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I have a couple of 5v servo Motors (this (big), and this (small)) powered by THIS I2C PWM expansion board.

Right now for testing I'm powering the thing with the +5v line of the raspberry pi, but fluctuations are too high for the pi so I want to power it from an external source. I'm running out of 5v power banks (already powering the Pi's with them) my question is .. would it be ok to power the PWM extension board (and there fore the servos) with 4 x 1.5v standard batteries? This leads to 6v, not 5v. Could I just plug a resistor in series, or even a diode, to bring it from 6 to 5?

My main concern is on the servos (the board keeps a line for the power on the servos and the logic will be powered with 3v3), will they overheat? will the current on the batteries be enough to move the servo? will they last just for some minutes?

Is there anything else I should think about?

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  • \$\begingroup\$ I've opened your link to the PWM board on Amazon and on the second product photo on the backside of the board it's clearly written "6V max". \$\endgroup\$ – Volodymyr Smotesko Jun 8 '17 at 11:52
  • \$\begingroup\$ And FIY, most cheap servos work just fine on 6V. \$\endgroup\$ – Volodymyr Smotesko Jun 8 '17 at 11:54
  • \$\begingroup\$ The title is misleading, you'd read it as 6x1.5 = 9V \$\endgroup\$ – Barleyman Jun 8 '17 at 12:11
  • \$\begingroup\$ you're rght, fixed to 4x 1.5. My point is not on the board, My question is on the servos, I'm worried they will over heat or whatever. \$\endgroup\$ – javirs Jun 8 '17 at 12:13
  • \$\begingroup\$ So the servos are actually really low power? That IC can safely drive only 40mA out of 5V which works out to only 0.2W. \$\endgroup\$ – Barleyman Jun 8 '17 at 12:46
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If you google for the datasheet (https://learn.adafruit.com/16-channel-pwm-servo-driver/downloads) you will find out that 6V is the "absolute maximum" VCC value.

No, you don't want to go there. Resistor is also a bad idea. You could use a humble diode to drop the voltage to ~5.3 volts however. Do bear in mind that the "standard battery" voltage hardly stays at 1.5v for long: http://eznec.com/Amateur/1.5_vs_1.2_Volt_Batteries.pdf

Rechargeable batteries are much better in that respect as they stay around 1.2V but they also have a higher full charge voltage. You could use 4x NIMH batteries with a diode - They start off at ~1.5v and quickly drop to 1.2v where they stay until nearly empty. That'll give you 4.8 - 0.7 = 4.1v logic output. That's probably enough for 5V logic,threshold for 74HC series is 3.7. If want extra margin, you could use a Schottky diode, which would result in min 4.4V and should keep the input voltage <5.6v.

EDIT looking at the adafruit schematic, I'd clarify you want the diode on the I2C connector VCC line, not the J1 +5V line which is OK with 6V.

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  • \$\begingroup\$ the logic is powered from the 3v3 from the pi, there is no problem there. I only need to power from the batteries the servos \$\endgroup\$ – javirs Jun 8 '17 at 13:33
  • \$\begingroup\$ @javirs Right and the servos are happy with 6V. You still probably want NIMH not alkaline as the latter would lose torque constantly. \$\endgroup\$ – Barleyman Jun 8 '17 at 13:46
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You are basically asking whether it is OK to apply 6 V power to a particular board.

That answer can only come from something called a datasheet. Look on the web site of the board manufacturer, and go back to whoever sold you the board. One or the other should supply you with, or give you a link to, this datasheet.

The datasheet is a collection of specifications. There will be two relevant ones to this question. One will be the Absolute Maximum power voltage. That's what you can't exceed and still expect the board to function afterwards. The other will be the Maximum Operating voltage. That's the maximum you can give it while running and for it to still work correctly.

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  • \$\begingroup\$ edited my question to be more exact, my question is on the battery live and the servo live \$\endgroup\$ – javirs Jun 8 '17 at 12:15

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