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Power Supply Add-On Circuit

The Dragon Rider printed circuit board includes an area for a power supply circuit that allows the board to be powered independently of the AVR Dragon.  This can be used when the Dragon is attached, as long as power from the Dragon is disconnected, or to run a completed project after the Dragon has been removed.  Given a suitable unregulated power input, up to 0.8 amps is available for the target circuit (about three times what the Dragon can supply) at voltages adjustable from 3.3 volts to above 5 volts.

Typical Parts Required

The table below lists the parts that are provided in the ECROS Technology add-on parts kit for the power supply.  Most of the parts are the sort of thing you may have in your spares drawer.  The comments column should help you find substitutes; refer also to the Technical Information section, below.

Qty. Description Ref. Des. Comments
1 2.1mm co-axial power jack J32 Source must be center positive!
1 3.3V, 0.8A regulator U7 1117-type, TO220
1 Heatsink, small horizontal - Optional, depends on voltage and current
1 1/4 inch, 4-40 machine screw - To mount regulator to PCB
1 4-40 hex nut - To mount regulator to PCB
2 100 µF capacitors C10, C12 Values not critical
1 0.1 µF capacitor C11 Usually required for stability
1 1 kohm trimmer R5 Voltage adjustment
1 470 ohm resistor R6 Sets LED current, about 6 mA at 5 volts
1 3mm (T1) LED, red LED1 Just about anything will do
1 Slide switch SW1 External power ON/OFF

Technical Information

Click here to open a window with the schematic of this add-on circuit.  This circuit is very similar to the LD1117-based Mini-PSU and more in-depth technical information is available on the pages for this product.

Unregulated DC input power should be applied at J32.  A 2.1 mm center pin coaxial jack is supplied in the parts kit for use with a compatible wall-wart style power pack.  The center pin must be positive; there is no protection against an incorrect input polarity, so be careful here.  To use flying leads to connect to a power source, solder the positive wire to the pad of J32's footprint that is closest to the regulator, U7, and the negative (ground) wire to the pad closest to the top edge of the PCB.  The pad close to the pillar and C10 is not connected.  The unregulated supply voltage should be at least 1.2 volts about the regulated target voltage you are looking for, but not greater than 15 volts.  For more on this subject, see the Technical Information section of the page on the LD1117-based Mini-PSU.

C10's purpose is to reduce ripple voltage on the unregulated supply.  Use any medium- or large-valued electrolytic capacitor with a working voltage at least as high as the unregulated supply voltage.  This component can be safely omitted if the power source is already adequately smoothed.

Most regulator ICs require a high-frequency decoupling capacitor at the input in addition to smoothing capacitors.  C11 performs this function and should not be omitted if you use the LD1117 supplied in the parts kit.  For other regulators, refer to their data sheet.

The regulator itself, U7, can be any regulator with input at pin 3, output at pin 2 and ground or adjustment at pin 1.  The circuit is particularly well suited to the 1117 family.  An LD1117V33 from ST Microelectronics is supplied in the parts kit.  Wire pin 1 to ground to get the voltage for which the regulator is rated (3.3 volts in this case) or install R5 to make the voltage adjustable.  More information on choosing regulators is available on the Mini-PSU Technical Details page.  The parts kit includes a small heatsink for the regulator.  The Dragon Rider PCB has a heat spreading pad and whether or not you need the heatsink depends on the average current you will draw and how much voltage the regulator will drop. 

As mentioned above, R5 can be installed with many regulators to make the output voltage adjustable.  The 1 kohm trimmer in the parts kit is intended for use with the LD1117V33 supplied to give output voltages from 3.3 volts to above 5 volts.  By shorting the wiper (the middle pad) to ground (the square pad), you will have a fixed output of 3.3 volts (or the rated voltage of whatever regulator you use).  For sensitive applications, where you want the absolute best regulation, you might want to connect a capacitor of about 10 µF between the wiper and ground as mentioned in some regulator data sheets.  There is no footprint on the Dragon Rider for this, so you will have to attach it under the board.

C12 is required or recommended with many regulators to improve the ability to deliver a steady output when the load current changes suddenly.  A medium-valued electrolytic capacitor with a working voltage at least as high as the regulated output voltage should be installed.  Omit this part only after checking the data sheet of your regulator and, even then, it is better to include it if your target circuit switches any substantial currents, such as operating a relay.

LED1 and R6 can be installed to provide a power ON indicator.  Make sure that the anode of LED1 (usually the longer lead) is closer to the top of the board, near the DB9 connector, J33.  If R6 is installed in the marked footprint, the LED will light when the unregulated power source is plugged in, whether or not SW1 is closed.  If you have the Dragon Rider mounted on an AVR Dragon, this would be a good warning that you should be careful not to close SW1 and SW2 at the same time.  To have LED1 light when target power is active, either from the power supply add-on via SW1 or from the AVR Dragon via SW2, stretch R6 so that the lower lead goes to the pad marked VTG.  You can do this even if you don't assemble the rest of the power supply add-on circuit, just to get a power ON indicator for the target circuit.

Switch SW1 connects power from the power supply add-on circuit to the target system.  Never close SW1 when target power is connected to an AVR Dragon.  Before closing SW1, make sure that SW2 is open (OFF), the Dragon is unplugged or some other measure has been taken to isolate the Dragon from target power.  The slide switch supplied in the parts kit is rated at 0.5 amps.  If your target circuit will be taking more current than this when you operate the switch, you might want to use a larger, separately mounted, toggle switch.

Parts Kit - Quick Start

If you assemble the components in the ECROS Technology Parts Kit to the Dragon Rider, LED1 will light when you apply the external power source and R5 will adjust the target voltage.  For modest current requirements, up to 300 mA, use a 6 volt 500 mA or 9 volt 300 mA wall-wart (or something similar) and you won't really need a heatsink.  For more current, you will need to be sure that the unregulated voltage stays at least 1.2 volts above the regulated voltage, allowing for ripple, so a 6 volt wall-wart at its rated load current will be OK for 3.3 volts but not for 5 volts.  Install the heatsink if the current times the voltage across the regulator comes to two watts or more.

For a fixed target supply voltage of 5 volts, a simple modification will get rid of the possibility of mis-adjusting R5.  Just omit R5 and run a wire from its wiper pad (the middle one) to the junction of LED1 and R6.  This works because a red LED drops about 1.7 volts, which is the difference between the rated 3.3 volts of the supplied regulator and the 5 volts you're looking for.  By raising the adjustment pin from ground to 1.7 volts, the output is raised from 3.3 to 5 volts.  If you want to do this after installing R5, just turn it fully clockwise (or use it to adjust the brightness of R5!).

Advanced Tips and Tricks

To keep the Dragon Rider cost and size down for the average user, it does not have complicated protection circuits to prevent power being applied from the add-on circuit or the AVR Dragon when the other is also connected.  It is anticipated that most users will either not use the power supply add-on or will only connect the external power source when the AVR Dragon has been removed.  If you want to work with both power sources together, you can easily adapt the design.  One idea is to install a pair of schottky diodes between each source and the target power line, VTG.  One can be installed in place of SW1 and the external power source unplugged or switched as needed.  The other can be installed near SW2 in the footprint of U6.  If this is done, power will come from whichever source is turned ON (or, actually, whichever has the higher voltage).  You can also use the power supply add-on circuit to power parts of your target system while the AVR Dragon powers the rest.  Of course, you must be careful of power sequencing so that having one on and the other off does not cause a problem.  This would be a good idea if you are switching relays or powering motors as the add-on could take the high-current loads which would then not introduce noise into the logic circuits.  Suitable changes to the components used would give voltages quite different from 3.3 or 5 volts, such as 12 volts for popular relays and solenoids.