servo connection bd of educ, rev b

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i have a bd of educ, rev b, dated 2000-2004. the boe bot manual says there is no 3 position sx, but there is such a sx on this board. also, it says to use only a 6 volt pack to power the servos but the servo headers are 5 volt regulated, vdd, as measured. i have been using a 9 volt battery to power these servos and haven't had any servo problems.

is the manual that wrong? i am looking at page 59, lower right. it also says to use the rev c rules for connection.

thanks,

jim··

Mike Green

I think there was an old Board of Education version that had a two position power switch, but all the recent ones have the 3 position switch. Somehow you've gotten a really old manual. There should be a jumper on the board to select servo power between the 5V regulated supply (Vdd) and the unregulated input (Vin). With a 9V supply you have to use the regulated 5V because 9V will damage the servos over time. It's much better to use a 6V or 7.2V battery pack and use Vin for the servos for several reasons. First, servos draw a lot of current and dropping 9V input to 5V at a peak current of over 1Amp, you'll get a lot of heating in the regulator. Second, running the servos off the Stamp's regulated supply may introduce noise into the circuit since motors are electrically very noisy. Third, the servos provide more power at 6V than 5V and even better at 7.2V (although that's at the limit of what should be used with them).

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thanks for the info. my manual is 2.2 and 2003-2004 copyright. jim

Mike Green

I looked at that page and see what you mean. What I said about running the servos off the regulated supply still holds. That's why they changed the board. I could guess why your board has a 3 position switch, but it would be just a guess.

I still recommend running the servos off Vin and using a Vin of 6-7.2V rather than 9V. If you can't do that, I'd just pay attention to how hot the regulator gets and possibly add additional heatsinking. If you're using a 9V radio battery, don't. They're expensive and have fairly low current capacity. They don't work well at the high current drains from a motor. You'd be much better off using a battery pack using AA cells (or even AAA cells)

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running off the stamp is for prototyping and testing.
i am actually going to run the servo off a 5v,5amp regulator i made and send the stamp signal to the reg board. it is being powered with a large 12v deep cycle marine battery. i'll be using the same ground for stamp and the regulator though. does that sound good?

jim

Mike Green

I'll probably work fine. My own preference would be to use a switching regulator to drop the 12V to 6V, run the servos off the 6V output, then use a low dropout regulator (like that used on the BoeBot) to provide 5V for the Stamp and any associated sensors.

I'd use one relatively heavy ground wire to the servo and a separate ground wire to the Stamp and its regulator.

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thanks again