Current MeasurementYou have a digital multimeter. It’s rated at 10 A max, but you would like to measure a current at about 30 A. How is that possible? Good news is that by using Ohm’s Law you can circumvent such limitation! Ohm’s Law describes the relationship between voltage (V), current (I) and resistance (R) in the form: V = I x R. This article will talk about how we can perform such measurement and also how to verify that our instrument is capable of handling such a task by using real examples. Some good practices and precautions will also be mentioned here.With just a slight manipulation of that equation, we can obtain I = V / R, which indicates to us that we can also derive the value of the current measurement if we know the voltage and resistance. From this, we observe that the current is inversely proportional to the resistance. So, if we could fix the resistance to some value using a resistor, we could then measure the voltage across that resistor to obtain the current. This resistor should be of a very small in order not to disrupt the circuit. This is termed as the shunt resistor and is placed in series with the circuit load as shown in Figure 1.

Figure 1 - Using a digital multimeter & shunt resistor to measure current

Let’s consider this real-life scenario - we are still interested to measure 30 A and the multimeter that you are using is for example, the Agilent U1253B 4.5 digit handheld digital multimeter, which has a maximum current rating of 10 A. Then consider some precision current shunts from Ohm-Labs. From the selection of shunt resistors, we identify one which is within the right range (rated greater than 30 A). Hence we select the CS-50 shunt resistor model with the following specs: 

Model Amps Ohms Output Accuracy

CS-20 20 0.05 1.0 V 0.1%

CS-50 50 0.01 0.5 V 0.01%

Table 1 - CS-50 shunt resistor specifications

Figure 2 - Image of CS-50 shunt resistor from Ohm-Labs.

CS-50 is a 0.01 Ohms shunt resistor with a 0.5 V output at maximum rated current of 50 A. Notice the fins in Figure 2? That's a heat sink to regulate the temperature on the resistor as things can get pretty heated up during operation.Back to Ohm’s Law, if we are measuring current of 30 A with shunt resistance of 0.01 Ohm, we should obtain:

Vmeasure = 30 A x 0.01 A = 0.3 V

This 0.3 V is definitely within the safety rating of the Agilent U1253B which is 1000 V. This proves that by using shunt resistors, we can measure current (30 A) that are greater than the maximum rating (10 A) of the multimeter.

Although ensuring that our measurement stays within the instrument’s maximum ratings is crucial, we also need to ensure that we have sufficient resolution to measure the smallest step change that interests us. For example, to detect every 0.1 A of change in our measurement, our multimeter must be able to measure voltage as small as: 

∆Vmeasure = ∆0.1 A x 0.01 Ohm = ∆1 mV

As an illustration, Table 2 below shows some values of Vmeasure as we vary the Imeasure at a resolution of 0.1 A about our target current value of 30 A. Notice that with every 0.1 A step of change, the voltage also change with a step of 1 mV or 0.001 V. This table is also useful to help convert the voltage measured to the corresponding current value.

Vmeasure Imeasure Rshunt

0.295

29.5 0.01

0.296

29.6 0.01

0.297

29.7 0.01

0.298

29.8 0.01

0.299

29.9 0.01

0.300

30.0 0.01

0.301

30.1 0.01

0.302

30.2 0.01

0.303

30.3 0.01

0.304

30.4 0.01

0.305

30.5 0.01

Table 2 - Imeasure vs Vmeasure

Referring to the datasheet of the Agilent U1253B under ‘DC Specifications’ section (see Figure 3 below), we zoom in on the 500 mV or 0.5 V range as our measurement falls within this and find that the resolution is 0.01 mV. This means that the Agilent U1253B is more than sufficient to resolve a 1 mV change.

Figure 3 - DC voltage specifications of the Agilent U1253B

Measure AC and DC Current Amps using a  Hall Effect Current Sensor Transducer

 There are three key advantages to using hall effect current sensors transducers:

1. They can be totally isolated from another high voltage electrical system which eliminate risk to delicate monitoring equipment and also minimizes safety concerns.  In other words this sensor only detects the magnetic field around the wire, there is no electrical contact between the sensor and the wire.   This is a nice advantage over using a current monitoring shunt precision resistor.

2. If your signal is too weak or you are not getting the resolution you want, you can simply loop the wire through the current clamp as many times as you want to double, triple, or quadruple the sensitivity or resolution of your sensor.  For example, if your current signal is only .03 Amps, you could loop the wire through the sensor 10 times and the signal would by 10X stronger and would appear as 0.3 Amps. 

3. Unlike the current shunt sense resistor which can have thermal temperature heat dissipation issues, the hall effect current sensor does not get hot.  Even when measuring 50 Amps!

 

Example Wiring Diagram For the CLSA2CD Sensor Sold  HERE on Amazon.com.

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The diagram above shows a 12 Volt DC wall adapter supplying voltage to a 8 Volt regulator.   The 7808 Volt regulator puts out a very stable DC voltage.  This is very important because the sensor outoput is only ~0.032 Volts per Amp that it measures, so if the voltage you are supplying the CLSA2CD is noisy, your data will get lost in the noise.   The ground is shared through out the circuit.   You could mount these components on a small piece of proto board like the one shown below.   Or if you want to get one already assembled take a look at the one sold at http://measure-current.com

 

APPLICATIONS

WIND SOLAR HYDRO

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HOW IT WORKS

 Below you will see a graph showing the relationship between current and sensor output voltage.   In this case, the sensors were mounted on a circuit board. 

The current sensor was supplied with 8 Volts DC from an on board regulator.  You can see that when there is no current flowing through the large red wire, the sensor simply divides its supply voltage in half (8V divided by 2 = 4.0 Volts output).

However, for every amp that flows through the wire, the voltage output from the sensor increased about 0.033 Volts. 

 The formula you could use to convert from Volts to Amps is as follows:

 

Measured Current  = (Vsensor - 4.0) / 0.033

So for example if your sensor puts out 5 Volts, then your measured current would be something like:

(5V - 4V)/ 0.033 = 30 Amps

 

For Purchase Information Go To  http://measure-current.com

EXAMPLE DATA FROM SENSOR - SOLAR PANEL / BATTERY CHARGING APPLICATIONBelow you can see real data from just one  sensor for a solar panel / battery charging application. The green trend line represents data from the sensor.   The red trend line has been converted to Amps using  the following  linear equation.   Amps = ( Vsensor - 4.0) / ~33mV

 As the sun comes up, current slowly flows into the batteries rising from 4 Volts DC to 4.5V DC.  This corresponds to a current range of Zero amps up to 16 Amps shown in red on the right side of the graph.

When someone in the home turns on an electric appliance then the current goes negative down to 2.6 Volts DC because it changes direction as it flows from the batteries into the House AC inverter which could be powering something like a refrigerator  or washing machine.  A value of 2.6 Volts DC output converts to a current value of ~ -45 Amps.  Assuming these batteries are setup in a 24 Volt configuration, then you could use this current measurement to approximate how much power your batteries are putting out.  In this case it would be about 1,080 Watt power output to the inverter.

(24V X 45A = 1080 Watts)

 Below is an example of how you can mount a this current transducer on a circuit board.  If you want to get order information for this device click HERE.

 

You can buy a hall effect transducer clamp off the shelf to measure AC and DC current for $50 to $400.  Or you can build your own. 

December 11, 2001 - Model ZXCT 1010 comes in SOT23 5-pin package, and measures load current with its high side sense resistor. Monitoring voltage across its shunt resistor and translating it into proportional output current, this current sense monitor IC uses 1 scaling resistor to convert proportional current into ground referenced output voltage. Features include separate pin to ground that produces output offsets to 30µV. Sense accuracy is rated to 1%.

Hauppauge, NY...In a compact SOT23 5-pin package, the new Zetex ZXCT 1010 provides an extremely simple and cost effective means of accurately measuring load current with a high side sense resistor. Monitoring the voltage across a shunt resistor andtranslating it into a proportional output current, this current sense monitor IC uses a single scaling resistor to convert this proportional current into a ground referenced output voltage, thereby eliminating disruption of the ground plane.

An enhanced version of the companion ZXCTl009 device, the ZXCT1010 high side current monitor IC features a separate pin to ground that improves the typical output offset from 500mV to 30µV. This makes the part particularly important for applicationswhere the sense voltage to be measured is very low.

Requiring less PCB space than alternative and more costly matched transistor pair solutions and providing an improved sense accuracy of 1%, the SOT23-5 packaged current monitor is ideally suited for portable battery powered equipment. Possessing a low quiescent current of just 4µA, the ZXCT1010 also operates over a broad input voltage range, from 20V down to as low as 2.5V.

Besides the full array of personal portable equipment, potential uses of the ZXCT1010 include automotive applications, low cost battery chargers and DC motor controllers requiring a simple, cost effective current measurement solution.

The 1,000 piece price is $0.54 each. Samples and start-up quantities are available immediately and delivery of production volumes is quoted at 6 weeks.

Selling Alloy shunt ResistorsFeatures:a.Power:3 watts.b.Tolerance wihin± 5% and±1%.c.TCR within 20ppm or 50 ppm/℃.d.Resistance down to 0.0003 Ohms.

e.Low Inductance.f.comply with ROHS,no lead.g.special requirement can be customerized.

Applications,a.Current sensingb.Feedbackc.Low inductanced.Surge and pulsee.High current applications for automotive marker.f.Frequency converters.g.Power modules.

Minimum Order: 1000 pieces