This document will walk you through the operations required to enable your cee to sink and source higher current than the 200mA supported out of the box.
Two hardware modifications and one software operation are required to modify the current sensor, add a power supply capable of sourcing more electricity, and inform cee of its improved capabilities.
WARNING - not only does this "void your warranty" but it could harm you, your computer, your cee, and probably a half dozen other things. If you're at all concerned, don't continue. At the very least, hit up your local hackerspace for advice.
CEE was designed for a variety of use cases including both analytical and qualitative exploration, additionally, we sought to keep the design and use of CEE as hassle free as possible, ruling out external power supplies. With these guiding notions, prototyping suggested that two channels at 200mA each would both meet the USB specified maximum current of 500mA, with a safety margin, and be close to ideal for daily use.
That said, you may need to interact with power-hungry devices such as solenoids, peltier panels or larger laser diodes, and adding a power buffer is a pain. Fortunately, the design of CEE makes modification a fairly straight-forward process.
In order to modify your cee to handle higher currents, you'll need to gather a few cheap supplies.
You will need a power supply capable of providing five volts with a current capability in excess of your target output. This power supply needs to be fairly high quality - it will be indirectly connected to your computer, and an electrically noisy supply may result in subpar performance, so don't try this with your spare $2 cellphone charger! We've had good luck with newer ATX power supplies, like you might find in a desktop computer.
WARNING - depending upon how your computer is made, it could be hurt by using an external power supply. If you're unsure, don't continue! If you're using a laptop, you can improve the odds by unplugging it when you use it with your modified CEE.
You'll need at least one high precision current sense resistor, surface mount size 0805 or 1206. For this walk-through, we'll use an 0805 0.0350Ω resistor, part number WSLP0805R0350FEA, available from Mouser.
You will also need a soldering iron capable of at least 700degF, solder, wire, and a good pair of tweezers.
The jist of this hack is that by putting a smaller resistor in parallel with the current sense resistor, the net resistance will be reduced. As per Ohm's law (V=I•R,) this will require more current to create the same voltage drop, measured by the CEE. This will let the CEE's current control continue to work as usual, while increasing the actual range of your hardware.
For this demo, we'll be increasing the current to 1A output on channel A. This requires a 0.0350Ω resistor to be paralleled wtih the 0.070Ω resistor (RS-A) already on the board, to result in a 0.02333Ω ohm resistor.
To calculate the resistor you need, you can use the following formula:
R = 1/(45*I/1.25 - 1/.07)
Alternatively, keep in mind the following figures:
Value Quantity Result Range Notes 0 0 .7000Ω ±0.2A nothing added .035Ω 1 0.233Ω ±1A this article .035Ω 2 0.140Ω ±2A maximum ?Ω ? 0.???Ω ±?A your value here!
"0.0350Ω resistors are your friend!"
There are two places you'll need to work with to pull off this hack.
We're going to start with the resistor part first.
Look at RS-A.
Stack your resistor on top of RS-A.
Hold the resistor in place on top of RS-A with your tweezers and solder each end in turn.
Next, look at the part of your board with three holes labeled "USB," "GND," and "EXT," respectively.
Using the tip of your iron, clear the solder joint connecting "USB" to "EXT."
This will separate the power supply for the output chips from the power supply of your computer's USB port.
WARNING - if you don't clear the solder joint ALL THE WAY you WILL hurt your computer. This is important. You may find it helpful to use solder wick.
If you're not sure, use a multimeter's continutity check mode or plug the CEE in and try to set a voltage - if it doesn't work, you're likely good to go.
Next, solder wires to "EXT" and "GND" to connect the device to your external power supply.
We've found it useful to use JST jumpers to keep the CEE removable from the power supply.
Put away your soldering iron and take a deep breath - the hard part's over.
After modifying CEE's hardware to support higher currents, you need to let the CEE know that it has more resources to work with.
Navigate to http://apps.nonolithlabs.com/editsenseresistor.
Fill in the boxes with the new resistor value. (0.07Ω||0.035Ω = 0.023Ω)
Click "Save to device."
Unplug and replug your device and pull up Pixelpulse.
If you were successful, the current range modification will be reflected in the current axis for the modified channel.
Congratulations! Go make awesome and tell us about it.
What if it didn't work?
- Email us! We're always glad to help. A few good pictures will help us debug.
- Make sure your power supply is outputting the right voltage. You can use the CEE to check by connecting a jumper from channel A or B to EXT with V in measure mode and I disabled. (think of a scorpion stinging itself)