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Adrenalynn
04-26-2008, 03:13 AM
I hope it's ok to cross-site post here, I'm looking for a "consensus" kinda answer.

Hi All,
I have an SSC-32 v2 interfaced to a Sabertooth 10 R/C
The Sabertooth 10 R/C provides a Battery Elimination Circuit - it pumps a pretty clean 5v back down the servo power wire. I don't see a DIP switch option to depower the BEC wire.
This seems to power the SSC-32 just fine. Neither the processor nor the regulator seem to heat up.

This leads to some questions:

Is it recommended/accepted procedure to power the SSC itself from the regulated BEC?
I assume that it's not kosher to power the servos from the BEC on the motor controller? That would be pulling the power through the regulator on the SSC, right?

Finally, would it over-volt the SSC to externally power it on VL w/+9v at the same time that the BEC circuit is active? Any back-current issues with putting external servo voltage on the SSC (+6v) for the servos, and use two spare channels for the Sabertooth - should I put the Sabertooth on the second bank and unjumper the banks from each other?

TIA for any advice!

Sienna
04-26-2008, 10:09 AM
From what I understand of the SSC, you really don't want to be powering it from a 5V BEC hooked into the servo pins.

For one thing, there is the power available from the Sabertooth itself. I couldn't find anything in the R/C version manual, but from the normal manual:

The 5v terminal is
capable of supplying 100 milliamps if the source battery is 12.6v or less. If the source battery is greater than 12.6 volts, the 5v terminal is capable of supplying 10 milliamps.
So, the power output is a little limited.

If your battery is <12.6v, then 100 milliamps is fine to drive the SSC32 logic. What you will need to due is unjumper the short between VL and VS1, then break the 5V from the sabertooth and plug it into VL, then remove the voltage regulator and bridge pins 1 and 3 of the removed voltage regulator. (if you don't remove the regulator, I believe it will take your 5V source and only provide 4.5V or similar to the Atmel, which seems close to brownout)

I wouldn't put any servos on VL personally. The only time I tried that even under no load I was browning out the processor.

Adrenalynn
04-26-2008, 10:47 AM
Thanks, Sienna.

I guess I didn't word the question clearly enough.

The Sabertooth is providing the power, which isn't being generated by the SSC-32.

Per my post:

"The Sabertooth 10 R/C provides a Battery Elimination Circuit [...]"

I have that little 3pg manual too, and it doesn't cover powering it from a BEC either.

The only 5v accessible source on the Sabertooth RC is at the servo pins.

I wasn't asking about putting servos on VL. "Any back-current issues with putting external servo voltage on the SSC (+6v) for the servos"

I'm sorry - my post must have been very poorly written. I'll reconsider the wording and repost.

Thanks though!

Sienna
04-26-2008, 11:27 AM
I am no electrician, but common sense would say that if you have a 6V source, and a 5V source, the 1V of difference will cause current to flow into the 5V source.

DE warned me not to connect my 6V and 5V BECs together on the high side, so I assume that the same would hold true for their H bridges.

Adrenalynn
04-26-2008, 11:41 AM
There's no 6v source. Just a regulated 5v source. According the the schematic, it appears that the servo power is isolated. If VL != VS jumper, then the board shouldn't get electronics voltage at all.

It would *appear* from the schematic that if VL=VS, VL goes straight to vo1 without passing through the LM2937 regulator. But that doesn't make sense against their warning of over-driving the regulator. I *could* see under-driving the regulator, which would explain the sags you observe. (note: I'm sourcing this from the manual's schematic)

According the the DS on the LM2937, it can't be convinced to output 6v anyway. I'll check that in a moment and report back, but: http://www.national.com/mpf/LM/LM2937.html


Output Current: 500 mA
Output Min: 12, 15, 8, 10, 5 Volt
Dropout Voltage: 0.5 Volt
Adjustable Output: No
Quiescent Current: 2 mA
So either way, near as I think we should be observing, it's spitting 5v, which would be the same voltage as the BEC on the Sabertooth R/C.

Adrenalynn
04-26-2008, 12:03 PM
That's interesting:

Powering VL from a 9v battery, I see 8.472v from a calibrated bench meter, on the servo power pins.
Powering from the BEC on the Sabertooth, I see 5.007v on the servo power pins. (no servos powered)

This was with VL=VS

With VL != VS jumper, I see no power to the logic (as expected) and 5.035v from the BEC, quiescent.

With both BEC and 9v battery in place, VL != VS, I observe 5.040v at the servo power, and 4.924v at the processor.

I haven't worked-up the courage to try that test VL=VS, because I'm not trusting the schematic entirely...

Hmm - one more test: with the BEC on, VL=VS, BEC providing logic power, power to the processor is 4.920v. So for sure it's feeding it's powering the regulator. 9+5 = 13 = more than would make the regulator happy.

So the only place I've come is that BEC powering logic = happy.
BEC + VL = bad.

I still don't know if I'm getting full BEC current to the servos, of if I'm sucking it down from the SSC's LM2937

Another thing we know: The BEC regulator in the Sabertooth is much cleaner than the regulator on the SSC, and I think the surest way of avoiding any gotcha would be to split-off the two banks, and VL, run the Sabertooth on one bank, servos on another, and a third source for logic... Other tests are potentially destructive, like VS1 = VS2, external 5v for servos, BEC still coming in from the Sabertooth.

Pending good data, I'm going to run VL != VS, VS2 != VS1, and three power sources...

LinuxGuy
05-24-2008, 08:38 AM
Is it recommended/accepted procedure to power the SSC itself from the regulated BEC?
Nope, sorry.


I assume that it's not kosher to power the servos from the BEC on the motor controller? That would be pulling the power through the regulator on the SSC, right?
Right. I never power servos from the BEC on the Sabertooth. In fact, it's not good to power anything off the BEC. I run separate batteries for servos and motors/Sabertooth. The only thing common between my SSC-32 and the Sabertooth is ground.


Finally, would .it over-volt the SSC to externally power it on VL w/+9v at the same time that the BEC circuit is active? Any back-current issues with putting external servo voltage on the SSC (+6v) for the servos, and use two spare channels for the Sabertooth - should I put the Sabertooth on the second bank and unjumper the banks from each other?
You don't want to have any power in common with the Sabertooth and SSC-32. You can power the SSC-32 from +9V, but not from the BEC. I have my Sabertooth 2x5 controlled from two channels of an SSC-32 at present, but might change that in the future to take advantage of other modes of control. It is connected to the same bank on the SSC-32 as servos.

8-Dale

Adrenalynn
05-24-2008, 11:52 AM
Thanks Dale. Old post, though, bumped by the spammer. I ended-up cutting the power wires on my servo cables.