Dedication to the Power Supply
So now that all the necessary (and some that aren’t) components have arrived, the build can commence! The new Soldering Station is warming and the Soldering Mat is in place (so I don’t burn her table) and Rum is at the ready.
First job is to mark out all the locations for the socket holes. For this, I’ve gone for a stepped drill bit. It’s a bit over-kill, but it was also cheap as chips in the local hardware store’s bargain bin.
The very first hole that I drilled was not a good one, if I’m honest. The front of the box appeared wider than it actually is. There’s a cut-out for the lid screws that wasn’t immediately clear from the front and, it being super important that I get the first hole drilled as soon as possible, with all of the caution gone with the wind.
Measure once, drill twice. Just as my equally impatient father taught me. You can see the boo-boo in the top centre of the pic. It was just too close to the radius of the lid-screw/weather sealing pocket. No matter, a bit of Dremel action and the M3 nut was awkwardly jammed into place. At a slightly odd angle.
So far, so good. In place is the two double USB’s, the main DC fuse and the little button I’ll use for flicking the LED’s on and off.
Equal parts YouTube Tutorials and Guesstimation
Next on the roster is a function test and output voltage reality check. The large metal plate on the bottom is what secures everything into the box, so I’ve drilled through this in strategic places for the (not supplied; off to the hardware store) screws to firmly fix down the supply itself. Not being missed is the plethora of very large, very conductive surfaces. To guarantee a clear path to earth, I’ve jumper-wired all the surfaces back to the 240vAC Earth. Hopefully, it’ll save my bacon if I drop a clanger. The box is also powered via an RCD to further negate my lack of experience. Considering this won’t exactly be IP (weather resistance) rated when it is complete, I’ll likely continue using the RCD while it is in use actually powering the telescope in anger too. Piece of mind when a glass of Rum inevitably lands on it on the dark. The voltage check was reassuringly spot on when tested with my multimeter. 12.00vDC. Good, it’s at least had one quality control test. At first I thought that the fan is duff when it didn’t spin up when the power was (gingerly and at a safe distance) first switched on. Turns out that the fan is only energised when the temperature reaches a certain limit. Around 40-43°C according to my inability to not know and digging out the digital thermometer.
The job is progressing nicely, with only one or two minor deviations from the plan. There was an unexpected trip back to Maplin when my curiosity got the better of me and I needed to know why you shouldn’t screw the cap onto the main fuse holder without a fuse being inside it. Experimentation quickly uncovered that there are only about 4 rotations of thread that to complete before the cap completely unscrews itself inside the body. Now the lid is below the last thread, occupying the space that a fuse was supposed to be doing. They should put a little note in the bag, warning you not to do this. Oh, they did.
Thanks to physics, a circle is the only shape that won’t fit through its own hole if it’s sized to sit atop it. This is why drain covers are circular
Shields. Anti-moist Shields.
The sub-system that I was most proud of is the Dew Shield one. Following some reading (I forget where) it’s actually not really possible to “dim” an LED reliably. They work at a specific voltage, under it they’re unpredictable and over it, they suddenly stop working. For good.
This is where something called PWM (Pulse Width Modulation) comes in. I wont pretend that I know what this is or how it works fully, but I will stagger through my understanding of it. LED’s are instant response on/off to current; in turn this means that they are capable of being turned on and off in short order. Very short. Many times faster than your measly human eyes can detect. If you were to adjust the pulses of power to a given LED on for, say, 60% of the time and off for [consults calculator] 40% of the time, then this will appear to your meagre homosapien eyes to being approximately 60% of its original brightness. Welcome to Modulating the Width of the Pulses. This is on the test at the end.
I know, you’re wondering how this applies to a warming jacket (Dew Shield), or why I’m even harping on about LED’s at all. Well, an LED and a heating jacket are ‘dimmed’ in much the same way: PWM. On for some of the time and off for the rest, modulating the total output. In my case, I can use a multi-colour (the ones that make colourful LED’s shift through the colour spectrum under you kitchen cupboard if your significant other made you fit them) work in place of a very expensive multi-channel Dew Shield Power Supply. All I have to do is to wire a common ground to the negative (that I cryptically used a red wire for) and wire the positive of each ‘channel’ to the Red Green and Blue terminals for 3 channel control. I paid £14.99 for this controller and it’ll apparently handle 3A per channel. [calculator consultation again]: 9 potential amps of warming power. Patent pending gloves anyone?
All that that I wrote just then was for naught. When assembling the system, I accidentally pushed down too hard on the terminal screws and shorted the solder-blob’d underside to the metal mounting plate. Popped the 10A main fuse and fried the RGB Dimmer. Yeh. I’m that stupid.
Back to Amazon and placed an order for x3 purpose-made Drok PWM Board. Grand total £9.90/board so £29.70 + £14.99 for the one I fired and its £44.69. Still way under a retail Dew Controller, but an unexpected cost nonetheless. Handily they come with built-in fuse protection (just visable in the photo behind the heat-sink). It is intended for DC motor control, so the supplied fuse was rated for 5A. I swapped this out for a 3A one, just to guarantee I dont accidentally go over the 20A limit on the main supply. And it comes with a tiny little display that gives you a visual read out of the duty cycle, displayed in %. Now I can have the mental stress of it not being on an even number! I could’ve spent less on it, but I really wanted that little display! Additionally, the boards are all now raised off of the metal plate on purpose-made plastic sticky-backed feet. You know, to counteract my stupidity.
Part 3 Coming soon!