Are all grounds created equal?

SteveWoodrough

Here is the scenario:

I'm succesfully controlling an RC car ESC with my STAMP through pin 13.· I picked PIN 13 since it's a lucky number and it has the 3 pin connector port at X5.

The ESC is connected to big supply battery (9.6 V 1600maH sorta big)· and the ESC has a BEC and a servo lead connection that normally goes to the RC RX module.·· When I powered the ESC with the 9.6 V battery and plugged the ESC servo connector into the PIN 13 area (X5) I was able to power the stamp.· I assume that the BEC voltage was "backfeeding" the STAMP board.· Everyting worked fine, but using the power supplied from the ESC seemed like a bad idea.

1. ·Could it be noisy and cause issues?·
2. If the power 9.6 V failed, could other bad things occur (loose steering control)?
3. Is there a voltage regulator on the Stamp that I'm bypassing?
4. etc.

All in all it·just did not seem like a good idea to provide power in a manner not consistent with the board designers intention.· Am I wrong?

Normally I power my STAMP with a 5 cell 6V NiMH pack through the standard power connection provided.· It seemed like an even WORSE idea to provide power through both the normal connection and whatever was flowing through the positive connection of the PIN 13 connector at X5.· I had visions of 2 positive voltages colliding and bad things happening.·It seemed to me that there would always be a discprepancy between the voltage supplied by the ESC and the voltage of the 6 V battery.·

So... I disconnected the positive lead comming from the ESC to the STAMP.· Now the Stamp gets it's power from the 6V battery, and the ESC and big honking RC car motor·are fed from the 9.6 V·battery.·

My concern is this:·

What about the ground wire comming from the ESC to the STAMP board?·

Which power source does that ground belong to?·

Does it matter, ground is ground right?·

Everything works fine, with the ESC positive disconnected, I just want to make sure there is not some hidden issue I'm overlooking.

Thanks Again for your time and wisdom!

Steve

allanlane5

Oh, I'm SO glad you disconnected the positive lead from the ESC to the BS2 -- that's exactly what you should have done.

Fortunately, you didn't move the 'jumper' on the BOE board, such that the "output of the regulator" appeared on the + supply line of the servo. If you had, and THEN you attached the 9.6 volt battery, you'd have smoked the BS2. Man, you were lucky.

Bottom line -- IF you leave the servo jumper in the "Vin" position, you COULD supply both the servo and the BS2 with the 9.6 volt supply. Frankly, 9.6 volts is a bit "hot" for the servo's, which are designed for 7.2 volts max. This means more 'sparking' and a 'shorter life' for the servo's -- but I don't know by how much. On the other hand, the ESC may be designed for 9.6 volts, so there'd be no problem there.

The problem with using the 9.6 volts this way is that IF, someday, you DID accidentally move that jumper, you'd fry the BS2. Bottom line -- the RIGHT way to do it is to do what you've done -- disconnect the ESC positive. And leave the grounds connected -- they MUST be connected for the BS2 to send the control pulse with a proper zero-volt reference on both sides.

SteveWoodrough

Thanks. On thing I did not make clear. The BEC (battery eliminator circuit) on the ESC supplies ~6V, roughly the same as my normal supply pack. I agree that 9.6 V is SUPER hot for a servo. I also, never liked the idea moving the jumper, so it stays at Vdd.

So I take it when it comes to this circuit with 2 batteries, a ground is a ground is a ground?

allanlane5

And yes, a ground is a ground is a ground, but they need to be connected to each other.