Friday, May 11, 2012

CryoBUG PCB Layout Revisited

Without fail there is always something that goes wrong at the last minute, or more specifically something that I overlooked. Well in this case it was the transformer's footprint and pin-out that I totally misrepresented. Apparently I need to do a better job at reading data sheets, because I mistakenly confused the transformer I wanted to use with one of a much lower VA rating, hence a much smaller size. Luckily I held off on having printed circuit boards made until I had all the parts in hand, and had checked them for physical fit against my layout.

So I needed to make room for a bigger transformer. This ended up taking my original 2.5" width to 3", and at the same time it took me out of the running for getting the ExpressPCB Miniboard deal. So here's what the new board layout looks like...

Rev 1.3 PCB Layout  -- Click Image to Enlarge --
I also took the opportunity to clean up some of the trace paths, and to add the Mytek Controls logo to the silkscreen layer. So the final board size is now 3.8" x 3.0". So just for grins, I used the compute board cost feature in the ExpressPCB software and here is what it came up with for production quality board cost.

Qty 2 Boards  = $284.22
Qty 10 boards = $321.94

Wow! That was quite expensive. So next I went to Futurlec's PCB manufacturing page and entered my data. Here is what they showed when calculating the price.

Qty 2 boards   = $  75.70
Qty 10 boards = $134.50
Shipping = $9-14 by Standard Post

So it looks like Futurlec will be making my boards.

Since there were some changes in the transformer pin-outs, and I made some adjustments in the LED drive resistors, including adding a resistor for the external On/Off switch's internal LED back lighting, I updated the schematic to Rev 1.3 as well.

Rev 1.3 Schematic (page 1 of 2)  -- Click Image to Enlarge --

Rev 1.3 Schematic (page 2 of 2)  -- Click Image to Enlarge --

Originally I thought I could purchase the switch I had in mind with a built-in resistor for 12V operation, but as it turns out this would have been a special order, so I opted for the one without the built-in limiting resistors. To accommodate this, a resistor (R19) was added to the board.

I should be ordering boards within the next couple of days.

R600a news

Well my source in England turned out to be a bust. They basically credited my PayPal account with no explanation given as to why the order got aborted. So I am once again searching about for a cheap source of iso-Butane.

Friday, May 4, 2012

CryoBUG Controller PCB Layout

I made a few more changes to the circuit design, so this now brings it up to Rev 1.2 (the design is now locked in). The changes mainly consist of adding another CD4013 Dual RS Flip-Flop IC chip and some discrete support circuitry to enable "Fault Identification" LEDs.

Rev 1.2 Schematic (page 1 of 2)  -- Click Image to Enlarge --

Rev 1.2 Schematic (page 2 of 2)  -- Click Image to Enlarge --

Page 2 of the schematic shows the additional latch circuits that retain the state of the last fault, and display which safety (via an LED indicator) caused the actual fault condition. Was it a high liquid line temperature fault, or an under/over pressure cutout? By knowing which condition caused the fault, it allows for a better response to correct the problem. Typically a high liquid line temperature would be caused by a clogged or blocked air cooled condenser, or perhaps a fan motor malfunction. A low pressure cutout would primarily be caused by a loss of charge due to a leak. And a high pressure cutout could be indicative of an excessive load condition.

I lumped the pressure faults together because quite frankly it was more easily accommodated with only one extra IC chip. And secondly, pressure faults will rarely be a problem. It is far more likely that an overheat condition may occur due to a neglected condenser cleaning.

These two fault LEDs will be mounted so as to be visible from the CryoBUG's control panel, along side the remote connector.

I was able to squeeze all of this circuitry on one 3.8"x2.5" MiniBoard (ExpressPCB's 3 for $51 deal). Although I'll probably opt to go with their MiniBoardPro which is a full production version having solder mask and silkscreen.

Here is the completed PCB design, although I still need to double check that I got everything connected, and that all the components will fit properly.

Rev 1.2 PCB Layout  -- Click Image to Enlarge --

When I'm ready to place my order, it's as simple as making a few selections in the program, give em my credit card info, and about 2 days later they're suppose to arrive at my doorstep. Pretty awesome!

BTW this controller can be set-up for either 115V or 230V operation just by changing the jumpers (W1-W3).