Measuring Power with a Fluke Scopemeter® 190 Series Application Note

Measuring power with a Fluke ScopeMeter® 190 Series Application Note

Although many electrical test tools are available to measure voltage, few can measure current, and even fewer are equipped to measure electrical power directly. Moreover, there is always the question of how to measure power in electronic systems that are not operating at mains frequencies. The Fluke ScopeMeter 190 Series has the answer.

Power measurements This elementary power measurement, using a DC-source for The prime parameter specified U1 and a resistive load RL for any electrical system is oper- requires little more than a multi- ating voltage. But that voltage in meter, which will usually be a itself has little meaning if, once digital multimeter (DMM). The applied, it doesn’t result in DMM is used first to measure the power to, for example, exert a voltage U1 and then to measure force, drive a machine or run a the current supplied to the load lighting system. Accurate meas- RL, from which power supplied urement of electrical power is to the load is calculated from for- therefore crucial for understand- mula (1). ing a system’s behaviour. Once the switch is closed, all Suppose we have an electrical values in this DC-system are circuit consisting of a voltage static and therefore the two source U1, a switch Sw and a measurements can be made suc- load resistor RL (see figure 1). By cessively using a single instru- closing the switch, the supply ment if so desired. voltage U1 is connected to the If the source is a low-fre- If the load isn’t purely resis- load RL and a current I will flow quency AC source, the same tive but also includes inductive as a result. Once this current is rules basically apply. We can or capacitive elements, a phase flowing, the load will heat as a measure the RMS value of the difference between the applied result of the power applied to it applied voltage and that of the voltage and the current will according to: current, and multiply these to get result that needs to be taken into the power handled by the load. account to determine the power. P = U1 * I (1) As long as we’re working at The relation now is: mains frequencies, most DMMs can do the job. P = U1 * I * cos ϕ (2) where j is the phase-angle in degrees between voltage and current. Multimeters are generally not equipped to make measurements on more than one input and therefore cannot measure phase angle. For these measurements, a more specialized instrument is needed, for instance a phase meter or an oscilloscope. Figure 1. Basic circuit loop in which voltage and current are measured.

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If power measurements are (instantaneous) power can be made more frequently, a power read off, at every point in time, meter like the Fluke 43B Power for instance using the cursors of Quality Analyzer would be the the Fluke 190 Series. most appropriate test tool. This instrument measures both volt- Example of a light age and current simultaneously dimmer and automatically takes the phase angle into account. For the As an example, we’ve applied occasional user, however, this the mains voltage to a so-called may be an expensive option and “light dimmer”. This is a thyris- a more generic tool like the tor-based power control device ScopeMeter would be preferred. in which current is allowed to flow only during a selected part Use of the Fluke of the mains cycle. The effective output voltage to the load (the ScopeMeter lights) can be controlled by All Fluke ScopeMeters have two changing the phase angle. The input channels and can measure measured voltage (see figure 2, voltage and current at the same trace A) shows that the output is Figure 3. Menu tree for setting up the waveform multiplication. time, whilst also measuring and active during approximately two- displaying the phase angle thirds of the time, or about 120 scaling can be modified. After between them. See also the sec- degrees during each half-cycle. pressing softkey F3 we can also tion “practical set-up hints” near During one-third of the time, the change the vertical position of the end of this publication for dimmer is ‘off’ and no output the resultant trace M. further details. voltage is supplied to the lights. If the voltage is non-sinu- If we change this phase angle, soidal or if the load isn’t purely the lights will brighten or dim. resistive, power measurements become too complex to be performed on a DMM. The best way to determine power under these conditions is to take a large number of current and voltage measurements over each cycle of the supply voltage. The measurements need to be made simultaneously on both signals. Each set of simultaneous measurements Figure 4. Voltage, Current and Power curves can then be multiplied to pro- on the Fluke 190 Series ScopeMeter. duce a corresponding set of data Figure 2. Output voltage and current of a points from which a curve can be dimmer driving a string of lights. Figure 4 shows the voltage constructed of the power han- (waveform A, in red), the current dling at successive moments in (B, in blue) and the multiplied time. In figure 2 we can also see curve (M, in green), representing The Fluke 190 Series the resulting current through the the power supplied to the lights. ScopeMeter is capable of per- light-bulbs (channel B, blue On the Fluke ScopeMeter 190 forming this specific function for curve). We can now set up the Series, the cursors can be used to you! ScopeMeter to calculate the measure the power at any point Included in the functionality of power applied to the lights (see in time, as shown in figure 5. the Fluke ScopeMeter 190 Series figures 3 and 4). is the ability to multiply individ- To do this select: ual curves (waveforms) to create SCOPE, a resultant curve. With this func- then F4 (= Waveform Options), tion, sets of samples from chan- next Mathematics, and Enter, nels A and B are multiplied to then A * B, and Enter. create a resultant curve labeled M. In other words, every time a Next we select a scaling that sample for channel A and a sam- we expect will keep the power ple for channel B is taken and curve on screen; this may also be written on screen, these are also changed later. The two input multiplied and displayed to cre- waveforms and the resultant (M) ate the resultant waveform M on will now be visible. If the result- screen. From this waveform, the ant is too small or too large, the Figure 5. The ScopeMeter’s cursor is used for power measurement at a specific point in time.

2 Fluke Corporation Measuring power with a Fluke ScopeMeter® 190 Series Here, the cursor is set at about all input signals (and which is the maximum peak of the power connected to the mains’ safety curve, and the reading tells us ground). In contrast, the that the lights are handling a ScopeMeter 190 Series has inde- peak of approximately 1.7 kW at pendently floating inputs. This that instant in time. allows for voltage measurements at voltage test-points as well as Power measurement in a across current-sensing resistors, switched-mode power which may be at different volt- supply (SMPS) age levels. See the ScopeMeter’s technical specification for full In electronic systems, the fre- Figure 7. Voltage and current waveforms are details. quencies of the signals are often used to create the power-curve. When used with current much higher and the waveforms clamps or with current sensing much more diverse than those in From the curves in figure 6, resistors in the network, the above example. we calculate the power that the ScopeMeter inputs can be set-up As an example, consider some transistor is handling by multi- to read amplitudes in amps waveforms from a switched- plying the two graphs, as in fig- directly. mode power supply (figure 6). In ure 7. Here also the To do so, select the input this system, the mains voltage is timebase-setting has been channel key (e.g.: “B”) and then rectified and filtered, resulting in changed to see the part of the softkey F3 (“Probe channel B”). A a DC voltage of about 350 V. This waveform that is of particular selection can then be made if the is then applied to a switching interest in more detail. input signal represents a voltage transistor that drives a step- On these curves, we can use or a current, or even a tempera- down transformer. the cursors of the Fluke 190 ture. What’s more, the sensitivity Series to measure the peak in of the current clamp or the cir- power handling, as is done in cuit’s sensing resistor can be figure 8. selected from a selection table, indicated in mV/mA (equivalent to V/A or simply Ω). If we have connected a 1 Ω current sensing resistor in our network and want to measure the voltage across it, the current sensitivity would be set at 1 V/A. If we include a 0.1 Ω resistor in our network to sense the current, Figure 6. Voltage and current handled by the then the sensitivity would be set switcher component in a SMPS. at 100 mV/A. If the voltage across that resistor is measured using the standard (10:1) voltage In figure 6 we can see the Figure 8. Cursor used to measure the peak of probe, the overall sensitivity voltage across the switching the power, handled by the transistor. would be 10 mV/A. By selecting transistor (curve A, in red) and this sensitivity from the menu, the current through the transistor From this we see that the the current that we read on the (curve B, in blue). The voltage switching transistor is handling a ScopeMeter screen directly indi- reaches peak values of over peak power of 123 W. During cates the true value. 400 V (see the 4 divisions of design of the SMPS, one has to amplitude at 100 V/div) and the be aware that such power peaks Total energy current has a peak-value of over may occur and the components 200 mA. A single cycle of this The total energy handled over need to be selected with this in time can be calculated by multi- converter’s signal takes approxi- mind. mately 26 µs, which means the plying the continuous power and operating frequency is around the time the system is active: 36 kHz. Note, however, that for a Instrument set-up given SMPS, the frequency may The ScopeMeter is unique in that W = P * t (3) vary with changes in the line its inputs are fully insulated from voltage and loading. one another, allowing direct con- The result is expressed in nection to electrical wiring, even watt-seconds (Ws), also known if this is at mains potential. Most as joules (J). The value in joules standard oscilloscopes have a can be rescaled into kWh, common ‘ground’ connection on whereby the inputs which has to be used 1 kWh = 1000 Wh = as a common reference point for 1000 * 3600 Ws = 3.6 * 106 Ws

3 Fluke Corporation Measuring power with a Fluke ScopeMeter® 190 Series Some ScopeMeter models also Practical set-up hints If no switch is provided, or if contain a function for calculating the DMM has no current-measur- the total power accumulated over The best way to measure a cur- ing capability, we may also add a period of time, which is then rent in a circuit loop is with a a current sensing resistor of a selected using the cursors. Power current clamp. These are com- known value Rs to the circuit, over time equals energy, and we mercially available for AC and for which needs to be small in value can read this on the ScopeMeter DC+AC measurements, and for compared with the load-resist- various current ranges. The Fluke as watt-seconds directly. ance RL (see figure 10). We can 80i-100s, for instance, measures now measure the voltage across DC and AC currents from 0.1 to this sensing resistor and calcu- 100A; the Fluke i1010 can even late the current from Ohms’ law. be used up to1000A AC and/or Adding the resistor is a one-time DC. modification, which is more con- These clamps eliminate the venient than repeatedly opening need to open up the circuit loop up the circuit loop. when making measurements and If Rs is more than 10 times provide good isolation between smaller in value than the load any ‘live’ wiring and the test RL, less than a percent of the instrument. When working on energy will be handled by the power circuits, this is definitely series resistance, and thus the the safest way to measure the error in the power measurement Figure 9. Power curve and energy current. measurement. that results from adding the If a current clamp is used for resistor will be less than 1%. measuring small currents and the Looking at figure 9, we can sensitivity of the clamp in insuffi- Conclusion see that between the two cur- cient, the effective sensitivity can sors, i.e. within a single cycle of be increased by feeding multiple Power measurements on low-fre- the mains voltage, an energy of turns of the wire through the quency linear systems can be 10.09 Ws is delivered to the clamp. The actual current is now performed using a DMM. The lights. A cycle of the mains in the measured current divided by measurement of power in elec- this case takes 20 ms. the number of turns. tronic systems where waveforms Per second this leads to a Sometimes, however, it’s not are more complex and frequen- power consumption of: so simple to cut the wiring of an cies often much higher than the existing circuit loop in order to mains frequency requires more 10.09 Ws / 20ms = include a current meter, for sophisticated tools. The Fluke 10.09 * 50 W*s/s = 505 W example when all wiring is part 190 Series of ScopeMeters are of a printed circuit board. A pos- well equipped to make these And over a time-span of one sible way to bring the current measurements, and can even hour, this equals a total energy meter into the loop of a low- make fast peak-power measure- consumption of: power circuit would then be to ments to determine the power set up the DMM for current handling of fast electronic com- 505 W * 1 h = 505 Wh = measurement and select the ponents, for instance in 0.505 kWh = 1.8 MWs highest current-range provided. switched-mode power supplies. Now connect the meter over the contacts of the on/off-switch. If the switch is left open, the meter will close the loop and read the current, while no wiring needs to be interrupted or modified. Fluke. Keeping your world up and running.

Fluke Corporation PO Box 9090, Everett, WA USA 98206 Fluke Europe B.V. PO Box 1186, 5602 BD Eindhoven, The Netherlands For more information call: In the U.S.A. (800) 443-5853 or Fax (425) 446-5116 In Europe/M-East/Africa (31 40) 2 675 200 or Fax (31 40) 2 675 222 Figure 10. Adding a current sensing resistor to In Canada (800) 36-FLUKE or the circuitry to allow current measurement. Fax (905) 890-6866 From other countries +1 (425) 446-5500 or Fax +1 (425) 446-5116 Web access: http://www.fluke.com