Power Distribution Box v2
I mentioned in another post that my dew heater controller had decided to self-combust and not work anymore.
I spent some time tracking down the parts and components that I needed to make a combined power distribution box and dew heater controller. It’s not anything fancy, by any stretch of the imagination. It’s cheap. That’s really it’s only saving grace. There are no fancy custom PCBs or 3D printing here. It’s certainly not a Pegasus AstroPower Box or a Wanderer Astro device…
Inside the box we have a variable output DC-DC convertor. This takes the 12v input and drops it to 5.01v. The 5v output is then fed into two parallel PWM controllers, whose output is attached to some panel-mount USB sockets. My USB dew heater straps will then plug into these sockets. Easy, eh?
Image 3 - Variable output DC-DC converter
No, not really. Tracking down the individual components was more difficult than I had planned as none of the local electronic stores carried anything remotely useful. After some discussions on IceInSpace I tracked down what I needed from eBay (at quite reasonable prices, too).
The DC-DC converter is overkill for what I am doing here. It’s rated to some crazy power output (like 300W or thereabouts). At most I’ll be pushing it to 20W if I have both dew heater straps running at full power. If the box gets noticeably warm then I can always drill some holes to allow for airflow. I’ll see how it goes…
Image 4 - PWM controller
One nice feature that wasn’t in the description of the PWM controller is the ‘clicky’ OFF position when the knob is rotated fully counter-clockwise. This means I don’t need to worry whether the socket is live or not if I’m not using it.
Of course, I will need to manually adjust the temperature of the heaters. Which means remembering to pause the imaging sequence, as touching the controller will ruin the current image being taken.
Image 5 - USB panel-mount socket
Oh, and the screw threads on the USB sockets are not M3, nor M2. They are the same threads as used by the old-school serial and parallel (DB9 / DB25) connecters. Remember them? If you’re under 40 years of age, probably not… Do you know how hard it is to find bolts with that thread size?
I originally bought a small project box from Jaycar but discovered it was going to be too small to house all the parts, so had to buy another, larger one. The joys of not properly thinking about just how much space I needed.
It’s frustrating how I needed to keep going back and buying yet more little bits and pieces to assemble this thing. PCB stand-offs, M3 bolts, more DC power connectors, drill bits, wire…
One thing I did do was purchase more 2.5mm DC power sockets, and a 2-pin microphone socket. This meant I didn’t have to destroy my old power distribution box, and I’ll be able to use the old one on the nights when I don’t need to run any dew heaters.
My old power distribution box mounts on the bottom of the scope’s dovetail bar. The new one didn’t have any flanges on the outsides of the box to duplicate this, so I have used a dovetail bar cable management clip bolted to the top of the new power distribution box as a way to suspend the box beneath the scope. It’s going to look odd, especially with my 4-port USB hub stuck to the bottom of the box by velcro. However, it’s the best I could come up with that didn’t involve going out and spending hundreds on (yet another) gadget.
As for why my original dew heater controller self-destructed, I worked out that the dew heater strap itself had developed a short-circuit in the connecting wire. Rather than spend more money on another heater strap that would eventually develop the same fault, I did some surgery to repair and reinforce the wire. It’s stronger now, but I suspect it will eventually wear and break again - in a different location.
Before anyone starts yelling about fuses and how there aren’t any present - relax. My mains powered 12v supply is limited to 7.5A, and the power cable I use with my portable power centre has a 5A fuse in it. IF anything does go south, it’s not going to be too huge a disaster. I hope. If you look closely at image 6 you’ll see plenty of space between all the cables, and plenty of heat-shrink insulation in there to keep everything safe.
What the burnt-out controller looked like
Image 1 - One burnt dew heater controller
Image 2 - One burnt dew heater controller
Some images of the completed power distribution box
Image 6 - What's inside the box?
Image 7 - 12DC Power Outlets
Image 8 - Dew Heater Sockets
