I recently had a customer ask if there was a way to use one of our AC sources to determine whether the load connected to its output was capacitive or inductive. Agilent’s 6811B, 6812B, and 6813B AC power source/analyzers are all capable of making many different measurements, several of which can be used to calculate the impedance of the connected load. To determine the impedance, the simplest measurements to use are the amplitude and phase measurements of the applied sine wave voltage and resultant current. Agilent’s AC sources can measure harmonic content of the voltage and current, both amplitude and phase, up to the 50th harmonic. From these measurements, you can calculate the impedance of the R, L, or C connected to the AC source output.
I grabbed a few sample parts from our lab area to demonstrate these measurements. First, I used a power resistor that was about 49 ohms. I applied a sine wave of about 20 Vac at 1000 Hz (0 phase) and used the built-in measurement capability to measure about 0.4 Aac at a phase (angle) of very near 0 degrees (the measurement was -7.01E-2 = -0.071 degrees). Of course, 0 degrees of phase between the voltage and current means the sine waves are in phase and the load is resistive as expected. See Figure 1.
The next test I did was with an inductor. Connecting it to the output of the AC source and once again applying about 20 Vac at 1000 Hz from the AC source, using the built-in measurement capability, I measured about 0.129 Aac at a phase of -88.66 degrees. The phase measurement of nearly -90 degrees confirmed that the load on the AC source was an inductor and the magnitude could be calculated to be about 25 mH which is what I expected (I measured it using an Agilent LCR meter). The series resistance in the line cord, clip leads, and inductor wire itself (I did not use remote sense) accounted for the non-ideal phase of -88.66 degrees instead of the ideal phase of -90 degrees. And the R was calculated from the AC source measurements to be about 3.6 ohms and agreed with external verification. See Figure 2.
Finally, I connected a capacitor to the AC source output and applied about 10 Vac at 1000 Hz. The AC source measurement system showed 0.633 Aac at an angle of 87.47 degrees indicating a capacitor was across its output. From these measurements, the capacitance and series R were calculated to be 9.88 uF and 0.711 ohms, consistent with externally verified measurements. See Figure 3.
So you can see that it is possible to determine the impedance (resistance, capacitance, or inductance) of a device connected across the output of an AC source when the right measurement capabilities are built into the AC source, as they are with the Agilent AC sources. These highly capable products not only make measurements like this easy, they also can easily create a large variety of AC output stimulus waveforms.
Friday, February 28, 2014
Use Agilent's BenchVue Software to Save Time
Hi everyone!
Agilent BenchVue Software
Naturally I am going to talk about the power supply potion
of BenchVue (this is a power supply blog after all). Presently the software supports the Agilent N6700 Modular Power Supplies and the Agilent E36xxA Basic Power Supplies. I do not know if many people remember but we
used to have a free software package for the E36xxA power supplies called Intuiilink. BenchVue is the modern successor to Intuiilink. I recently checked BenchVue out on my E3646A
DC Power Supply.
The E3646A is a basic two output DC Power Supply. BenchVue communicates with it using GPIB (in my case my Agilent 82357 USB/GPIB converter). Take a look at the below picture:
You can program all of the basic settings (even for multiple
channels) on your power supply on one screen. If you used the front panel to do all of these
entries it would take quite a while to navigate through all of the menus. BenchVue saves you time by putting all of this in one place. The other neat thing that I noticed is that you can get the voltage and current readback from both channels on the screen at the same time. You can't see both channels on the front panel, you have to manually switch.
The other main feature of BenchVue that can save you some time is the
built in datalogger. For those of you that are unfamiliar with the term datalog, this is when an instrument takes measurements at a predetermined time interval and stores them in a file that you can look at later. With BenchVue, you can set a datalog up to take a measurement as fast as every second. You can then take the
stored values and export them to Matlab, Microsoft Excel, Microsoft Word, or to
a CSV file.
You could write your own datalog program but the beauty of BenchVue is that you can set the log up by entering a few values and then just press the "Run" button to make it go. You also do not have to worry about formatting
the data or creating files since BenchVue does all of that for you. You basically just run it, then export the data and then you can view it in your chosen format.
Here is a sample thirty second datalog where I logged the voltage on Output 1:
After it was complete, I just hit the "Export" button, chose Microsoft Excel and I got the following:
Pretty cool!
That's my short intro to the new Agilent BenchVue software. Go download it. Let us know any thoughts you have in the comments section.
Monday, February 24, 2014
How to test the efficiency of DC to DC converters, part 2 of 2
In part 1 of my posting on testing the efficiency of DC
to DC converters (click here to review) I went over the test set up, the
requirements for load sweep synchronized to the measurements, and details of
the choice of the type and set up of the current load sweep itself. In this
second part I will be describing details of the measurement set up, setting up the
efficiency calculation, and results of the testing. This is based on using the
N6705B DC Power Analyzer, N6782A SMUs, and 14585A software as a platform but a
number of ideas can be applicable regardless of the platform.
Figure 1: Synchronized measurement and efficiency
calculation set up
The synchronized measurement and efficiency calculation
set up, and display of results are shown in Figure 1, taking note of the
following details corresponding to the numbers in Figure 1:
- In the 14585A the data logging mode was selected to make and display the measurements. The oscilloscope mode could have just as easily been used but with a 10 second sweep the extra speed of sampling with the oscilloscope mode was not an advantage. A second thing about using the data logging mode is you can set the integration time period for each acquisition point. This can be used to advantage in averaging out noise and disturbances as needed for a smoother and more representative result. In this case an integration period of 50 milliseconds was used.
- To synchronize the measurements the data log measurement was set to trigger off the start of the load current sweep.
- Voltage, current, and power for both the input and output SMUs were selected to be measured and displayed. The input and output power are needed for the efficiency calculation.
- The measurements were set to seamless ranging. In this way the appropriate measurement range for at any given point was used as the loading swept from zero to full load.
- A formula trace was created to calculate and display the efficiency in %. Note that the negative of the ratio of output power to input power was used. This is because the SMU acting as a load is sinking current and so both its current and power readings are negative.
With all of this completed really all that is left to do
is first start the data logging measurement with the start button. It will be
“armed” and waiting from a trigger signal from the current load sweep ARB that
had been set up. All that is now left to do is press the ARB start button.
Figure 2 is a display of all the results after the sweep is completed.
Figure 2: DC to DC Converter efficiency test results
All the input and output voltage, current, and power
measurements, and efficiency calculation (in pink) are display, but it can be
uncluttered a bit by turning off the voltages and currents traces being
displayed and just leave the power and efficiency traces displayed. This
happened to be special DC to DC converter designed to give exceptionally high
efficiency even down to near zero load, which can be seen from the graph. It’s
interesting to note peak efficiency occurred at around 60% of full load and
then ohmic losses start becoming more significant.
Labels:
CC mode,
constant current,
DC electronic load,
DC Power Analyzer,
DC/DC converter,
digitizer,
electronic load,
ELOG. datalog,
N6705B DC Power Analyzer,
N6782A,
power efficiency,
Usage
Thursday, February 20, 2014
How to test the efficiency of DC to DC converters, part 1 of 2
I periodically get asked to provide recommendations and
guidance on testing the efficiency of small DC to DC voltage converters. Regardless
of the size of the converter, a DC source is needed to provide input power to
the converter under constant voltage, while an electronic load is needed to
draw power from the output, usually under constant current loading. The load
current needs to be swept from zero to the full load current capability of the
DC to DC converter while input power (input voltage times input current) and
output power (output voltage times output current) are recorded. The efficiency
is then the ratio of power out to power in, most often expressed in a
percentage. An illustration of this is shown in Figure 1. In addition to sourcing
and sinking power, precision current and voltage measurement on both the input
and output, synchronized to the sweeping of the load current is needed.
Figure 1: DC to DC converter efficiency test set up
One challenge for small DC to DC voltage converters is
finding a suitable electronic load that will operate at the low output voltages
and down to zero load currents, needed for testing their efficiency over their
range, from no load to full load output power. It turns out in practice many
source measure units (SMUs) will serve well as a DC electronic load for
testing, as they will sink current as well as source current.
Perhaps the most optimum choice from us is to use two of
our N6782A 2-quadrant SMU modules installed in our N6705B DC Power Analyzer
mainframe, using the 14585A software to control the set up and display the
results. This is a rather flexible
platform intended for a variety of whatever application one can come up with
for the most part. With a little ingenuity it can be quickly configured to
perform an efficiency test of small DC to DC converters, swept from no load to
full load operation. This is good for converters of 20 watts of power or less
and within a certain range of voltage, as the N6782A can source or sink up to 6
V and 3 A or 20 V and 1 A, depending on which range it is set to. One of the
N6782A operates as a DC voltage source to power the DUT and the second is
operated as a DC current load to draw power from the DUT. A nice thing about
the N6782A is it provides excellent performance operated either as a DC source
or load, and operated either in constant voltage or constant current.
An excellent video of this set up testing a DC to DC
converter was created by a colleague here, which you can review by clicking on
the following link: “DC to DC converter efficiency test”.
The video does an excellent job covering a lot of the
details. However, if you are interested in testing DC to DC converters using
this set up I have a few more details to share here about it which should help
you further along with setting it up and running it.
First, the two N6782A SMUs were set up for initial operating
conditions. The N6782A providing DC power in was set up as a voltage source at
the desired input voltage level and the second N6782A was set to constant
current load operation with minimum (near zero) loading current.
Note that the 14585A software does not directly sweep the
load current along the horizontal axis. The horizontal axis is time. That is
why a time-based current sweep was created in the arbitrary waveform (ARB) section
of the 14585A. In that way any point on the horizontal time axis correlates to
a certain current load level being drawn from the output of the DUT. The ARB of
course was set to run once, not repetitively. The 14585A ARB set up is shown in
Figure 2.
Figure 2: Load current sweep ARB set up in 14585A
software
This ARB sweep requires a little explanation. While there are a number of pre-defined ARBs,
and they can be used, an x3 power formula was chosen to be used
instead. This provided a gradually increasing load sweep that allowed greater
resolution of this data and display at light loads, where efficiency more
quickly changes. As can be seen, the duration of the sweep, parameter x, was
set to 10 seconds. As a full load current needed to be -1 A, using the actual formula
(-x/10)3 gave us a gradually
increasing load current sweep that topped out at -1A after 10 seconds of
duration. The choice of 10 seconds was arbitrary. It only provided an easy way
to watch the sweep on the 14585A graphing as it progressed. Finally, a short
(0.1 second) pre-defined linear ramp ARB was added as a second part of the ARB
sequence, to bring the load current back to initial, near zero, load conditions
after the sweep was completed. This is shown in Figure 3.
Figure 3: Second part of ARB sweep to bring DUT load
current back to initial conditions
I hope this gives you a number of insights about creative
ways you can make use of the ARB. As there is a good amount of subtle details on
how to go about making and displaying the measurements I’ll be sharing that in
a second part coming up shortly, so keep on the outlook!
Labels:
CC mode,
CV mode,
DC electronic load,
DC Power Supply,
DC/DC converter,
electronic load,
N6705B DC Power Analyzer,
N6782A,
power efficiency,
SMU,
Usage
Friday, January 31, 2014
More Information on the Awesomeness of binary data
Hi everybody!
This month I am going to do a build on one of Ed's posts from this month. It was titled "Using Binary Data Transfers to Improve Your Test Throughput". If you have not read it, go ahead and click on the link. I'll be here when you get back.
This month I am going to do a build on one of Ed's posts from this month. It was titled "Using Binary Data Transfers to Improve Your Test Throughput". If you have not read it, go ahead and click on the link. I'll be here when you get back.
I wanted to reiterate how drastic the difference is between
using these two interfaces when you are reading large amounts of data. I did some bench-marking a little while ago and I wanted to share it now with everybody.
Please note that these were quick tests that I did and in no way
are official numbers. In fact if you see
anything wrong with my methods, please comment.
The first thing that I will talk about is my method. I did the test with a N6700B MPS Mainframe and a N6781A SMU module. I wrote a program that set up the module to source 5 V and then take an array of voltage measurements. I set it for the maximum number of measurement points (524288 points) with the fastest sample rate (though for this experiment the sample rate does not really matter). Before I did the reading of the data from the N6700B to my PC I started a programmatic stopwatch and stopped the stopwatch after the reading was complete. I looped 20 times and took the average.
One thing that I would highly recommend is to use the Agilent VISA-COM IO library. The VISA-COM library offers a ReadIEEEBlock function that makes reading binary data really easy for you.
The screenshot below shows the relevant loop and the calculation. This program was written in VB and I used LAN to communicate with the instrument.
The other important piece that this is not showing is that I am setting the data format to real using FORM REAL command. When you use ASCII, the command is FORM ASCII (this is also the default setting).
You can see the commented out ReadString command that I swapped in when I used the ASCII data format. You can also see my extremely professional (and useful) "I am on line" counter that I put in so that I knew my program was looping correctly.
So now for the times. ASCII format took around 100 s to read back all 524288 measurements into a string. When I switched to the binary format, it took under 5 seconds. As you can see, that is a very drastic difference and if you are reading back a lot of data from an instrument that supports the binary format, you really need to use it.
I also did a few other experiments. I changed the total number of points down to 1000. The binary format took a little under 20 ms to read the data and the ASCII format took about 125 ms. The last test that I did was 3 data points. The binary format took a little less than 15 ms and the ASCII format took under 5 ms to make the measurement. So you can see that as you read less and less data back, the ASCII format does catch up to the binary format and even exceed it.
Moral of the story is that if it is something more than a few points to read back, use binary because it will save you a ton of time.
That's all I have this month and I will be back next month!
Wednesday, January 29, 2014
Using a DC Power Supply to Regulate Energy
In a previous 2-part posting I talked about what power
and energy is (part 1 – energy) (part 2 – power). It is pretty straight-forward thing to do to
use a DC power supply for regulating voltage or current. Constant voltage (CV)
and constant current (CC) regulation are standard features of most all DC power
supplies used in testing. However, what if you have an unusual application
calling for applying a fixed amount of energy to your device under test (DUT)?
For example, adding a fixed amount of energy to a calorimeter or chemical
process, or testing the must (or must not) tripping energy of a fuse, or
circuit breaker, or squib or detonator perhaps?
When the resistance of a device remains constant, it is
relatively straight-forward to apply a fixed amount of energy to a DUT. By
applying a fixed voltage or current, the power in the DUT remains constant. Then
the energy is simply:
E = (V2/R)*t = (I2*R)*t
Where E is the energy in watt-seconds or joules, V is
voltage in volts, R is resistance in ohms, I is the current in amps, and t is
time in seconds. All you now need to do is apply the constant voltage or
current for a pre-determined amount of time and you will then be delivering a
fixed amount of energy to your DUT.
Many times however, a lot of DUTs do not maintain
constant loading. The may have a dynamically varying loading by nature or its
resistance dramatically increases as it heats up. How do you regulate a fixed
amount of energy to your DUT under these circumstances? One possibility is to
use one of a few specialized power supplies on the market can regulate their
outputs with constant power. As the DUT’s loading decreases or increases the
power supply will adjust its output accordingly in order to maintain a constant
output power delivered to the DUT. Again
then, by applying this constant power for a pre-determined amount of time you
will then be delivering a fixed amount of energy to your DUT.
Still, for DUTs that do not maintain constant loading, it
is very often not desirable, or outright unacceptable, to apply constant power
sourcing.. It may be you can only apply a fixed voltage or current to your DUT.
What can you do for these circumstances? Time can no longer remain a fixed
value when trying to regulate a fixed amount of energy. The solution becomes
quite a bit more complex, as depicted in Figure 1.
Figure 1: Regulating a fixed amount of energy to a DUT
Putting the solution depicted in Figure 1 into practice
can prove challenging. The watt-hour meter needs to provide a trigger out
signal when the desired watt-hour (or watt-second) threshold level is reached.
This becomes even more challenging if this response time required needs to be
just fractions of a second for this set up. More than likely this may become a
piece of customized hardware.
Interestingly this very set up can be programmatically
configured within our N6900A and N7900A series Advanced Power System (APS)
power supplies. These products have Amp-hour and Watt-hour measurement
integrated into their measurement systems. Not only can you measure these
parameters, there is a programmable way to act on them in a variety of ways as
well, which is the expression signal routing. Logical expressions can be
programmed and downloaded into APS, which then acts on them at hardware-level
speeds. Creating and loading the signal
routing expression into the APS unit is simplified by using the N7906A Power
Assistance software, which let me do it graphically, as shown in Figure 2.
Figure 2: Graphically developing and loading an energy limit setting into an Agilent APS unit
In Figure 2 a threshold comparator was set to generate a
trigger output at a level of 0.0047 watt-hours. This trigger was then routed to
the output transient system, to cause the output to transition to a new output
level when triggered. I had entered in zero volts as the triggered output
level. Thus when the watt-hour reading reached its trigger point, the output
went to zero, cutting off any more power and energy from being delivered to the
DUT.
The SCPI command set for this signal routing expression
is also generated from this software utility by clicking on “SCPI to clipboard”.
This saves on the effort generating the code manually if you are incorporating
the expression into a larger test program. For this expression the code
generated is:
:SENSe:THReshold1:FUNCtion WHOur
:SENSe:THReshold1:WHOur 0.0047
:SENSe:THReshold1:OPERation GT
:SYSTem:SIGNal:DEFine EXPRession1,"Thr1"
:TRIGger:TRANsient:SOURce EXPRession1
To test things out a 1.18 ohm resistive load was used to
draw 84.75 watts for a 10 volt output setting. The output cut back to zero
volts at nearly 200 milliseconds, as expected. This is shown in the
oscilloscope capture in Figure 3.
Figure 3: APS output for an 84.75 watt load and energy limit set to 0.0047 watt-hours
The load power was then doubled by increasing the output
voltage to 14.142 volts. The APS output cut back to zero volts in half the
time, delivering the same amount of energy, as expected. This is depicted in
the oscilloscope capture in Figure 4.
Figure 4: APS output for a 169.5 watt load and energy limit set to 0.0047 watt-hours
Labels:
Advanced Power System,
APS,
constant current,
constant voltage,
energy,
kilowatt-hours,
programming,
triggered output transient,
Usage,
watt-hour meter,
watt-hours,
watt-seconds,
watts
Friday, January 24, 2014
Using Binary Data Transfers to Improve Your Test Throughput
From time to time I have shared here on “Watt’s Up?” a
number of different ways the system DC power supply in your test set up impacts
your test time, and recommendations on how to make significant improvements in the
test throughput. Many of these previous posts are based on the first five of
ten hints I’ve put together in a compendium entitled “10 Hints on Improving
Throughput with your Power Supply” (click here for hints 1-5).
Oscilloscopes, data acquisition, and a variety of other
test equipment are often used to capture and digitize waveforms and store large
arrays of data during test, the data is then downloaded to a PC. These data
arrays can be quite large, from thousands to millions of measurements. For
long-term data logging the data files can be many gigabytes in size. These data
files can take considerable time to transfer over an instrument bus, greatly
impacting your test time.
Advanced system power supplies incorporating digitizing
measurement systems to capture waveform measurements like inrush current are no
different. This includes a number of system DC and AC power products we
provide. Even though you usually have the choice of transferring data in ASCII
format, one thing we recommend is instead transfer data in binary format.
Binary data transmission requires fewer bytes reducing transfer time by a
factor of two or more.
Further details about using binary mode data transfers
can be found in hint 7 of another, earlier compendium we did, entitled “10
hints for using your power supply to decrease test time” (click here to
access). Between these two compendiums of hints for improving your test
throughput I expect you should be able find a few different ideas that will
benefit your particular test situation!
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