Wall Charger for BS HomeWork Board

ddc9999

Hi, I bought a Basic Stamp 2 Homework Board Rev E and I have been happy with it so far, but now I would like to also use a wall charger with it, similar to the option with the board of education. I was thinking about getting a 9VDC Wall Wart and directly hooking that up to the 9V batter connection on the homework board. Is this asking for trouble in terms of destroying the micro controller or just not working, or can I proceed according to my plan? Thanks for the help.

Mike Green

You can certainly do that (use a 9V "wall wart" battery eliminator). Make sure that it can provide enough current (a couple of hundred mA ideally - 150mA is ok). If you're adept at soldering and handling wires, you could get a 7.5V battery eliminator and graft on a 9V battery clip that would fit the Homework Board. Make very sure that you've got the polarity of the battery clip correct. Putting it on backwards can indeed cause damage to the Homework Board. Use a multimeter to check the voltage at the battery clip and compare the polarity with the markings on the Homework Board. The lower voltage will work fine and there'll be less heat to dissipate.

ddc9999

Appreciate the help man!

ddc9999

I have a couple extra questions along the same theme.· Why do you suggest that I use a 7.5 VDC wall wart instead of a 9VDC one?· Does it not matter which one I use due to the STAMP's voltage converter?· Also, if I then use a 7.5V adapter, my Vin will be 7.5V and not 9 V correct?

Thanks for the help!

Mike Green

1) The Homework board uses a 5V regulated power supply (Vdd) internally. The regulator requires a minimum input (Vin) of 5.5V to function properly. Anything over 5V is dissipated as heat. Power = voltage x current, so, if you have some circuitry attached to the Homework Board that uses 200mA and you operate it with Vin = 9V, you're producing (9V - 5V) x 0.2A = 4V x 0.2A = 0.8W of heat. That's not a huge amount. It's less than a Watt, but the regulator will get quite hot to the touch. If you operate the Homework Board at 7.5V, that's (7.5V - 5V) x 0.2A = 2.5V x 0.2A = 0.5W, quite a bit less heat. You can even operate the Homework board at 6V. The BoeBot uses a similar regulator and operates very happily with a 4 x AA alkaline battery pack at 6V.

2) Yes. Vin is whatever voltage your power supply will furnish.

ddc9999

Thanks Mike! Didn't think of the heat aspect.

ddc9999

Got it to work and nothing got fried. I bought a wall wart from Radioshack that has several outputs so I can use the 6, 7.5, or 9V output which is handy cause it gives me access to different Vin's. Wired this up to a switch as well so I can quickly connect/disconnect power. Now I'm on to the next project. I'm gonna try and make a CV and maybe CC as well power supply.

kleekru

@ddc9999 Do you have a picture of how you wired this up? Seems basic enough but I may have made the mistake of removing the battery contacts to the board and tried to solder new connections. It isn't getting power at all now... I was careful not to cross polarity... But now I'm wondering if I was not careful enough.

erco

Classic danger here. "Everyone knows" that a 9V battery clip has 2 wires: the red goes to positive and the black goes to negative. That's true when you are connecting a 9V battery to a circuit.

BUT everything is reversed when you are making a 9V battery eliminator to connect to an existing 9V battery connector. Per Mike, you must test everything with your meter and proceed very cautiously.