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Servo error on micro maestro: CR2032 Battery?


#1

Hello. I am running a SG51R micro servo and LED off of a micro maestro. If I run the program off of a 4 AA cell pack (measuring 4.8V) I am not seeing issues. If I run it off of a two CR2032 (measuring 5.78V) pack then the LED’s will run, but the Servo will not and I get a red light on the board. Any thoughts on what may be causing this and possible solution?.

BTW: The batteries where selected as I am extremely limited on space.

Component info is below:

http://www.towerpro.com.tw/product/sg51r/


#2

Thinking about trying out a 7.4V 300MAH LIPO thinking maybe the battery is the problem. Any thoughts on what else I can look at? Would it be ok to run a 7.4V battery on the Micro Maestro? Are outputs controled to 6V or would 7.4V pass through?


#3

Desperate for an answer. Can either of you help me? @nathanb @AmandaS @JonathanKosh ??


#4

Hello.

Thanks for posting those datasheets. In general, motors tend to draw more power than control boards or small LEDs and it sounds like the coin cells you are trying to use cannot provide enough power to keep the voltage high enough to run the Maestro when the servo is also powered. The second Y axis (IR, ohms and the green line on the graph) in the “Pulse Discharge Characteristics” graph in the lower right hand side of the battery datasheet shows how the internal resistance of the cells varies as the battery discharges. In this case, the IR is about 10 ohms when the battery is fresh and increases up to about 25 ohms just before the useful capacity of the cell is passed. You can think of the internal resistance of a battery as a resistor in series with an ideal cell that will produce a voltage drop from the nominal cell voltage at the output terminals of the battery as current is drawn through the internal resistor. So to use that internal resistance to determine what the voltage of your two coin cells might drop to when trying to supply the 0.5A to 1.5A a small servo like the one on mentioned might require:

First, determine the open circuit voltage of the two cells:

3.0V x 2 = 6.0V

Since each cell has this internal resistance effect in series with it, we have to add the two together to get the total in series resistance:

10ohms + 10 ohms = 20 ohms

To determine the voltage drop across the battery internal resistance when a load much smaller than your motor tries to draw 0.1A through it, we can use Ohm’s law:

V=IR
0.1A * 20ohms = 2.0V

So we can subtract the voltage drop across the internal resistance from the nominal cell voltage to see what the terminal voltage would be under a small 0.1A load:

6.0V - 2.0V = 4.0V

So even with a small 0.1A load, those two cells cannot deliver enough power to keep their output voltage above the 5V minimum operating voltage of the Maestro.

Generally, we expect the rechargable LiPo batteries commonly available for RC hobbyists to be able to provide more power for a given size than a coin cell. Those types of batteries commonly have a “C” rating (10-20C is common) that you can use with the batteries capacity to see how much current you might expect the battery to be able to deliver (the actual units of the “C” rating are per hour or /h). So if the 300mAh battery you mentioned (300mAh = 0.3Ah) has a 10C rating, you might reasonably expect the battery to be able to provide a peak current of about 3A (0.3Ah * 10/h = 3A). So, to summarize, it might be possible to find a 300mAh LiPo battery that can power your servo.

You might look for a tutorial on Kirchoff’s law and Ohm’s law for more about how to look at current flow though a circuit and how to calculate voltage drop across resistive loads in the circuit.

-Nathan


#5

Thank you for the reply @nathanb . I was hoping that wasn’t the issue, but it looks like I will have to switch the input. I have a lot of reading up to do to educate myself. Your walk-through was perfect and appreciated. The battery I purchased is 30C (or 9A??). I assume that is enough.

Is the output voltage regulated to 6V or will it pass through the 7.4V? I am not sure how much I want to be sure I do not put in too much voltage and burn out the LEDs and servo.

If that wasn’t enough: How would I determine my battery life with constant LED (5) and periodic servo movement (90° every hour)

https://lighthouseleds.com/0402-pico-smd-wide-angle-12v-pre-wired-led-warm-soft-white-ultra-bright.html

Again: your help is appreciated.

-Jason


#6

The Maestro does not provide regulated power (or any other type of power) to the servo headers. You can read more about the power system on the Maestro in the Powering the Maestro section of the User’s Guide.

The easiest way to determine your battery life might be to measure how much current all of those devices draw from the battery using a multimeter. If all of your devices draw 100mA, than you could expect about 3 hours of runtime from a 300mAh battery. If they draw 600mA, than you could expect about 0.5 hours of runtime. You can read more about this in the Understanding battery capacity: Ah is not A post on our blog.

-Nathan


#7

Thanks @nathanb. On to the next issue… The servo problem has been solved and is running smothly. With that being said I know have an issue with the LED’s. They are on all the time with a dim glow. When turned on in the program they brighten. There is no true on and off. I’m not sure if this has to due with the increase in voltage or not… A 4AA cell pack pack produces a current of 0mA when it is off and ~1.4 when it is on. The LiPo produces .4mA when it is off and ~3mA when it is on. I assume that the .4mA is what is giving me the problem. Do you have any thoughts or even better a solution? Thank in advance.

-Jason


#8

Can you post pictures of your Maestro and LEDs that show how they are connected?

-Nathan