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SBBPWR2 Revised design for Breadboard power supply module

SBBPWR2In a previous article we introduced the first uCHobby designed kit. The SBBPWR1 is a handy power supply module which plugs directly into a solderless breadboard. The previous article detailed the the overall design process for this kit. The SBBPWR1 has been received well and we got several good suggestions for improvements.

This article describes the improvements for the second revision of the kit. The new version is called SBBPWR2.

OnBreadboard

Changes:
Picture above is the new SBBPWR2 board with the SBBPWR1 shown below. The pictures are not at to scale but you can tell that the SBBPWR2 is wider and includes some header pins along the center area. Both boards have the outer header pins that pick up power rails on the breadboard. On the older board the VIN and VADJ signals were brought out to two inner headers. The new board replaces these two inner headers with four (4) headers that plug in like an IC would. This change adds stability to the board and provides an option for use on breadboards that don’t have the correct spacing on the power rails.

Larger Board Size:
The board is wider to accommodate more header pins in the center area.

Voltage Selection Jumper:
Several comments compared the SBBPWR1 board to some other similar kits which has a switch to change the voltage. We considered changing the power switch to one of these three position switches so you could select between off – 3.3 and 5V but opted to put a jumper on the board instead. The LM317 VADJ voltage divider is made up of 3 resistors on the SBBPWR2, one of which can be shorted with a removable jumper. We supply the resistors to make this select between 3.3 and 5V with 3.3 as the jumper off selection.

The idea here is that you would not damage your 3.3V parts when you meant to turn off the power supply. The voltage selection on the power switch just did not seem safe to us. With the Jumper you can decide which voltage you need in your project and then use the simple on/off power switch safely.

You can change the voltages and use the jumper for selection as well. Maybe you want 9V/5V or 12V/6V. You just need to install the correct resistors to make this happen.

SBBPWR2Bottom Inner Header connections:
The original board was a bit unstable, it wobbled on the connection headers so we placed some header connections along the center. This change makes the board stay put very well.

Support for other power rail spacing:
There is some variability in the spacing of of the power rails on breadboards. Most of the ones I have seen work fine with spacing we chose, but some of the less-expensive models such as those from Radio Shack have a slightly different spacing. The SBBPWR1 was not convenient for those boards, as you could use a single header set to connect to power rails on one side only. To address this issue a set of header connections were included along the center for the SBBPWR2. You could leave off the outer power headers and use the inside set on these boards. You have to place your power rail jumpers then plug in the SBBPWR2.

Moved the VIN and VADJ signals:
The original inner two connection headers for VIN and VADJ were moved to the center area so they could plug in much like an IC would. Moving these connections make the board more stable. The VIN signal provides access to the filtered and polarity-adjusted input voltage. This is ideal for applications where you don’t need regulated voltage for something like a motor. The VADJ signal is the adjust pin on the LM317 regulator. You can use this signal to make a current source or to supply an external voltage adjustment.

Fixed Lead Spacing Problems:
The lead spacing for the DC power jack and the large filter cap (C1) were tight on the SBBPWR1.

Higher Voltage on C1:
The input filter cap has changed. The SBBPWR1 used a 25V cap that was not easy to find. The SBBPWR2 uses a more common 50V cap which is better suited to this application. The LM317 can take a maximum of 40V on its input so the 25V cap was limiting the options. Now you should be able to use higher voltage wall warts.

Larger Pads on many parts:
Several of the pad sizes were increased for SBBPWR2. Where ever possible the pad size was increased and where spacing was tight we made the pads oval. This makes the board much easier to solder. We want this to be a good first kit and some of the lead spacing created a soldering challenge. The SBBPWR2 is very easy build now.

Connection/Inspection holes:
We had this idea that some larger holes next to the inner header pins would make it easier for someone to line up the SBBPWR2 with power jumpers used under the board. This does work but you need a bright light under the board edge to see well. But there was a great unexpected benefit with these holes. Jumper wires pass though the holes and plug into the breadboard just fine. If you do not mind seeing the jumpers, you can easily pickup the inner signals just by plugging your jumper wire though the hole. It really does work well.

Comments Please:

  • What do you think of the changes?
  • Any more suggested improvements for SBBPWR3?
  • Do you have one of these modules, SBBPWR1 or SBBPWR2? If so, let us know what you think…

Posted in Development Tools, Ideas, Parts, Workshop Tools.

14 Responses

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  1. Daniel said

    Very nice. The larger pads will help those rusty people put this thing together.

  2. Thomas said

    > The SBBPWR1 used a 25V cap that was not easy to find. The
    > SBBPWR2 uses a more common 50V cap which is better suited
    > to this application. The LM317 can take a maxim of 40V on
    > it’s input so the 25V cap was limiting the options. Now
    > you should be able to use higher voltage wall warts.

    > What do you think of the changes?

    Well, it is not the cap that limits the input voltage. It is the heat the LM317 can disperse without additional cooling. At 25V input and 5V output, and assuming your circuit just takes 100mA the LM317 has to transform (25V – 5V) * 0.1A = 2W into heat.

    Now, with

    * a terminal resistance of 50 C/W (datasheet) without external cooling

    * a maximum junction temperature of 125°C (datasheet), and if you like to have working parts for some longer time you better just calculate with 100°C, and

    * assuming a rather generous maximum ambient temperature of just 25°C

    you find that the 317 can just dissipate

    (100°C – 25°C) / (50 C/w) = 1.5W

    without extra cooling. And if you really want go for the maximum junction temperature of 125°C you get

    (125°C – 25°C) / (50 C/W) = 2W

    So with my above example (25V in, 5V out, 100mA current) you have already reached the point where you should consider a heat sink if you are a conservative engineer and not a gambler :-)

    To summarize, it is not a good idea to use the board with higher voltage wall warts. The LM317 needs at least 3V difference between in and out voltage. With 5V difference you are on the safe side and you can draw 300mA at 5V output.

  3. dfowler said

    Thomas,

    Very good analysis there. I agree that the higher voltage may make it easy to overheat the regulator. One thing to consider is that we also suppor AC input so a higher voltage margin could be necessary.

    The Regulator is overtemp protected and it is mounted so that a heatsink can be included very easialy. If someone needed to use a higher voltage wall wart with significant disapation in the regualator they should include a heatsink.

    Maybe I should take your sugestion and create a quick article showing the relationship between heat disapation and voltage drop vrs current. This could include details about some heatsinks and their applicaiton. Sounds like a fun article to do.

  4. Nice product. Love the price!

  5. jalspach said

    This is great, especially for a person just getting (read: me) started. I am not sure how well it would work but it would be cool to see something similar that would provide clocking. a small board that provides a few selectable frequencies running along side this module would allow a person who is new to microprocessors to get started right away.
    Thanks again and I love the site.

  6. Alan said

    Great revision. I too noticed that the original SBBPWR1 board that I owned was both wobbly and barely could fit in right with the bread board I have, it looks like from the pictures that v2 solves those issues. Although this may not be totally necessary considering the power limitations on this board, v3 should be slated with just a little more room between the LM317 and the small tantalum (or is it ceramic) cap so that the LM 317 can have a small heat sink attached to it. I noticed with the V1 board, the LM317 heated up incredibly fast regardless the load or draw it was taking and it is near impossible to fit a heat sink into the picture on the original board. Other than that, great product for people who work with LEDs alot or other low voltage gadgets and also a great first soldering experience (well… it’s not my first time soldering). But from a first timers perspective, larger solder pads does sound like a good idea. Even though I have soldered for many years, I found it slightly difficult to solder the rectifier on the v1 board due to the smallest solder pad I have ever seen. Another consideration for v3 is to actually include a potentiometer position on the board in place of R1 so you do have your infinite adjust-ability on board. The last thing I can think of is to include a said of pin headers (3 pairs of 2) and 3 jumpers with V3 so that a user can configure the LM317 supply to run in either constant current, constant voltage, or adjustable voltage mode (an idea I am snagging off of one of my early attempts at a bread board power tap unit). Also it would be nice if someone could start compiling data about different bread boards by different manufacturers and tell which bread board power supply board (v1 or v2 at this time) works with it. I know for a fact it was a real tight squeeze for the v1 board to work on the Jaemco JE24 bread board. Sorry to write an essay: but one last note: if you need to keep a few spare DF01M rectifier chips around, Vishay semi conductor is kind enough to give samples (at most 10) of this rectifier and several other similar ones for free at their web page. This tid bit of news is intended as a friendly note, in other words don’t go running up any time and just start sampling like crazy from Vishay, only do it when necessary and you know that you are prototyping or designing a new project and would rather only get the number of parts you need for free rather than having too many parts and not ending up using them.

  7. dfowler said

    Alan,

    The regulator should not get hot unless you are drawing substantial current from it. My testing shows that you can draw 300mA with a 6V drop on the regulator continously wihtout damage. The regulator does have overtemp and short circuit protection.

    On the heat sink question, the regulator is mounted flush with the edge of the board and there should be plenty of space for the typical TO-220 heat shinks.

    The adjust pin is broght out to a header which plugs into the breadboard. This provides stability and allows access so that you can make the module into a continously varable supply or current source.

  8. Alan said

    Oh, also I am talking about current draws between 350mA and 1400mA (led drive current)

  9. dfowler said

    Alan,

    I thought you were saying that it was getting hot without a load. That should not happen. For more the about 300mA with 6V drop on the regulator you will need a heat sink. This should be easy to add as the regulator is mounted flush with the board edge to allow for this.

  10. schill said

    Just a brief warning on a mechanical issue. Make sure that you check your breadboards before installing the support headers in both the center and outside locations – even if the width is correct. In a lot of the breadboards I’ve seen and purchased recently, the holes in the middle do not align with the bus holes (+ and -). They are off by a 1/2 hole. If you install both sets of headers, you will not be able to plug the board into your breadboard.

    There will also be problems with the breadboards sold by Parallax (which are out of stock, anyway). These have 6 holes on either side of the central gutter, making them wider than typical breadboards. Other than that, I think the change from 5 to 6 holes is a great idea – you always seem to need one more of anything and it also helps when you are plugging in chips that are wider than 0.3″.

  11. I don’t mean to hijack your webpage. But I also designed a breadboard power supply, with positive and negative output voltage.
    Around one month ago, I came across the ideia of breadboard power supplys, and I think I also used yours as inspiration.
    Schematics and more information can be found here:

    http://hazard.dynip.sapo.pt/joomla/index.php?option=com_content&task=view&id=47&Itemid=58&limit=1&limitstart=1

  12. Very good idea. This is a very useful project for accelerating the development of prototypes using breadboards.

  13. Mr. Meval said

    Very good idea and thank you for the work!

    A heat sink is easy to make, I’ve got one torn out of a laptop that would do well. It’s mean to epoxy onto a D-Pak but drilling it would be easy. I also have a heat pipe from a Dell with fan. I can’t think of anything else to use it for. :)

  14. alan said

    Dave,
    Quick question for C3, which hole should the negative lead of the capacitor go into? According to the Curious inventor instructions the negative side should face outward, but in all of the pictures I have seen of the new power supply board, the positive side faces outward? (Is this a mistake on the silkscreen?