Programmable DC Electronic Load Applications

Programmable DC electronic loads are useful for a variety of test and measurement applications including battery testing and characterization as well as testing power supplies and solar panels. We will discuss these three types of applications in this article.

A programmable DC electronic load is ideally suited for testing any battery, especially rechargeable lead-acid, NiMH, NiCad, and Li-ion battery packs. The ability to set a minimum voltage level enables unsupervised discharge of the battery pack under test. The type of battery you are testing will determine the parameters you will program into the electronic load. These parameters include end of discharge voltage and discharge current. Some higher-end electronic loads feature an automatic battery testing feature that automates battery testing, but you can use any DC electronic load for this operation.

DC power supply testing is the most common use for an electronic load. Voltage and current output values can be verified as well as current limit and constant current operation. You can easily measure power and regulation with an electronic load. If the load has a computer interface, you can log temperature and time variance information for future reference or for documentation requirements.

As solar power becomes more affordable and popular, DC electronic loads have become an indispensable tool for characterizing and testing photovoltaic solar panels. For this type of test the load should be capable of constant voltage operation, which enables you to step through the output voltage value while measuring the resultant output current. Constant voltage operation provides you with a current versus voltage (I-V) curve that shows all of the solar panel’s characteristics, including the maximum power point at which the solar panel is operating most efficiently.

Although we have outlined several common uses for a programmable DC electronic load, this list is by no means exhaustive. Transient load testing and power supply recovery are also possible with some of the more complex electronic loads available at Circuit Specialists.

A Look Back at Circuit Specialists

When Circuit Specialists was established in Darrel and Leon Thorpe’s garage in Scottsdale, Arizona, all the way back in 1971, no one expected us to be where we are now.

By September of 1971 Circuit Specialists had moved out of the garage and into a small office and began publishing a mail order catalog called “Semiconductor SuperMart.” This catalog featured resistors, capacitors, diodes, chokes, coils, and integrated circuits (which had just been invented) as well as other products for electronics hobbyists and do-it-yourselfers.

In the Fall of 1972 Circuit Specialists expanded into the brick-and-mortar world by opening a retail outlet in Tempe, Arizona. Circuit Specialists became a Lafayette Radio franchise and was located right next door to a Radio Shack. In addition to the electronic parts line from the “Semiconductor SuperMart,” Circuit Specialists offered quality stereo equipment, police scanners, and CB radios courtesy of Lafayette Radio. We soon became a regional distributor. One of our current employees who joined Circuit Specialists in 1991 was even a regular customer, buying his first police scanner from Leon Thorpe back in the early ’70s.

In 1987 Circuit Specialists bought its first computer for inventory and accounting. In just a few years we started offering products online via the new realm of the Internet. Soon after that Circuit Specialists moved into the twelve thousand square foot retail storefront and warehouse in Mesa, Arizona where we continue to do business today.

Circuit Specialists printed countless paper catalogs over the years, but with the exponential growth of the Internet we decided to stop printing paper catalogs in the early 2000s and have become primarily an online retailer, although our retail storefront is still open for the convenience of our local customers.

In order to increase our product line Wayne Thorpe, then president of Circuit Specialists, made his first trip overseas to initiate direct importation of electronic equipment and supplies in 1986. By 2001 Circuit Specialists had opened an international office overseas, which enabled us to rapidly expand into soldering equipment, power supplies, and test equipment. Circuit Specialists is able to keep the price of our products remarkably low by cutting out all the middlemen.

We usually make at least one or two trips overseas each year to source new products and to maintain the close contact that we have established over the years with our suppliers. We feel this is an important part of doing business, as it allows us to have a much closer relationship with our suppliers, which benefits our customers.

For over forty years Circuit Specialists has been selling quality electronic components, equipment, and accessories and we will continue striving to provide you with the finest products at the lowest prices.

We continue to operate from our location in Mesa, Arizona, offering our customers the benefit of dealing with an established US-based company and helping them avoid the pitfalls that commonly occur when dealing with overseas sellers themselves. In addition to our large selection of electronic components, we also continue to offer our educational electronic lab kitting service for schools.

Power Supply Design: Switch-Mode vs. Linear

DC power supplies are available in either switch-mode (also called switching) or linear designs. While both types supply DC power, the method used to produce this power is different. Depending on the application, each type of power supply has advantages over the other one. Let’s look at the differences between these two technologies as well as each design’s respective advantages and disadvantages.

A switch-mode power supply converts the AC line power directly into a DC voltage without a transformer, and this raw DC voltage is then converted into a higher frequency AC signal, which is used in the regulator circuit to produce the desired voltage and current. This results in a much smaller, lighter transformer for raising or lowering the voltage than what would be necessary at an AC line frequency of 60 Hz. These smaller transformers are also considerably more efficient than 60 Hz transformers, so the power conversion ratio is higher.

A linear power supply design applies the AC line voltage to a power transformer to raise or lower the voltage before being applied to the regulator circuitry. Since the size of the transformer is indirectly proportional to the frequency of operation, this results in a larger, heavier power supply.

Each type of power supply operation has its own set of advantages and disadvantages. A switch-mode power supply is as much as 80% smaller and lighter than a corresponding linear power supply, but it generates high-frequency noise that can interfere with sensitive electronic equipment. Unlike linear power supplies, switch-mode power supplies are able to withstand small losses of AC power in the range of 10-20 ms without affecting the outputs.

A linear power supply requires larger semiconductor devices to regulate the output voltage and therefore generates more heat, resulting in lower energy efficiency. A linear power supply normally operates around 60% efficiency for 24V outputs, whereas a switch-mode power supply operates at 80% or more. Linear power supplies have transient response times up to 100 times faster than their switch-mode counterparts, which is important in certain specialized areas.

In general, a switch-mode power supply is best suited for portable equipment, since it is lighter and more compact. Because the electrical noise is lower and easier to contain, a linear power supply is better suited for powering sensitive analog circuity.

Switch-Mode Power Supplies

Starting at $27.95

Switching power supply

Compact, lightweight, and efficient. Shop now >

Linear Power Supplies

Starting at $49.00

Switching power supply

Low ripple and noise and high reliability. Shop now >

Advantages of a Closed-Loop Stepper Motor System


closed-loop step motor system combines the advantages of servo motor and stepper motor technologies. Functionally, a closed-loop stepper motor system will run much more smoothly and with less resistance than a standard stepper motor setup. Since a closed-loop system provides feedback and control as well as short transient and free oscillation times, the closed-loop system will not lose or gain steps.

A closed-loop stepper motor system, such as the 86BHH76-860H, may be the best option when the application requires improved energy efficiency and smoothness of operation, especially at high loads. In addition, a closed-loop system has the advantage over servo motor systems of higher torque at low RPMs. Additional benefits include short transient times, less packaging, accurate/correct positioning using feedback from encoders integrated into the motor(s) to the controller, and comparatively low prices.

Wayne Thorpe

Using an Arbitrary Waveform Generator & a Digital Storage Oscilloscope to Create a Transistor Curve Tracer

You can use an arbitrary waveform generator and a digital storage oscilloscope (DSO) to create a transistor curve tracer that you can then use to test and characterize semiconductor devices. I will describe a method to produce current vs. voltage (I-V) curves of any semiconductor device using a Siglent SDG 1000 series waveform generator and a Hantek DSO 5000 series oscilloscope.

Set the waveform generator to generate a staircase waveform at a frequency of 120 Hz. This frequency coincides with the frequency produced by a full wave–rectified AC line voltage. The full wave–rectified signal provides the collector (drain) to emitter (source) power, which is displayed on the horizontal axis. The staircase waveform provides the base (gate) driving signal for the device under test.

The collector (drain) current is displayed on the vertical axis of the oscilloscope by developing a voltage across a current-sensing resistance. You will use the oscilloscope in X-Y mode rather than the traditional voltage-time (Y-T) display mode. You must also change the persistence of the display to provide an easier-to-view and more meaningful display.

curve_tracer_1 The schematic of the test setup and an example of the resulting display are shown below for an NPN or N-channel device. Simply invert the polarities for a PNP or P-channel device.


This example demonstrates a method for generating and displaying the current vs. voltage (I-V) curves of a transistor (either BJT or FET). This task was performed using the Siglent SDG1025 25MHz arbitrary waveform generator and a Hantek DSO5202B 200MHz digital storage oscilloscope as well as a handful of readily available components.

Buyer Beware: Overseas Sellers

It is very tempting to buy an expensive piece of test equipment from an online dealer located in China, since the initial cost for a comparable piece of equipment is typically lower than in countries like the United States. However, it can cost you much more in the long run. Online shopping has become extremely popular in recent years, thanks in large part to convenience — but with convenience comes greater individual responsibility. Shoppers don’t always realize that a seller may be nothing more than one person with no knowledge of test equipment selling or reselling products. Sellers like these have no idea how to use the products they’re offering and, thus, cannot provide any technical support whatsoever.*

If you’re buying simple items like a cell phone case or a USB flash drive, it sometimes makes sense to buy the cheapest product you can find, typically from an overseas seller. If the product fails you can just buy another equally cheap one instead of sending it overseas for repair or replacement. You (hopefully) won’t need technical support for a cell phone case, so it sometimes makes financial sense to buy whatever’s cheapest and hope for the best. But complex, expensive pieces of test equipment built from thousands of discrete components are another story altogether.

You can buy, for example, a cheap digital storage oscilloscope directly from a seller in China. But do you really know who you’re buying from? How long have they been in business? What kind of support do they offer their customers?  The fact of the matter is you might be dealing with a single person who is buying a product directly from the manufacturer, marking up the price, and shipping it overseas. That person might even be reselling a used or refurbished product without your knowledge.

More importantly, what happens when you buy a cheap piece of test equipment from an overseas seller only to discover you need technical support? You may have out-of-the-box issues that you could easily address yourself with a little guidance, or you may run into a serious soft- or hardware issue that requires professional repair. Unfortunately, sometimes you’ll return an item only to find that you need to return the replacement as well. And let’s not forget how easily packages are damaged in transit. These kinds of things have happened to all of us at some point.

Consider the logistics of shipping a product back overseas for repair or replacement — that is if the vendor even provides repair or replacement service. Shipping delicate electronic equipment overseas is more complicated than you might imagine. In addition to overseas shipping costs, you may have to pay import charges and other fees. Most US-based businesses will at the very least cover return shipping costs, but an overseas seller may expect you to foot the bill for return shipping. Keep in mind that it takes a long time to return an item to a seller on another continent. Simply contacting the vendor and waiting for a response can take days or even weeks.

Also consider that if the product you ordered is defective or was damaged in transit, many overseas dealers will not take responsibility, forcing you to deal with the manufacturer (more on this below) or shipping company on your own. In contrast, most established businesses in the United States insure their shipments, so if something is damaged in transit it may be replaced at no charge to you. Some businesses will issue a call tag to pick up products that are dead on arrival and may ship a replacement unit to you, even before they receive the defective item.

Dealing with an established business in the United States is the way to go when purchasing products like electronic test equipment. First of all, you don’t have to worry about the language barrier. In addition, an established business will provide support for the products they sell. That support alone can be as valuable as the product itself, even if you never run into a problem with the product. An overseas seller who doesn’t know anything about the product will not be able to help you with even the simplest questions you may have.

Another thing you should consider is service on a unit that is no longer under warranty. Some companies will not even work on units that weren’t purchased from them, and some manufacturers do not offer service on units that were sold in another country; for instance, some North American brands do not recognize their own products when they’re sold from China, even though that product was manufactured on the same assembly line (this has been a common issue with camera equipment for many years).

Finally, consider this: Counterfeit goods are very common in China. Many of these products not only look and feel the same as their name-brand counterparts, some even have the same logo and model number on them as the actual product. However, most of these counterfeits are poorly built and may be very different inside than the piece of test equipment you thought you were buying. Reputable US businesses that import products from China do not sell counterfeit products and will stand behind their entire product line.

With all of this in mind, you should thoroughly consider the true cost of an item and its actual overall value when deciding whether or not to risk buying a cheaper product from an overseas seller. Regardless of the sticker price, you may end up paying much more in the long run. Know what you’re really buying and shop smart.



*Furthermore, these sellers may not know much English, making it difficult to even find out that they can’t help you with any technical problems.


Ten Measurements Using an Oscilloscope

1) Measuring and Viewing Voltage Waveforms

You can measure and view either DC or AC signals up to the oscilloscope’s BW using the standard voltage vs time mode of operation. Adjust the vertical setting to display the complete waveform and determine the value by counting the vertical divisions and multiplying by the vertical scale factor. Most digital storage oscilloscopes have a measurement menu that directly displays values without forcing you to count graticules.

2) Measuring and Viewing Current Waveforms

You may view current values and waveforms using an external low-value shunt resistor. Use Ohm’s law* to determine the correct scale multiplier for the measurement. The current shunt should be connected to the “low” side of the power.

*I = E / R

3) Measuring Frequency

You can perform frequency measurements by displaying the signal waveform on the oscilloscope’s screen and adjusting the horizontal time base value until you see at least one full cycle on the display. Measure the time value for one cycle and determine the frequency using the equation Freq = 1 / time.

4) Measuring Rise Time of a Pulse

You can determine the rise time of a pulse signal in a manner similar to frequency. Adjust the horizontal time base to display the rising edge of the pulse. Rise time is defined as the time between 10% and 90% of the amplitude.

5) Measuring Capacitance

You can estimate capacitance using a simple RC circuit and noting the phase difference between applied and resultant voltage across the capacitor using both vertical channels of the oscilloscope. The phase difference is noted and the capacitance is calculated using the following formula (α is the phase angle and Z is the impedance):

Capacitance = -1 / (2 x π x frequency x Z x sin(α))

6) Measuring Amplifier Gain

You may measure the gain or amplification of a circuit using both channel one and channel two of the oscilloscope. You will monitor the input signal with one channel and the output signal with the other. The difference between the amplitudes of these two signals indicates the gain.

7) Measuring Cable Length (TDR)

You may use a simple Time Domain Reflectometer (TDR) to determine the approximate length of a cable. A single fast-rising pulse from a pulse generator is simultaneously applied to the cable and the oscilloscope’s vertical channel. The time required for the pulse to travel to the end of the cable and reflect back again is a factor of both the cable length and the cable’s dielectric constant. The formula for measuring cable length is:

Length = (velocity of propagation x time) / 2

8) Measuring Differential Signals

You can measure differential signals, such as on a twisted pair cable, by using both of the oscilloscope’s vertical channels simultaneously. Use the MATH operation CH1-CH2 if both vertical channels are set to the same scale factor.

9) Measuring Signal Spectrum (FFT)

You can use the MATH operation FFT to view a waveform in an amplitude vs frequency representation. This is a simplified spectrum analyzer-type measurement and is useful for determining the frequency components of a periodic signal.

10) Measuring Duty-Cycle of a PWM Signal

You can determine the duty cycle of a PWM signal by displaying one complete cycle on the oscilloscope’s screen, which will enable you to determine the width of the positive portion as well as the width of the negative portion. Then you calculate the duty cycle with the following formula:

Duty cycle = (pulse high / (pulse high + pulse low) x 100%

TIP: Using the VDV501-089 Distance meter with unknown wire types.

Technicians and installers are often faced with the question of `how long is this wire?’ If the wire is already installed or you only have a partial spool it can be a difficult task. Often times without the proper tools your `best guess’ is all you can do.

Now with the tools available today, such as the VDV501-089 Distance Meter from Klein Tools. The VDV501-089 is a handy digital tool that can measure many types of wire either in place or while still on the spool. Frovdv501-089-am Coax to network and telephone twisted pair cables, it is a simple job to connect the VDV501-089 to one end of the cable, set the meter to the cable type and read the cable length right on the device.

If you have a cable that is not one of the preset values on the VDV501-089 you can setup the meter in either of two ways. If you know the capacitance per foot value of the cable you can enter that into the USER setting of the meter and then measure the cable. You can also use a known length of the desired cable and set the capacitance value using the up/down arrow keys until the meter reads out to the known length of the test cable. Note the capacitance value for future uses and when you check the same type of wire it will give you an accurate length. vdv501-089-b

Another handy item to have with the VDV501-089 is a 6 pin modular coupler. This will allow you to test telephone cables that already have modular connectors installed. (an 8 pin coupler would be used to test many network cables.)

Ten Uses for an Autotransformer, or Variac

1) Testing a repaired electronic device.

After performing a repair on a piece of electronic equipment, it is best to slowly power up the device to avoid burning up the replaced parts. You will often discover multiple part failures one at a time. Using the Variac to slowly power up the repaired unit can prevent damaging the replacement parts.

2) Powering up an old radio or amplifier.

When powering up an old radio or amplifier that has not been used for an extended period of time, it is best to apply reduced voltage power to the unit in stages in order to re-condition the electrolytic capacitors.

3) Roasting coffee beans or nuts.

To obtain the freshest coffee possible, coffee connoisseurs prefer to roast and grind their own coffee beans. You can use a Variac to adjust the roasting temperature to the ideal value.

4) Adjusting the temperature of a resistance-type heater.

Resistance-type heating elements can be adjusted by varying the AC power applied.

5) Hot-wire cutting of Styrofoam.

Styrofoam and other types of foam are typically cut to the desired shape using a hot wire cutting system. You can use a Variac to adjust the temperature of the hot wire to the optimal value required for cutting.

6) Dimming incandescent lighting.

Incandescent lights may be dimmed by varying the applied AC voltage. This is not possible with newer LED or fluorescent lamps.

7) Changing fan speed over a narrow range.

The speed of some types of small AC motors may be varied over a narrow range by changing the applied AC voltage. A problem with this application is that the torque of the motor is also reduced which may result in the fan or motor stalling out. Also note that this technique cannot be used with all electric motors.

8) Compensating for line voltage drop in AC power applications.

Long power line runs can often result in voltage drops across the lines, especially in high current draw applications like hot tubs or spas. You can use a Variac to compensate for this voltage loss.

9) High-current unregulated DC power supply.

A high-current adjustable unregulated DC power supply can be constructed using a Variac, high current rectifier, and filter capacitor. Do not attempt this without being aware of the possible safety concerns when using a Variac. Since a Variac is not isolated from the AC power lines, the AC power is directly accessible. The ideal configuration would use an isolation transformer to isolate the AC power mains from the application.

10) Adjusting the voltage of an AC power supply.

The output of an AC power supply, such as an electric train transformer, can be adjusted with a Variac.

Getting Started: The Aardvark HD3M Wireless Inspection Camera

Unlike many IP-based inspection cameras that connect to a wireless access point, the Aardvark HD3M wireless inspection camera is its own access point to which your computer or mobile device directly connects. This allows you to use the HD3M in areas without wireless access, such as a construction site or even a campsite. Because it is its own network, the HD3M also provides greater security, so other people will not see what is on the inspection camera.


You will need to download the Android or iPhone software from the Google Play or Apple App store. The Android version is used in the following example, but the iPhone version is quite similar. First you will go to the Google Play Store and search for the “WiFi Endoscope” app and then install the app to your device. (Note: Sample screens were taken from a Samsung Galaxy S-III running Android version 4.3.)


Once the app is installed the WiFi Endoscope app icon should appear on your device’s homescreen.


At this point you will need to disconnect from the access point or wireless data connection you are currently on and connect to the camera. This initially causes confusion as you must disconnect the computer or mobile device from the access point to which you usually connect and then reconnect it to the wireless inspection camera‘s “WiFi_Endoscope” access point for it to work.


You should also disable your “Auto Network Switch” setting. This will help maintain the connection if the signal level varies as you move the camera around while in use.


Remember that the default wireless SSID for the Aardvark HD3M is “WiFi_Endoscope” and the default password is “00000000” (eight zeroes). Both can be changed, if desired, from within the Windows software. (More on the Windows software below.)


Once you are connected to the camera you will open the WiFi Endoscope application.


The next screen shows the link to the camera. Click on “WiFi Endoscope” to select the camera you’ll be using.


The main viewing screen will appear. From this screen you can choose to capture a photograph or record video.


The app will save the photos or video files to your computer or mobile device. Depending on your device you can view, email, or transfer these files to another device.

Once you have finished using the HD3M you will want to reconnect to your normal access point or wireless data provider.


The Windows-based software for the camera is called “Smart Camera” and is included on the CD that came with your inspection camera.


As with the Android and iPhone software, you must disconnect your computer from your normal wireless access point and connect to the camera’s WiFi access point, “WiFi_Endoscope.”


Once connected to the camera you will run the “Smart Camera” software.


You can take still pictures or record video that has been saved to your computer with this software.


You can also change the resolution of the camera as well as the camera’s WiFi SSID and access password.



As with the Android & iPhone software, once you are finished using the camera you will need to disconnect from the camera’s access point and reconnect to your normal WiFi access point.

Another unique feature of the Aardvark HD3M is Bridge Mode. Once connected to the inspection camera you are able to “bridge” the inspection camera’s WiFi connection to another WiFi access point and then to wherever that access point is connected. You will select the access point to which you wish to connect. Click “Join Network” and follow the login prompts.

On the next screen you will click “Save & Apply” to establish the bridged connection. To connect through the Bridge Mode access a web browser on your computer or mobile device and then access the camera’s default address at “″. Select “Network/Bridge Mode” in the top menu.


Note: Be sure you select a unique network address in Bridge Mode. If the access point to which you are connecting uses the same address you can change the camera’s address in the “Network/LAN” menu. In this example the IP address has been changed to because was already in use.


Once connected in Bridge Mode you can access the Internet or any functions of the access point to which you are connected, enabling you to browse the Internet, networked printers, or shared network hard drives.


You can also access the video stream from the camera from other computers.


There are many options to explore once the HD3M inspection camera is connected to your network in Bridge Mode.

Simply smart circuitry since 1971.