Power Supply for Homework Board - Updated
Whit
Posts: 4,191
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?
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?
Comments
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.
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?
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.
Better? or should I just stick with 9 volt, 450 mA? I will check the actual output voltage before I cut off the connectors.
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.
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.
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?
Why is it that a mess-up teaches better than a success?
Thanks Peter!
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.
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.
I am providing photos of my latest accomplishment. A Stamp1 running a program who participates in number exchanges with a TI83Plus calculator.
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,
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.
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.
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.
Welcome to the forums! Great bunch of people here. No question is a dumb question.
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.
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...
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.
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