Author Topic: Why so hard to find Servo power specs?  (Read 1037 times)

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Offline GrooveHolmesTopic starter

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Why so hard to find Servo power specs?
« on: May 23, 2011, 05:59:43 PM »
Trying to build a custom power rail(s) but am having a bugger finding the max amperage or wattage on the Hitec 322HD servo. I would have figured this was need to know info for board builders...  ???

To figure out the resistor values in the voltage regulator voltage divider circuit, I need to know either the amps and/or wattage of the Vout circuit (from VR). Cannot do this without the A or W of the servos on the line right?!


Thanks!
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Offline rbtying

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Re: Why so hard to find Servo power specs?
« Reply #1 on: May 23, 2011, 06:45:10 PM »
Trying to build a custom power rail(s) but am having a bugger finding the max amperage or wattage on the Hitec 322HD servo. I would have figured this was need to know info for board builders...  ???
A good estimate is usually about 1A per servo @ 6v.  They're not often published since RC servos are marketed towards RC enthusiasts, who generally have highly overspecced SMPSs that will run just about any servo, and use premade boards/receivers.  As a board builder, you are among the minority - and of that minority, most will just measure the current through the servo at stall. 

[...] resistor values in the voltage regulator voltage divider circuit [...]
This sounds foreboding... Motors should just about never be powered from a linear voltage regulator, or even worse a resistive voltage divider - the currents are too high.  At 1A per servo motor, you could maybe run one servo per regulator, and even that will cause inordinate amounts of heat buildup.  Power it from unregulated 6v or with a switching power supply rated for several amps. 

Offline GrooveHolmesTopic starter

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Re: Why so hard to find Servo power specs?
« Reply #2 on: May 23, 2011, 08:25:44 PM »
Hmmm thank you, that's a lot of good info.

I'm not using a resistive divider to lower the voltage. I'm using an LM217 LDO Adjustable Voltage Regulator, which uses a voltage divider across the output and adjust pins to set the output voltage ([email protected]).

I'm into protoyping, so I am learning to build 'machines' not just bots. I need to be able to build 'one off' boards for dedicated projects (e.g. Terrarium environmental control systems, smart cat toys, IR automatic toilet flushing devices, cleaning bots, etc). Ultimately, that board will be built around the 1284P.

So right now I'm just tinkering with creating my own power rails due to all the sensors.

Ideally, I would like to work with only two types of power sources; 7.2V batteries for the mobile projects, and 12V (various amp) DC wall power supplies for 'machines'.

The plan was to create a power supply board that took the input from the battery and then split it into a few standardized blocks for universal usage: 6V - servos, 5V - MCs and general electronics, 3.3V - sensors, and 1.8V again for certain sensors.

In trying to figure out overall capacitance and wattage, but...
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Offline rbtying

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Re: Why so hard to find Servo power specs?
« Reply #3 on: May 23, 2011, 08:32:54 PM »
The LM217 is unsuitable for high loads like servos (it's a linear regulator).  If you're willing to wait for shipping, these are much more suitable - high current output, and they're pretty cheap for switching regulators. 

Some things to consider:  Power loss through a linear regulator can be approximated with (input voltage - output voltage) * current:

Assuming two servos, you will have a ~2A draw.  With a 7.2v battery, that calculates out to be (7.2v - 6.0v) * 2A = 2.4W.  With a 12v power supply, it is (12.0v - 6.0v) * 2A = 12W!  The heat dissipation of the LM217 won't be able to handle either for long. 

 


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