Better pictures of yet another batch of Osh Park PCBs reflowed with solder paste applied using Osh Stencils metal stencils.
Here are Chip McClelland’s latest Trail Counter and Solar Charger boards, together with a shot of the stainless steel stencil now offered by OSH Stencils. The metal stencil makes it hugely easier to get perfect pasting and confirms that the curve that comes with polymide stencils has been at least one root cause of that type being so challenging to use. There was no paste bleed through to the back of the stencil, and the fine pitched power controller and Simblee BLE chip lands were perfectly formed. To get a sense of the cost difference some existing board specs were plugged into the OSH Stencils web site: a roughly five square inch battery charger, a three square inch decade counter/divider, and sub-square inch 6-axis breakout board. The steel stencils would be almost exactly double the cost of polymide, averaging about $12 each. By the way, multiple PCB designs can be grouped within one stencil. This can greatly reduce the cost difference depending on the areas involved.
These are OSH Park boards: very deep blue/purple, sitting on a sky blue antistatic mat. This is a tradeoff with the very bright, high color temperature LED flood lights used to illuminate the bench.
Paul MacDougal will talk about arranging communication between these two popular single board computers, and Alex Davis will have a Raspbery Pi with Retropie and MAME to show. This will be followed by the usual show and tell and chat session. Bring your projects, ideas and questions.
We’ll be meeting in EBI room 1007, next door to the one for previous months (and the same as the one TAR is meeting in). This room will be stable through April.
Details and maps here.
Also, we took advantage of TI’s special offer of MSP432 Launchpads. They’ll be available at the meeting for $5. (They will also be at the Splatspace open meeting tomorrow evening.)
This month’s meeting will be at NCSU Centennial Campus Engineering Building III: Meeting Details
The August meeting will be in three parts:
- Ohm’s Law Modeled With Fluid Flow by Ryan Schuster
- Transistor Tips by Shane Trent
- A Programmable Bench Test Instrument Oriented Toward Power Measurements by Chip McClelland
The last part of the meeting will be the usual open forum. Bring your show and tell items, problems needing help, etc.
This is a constant current dummy load made from one of Shane Trent’s PCBs like those given away at a recent TriEmbed meeting. As mentioned on the email list, this PCB is a “fixed” version of a design from a “Sleepy Robot” blog of a guy named Wittenberg, which is itself a derivation of an original design by Dave Jones of EEVblog. Wittenberg had made available gerbers for his design (in early 2012) that were unfortunately defective, and he didn’t allow for two way communication, forcing Shane to go to great lengths to correct the gerbers and get a run of PCBs fabricated. Shane’s blog article covers all this in depth and has a link to the corrected gerber files in zip format.
Fast forwarding to the present, here’s a recent tutorial by Dave going into depth about battery measurements. Viewers will just have to put up with the axe-grinding, horse-beating treatment of a “battery life extender” Kickstarter that pushed Dave’s buttons. Apart from this, it’s an excellent treatment and a fantastic “essential subset” spreadsheet tutorial for folks that just want a hint about how to do cool things like the graph-making done in this video.
I assembled and tested a second of the PCBs recently. It will sink up to one amp at up to around the 60 volt limit of the FET used (MTP3055VL) HOWEVER, unless you like to see magic smoke the 18 watt thermal limit of the FET/heatsink assembly has to be honored. So at a full ampere the voltage limit is around 18, and at that load be sure to avoid touching the transistor! At one ampere the shunt resistor will be operating at it’s rated dissipation limit and will also be very hot. To summarize, this load has to be kept at an amp or less and at 18 watts of power dissipation or less. (Note: the shunt resistor is temporarily 5% tolerance due to an ordering blunder. That will be fixed.)
I’ve decided to make it available for borrowing by TriEmbed meeting attendees who can guarantee it’s return by them or their designee at the following month’s meeting. The transistor is not expensive and it won’t be any big disaster (just embarrassing) if it’s accidentally destroyed, but blowing the traces off the PCB will be frowned upon (joke). So this (and perhaps some of the TriEmbed contact cards Paul made, hallway signs, etc) could be part of a shared resource that could expand over time.
The “UI” is currently two voltmeter test points, with the unit showing the load current as a one to one mapping from amperes to volts. A digital display with simple USB serial (current and “external voltage” aka battery voltage) logging output and some temperature compensation/auto-calibration is planned, but it would be straight forward to tie the test points to something like an Arduino analog pin or two.
Remotely controlling and/or making the current limit programmable would be a bit harder, but a properly coordinated hack to provide an alternative control mechanism would be OK with me and make for a fun project for somebody.
Here are all the design-related links in one place:
Here are some BOM changes:
- The load connection is just four bare plated through holes intended to get some wire loops. These Newark 12H8386 screw terminals solder in and work well.
- As mentioned, a momentarily loose screw resulted in this Mouser part 660-MF1/4DCT52R10R0F five watt resistor being substituted for the default 10 1/4 watt resistors. The bad news is this resistor has a 350ppm/C coefficient as well as being only 5% tolerance. A better choice than either might be a pair of three watt, two ohm 1% Vishay resistors such as Mouser 71-RS02B2R000FE12. These have 50ppm/C coefficient so there would be about another 1/2% error at the point you could boil water on them.
I recently added an app to my smart phone and, as a side effect, added another app, and that led me to stumble upon the web site http://everycircuit.com, which provides electronic circuit editing/simulation/presentation tools together with commercial and crowd sourced example circuits.
To check this site out, try this link to the user-submitted rectifier circuit example above inside an instance of the Google Chrome browser on a desktop or laptop computer:
Now click the moving squiggle in the upper left. It expands to a pretend oscilloscope showing voltage and current vs time for one part of the circuit.
Now click the “edit this circuit” link. There are tools for modifying this circuit.
Cool, heh? The main site page is here.
Yes, again, this tool isn’t supported in many, many settings that readers of this blog find themselves in. It’s completely dependent on Chrome. On behalf of the company owning this site (that I just found: I have no connection to it), I’m sorry it isn’t more portable.
By the way: Anybody reading this is welcome to submit their impressions of this or anything else here as comments below. TriEmbed meeting attendees or their online friends (such as in Vermont) are invited to request a blog account if they have interest in adding content to this site via blog postings, additional project pages, etc. Your access will be proportional to how well you’re known to the admins and editors (but we’re always looking for new editors and at least one additional admin!) It takes about 10 minutes with somebody who is WordPress fluent to learn how to add content.
Ford, Chrysler, RAM, Dodge, and Scion have embraced an open standard for conveniently recharging portable devices that appears to be more effective and just easy to use as inductive charging systems. It’s called Open Dots.
The basic idea is to use a set of four parallel conductive strips to carry positive and negative voltages (or +V and ground, depending on your point of view) and have the package of a device to be charged connect to the charging strip automagically just by resting on it. The device to charge has a pattern of four conductive “dots” on its case that will properly connect with the charging pad in any orientation. This scheme was invented for recharging toys in 1963.
I’m sharing this as a potentially handy way to deal with the general problem of recharging battery-operated gadgets. It would take a fair amount of work to implement the pieces and parts involved with the actual electrical connections, but based on the specification this is on the other end of the scale from rocket science and one would hope that the basic components may be or become cheaply available if the auto industry is involved .
As far as I can tell from their web site anybody could freely use these circuits and connector specs without consequences. (In order to sell something using the Open Dots logo one would need to execute and abide by a member agreement. But you do not need to get within a mile of this logo and could simply use the specs and reference circuits freely until you start selling a ton of stuff and see an advantage to becoming “official”.)
(Open Dots logo used without permission.)