Extra power for Analog Discovery Experiments
We have been getting many questions with regard to the limited current and fixed voltages available from the built in user power supplies provided by the Analog Discovery Module. The fixed +/- 5 Volts and maybe up to 50 mA is OK for the simple Lab activities the students do in their early Sophomore and maybe Junior level lab classes but to do interesting projects, students may need more current.., and the Analog Discovery does not put out enough. Professors have asked, we need our students to be able to make use of this for multiple semesters if possible, and increasingly more sophisticated projects.
Do not take any of the following information an official endorsement of any of these products and these suggestions are provided as examples only.
The solution to this problem can of course take any number of directions. An obvious solution is to turn to the old stand-by of the traditional lab bench top power supplies electrical engineering labs have included for years. This approach however conflicts with the portable “anywhere anytime”, low cost, students own the hardware concept embodied in the Analog Discovery. Some lower cost “bench” power supplies targeted at the experimenter market are available. One example is this switch selectable variable supply, figure 1, from parts-express.com (part number 120-536). The rotary switch selects between six common voltages 3V, 4.5V, 6V, 7.5V, 9V, and 12V. The cost for single units is $19.40 which is not out of the range for a student to purchase one or more of these.
Figure 1 Regulated Power Supply 3-12 VDC 2A
An alternative that certainly fits the anywhere, anytime, low cost requirement would be to use batteries to supply the needed higher voltages and currents. Standard 9 volt batteries and connectors are inexpensive and readily available. Holders for multiple (from two to eight) 1.5 volt AA cells are also common and relatively inexpensive. The major drawback with using conventional batteries is that they run down over time and need to be replaced. This problem can be solved to some extent by buying rechargeable versions of these batteries.
Here are a few examples of where to find battery adapters:
Digilent offers 2 cell and 4 cell AA battery holders for $0.95 and $1.45 respectively.
Radio Shack offers 24 different battery holders for AAA to D cells and 9V batteries.
Jameco supplies over 40 different battery holders for various types.
A third alternative that falls somewhere in between these two solutions is to use one of the common wall plug power adapters (so called “wall wart”). These power adapters come in a wide range of DC or AC output voltage and current ratings from 3.3V to 18V, and from 200 mA to 3 or 4 Amps. The cost ranges from as little as $3.00 to maybe $8.00 for the highest power versions. Aside from needing to be plugged into the wall for power they fit into the anywhere, anytime, students owns the hardware model. Figure 2 shows what a typical “wall wart” looks like.
Figure 2 Typical “wall wart” power adapter
The simplest of these wall adapters are step-down transformers with just rectifiers and rudimentary filtering that supply unregulated DC voltages. Others, which cost a little more, are switching power supplies with regulated DC output voltages, mainly 5V. The problem with these wall adapters is their outputs are not adjustable, are one polarity and probably unregulated. Jameco (www.jameco.com) and Parts Express (www.parts-express.com) offer a wide assortment of wall power adapters.
Voltage regulator circuit boards designed for use with solder-less breadboards are available such as the one shown in figure 3. These generally supply only one fixed 5V output voltage, or perhaps can be switched between 3.3V and 5V. Adjustable versions are also out there but often provide only one, or maybe two, positive output voltages.
Figure 3 Breadboard power supply.
For really interesting and sophisticated mixed signal projects positive and negative supply voltages are needed. Perhaps one of the first projects for the students should be to design and build a regulated, adjustable, dual voltage power supply? The ADP667, +5 V fixed or adjustable Low-Dropout linear voltage regulator (Jameco Part no. 1803518) would be a possible choice for a positive supply. The ADM660, CMOS switched-capacitor voltage converter (Jameco Part no. 1802259) would be a possible choice to generate a negative supply from a positive supply (such as the ADP667).
Alternatively, a switching power supply could be built around the ADP2300, a 1.2A, 20V, 700 kHz nonsynchronous step-down regulator as shown in figure 4. By adjusting the resistor divider feeding back to the FB pin the output voltages could be a made variable.
Figure 4 Dual +/-5 supply schematic (taken from CN-0183)
Still another possible source for additional power would be to tap into a second USB port on the students computer. Standard USB ports can supply +5V and up to 500mA of current. You have to be careful using a USB port to power your experiments and accidental shorts may cause the computer to crash or even damage the USB port power supply. Also remember that the ground connection of the second USB port is the same as the ground from the USB port the Analog Discovery is plugged into.
Higher voltage split + and – supplies can be generated from the fixed +5 volts from a USB port using a topology like the one shown in figure 5, based on the ADP161X family of SEPIC-Ćuk DC-DC converters. This topology consists of an unregulated Ćuk converter tied to the same switching node as a regulated SEPIC converter. This combination results in two supplies that track each other very well under all but a 100% load mismatch.
Figure 5 Creating Split Rails from a Single Input Voltage
(taken from AN-1106)
Changing the RF1B and RF2 resistor ratio will change the + and - output voltage.
Short of building a variable power supply from scratch here is one I came across on the web:
Figure 6 Buck-Boost adjustable power supply
The Voltage Regulator shown in figure 6 has an adjustable 1.2VDC to 30VDC output. This regulator uses a SEPIC buck-boost type design, so that the output voltage can be both higher or lower than the supplied input voltage. The price listed is $12.99. For safety and ease of use you would probably want to put this into some kind of enclosure with an on/off switch. The screw driver trim pot might be a little awkward to use but could probably be replaced with a conventional pot with a knob.
Voltage regulator integrated circuits can be found at suppliers like:
As always I welcome comments and suggestions from the user community out there.