Since I decided to use a horizontal layout for my control panel as opposed to a vertical one, I have a bit of work space available on the top. Since I usually use my iPad Mini for keeping track of beer recipes, I figured it would be a good space to set up a little iPad work station.
iPad And iPad Mini Stand
The stand I chose for the top of my control panel is the Skadoosh iPad stand. It’s a precision machined, fully adjustable stand that is compatible with many of the popular iPad cases. Once you set a new position, you can enable a locking mechanism to keep it there, which is perfect as you start working. There’s even a slot at the back where you can route a USB cable for charging.
Now if only there was some where to plug it in while working.
Built in USB Charging Station
With one 5500W heating element and both pumps active, the control panel draws roughly 23.3 amps. Most electrical code stipulates that a circuit can only draw a maximum of 80% of the total current available on a circuit. Therefore a 30 amp circuit only has 24 amps available to work with. Since we are already using 23.3 amps, there are still 700mA available.
Since I often brew with my iPad mini nearby (for reading recipes and for playing music), I thought it would be cool to incorporate a USB charging station directly in the control panel. That way on brew day I could simply set up my iPad mini on top of the control panel, leaving it connected to the built-in USB port for the duration of the brew session. Sure, I could probably just plug it into a wall socket somewhere and charge it that way. But adding a USB charging station allows for a more elegant solution to keep the iPad charged while using it.
By default a well-behaved USB device plugged into a USB port will only draw 100mA. That’s enough for many small devices, like flash drives, but not enough for most tablets or cell phones. In fact, an iPad plugged into a 100mA charging port will typically display a message saying that the device isn’t compatible.
To get around that, the USB device can either negotiate (via the USB data pins) a higher charger rate, or those data pins can be configured in a certain way so the USB device can detect the maximum current. The USB specification addresses this form of charging station:
Apple’s iPod and iPhone chargers indicate the available current by voltages on the D− and D+ lines. When D+ = D− = 2.0 V, the device may pull up to 500 mA. When D+ = 2.0 V and D− = 2.8 V, the device may pull up to 1 A of current. When D+ = 2.8 V and D− = 2.0 V, the device may pull up to 2 A of current.
We have 700mA available, but that current is relative to the 240V circuit. When using a down-converting power supply with a transformer the voltage will go down on the far-side of the transformer, but the available current will go up. So while the 5V line may supply 2A of current (10W), at 240V that’s the equivalent of 41.6mA for the same amount of power (10W). So we have more than enough power left with our 700mA available (168W) for a full 5V/2A charging line.
To let devices know that there are 2000mA available we need to set D+ to 2.8V and D- to 2.0V. Once detected, Apple iOS devices will know that they can draw up to 2000mA of current.
I recently purchased a 5V power supply for use in my control panel back when I was considering using LCD-based voltage and current meters. Since I ultimately decided to use analog meters, the power supply wasn’t needed any more. But it turns out that the 5V/2000mA output on the power supply is perfect for powering our USB charging station.
I picked up a cheap 4-pin USB header on eBay that will be connected to the chassis. Pin 1 will be connected to the 5V output on the power supply, and pin 4 to its ground. Both pins 2 and pins 3 need to be configured to supply 2.8V on the D+ line and 2.0V on the D- line.