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Power Supply for Homework Board - Updated — Parallax Forums

Power Supply for Homework Board - Updated

WhitWhit Posts: 4,191
edited 2016-06-10 11:20 in BASIC Stamp
I have a friend who just ordered a BASIC Stamp Activity Kit (with a Homework Board, which uses a a 9 volt battery).

I wanted to build him a "power supply" using a wall-wart and a 9 volt battery connector. I have the following choices and was wondering what would work best:

1. 9v, 450 mA
2. 9.5v, 3.3A
3. 10v, 800 mA

Will this work okay? Is #1 under spec? Are #2 (max Amp rating) or #3 (voltage) too much? Any suggestions, pitfall or warnings? The Parallax version power supply for BOE type boards is 7.5 volts and 1.5 A. Should I start with that or is just using a 9 volt battery really the best way to go?
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Comments

  • Hal AlbachHal Albach Posts: 747
    edited 2016-06-08 13:06
    I would go for curtain #2 only because as your friend progresses he (she) is going to want to do more than blink lights and move on to servos & small motors. Having a decent amount of current available will prevent intermittent Stamp brownouts and resets. Just because a power source is capable of delivering 3.3 Amps does not mean that the maximum current will flow, rather only what the circuit requires per Ohm's Law, I = E/R.
    Personally, I almost never use 9V batteries because 1) they are expensive and 2) they decline rapidly when drawing even modest current from them. The only place I ever use them is in my meters.
  • Peter JakackiPeter Jakacki Posts: 10,193
    Whit wrote: »
    I have a friend who just ordered a BASIC Stamp Activity Kit (with a Homework Board, which uses a a 9 volt battery).

    I wanted to build him a "power supply" using a wall-wart and a 9 volt battery connector. I have the following choices and was wondering what would work best:

    1. 9v, 450 mA
    2. 9.5v, 3.3A
    3. 10v, 800 mA

    me wonders why 450mA is "under spec" as 9V batteries just do not have any oomph at all, let alone 450mA!

    me thinks that since the Stamp is 5V and all it is using is a liner regulator that you are better off with a 6V supply anyway, it would be much kinder on the LDO regulator.

    Then again 5V power packs are plentiful and you could just run it straight to the 5V rail or even jumper the USB +5 across, much easier. What kind of loads would you run from a 9V battery anyway?
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    Peter is right. A good Duracell 9V barely has 200 mA of useable current. A 450 mA P/S more than makes up for a 9V battery and quite frankly if you're drawing more than 450 mA from the VDD pin the on-board regulator isn't going to handle it anyway. At that point you really should consider a more robust board. The HomeWork Board is designed to operate from a minimal battery power source on a desktop for students. So a 9V, 450 mA P/S is a huge upgrade. :cool:
  • WhitWhit Posts: 4,191
    Thanks for the imput everyone. I have some 7 volt supplies (maybe a 6 volt) and will check the rating for mA. If nothing is better, I will go with option 1.

    By the way, that it why I listed it as option 1 - wanted to check my thinking for I built the "power supply"

    A further note - I will be careful about the polarity of the connection so that positive and negative to go to the 9 volt battery connector in the proper orientation.
  • WhitWhit Posts: 4,191
    Okay - Last option. I found a 7.5 volt, 250 mA adaptor.

    Better? or should I just stick with 9 volt, 450 mA? I will check the actual output voltage before I cut off the connectors.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    I'd go with the 9V @ 450 mA and solder a barrel plug onto it. :nerd:
  • WhitWhit Posts: 4,191
    Thanks Chris - I was going to use a 9 volt battery connector since I want to power a Homework Board. I guess I could attach a female barrel plug to the 9 volt battery connector and the matching male barrel plug connector to the wall-wart, to make it universally usable - but that seems overly complex.

    Probably will go with

    9V @ 450 mA Adaptor --> 9V Battery Connector - soldered and shrink wrapped. To the Homework Board it will "look" like a battery of correct polarity.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2016-06-08 17:58
    Whit, yeah, that should have read "Solder a battery snap onto it". :innocent:

    Radio Shack "used" to sell a 9V battery saver, which was essentially a 9V wall adapter with a 9V battery snap on it. Of course who knows these days.
  • WhitWhit Posts: 4,191
    Chris Savage wrote: »
    Radio Shack "used" to sell a 9V battery saver, which was essentially a 9V wall adapter with a 9V battery snap on it. Of course who knows these days.

    Sad, but true.

    I will post a photo of the finished product... Thanks again to all for the help and suggestions.

  • WhitWhit Posts: 4,191
    edited 2016-06-09 02:44
    Was checking the voltage on the Wal-warts before assembly - They were really high!

    The adapter listed to output 9 volts, 450 mA - reads 15 volts - is this normal for no load? Is it too high?
    A 9 volt, 200 mA - reads 18.1 volts
    A 6 volt, 1.2A - reads 9.8 volts
    and a 7.2 volt, 250 mA - reads 14.30 volts

    So, most are 50% high, one double the stated output...

    Different meters gave similar results - am I nuts - have I never know that these units were that over?
  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2016-06-09 03:09
    I was going to say "bear in mind..." but I see you have discovered that those unregulated supplies are not so good, they are spec'd for when they are running at the rated load. However you are better off with a supply that has a lower voltage as 9V is still a bit high for a 5V linear regulator. All that extra voltage means more heat. Remember though that the all the board itself needs is 5V so 9V is not going to give you a "better" 5V.
  • WhitWhit Posts: 4,191
    edited 2016-06-09 03:54
    Going to keep looking - as always, something I thought would be simple turns into a bigger project - l am learning a lot though!

    Why is it that a mess-up teaches better than a success?

    Thanks Peter!
  • Peter JakackiPeter Jakacki Posts: 10,193
    Whit wrote: »
    Why is it that a mess-up teaches better than a success?

    Too true, a success can make us overconfident and rest on our laurels while the mess-up really makes us think and remember, and remember to think!

    Personally I would just strap the USB 5V to the board, I have lots of boards wired like this even the original Prop demo board and have never had any problems and it is so convenient. Why oh why they designed a board with a 9V battery is beyond me as I can buy little 1/2 cell to USB 5V converters on ebay for a song but an on-board boost chip would have allowed AA cells if they really wanted "safe" battery operation.
  • ercoerco Posts: 20,256
    edited 2016-06-09 05:17
    Careful, Whit! You're setting yourself up for the classic reverse polarity mistake. 9V battery snaps, red wire is positive, black white is negative, right?

    WRONG! When you're wiring up a "battery eliminator" as you describe, everything reverses. If you don't believe it, then hook up just a bare 9V connector to the HW board, then use an ohmmeter to verify that the black wire is connected to Vin on the board, and the red wire goes to ground.

    It's easy to make that miSteAk and release the magic smoke. Don't be that guy!

    Edit: Looking back, I see you did mention that you were aware. Phew! No Homework boards were harmed in the making of this battery eliminator. :)
  • Buck RogersBuck Rogers Posts: 2,181
    Hello!
    I am providing photos of my latest accomplishment. A Stamp1 running a program who participates in number exchanges with a TI83Plus calculator.001.JPG002.JPG019.JPG018.JPG Basically I tracked down a previously wired to a barrel connector 9v snap connector. It is connected to a pair of cross-wired ones who have a switch stuck in one. One end is attached to that Stamp, and the power supply is (sadly) a garden variety 9v. And it is using RS Adaptaplugs to connect the whole business together. However the battery box and the female barrel connector were bought from Tinkersphere also in NYC, since RS kept failing to deliver on the battery holder.

    Incidentally I owe both you erco, and Mike Green, if you both ever visit NYC meals at Katz's Deli for good advice regarding wiring the snaps. It was originally wired for a HWB that I bought about three years ago,

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  • kwinnkwinn Posts: 8,697
    erco wrote: »
    Careful, Whit! You're setting yourself up for the classic reverse polarity mistake. 9V battery snaps, red wire is positive, black white is negative, right?

    WRONG! When you're wiring up a "battery eliminator" as you describe, everything reverses. If you don't believe it, then hook up just a bare 9V connector to the HW board, then use an ohmmeter to verify that the black wire is connected to Vin on the board, and the red wire goes to ground.

    It's easy to make that miSteAk and release the magic smoke. Don't be that guy!

    Edit: Looking back, I see you did mention that you were aware. Phew! No Homework boards were harmed in the making of this battery eliminator. :)

    Those 9V clips are a hazard even when everything is wired correctly. So easy to place the battery or battery eliminator clip on the project the wrong way. Even if it won't snap in place just that short contact time can do a lot of damage. Better to use a connector that avoids those kinds of mistakes.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    erco wrote: »
    Careful, Whit! You're setting yourself up for the classic reverse polarity mistake. 9V battery snaps, red wire is positive, black white is negative, right?

    WRONG! When you're wiring up a "battery eliminator" as you describe, everything reverses. If you don't believe it, then hook up just a bare 9V connector to the HW board, then use an ohmmeter to verify that the black wire is connected to Vin on the board, and the red wire goes to ground.

    It's easy to make that miSteAk and release the magic smoke. Don't be that guy!

    Edit: Looking back, I see you did mention that you were aware. Phew! No Homework boards were harmed in the making of this battery eliminator. :)

    erco, I had previously discussed how common it was to wire a battery snap backward on a P/S with Whit via email. I did it once back in the 80s and never did that again. The few times since that I built a "9V Battery Saver" supply I even tested everything with my meter just to be sure. Better safe than sorry.
  • ercoerco Posts: 20,256
    Agreed, it's a rite of passage to make that error with 9V snaps. As Whit said, we learn more quickly from our failures than our successes.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    erco, I remember at the time it taking a few minutes to figure out what was going on. I had the black connected to the ground and the red connected to the positive voltage. At the time it almost didn't make sense. :innocent:
  • MikeDYurMikeDYur Posts: 2,176
    Hal Albach wrote: »
    I would go for curtain #2 only because as your friend progresses he (she) is going to want to do more than blink lights and move on to servos & small motors. Having a decent amount of current available will prevent intermittent Stamp brownouts and resets. Just because a power source is capable of delivering 3.3 Amps does not mean that the maximum current will flow, rather only what the circuit requires per Ohm's Law, I = E/R.
    Personally, I almost never use 9V batteries because 1) they are expensive and 2) they decline rapidly when drawing even modest current from them. The only place I ever use them is in my meters.

    My next multimeter will have auto-power off, I have wasted a few 9 volt's forgetting to hit the switch. Remember when they were called 9 volt transistor batteries, when the pocket transistor radio was invented.
  • ercoerco Posts: 20,256
    6 or 7 volts from the adapter should do it. 9 honest volts might damage a servo. The HW board experiments connect a servo directly to a 9V battery, which is only safe because the 9V battery voltage drops due to high current demands.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    On newer versions the regulator is beefier so you can use VDD for the servo power.
  • edwinadamsedwinadams Posts: 1
    It's good to be a Whit mentee as I step into this hobby. Thanks, Whit, for the connection to this incredible league of extraordinary roboticists. I'm looking forward to getting started with my BSC! Whit was kind enough to help me with a box, meter, and organization to start right! And I'm looking forward to the new power supply. Many thanks Padré!
  • PublisonPublison Posts: 12,366
    edwinadams wrote: »
    It's good to be a Whit mentee as I step into this hobby. Thanks, Whit, for the connection to this incredible league of extraordinary roboticists. I'm looking forward to getting started with my BSC! Whit was kind enough to help me with a box, meter, and organization to start right! And I'm looking forward to the new power supply. Many thanks Padré!

    Welcome to the forums! Great bunch of people here. No question is a dumb question.



  • xanaduxanadu Posts: 3,347
    I built a set of 10 of these and they work great.

    2S 500mah LiPo
    1 amp fuse
    9v clips w/ side leads (eBay)

    For larger projects and more experienced users the fuse can be swapped with a larger size. To save some money I used female spade crimp connectors for the fuse holders. In the past I've found the battery clips that didn't have side leads would usually rub the PCB and eventually disconnect or short out inside of the clip.
  • WhitWhit Posts: 4,191
    Welcome Edwin! and thanks to all - the search continues.

    Thanks also to erco and all those who mentioned the release of the magic smoke - Edwin and I covered that yesterday when discussing the power supply polarity issue! Made the mistake before on something...

    Photos to finished product to come...
  • WhitWhit Posts: 4,191
    edited 2016-06-10 02:04
    Update - Plan B - Build and Photos on my blog - http://roboticsunderthestole.blogspot.com/2016/06/usb-power-supply.html

    USBPS10.jpg

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  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2016-06-10 14:52
    Whit,

    I'm surprised that worked. Here's why...without the USB device requesting more, the port should only be giving you about 90 mA. The servo should be drawing more than that with the board and LED. I know on my old IBM ThinkPad I would have gotten a message saying the port exceeded the current level. I think it would have shut it down. You must have a more forgiving system. :innocent:
  • WhitWhit Posts: 4,191
    edited 2016-06-10 21:42
    Chris,

    I am not using a USB port on my PC, but an external charger - a CyberPower with at stated output of 5 Vdc and 1000 mA (my iPhone charger lists the same output). The info I had was from here - https://discussions.apple.com/docs/DOC-3511

    The voltage output is easy enough to measure, and I could try to measure the current too I guess, but I just gave it a shot... and it worked.

    Edit - just measured amperage with the same program running as is shown in the video. Set up is per the WAM text, with an LED, 470 ohm resistor and standard Parallax servo - the highest reading I got was 178 mA - so under 200 mA always - works great.

    Are angels watching over me?
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edwinadams wrote:
    It's good to be a Whit mentee as I step into this hobby. Thanks, Whit, for the connection to this incredible league of extraordinary roboticists. I'm looking forward to getting started with my BSC! Whit was kind enough to help me with a box, meter, and organization to start right! And I'm looking forward to the new power supply. Many thanks Padré!
    Wow! You're a lucky guy to have Whit as a mentor! Welcome to the Parallax Forum, Edwin!

    -Phil

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