated together on a single station, the total resistance will be less than the resistance of the lowest solenoid. Example: if a 52 solenoid and a 24 solenoid are combined on the same station, the total resistance is about 17, less than the resistance of the lowest solenoid (24.)

Using The Multimeter

The following instructions have been written specifically for the Radio Shack catalog No. 22-179 Digital Multimeter.

AC volts - Turn the dial to the AC V position, AC will show in the lower left corner of the display. Touch the meter probes to the circuit that you wish to measure. NOTE...When the mV is active to the right of the display, divide the reading by 1000. Example, the display shows 061 mV, the actual measurement is 0.061 volts.

DC volts - Turn the dial to the DC V position. DC voltage has a positive (+) and a negative (-) terminal. The red lead is (+), and the black is (-). If the leads are reversed, the display will show a minus (-) sign to the left of the reading. NOTE...When the mV is active to the right of the display, the reading must be divided by 1000. Example, the display shows 061 mV, the actual measurement is 0.061 volts.

Resistance - To prevent meter damage, it is very important that the circuit power be turned off when measuring resistance. Turn the dial to the K symbol (resistance), .OL M will show in the display. This indicates infinite resistance, an open circuit. Touch the probes to the circuit you wish to measure. The display indicates the resistance.

NOTE...When the K is active to the right of the display, the reading must be multiplied by 1000. When the M is active to the right of the display, the reading must be multiplied by 1,000,000. Example, the display shows 41.2 K, the actual measurement is 41,200 ohms.

NOTE...When measuring resistance from the controller, you are measuring the resistance of the wires leading to the solenoid through the wire connector, the solenoid, the second wire connector, and the wire leading back to the controller. Resistance of the control wires do not add much to the total resistance. UFUL 14 is approximately 2.5 ohms per 1,000 feet, 16 gauge UFUL wire is approximately 4.0 ohms per 1,000 feet, and 18 gauge UFUL multi-conductor wire is approximately 6.5 ohms per 1,000 feet.

How To Interpret Display Characters

Voltage

0.0126 volts, or 12.6 millivolts
Multiply the reading by 0.001 when the MV is active.

Resistance

1,004 ohms
Multiply the reading by 1,000 when the K is active.

Knowing how to operate a meter will save considerable time when testing the wiring on a job site. It is possible to inspect the solenoid and field wiring condition from the controller. The procedure is as follows:

Resistance Measurements

1. Disconnect the common wire(s) from the terminal strip on the controller.

2. Turn the dial to K.

3. Connect one of the meter leads to the common wire, not the controller common terminal.

4. Touch the second meter lead to each of the station terminals, and record the resistance readings. Compare your readings to the acceptable range of 20 - 60 ohms.

5. If the measurements are within the acceptable range (20 - 60 ) then the electrical circuit for that station is good. NOTE...This test only inspects the condition of the wiring, the station may not operate properly because of controller and/or valve problems.

6. If the resistance range is below 20 ohms (a short), proceed to the valve and disconnect the solenoid from the field wires. Test the resistance of the solenoid only. If the measurement is still low, then the solenoid must be replaced. If the solenoid resistance is acceptable, then the short is in the field wiring itself (two solenoids connected to the station can also produce a low reading). Wire tracing equipment should be used to locate the problem.

7. If the resistance is above 60 ohms (an open), as in step 7, test the solenoid without the field wires connected. Replace the solenoid if its resistance is above 60 ohms. More than likely the solenoid will test within proper limits of between 20 - 60 ohms. If the solenoid is good, cut out the wire connectors and connect the station and common wires together at the valve location. From the controller, re-test the resistance without the solenoid in the circuit. The resistance should now read very low, possibly 5 ohms or below since only the resistance of the field wires is being measured. If the resistance is this low, then the problem was a faulty wire connector. Install new waterproof wire connectors on the existing solenoid and test the resistance again at the controller.

If the resistance is still high when the common and station wires are connected together, then there is an open somewhere between the valve and the controller, possibly caused by a faulty wire or wire connector. This fault can only be found with the use of wire tracing equipment.

8. This final field wiring test will determine shorts directly into the earth. In addition to the common still being disconnected, remove each of the station wires from the controller. Connect one of the meter leads to a piece of bare wire wrapped around the metal shaft of a screwdriver. Insert the screwdriver into the ground (it may be necessary to wet the ground to assure a good connection). Touch the second lead to the station wires and the common one at a time. Each of these measurements should be above 700K (700,000) ohms. A resistance reading of below 700,000 would indicate a section of the wire has a nick in the insulation and is making contact with the earth. Wire tracing equipment should be used to locate the problem.

9. Transformers can also be tested using a multimeter. Connect the meter leads to the primary winding, that is, the transformer input wires or plug in connectors. You will either get a resistance reading or an open. A resistance reading shows the internal windings are intact, an open indicates the transformer's internal fuse is bad and the transformer must be replaced. The secondary winding, the transformer output, is tested the same way. Connect the meter leads to the output wires. An open indicates that the transformer must be replaced. The 20 - 60 ohm rule does not apply to transformers. It is common to find resistance as low as 3 ohms.

10. A multimeter may also be used to determine what type of battery should be used in a solid-state controller. The controller should be powered up and the battery removed from its connector. Rotate the dial to the DC V setting. There are two battery connectors, a large one (female) and a smaller one (male). Touch the red probe to the large connector, the black probe to the small connector. If the controller is designed to accept an alkaline battery, the reading will be close to zero volts. If it is designed to accept a rechargeable NiCad battery, the reading will be between 7 and 13 volts DC. Never use an alkaline battery in a controller designed to accept a rechargeable battery.

Note...Do not use rechargeable alkaline batteries in solid-state controllers!