Extra power supplies for Active Learning Modules (M2k , M1k)

We have over the years gotten many questions with regard to the limited current and voltages available from the built in user power supplies provided by the Active Learning Modules ADALM1000 and ADALM2000. Trade-offs in the design of these two modules were made to constrain both the parts cost and power budget to allow operation from USB supplied power. While the ADALM2000 allows powering the board from an external +5 V supply this was done mainly to allow stand-alone portable or wireless operation not to increase the user supplied voltages or current.

The adjustable user supplies of the M2k supplying up to +/- 5 Volts and up to 50 mA of current is OK for many of the simple lab activities the students do in their early Sophomore and maybe Junior level lab classes but to do really interesting projects, students may need higher voltages or more current. The M1k can source and sink more current, up to 200 mA, from its power rails but only at +2.5 and +5 V. Professors have asked, "We need our students to be able to make use of these modules for multiple semesters if possible, and an increasingly wider range of sophisticated projects" (meaning higher supply voltages).

One quick note of caution before proceeding, the M1k input voltage range is limited to only 0 to 5 V. Before building any circuits that operate from power supplies outside the native 0 to 5 V range of the M1k we need to protect the analog inputs when in Hi-Z mode and extend the usable range of input voltages. This earlier Blog post covers techniques to address this limitation.

To further address the power supply deficiencies of the Active Learning Modules, the newest production run of the ADALP2000 Analog Parts Kit now contains even more power supply related components. In addition to the original ADP3300 3.3 V LDO regulator a few more parts that might help extend the power supply options have been added.

The first is simply a micro USB break-out board that can be plugged into the solderless breadboard. Using another USB cable (not supplies in the parts kit) plugged into a spare USB port (or USB charger) can provide an additional +5 V supply up to the current limit of the USB port.

Connecting your experimental circuits to an extra USB port on your computer can be hazardous and extreme care should be taken to not accidentally damage the computer or USB port power supply.

It is important to note here, when using a spare USB port on the same computer that the M1k or M2k is plugged into, the ground side of the USB break-out board will be the same ground (i.e. shorted to) as the ground pin on M1k, M2k.

Micro USB break-out board

A much safer option is to use a separate "wall wart" USB charger. Most all of these USB chargers (or any wall wart for that matter) are likely isolated so that the grounds will not be connected to anything else. The two leads, + and – can float and be attached to just about any node in your circuit. Most USB wall chargers with just two prong plugs will be isolated (for safety reasons because it could be plugged into the wall either way around). To be doubly sure you can use an Ohmmeter or continuity tester to check if either of the output pins has a path back to the AC plug.

The second new power supply related component is the isolated μModule (Power Module) DC/DC Converter break-out board [[http://www.analog.com/en/products/power-management/umodule-regulators/isolated-umodule-converters/ltm8047.html#product-overview]] that can take a 3.1 V to 32V input. A trim pot on the breakout board allows the output voltage to be adjusted from 2.5V to 12V. Because the output voltage pins are fully isolated from the input pins the voltage can be either positive or negative depending on which output pin is connected to ground. This isolation feature can be very useful when powering the module with +5 V using the USB break-out board plugged into a spare USB port or external charger.

Isolated μModule DC/DC Converter break-out board

The output current from the module can be as much as 440 mA (when VOUT = 2.5V).

The third new power supply related component added to the kit is the LT1054 Switched-Capacitor Voltage Converter with Regulator [[http://www.analog.com/en/products/power-management/inductorless-charge-pump-dc-dc-converters/regulated-step-up-charge-pumps/lt1054.html]] which can be used with just two capacitors as a voltage inverter to make   -5 V from +5 V or with two additional diodes as a voltage doubler to make approx. +9 V from +5V. Using the LT1054 to generate -5V from the +5V supply from the M1k for example is particularly useful because the M1k lacks a built-in negative supply.

The fourth new power supply related component is the LT3080 [[http://www.analog.com/en/products/power-management/current-sources/lt3080.html]] adjustable linear voltage regulator (LDO) which can be programmed using a single resistor to set the output voltage from 0 to VIN - 1.2 V (i.e VIN must be at least 1.2 V greater than VOUT). It can source up to 1.1 A (with the appropriate heat-sink). The TO-220 package version is supplied in the kit which has staggered leads that need to be bent slightly to fit into a solderless breadboard. Inserted on the breadboard in the photo below is a LT3080 with the leads bent and bolted to a small heat-sink. A 9 V battery supplies the input voltage and the 500 KΩ pot will adjust the output voltage from 0 to 5 V. The 9 V battery holder has a built-in on-off switch is handy.

LT3080 inserted in breadboard with heat sink

What if I didn't bother to purchase the ADALP2000 parts Kit?

If you are a registered user of myAnalog you can request samples of some of these parts in bread-board friendly through-hole packages. Samples of the LT1054 DC-DC converter in the 8-pin PDIP package are available. Samples of the ADM660 CMOS switched-capacitor voltage converter (similar to the LT1054 but not part of the ADALP2000) in the 8-pin PDIP package are also available.

Another IC with switched capacitor DC-DC converters built in is the ADM202E (and other similar parts in the same family like the ADM237L, LT1081). While the ADM202E is generally used to interface CMOS logic levels to RS232 serial ports it contains two DC-DC voltage converters. The first converter is used to boost (double) the +5V input power supply to +10V and the second converter is used to invert the +10V to produce -10V. These DC-DC converters are not capable of supplying large output currents, up to 20 mA combined total from both rails, but can supply enough current to power experiments and circuits using a few low power op-amps. These parts are also relatively low cost and samples are available in breadboard friendly PDIP packages. The LT1026 is just the dual DC-DC converter part from these RS-232 interface chips. It is in available in an 8 pin PDIP and can output +/- 9V from a +5 V supply. The combination of the two DC-DC converters in one package is convenient over using two separate ADM660 converters to do the same thing. It might seem like a waste to not use the Rx and Tx parts of this IC but they might actually come in handy in that the ADALP2000 kit does not come with any CMOS Schmitt trigger inverters.

*Do not take any of the following information as an official endorsement of any of these products or suppliers and these suggestions are provided as examples only.

The solution to the power supply problem can of course take any number of directions. Batteries are a simple and obvious alternative that certainly fits the anywhere, anytime, low cost requirement to supply the needed higher voltages and currents. And batteries are inherently isolated so can float above or below any node, ground or other power rail. Standard 9 volt batteries and connectors are inexpensive and readily available. Holders for multiple (from one to eight) 1.5 volt AA or AAA 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. Batteries also do not supply a constant voltage as they discharge (a reason for ADI to include the linear voltage regulators in the parts kits). The problem of having to replace dead batteries can be solved to some extent by buying rechargeable versions.

Another obvious solution is to turn to the old stand-by of the traditional bench top power supplies, figure 1, electrical engineering labs have included for years. Expensive and not exactly portable.

Figure 1, Bench Power Supply approx. $100

This approach however conflicts with the anywhere, anytime, low cost, students own the hardware concept embodied in the Analog Devices Active Learning program. Some lower cost power supplies targeted at the experimenter market are available. One example, in figure 2, is this switch selectable variable supply, sometimes called a "battery eliminator". The slide switch selects between six common (battery) voltages 3V, 4.5V, 6V, 7.5V, 9V, and 12V. The cost is $13.95 which is not out of the range for a student to purchase one or more of these.

Figure 2, Selectable regulated power supply 3-12 VDC at 1 A

A third alternative that falls somewhere in between these two solutions is to use one or more of the common wall plug power adapters (so called “wall wart”). These power adapters come in a wide range of fixed DC or AC output voltage ratings from 3.3 V to 18 V and current from 200 mA to 3 or 4 Amps. The cost ranges from as little as $1.00 for a +5 V at 1.0 Amp USB charger from the Dollar Store, or $3.00 to $8.00 for the higher voltage and current versions. Aside from needing to be plugged into the wall for power they fit into the anywhere, anytime, students own the hardware model. Figure 3 shows what a typical “wall wart” looks like. Some will have the standard coaxial barrel style DC connectors and some will have USB connectors.

Figure 3 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 fixed DC output voltages, 5V is common. The problem with these wall adapters is their outputs are not adjustable, are one polarity and might be unregulated.

One thing I've found interesting to do is to take the DC-DC buck regulator circuit board out of one of these 12 V Car USB charger adapters, figure 4. The four that I've opened up all had very similar designs that used a resistor divider to set the output voltage. Enterprising experimenters could replace this resistor divider with a potentiometer to make the output voltage adjustable. A really older one of these I took apart just had two transistors, one a power device, two different Zener diodes and a switch to select between 9 V and 6 V for the output.

Figure 4, Car charger adapter plug

In the photo in figure 5 I show the internal circuit boards for 4 of these car charger adapters. I've mounted one of them to another proto-board and added a barrel connector on the input so it can be powered from a wall wart or 9V battery. I've also added a pot and slide switch to select between either a fixed or adjustable output voltage. Pins on the board allow it to be plugged directly into the power / ground rails of a solderless breadboard.

Figure 5, 12V charger internals

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? Of the parts from the ADALP2000 Kit, the LT3080 adjustable Low-Dropout linear voltage regulator would be a possible choice for a positive supply. The Isolated μModule (Power Module) DC/DC Converter break-out board and the LT1054 (or ADM660 CMOS) switched-capacitor voltage converter would be possible choices to generate a negative supply from a positive supply.

As always I welcome comments and suggestions from the user community out there.

Doug

Links to places to get battery holders, wall adapters and other neat stuff.

https://www.allelectronics.com/

http://www.goldmine-elec-products.com/

http://www.bgmicro.com/

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