servo controller power issues

SelfParadoxical

I recently bought Parallax's servo controller and in trying to figure out what kind of power source I would need for the servos themselves (since the manual tells you to use a seperate source) I came across these two forum posts:

http://forums.parallax.com/forums/default.aspx?f=10&m=62709

http://forums.parallax.com/forums/default.aspx?f=10&m=56655

and assuming that what they say is true; that standard servos can draw up to 500mA, then I must question where exactly parallax would suggest that I find a power source to hook into the + and - Vin port on the PSC that would supply enough amperage to actually be able to run 16 servos simultaneously as the product advertises?

allanlane5

Typically the 'control' wire (usually white, in the three-wire Servo cable) is the one that is 'pulsed'. The Black wire is typically ground, it's very important that all grounds be connected together.

The 'Red' wire can go to a separate +5 or +6 regulated power supply. So, for 16 servos, you'd need 8 amps. This can come from 8 7805 regulators (1 amp each). There do exist regulators which can put out 5 amps (I think it's a TO-3 form factor), so two of those could do it.

The "Vin" of the PSC I am pretty sure is NOT supposed to supply all the current to the servos, only to the PSC unit. But as you see above, the PSC does not have to POWER the servo's, only send the control signals.

This is an on-going problem in robotics. You want to use a small, low-power microprocessor to control lots of mass and power (a robot with arms and wheels). Thus you need some way to power the motors with lots of current, as well as powering the micro which needs 'clean' power (not much ripple).

Chris Savage

The power for the PSC itself would normally come from the 3-conductor servo wire connected to, for example, the X4 or X5 jack on the BOE.· As for the servos, there is a screw terminal header which many people connect to a 7.2V Rechargable Battery Pack.· This should provide adequate power for most applications.· Let's remember that 500mA is upper range for the servo, not necessarily it's continuous use consumption.· Given that, and the fact that you rarely have 16 servos running all at the exact same time using max current, you shouldn't need a really heavy-duty P/S.· Just remember, max input voltage on the servo power input is 7.5VDC.



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SelfParadoxical

Thanks for the answer Chris, most appreciated.
Out of curiousity, supposing that I want to use an AC power supply instead of a rechargeable battery pack, would a standard 6v 1800mA ac adapter also be sufficient (and for that matter, safe for the electronics), or is it a safer bet to stay with the rechargeable battery pack?

Chris Savage

·· As long as the output is DC and within the input voltage limits, you should be okay.· Bear in mind that some "Wall-Warts" can produce an output voltage somewhat higher than their rated voltage·when they don't have a·sufficient load on them.· I would confirm the output voltage with a meter before connecting it, just to be sure, since the adapter isn't something rated for use with the PSC.· Just a precaution, really.

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