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Using a digital multimeter to measure array voltage.

AC electricity can be measured by using the meter's normal probes, with the meter in series in the circuit, or by using an external probe that clamps around the wire. This probe uses the induced magnetic field generated by alternating current to measure the amperage. While not quite as accurate as running the circuit through the meter, AC clamp-on meters allow the measurement of amperage above the standard 10 amps that most DMMs can measure directly. The AC clamp-on current probes are usually inexpensive. Once again, "true rms" DMMs must be used on modified square wave inverters to get an accurate measurement.

DC Amps

Measuring DC amperage is much the same as measuring AC amperage. Two ranges are available with the DMM and both are fused. The polarity of the current is indicated by the DMM—it tells you which direction the electrons are flowing.

DC clamp-on probes are available to measure DC amperage by clipping around the wire. The situation is similar to the AC clamp-on current probes—they are less accurate, but more convenient since you don't have to open up the circuit and run the circuit through the meter, and they allow you to measure higher amperages. DC clamp-on probes are relatively expensive—an accurate one will probably cost more than the DMM itself.

Frequency

Many of the more expensive DMMs will also measure frequency. This function is great to fine-tune your 120/240 VAC generator and make sure that it is running at exactly 60 cycles (Hz). It can also be used to do esoteric things, like calculate the rpm of your wind generator.

DMM Purchasing Tips

Many, many makes of DMMs are available. Prices start at under US$50 and go to well over US$300. In general, the more you pay, the higher the accuracy and the greater the number of functions the DMM will have. I usually recommend that novices start with an inexpensive meter, and once they have learned to use it, upgrade to a more expensive instrument.

Here are some ideas to help you find a good DMM. Look at the mechanicals on the DMM—the switches, jacks, and plugs. These mechanical items most often fail first. (This is from experience—I've owned dozens of DMMs over the years.) Check out the ruggedness of the case—is it light and flimsy, or heavy and durable? Investigate the fuses used for current protection in the meter—are they easy to replace and are the fuse types common? What type of battery does the DMM use? Most use a 9-volt transistor radio battery, but some use AA cells. An unusual battery can be a pain over the years.

In terms of brands, the inexpensive meters marketed by Radio Shack are adequate for beginners. If you are willing to spend more than US$200, consider those made by Fluke. The Flukes are my all-time favorite, and I currently own three of them. The better Flukes have all the features described above, and a few more besides. I particularly like their "average" function, which allows taking measurements for up to 42 hours, and then gives a true arithmetic average of the data—very useful. Flukes are ruggedly built and should last for many years with careful handling and storage.

AC and DC clamp meters allow you to measure system amperage without modifying any wiring.

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Measuring Amperage

Resistance/Continuity

You will probably only need to measure resistance if you get into electronics design or troubleshooting. But the continuity function of a DMM is frequently used in troubleshooting electrical systems. It can tell you that the circuit is complete, and that all wiring and connectors are continuous. Many meters have an audible continuity function that sounds a tone when there is continuity between whatever points the two probes are touching.

AC and DC clamp meters allow you to measure system amperage without modifying any wiring.

new and fresh from the factory, that are reverse polarized. (The terminal marked positive is really negative and the terminal marked negative is really positive.) Such batteries should be immediately returned and not wired into the battery bank.

As I assemble the series strings of batteries that make up the pack, I measure the voltage of each resulting string. I want to see if the numbers add up and if everything is properly polarized. For example, in a 24-volt nominal battery bank made up of individual 6-volt batteries, four batteries will be connected in series (6 x 4 = 24). I test to make sure that each string is at 24 volts nominal, and that the positive and negative poles of the resulting battery string are correct. Finally, when all the series strings are wired into parallel, I check the resulting voltage and polarity of the completed battery bank before wiring it into the system.

Checking Inverter Polarity, AC Output Voltage & Frequency

The first check to make before installing an inverter is

Using a DMM with PV Systems to make sure that the battery input polarity is correct.

DMMs are particularly useful when installing solar- While the main battery/inverter breaker is still in the off electric systems. They can make quick checks for polarity, position, check the polarity on the inverter input cables by which is of prime importance. If controllers and inverters measuring the DC voltage between the battery side of the are installed with the polarity reversed, they can be breaker, and the negative bus bar. Connecting an inverter instantly damaged. in reverse polarity will kill it dead, immediately. Double-

When wiring a PV array, I check each and every check this—inverters are expensive, and connecting module before adding it to the array. With the module them reverse polarity is not covered in any inverter facing the sun in full light conditions, I check the warranty I've ever encountered. (The same goes for module's open circuit voltage (Vqc) by attaching the charge controllers.)

DMM's positive probe to the module's positive terminal Once the inverter is connected and operating, use the or lead, and the negative probe to the PV's negative DMM to measure its AC voltage and frequency. Do this terminal or lead. The voltage reading on the meter should immediately to make sure it is functioning properly. Voltage match the Voc specification listed on the back of the should read 117 VAC plus or minus 5 VAC. Frequency module if the measurement is taken in full sunlight. I also should read 60 Hz, plus or minus 0.2 Hz. make sure of polarity at this stage, since some PVs are not marked clearly as to which terminal is positive and which Checking Engine Generator Voltage &

is negative. Frequency

Next, I check the short circuit current by changing the Use the DMM to check both generator voltage (rms and meter's setup. I put the DMM into high current DC mode peak), and generator frequency. RMS voltage should be

(usually this is 10 amps for most meters) and attach the between 110 VAC and 120 VAC. This depends on generator probes to the positive and negative terminals or leads of the loading, so make sure that the generator is at least at half

PV, and to the meter as shown in the illustration. This short load before making this measurement. Peak AC voltage circuits the PV through the meter and allows a measurement should be around 164 VAC, but no lower than 150 VAC.

of its short circuit current (Isc). This specification is also Low peak voltage will make your battery chargers work printed on the back of the module. poorly. Frequency should read 60 Hz, plus or minus 0.2

While I've never discovered a new PV that didn't meet Hz. While the generator is under its typical load, adjust the the Voc and Isc specs, I've often used this procedure to test generator's throttle control to bring the frequency as close older modules and to troubleshoot arrays. When installing to 60 Hz as possible. large arrays, remember that a clamp-on DC current probe will be necessary to measure the full current of the array. DMMs Are Your Eyes...

Performing these tests on a new array makes sure that Using a DMM at all stages of system installation ensures all the elements are functioning properly and that all the a working system, and no catastrophes or surprises.

polarities are correct. Where the DMM really shines is doing diagnostics and troubleshooting, but that's another, far deeper subject than

Checking Battery Polarity this beginning article can cover. If you want to know more

I perform a voltage measurement on each battery before I about DMMs and how to use them for troubleshooting, wire it into the battery pack. I have seen batteries, brand please see my article in HP60.

Measuring Open Circuit Voltage mV n -MV /C mA V ^ X A

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