A technical challenge!

A project log for MoonLITE

Interactive lighting elements for the mobility impaired. Designing a power supply for architectural scale interactive EL installations

foolfool 08/27/2017 at 04:300 Comments

So far we've defined an assistive technology product for integrating interactive lighting into spaces for the mobility impaired. Now we can start hacking to figure out what needs to be done in order to get this implemented!

The main challenge here is that the on-board power supply on the Whoa Board is based on the MIC4833.

The way that it works is by first generating a 110 volt DC signal (from 5v, using a high frequency boost circuit), and then generating an alternating current out of that by passing the DC signal through two alternating half bridges (which we have set to oscillate around 700Hz).

We chose because it was quiet (ours is one of the only EL drivers that doesn't make a 1-2KHz buzzing noise), but is limited in it's power output, and cannot power EL elements on an architectural scale.  Fortunately, we manufactured the whoa board with the ability to accept an external power supply!  

The way it works is as follows:  At the core of the Whoa Board sits a fast high voltage switch (that we control using a bit-banged serial protocol). In order for the switch to switch, it needs a "min" and "max" voltage reference as an input to it's level shifters.  

We obtain this reference for an external supply by disconnecting the onboard supply (to protect it) and then by passing it through the coupling circuit at the top of this post.

Note: If you forget to flip the dip switches and connect and external supply, you will probably fry the onboard EL drive circuit (and maybe something else as well).  

The ground marked optional is something that we've found helps protect the board, but is not currently integrated.  We are happy to provide instructions on how to add this for anyone interested.  

Finally, note that the touch sensing doesn't work on "first edition" boards with the external supply without a bit of rework.  Again, if you have a board and want more insight, happy to answer questions. 

The onboard switch also has a few additional constraints which end up presenting challenges.  First off, it is rated for a peak current of 3A, which is worth bearing in mind because EL loads are capacitative.  Secondly, it cannot safely bear a peak reference voltage of more than 150V.  

Now that we have this circuit, we are ready to define our hack!

Namely, we set out to create a power supply capable of driving a capacitative load at ~100V and ~1KHz, that can safely output ~5 watts, and is also compatible with the above constraints!