Measure Dc Amps

Fluke 381 Clamp Meter Wins Gold Award in the ACHR-The NEWS 2011 Dealer Design ...

The 127 entries in the 2011 awards were judged by a panel of independent HVACR contractors selected by the editorial staff of The NEWS. The panel reviewed each entry and judged it on its ease of installation and use, maintenance, application and repair, and competitive difference. Gold, silver, and bronze winners were selected in each category.

"The Dealer Design Award recognition for the innovative 381 Clamp Meter confirms what we hear all the time: that technicians want a meter that makes a real difference in the way they do their jobs," said Clement Feng, Vice President Global Marketing, Fluke Corporation. "With its unique detachable display and iFlex probe, the 381 delivers the performance and flexibility HVACR technicians need to help them get their work done quicker and more easily."

This is the fourth major recognition this year for the Fluke 381. The meter also won the EC&M 2011 Product of the Year Award in the portable measurement equipment category, the 2010 Control Engineering Engineers' Choice Awards in the portable measurement equipment category, and 2010 Plant Engineering Product of the Year Silver Award in the Maintenance Tools and Equipment category.

About the Fluke 381 Clamp Meter The first clamp meter with a wireless display, the Fluke 381 allows one technician to do the work of two, by leaving the clamp in place and taking the measurement readout to another location up to 30 feet away. The accompanying iFlex flexible current probe allows the user to measure large, inflexible, or hard-to-reach conductors up to 2500 amps. And the clamp meter itself allows electricians to take all of the most common measurements required by today's complex systems, quickly, without complication, and with extremely high accuracy. That means fewer tools, less hassle, faster results. The U.S. manufacturer's suggested retail price for the Fluke 381 is $499.95.

Fluke Corporation Founded in 1948, Fluke Corporation is the world leader in compact, professional electronic test tools. Fluke tools deliver the testing and troubleshooting capabilities that are critical to keep commerce and industry running smoothly.

How to measure dc current with a microcontroller? :Embedded Lab

Microcontrollers usually don’t have specific ports for measuring currents, but they do have ADC channels through which you can measure analog voltages of a certain range. This means a dc current can be indirectly measured by a microcontroller’s ADC channel by first converting the current into voltage. The simplest way of doing this is to place a resistance in series with the current path and measure the voltage drop across it. But hold on, if you place an additional resistance in the circuit, it will affect the original current. Therefore, we need to use a very small value resistance so that it’s effect in the circuit current won’t be significant.

Resistors with values less than 1 Ω are available in electronics stores. Depending upon the amount of current in the circuit, you need to choose proper power rating for the resistor. Suppose, if you pick 0.47 Ω, and the maximum current in the circuit is about 2 A, then the resistor should have the capacity of dissipating 4 x 0.47 ≈ 2 Watts of heat.

You can also make a small value resistance by yourself. Yes, by simply winding a copper wire into coil. I have made one from a 5 ft long solid copper wire (22 AWG) with plastic insulation on outer side, as shown below.

Now lets measure its resistance. The resistance can be measured directly with a digital multimeter. My digital meter shows its value equal to 0.3 Ω. This measurement may have higher uncertainty as it is very small and most multimeter does not show values beyond 1 decimal digit. The resistance can also be measured using Ohm’s law. Connect a 47 Ω resistor in series with the coil resistance (Rs) and supply a 5V power as shown below. Next, measure the voltage across Rs and current through it separately using the multimeter. In my case, I found the measured voltage and current values to be 24.1 mV and 84.3 mA, respectively. This gives the resistance of the coil is about 0.286 Ω.

Now, suppose that the range of current to be measured using this coil resistance is from 0-2 A. Then the voltage drop across the coil resistance will be somewhere from 0 – 0.57 V. Because of its low dynamic range, this voltage signal may not be accurately measured with a microcontroller’s ADC module.So this requires some sort of scaling voltage. One way to do this is to use an operational amplifier circuit, as shown below.

In the circuit, Rs is the current low value sense resistor (our resistance coil) that is connected in series with the load resistance. Our goal is to derive the charge current (I). The low voltage drop across Rs is amplified by the non-inverting amplifier. The amplifier gain is set by resistors Rf and Ri. For f = 10 K, and Ri = 1.3 K, the amplifier gain would be about 8.7. This is sufficient to linearly scale Vs (0 to 0.57 V) Vo (0 - ≈ 5 V). Now you have 0-5 V voltage signal that corresponds to 0-2 A current through Rs. This voltage signal is now more appropriate for ADC conversion with Vref = 5 V.

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