Quad Bench Power Supply

The need for a bevy of equipment for building and testing presents a problem: how to deliver an adequate power supply while keeping workbench clutter to a minimum. Brian decided to tackle this classic engineering conundrum with a small, low-capacity quad bench power supply.

To the right of the output Johnson posts are the switches that set the polarity of the floating supplies—as well as the switch that disconnects all power supply outputs—while leaving the unit still powered up.

To the right of the output Johnson posts are the switches that set the polarity of the floating supplies—as well as the switch that disconnects all power supply outputs—while leaving the unit still powered up.

In “Quad Bench Power Supply,” Millier writes:

I hate to admit it, but my electronics bench is not a pretty sight, at least in the midst of a project anyway. Of course, I’m always in the middle of some project that, more often than not, contains two or three different projects in various stages of completion. To make matters worse, most of my projects involve microchips, which have to be programmed. Because I use ISP flash memory MCUs exclusively, it makes sense to locate a computer on my construction bench to facilitate programming and testing. To save space, I initially used my laptop’s parallel port for MCU programming. It was only a matter of time before I popped the laptop’s printer port by connecting it to a prototype circuit with errors on it.

Fixing my laptop’s printer port would have involved replacing its main board, which is an expensive proposition. Therefore, I switched over to a desktop computer (with a $20 ISA printer port board) for programming and testing purposes. The desktop, however, took up much more room on my bench.

You can’t do without lots of testing equipment, all of which takes up more bench space. Amongst my test equipment, I have several bench power supplies, which are unfortunately large because I built them with surplus power supply assemblies taken from older, unused equipment. This seemed like a good candidate for miniaturization.

At about the same time, I read a fine article by Robert Lacoste describing a high-power tracking lab power supply (“A Tracking Lab Power Supply,” Circuit Cellar 139). Although I liked many of Robert’s clever design ideas, most of my recent projects seemed to need only modest amounts of power. Therefore, I decided to design my own low-capacity bench supply that would be compact enough to fit in a small case. In this article, I’ll describe that power supply.

MY WISH LIST

Even though I mentioned that my recent project’s power demands were fairly modest, I frequently needed three or more discrete voltage levels. This meant lugging out a couple of different bench supplies and wiring all of them to the circuit I was building. If the circuit required all of the power supplies to cycle on and off simultaneously, the above arrangement was extremely inconvenient. In any event, it took up too much space on my bench.

I decided that I wanted to have four discrete voltage sources available. One power supply would be ground referenced. Two additional power supplies would be floating power supplies. Each of these would have the provision to switch either the positive or negative terminal to the negative (ground) terminal of the ground-referenced supply, allowing for positive or negative output voltage. Alternately, these supplies could be left floating with respect to ground by leaving the aforementioned switch in the center position.

This arrangement allows for one positive and two positive, negative or floating voltage outputs. To round off the complement, I added Condor’s commercial 5-V, 3-A linear power supply module, which I had on hand in my junk box. Table 1 shows the capabilities of the four power supplies.

I wanted to provide the metering of voltage and current for the three variable power supplies. The simultaneous voltage and current measurement of three completely independent power supplies seemed to indicate the need for six digital panel meters. Indeed, this is the path that Robert Lacoste used in his tracking lab supply.

As you can see, there are four power supplies. I’ve included all of the information you need to understand their capabilities.

As you can see, there are four power supplies. I’ve included all of the information you need to understand their capabilities.

I had used many of these DPM modules before, so I was aware of the fact that the modules require their negative measurement terminal to float with respect to the DPM’s own power supply. I solved this problem in the past by providing the DPM module with its own independent power source. Robert solved it by designing a differential drive circuit for the DPM. Either solution, when multiplied by six, is not trivial. Add to this the fact that high-quality DPMs cost about $40 in Canada, and you’ll see why I started to consider a different solution.

I decided to incorporate an MCU into the design to replace the six DPMs as well as six 10-turn potentiometers, which are also becoming expensive. In place of $240 worth of DPMs, I used three inexpensive dual 12-bit ADCs, an MCU, and an inexpensive LCD panel. The $100 worth of 10-turn potentiometers was replaced with three dual digital potentiometers and two inexpensive rotary encoders.

Using a microcontroller-based circuit basically allows you to control the bench supply with a computer for free. I have to admit that I decided to add the commercial 5-V supply module at the last minute; therefore, I didn’t allow for the voltage or current monitoring of this particular supply.

THE ANALOG CORE

Although there certainly is a digital component to this project, the basic power supply core is a standard analog series-pass regulator design. I borrowed a bit of this design from Robert’s lab supply circuit.

Basically, all three power supplies share the same design. The ground-referenced power supply provides less voltage and more current than the floating supplies. Thus, it uses a different transformer than the two floating supplies. The ground-referenced supply’s digital circuitry (for control of the digital potentiometer and ADC) can be connected directly to the MCU port lines. The two floating supplies, in addition to the different power transformer, also need isolation circuitry to connect to the MCU.

Figure 1 is the schematic for the ground-referenced supply. As you can see, a 24VCT PCB-mounted transformer provides all four necessary voltage sources. A full wave rectifier comprised of D4, D5, and C5 provides the 16 V that’s regulated down to the actual power supply output. Diodes D6, R10, C8, and Zener diode D7 provide the negative power supply needed by the op-amps. …

The ground-referenced power supply includes an independent 5-V supply to run the microcontroller module.

The ground-referenced power supply includes an independent 5-V supply to run the microcontroller module.

MCU AND USER INTERFACE

As with every other project I’ve worked on in the last two years, I chose the Atmel AVR family for the MCU. In this case, I went with the AT90S8535 for a couple of reasons. I needed 23 I/O lines to handle the three SPI channels, LCD, rotary encoders, and RS-232. This ruled out the use of smaller AVR devices. I could’ve used the slightly less expensive AT90LS8515, but I wanted to allow for the possibility of adding a temperature-sensing meter/alarm option to the circuit. The ’8535 has a 10-bit ADC function that’s suitable for this purpose; the ’8515 does not.

The ’8535 MCU has 8 KB of ISP flash memory, which is just about right for the necessary firmware. It also contains 512 bytes of EEPROM. I used a small amount of the EEPROM to store default values for the three programmable power supplies. That is to say, the power supply will power up with the same settings that existed at the time its Save Configuration push button was last pressed.

To simplify construction, I decided to use a SIMM100 SimmStick module made by Lawicel. The SIMM100 is a 3.5″ × 2.0″ PCB containing the ’8535, power supply regulator, reset function, RS-232 interface, ADC, ISP programming headers, and a 30-pin SimmStick-style bus. I’ve used this module for prototypes several times in the past, but this is the first time I’ve actually incorporated one into a finished project. …

eded to operate the three SPI channels and interface the two rotary encoders come out through the 30-pin bus. As you now know, I designed the ground-referenced power supply PCB to include space to mount the SIMM100 module, as well as the IsoLoop isolators. The SIMM100 mounts at right angles to this PCB; it’s hard-wired in place using 90° header pins. The floating power supplies share a virtually identical PCB layout apart from being smaller because of the lack of traces and circuitry associated with the SIMM100 bus and IsoLoop isolators.

The SIMM100 module has headers for the ISP programming cable and RS-232 port. I used its ADC header to run the LCD by reassigning six of the ADC port pins to general I/O pins.

When I buy in bulk, it’s inevitable that by the time I use the last item in my stock, something better has taken its place. After contacting Lawicel to request a .jpg image of the SIMM100 for this article, I was introduced to the new line of AVR modules that the company is developing.

Rather than a SimmStick-based module, the new modules are 24- and 40-pin DIP modules that are meant to replace Basic Stamps. Instead of using PIC chips/serial EEPROM and a Basic Interpreter, they implement the most powerful members of Atmel’s AVR family—the Mega chips.

Mega chips execute compiled code from fast internal flash memory and contain much more RAM and EEPROM than Stamps. Even though flash programming AVR-family chips is easy through SPI, using inexpensive printer port programming cables, these modules go one step further by incorporating RS-232 flash memory programming. This makes field updates a snap. …

The user interface I settled on consisted of a common 4 × 20 LCD panel along with two rotary encoders. One encoder is used to scroll through the various power supply parameters, and the other adjusts the selected parameter. The cost of LCDs and rotary encoders is reasonable these days. Being able to eliminate the substantial cost of six DPMs and six 10-turn potentiometers was the main reason for choosing an MCU-based design in the first place.

Brian Millier’s article first appeared in Circuit Cellar 149.

SIMPLE SWITCHER Nano Modules

Texas Instruments has introduced four new SIMPLE SWITCHER nano power modules for space-constrained applications. The compact 17- and 5-V modules expand TI’s SIMPLE SWITCHER module portfolio to address 100-mA to 2-A industrial designs, such as servers, factory automation, test and measurement, and network security cameras.TI-Nano2jpg

TI’s 17-V, 0.65-A LMZ21700 and 1-A LMZ21701—as well as the 5-V, 1-A LMZ20501 and 2-A LMZ20502 DC/DC power modules—achieve an overall solution size of up to 40% smaller than a discrete implementation. The modules combine high efficiency with high density and reduce EMI, even while operating at low power. All four modules enable designers to easily add more features and functionality to their systems in a smaller form factor, while speeding time to market.Watch a demonstration on how to create a high-density, multi-output design.

Key features and benefits:

  • Small solution sizes reduces board space by 40% when compared to discrete solutions.
  • Low component count simplifies design and increases system reliability.
  • Modules provide effective power management over the entire operating range.
  • Low output ripple at less than 10 mVPP for noise sensitive rails.
  • Low EMI complies with the CISPR 22 (Class B) radiated and conducted electromagnetic interference standard.
  • Modules enable easy implementation of multiple power rail sequencing using Power Good pin.

The four nano modules are available now in volume production. The LMZ21700 and LMZ21701 cost $1.55 and $1.75, respectively, in 1,000-unit quantities. The LMZ20501 and LMZ20502 cost $1.55 and $1.90, respectively, in 1,000-unit quantities.

Source: Texas Instruments

Industry’s Smallest Dual 3A/Single 6A Step-Down Power Module

Intersil Corp. recently announced the ISL8203M, a dual 3A/single 6A step-down DC/DC power module that simplifies power supply design for FPGAs, ASICs, microprocessors, DSPs, and other point of load conversions in communications, test and measurement, and industrial systems. The module’s compact 9.0 mm × 6.5 mm × 1.83 mm footprint combined with industry-leading 95% efficiency provides power system designers with a high-performance, easy-to-use solution for low-power, low-voltage applications.INT0325_ISL8203M_Intersil_Power_Module The ISL8203M is a complete power system in an encapsulated module that includes a PWM controller, synchronous switching MOSFETs, inductors and passive components to build a power supply supporting an input voltage range of 2.85 to 6 V. With an adjustable output voltage between 0.8 and 5 V, you can use one device to build a single 6-A or dual output 3-A power supply.

Designed to maximize efficiency, the ISL8203M power module offers best-in-class 15° C/W thermal performance and delivers 6 A at 85°C without the need for heatsinks or a fan. The ISL8203M leverages Intersil’s patented technology and advanced packaging techniques to deliver high power density and the best thermal performance in the industry, allowing the ISL8203M to operate at full load over a wide temperature range. The power module also provides over-temperature, over-current and under-voltage lockout protection, further enhancing its robustness and reliability.

Features and specifications:
•       Dual 3-A or single 6-A switching power supply
•       High efficiency, up to 95°
•       Wide input voltage range: 2.85 to 6 V
•       Adjustable output range: 0.8 to 5 V
•       Internal digital soft-start: 1.5 ms
•       External synchronization up to 4 MHz
•       Overcurrent protection

The ISL8203M power module is available in a 9 mm × 6.5 mm, QFN package. It costs $5.97 in 1,000-piece quantities. The ISL8203MEVAL2Z evaluation costs $67.

Source: Intersil

Expanded Multi-Range Programmable DC Power Supply Offerings

B&K Precision recently announced the 9200 Series, which is its newest multi-range programmable DC power supply line. The series includes four 200- to 600-W models that can deliver power in any combination of the rated voltage and current up to the maximum output power of the supply. With voltage and current ranges up to 150 V and 25 A, the programmable DC power supplies are well suited for electronics manufacturing, R&D, and more.BK-9201_front

 

Multi-range power supplies provide greater flexibility than traditional power supplies. The supplies can provide any combination of higher voltage or higher current along a maximum power curve. This design helps save both bench space and cost by eliminating the need for having multiple power supplies on your workbench.

 

On the front panel, the 9200 Series features a high-resolution, 1 mV/0.1 mA display, output on/off control, and a handy user interface with numerical keypad, cursor keys, and rotary control knob for adjusting voltage and current settings. The power supplies also provide internal memory storage to save and recall up to 72 different instrument settings.

 

For programming and remote control, the 9200 Series offers list mode programming, remote sense, and standard USBTMC-compliant USB, RS-232, and GPIB interfaces supporting SCPI commands. Remote control software is provided for front panel emulation, execution of internal and external program sequences, and logging measurements via a PC. The application software can also be integrated with National Instrumnets’s Data Dashboard for LabVIEW app (available for Android and iOS), allowing users to create custom dashboards on smartphones and tablets for additional monitoring functions.

 

the 9200 Series models are all backed by a standard 3-year warranty and list at the following prices:

  • 9201 (60 V, 10 A, 200 W): $680
  • 9202  (60 V, 15 A, 360 W): $880
  • 9205 (60 V, 25 A, 600 W): $1,380
  • 9206  (150 V, 10 A, 600 W): $1,495

Testing Power Supplies (EE Tip #112)

How can you determine the stability of your lab or bench-top supply? You can get a good impression of the stability of a power supply under various conditions by loading the output dynamically. This can be implemented using just a handful of components.

Power supply testing

Power supply testing

Apart from obvious factors such as output voltage and current, noise, hum and output resistance, it is also important that a power supply has a good regulation under varying load conditions. A standard test for this uses a resistor array across the output that can be switched between two values. Manufacturers typically use resistor values that correspond to 10% and 90% of the rated power output of the supply.

The switching frequency between the values is normally several tens of hertz (e.g. 40 Hz). The behavior of the output can then be inspected with an oscilloscope, from which you can deduce how stable the power supply is. At the rising edge of the square wave you will usually find an overshoot, which is caused by the way the regulator functions, the inductance of the internal and external wiring and any output filter.

This dynamic behavior is normally tested at a single frequency, but the designers in the Elektor Lab have tested numerous lab supplies over the years and it seemed interesting to check what happens at higher switching frequencies. The only items required for this are an ordinary signal generator with a square wave output and the circuit shown in Figure 1.Fig1-pwrsupply

You can then take measurements up to several megahertz, which should give you a really good insight for which applications the power supply is suitable. More often than not you will come across a resonance frequency at which the supply no longer remains stable and it’s interesting to note at which frequency that occurs.

The circuit really is very simple. The power MOSFET used in the circuit is a type that is rated at 80 V/75 A and has an on-resistance of only 10 mΩ (VGS = 10 V).

The output of the supply is continuously loaded by R2, which has a value such that 1/10th of the maximum output current flows through it (R2 = Vmax/0.1/max). The value of R1 is chosen such that 8/10th of the maximum current flows through it (R1 = Vmax/0.8/max). Together this makes 0.9/max when the MOSFET conducts. You should round the calculated values to the nearest E12 value and make sure that the resistors are able to dissipate the heat generated (using forced cooling, if required).

At larger output currents the MOSFET should also be provided with a small heatsink. The gate of the FET is connected to ground via two 100-Ω resistors, providing a neat 50-Ω impedance to the output of the signal generator. The output voltage of the signal generator should be set to a level between 5 V and 10 V, and you’re ready to test. Start with a low switching frequency and slowly increase it, whilst keeping an eye on the square wave on the oscilloscope. And then keep increasing the frequency… Who knows what surprises you may come across? Bear in mind though that the editorial team can’t be held responsible for any damage that may occur to the tested power supply. Use this circuit at your own risk!

— Harry Baggen and Ton Giesberts (Elektor, February 210)

Member Profile: John Peterson

John Peterson

John Peterson

Location: Menlo Park, CA

Education: BS and MS, University of Utah

Occupation: Software Developer

Member Status: John has been a subscriber since 2002.

Technical Interests: His interests include user interfaces for embedded systems, field-programmable gate array (FPGA) development, and embedded Internet development.

Most Recent Embedded Tech-Related Purchase: John recently purchased a power supply for one of his designs.

Current Projects: He is currently working on a custom light controller for strings of progammable LED lights.

Thoughts on the Future of Embedded Technology: John feels that smartphones have raised everybody’s expectations for how we interact with everyday things (e.g., cars, appliances, household control, etc.). “Either the phone becomes the interface (via the network) or the gadgets need touchscreen displays,” John said.

Engineer’s “Harlem Shake” Meme

In an blog posted today on the Phoenix New Times site, Troy Farah asks: “Harlem Shake vs. Gallon Smashing Prank: Which Meme Will Destroy America First?” Well, both have caused a lot of problems for smashers and shakers in the United States. We read a recent report about the possible legal issues facing some gallon smashers. And CNN.com posted a story on March 1 about the FAA’s probe into a recent “shake” on a plane. With negative results such as these, it’s clear that the Smash and the Shake are bidding for the most tile of “most destructive.”

Where does the engineering community stand on these pranks? Well, we have not seen an electrical engineer, robot, or microcontroller-based system smashing a gallon of milk to get a laugh. (Thankfully! We don’t endorse it.) But we did recently seen an engineer’s take on the Harlem Shake.

And so the meme continues.

Be sure to check out Dave Jones’s EEVblog video about the rocker.