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Topic: Trouble with power (Read 3067 times) previous topic - next topic


I suppose this is more of a general electronics question, however maybe someone here would be able to help me.

When my robot was working (before i built the new body) i had the ultrasonic sensor and the servo running off a 4.5v pack (3 AA). I know they both need more power then that but it worked anyway lol. So i have been messing around with making resistor voltage dividers so that i can not use as many batteries. To start off i am just using a 9v to see if i can get the servo working.

I figured that it ran of less then 4.5v before so i used 1k ohm 1/4 watt resistors in a series to reduce the voltage of the 9v to 4.5v. The multimeter reads 4.5v but the servo won't work. However if i hook it up to the 3 AA then it works fine.

Any ideas as to why it's not working?


Sep 01, 2010, 10:31 pm Last Edit: Sep 01, 2010, 10:46 pm by AWOL Reason: 1
So i have been messing around with making resistor voltage dividers so that i can not use as many batteries

That's a way to use more batteries, because you'll simply waste power as heat.
Use a proper regulator.

The multimeter reads 4.5v but the servo won't work

And how much current does the multimeter say is trickling through the servo?

Also, it's been said here many times before, the usual PP3 9V battery is a truly rubbish way of powering a servo.
"Pete, it's a fool looks for logic in the chambers of the human heart." Ulysses Everett McGill.
Do not send technical questions via personal messaging - they will be ignored.
I speak for myself, not Arduino.


So you want to drop 4.5 volts through a 1k resistor.  To do this the load (your servo) must take 4.5ma.

Now I might be wrong but I don't think you'll find a servo that can operate at such a low current.

As per the previous comment, if you want a fixed voltage out of a battery of higher voltage then you use a voltage regulator chip (such as a 7805) not a resistor.

You could use a resistor but it offers only one predetermined voltage drop at one current only in accordance with Ohm's law.  V=IxR


Sep 02, 2010, 12:06 am Last Edit: Sep 02, 2010, 12:06 am by Grumpy_Mike Reason: 1
The multimeter reads 4.5v but the servo won't work

Does the multimeter read 4.5V with the servo's attached?

Thought not. You can't control voltage like this. Look at a switching DC/DC converter like this one:-


Another suggestion: Build a 9V battery pack using 6 AA cells, and split off 6V at the 4-cell "midpoint", and use the 6V to power the servos (servos are typically rated to use from 4.8 volts to 6 volts - you get maximum torque from your servos at 6 volts, btw), and the 9V to power the Arduino.
I will not respond to Arduino help PM's from random forum users; if you have such a question, start a new topic thread.


Thanks a lot for all the help guys. I'm going to try the voltage regulators now i just need to find a big honkin battery pack. I want to expand on the bot later but as of now i need power for a servo (5v), ultrasonic sensor(5v), motor driver (10v, but i get 5 of it from the arduino), and the arduino(9v). So all together 24v. Not to mention when i add the rest of what i want with another motor driver 4 more servos and another sensor lol.


Would this one work?



sorry for the double post,

while this isn't rechargeable would 2 of these work?



in a word   no

you seem to be under the misunderstanding that all your devices are somehow wired in series, hence your 24 volts.

devices are wired in parallel so you pick your highest voltage and work from there.   i suggest you get a 12volt sla battery rated at say 3ah and a 5 volt regulator,   this willgive you 12 volts for the arduino and 5 volts for all the rest.   you'll also need a coupleof output capacitors for the regulator to prevent  the risk of oscillation   say 0.1mf and 10mf, both rated at 25volts.



wow, so now i feel super stupid. I promise i will get this eventually lol it's just a matter of time.

So i never thought about the components being wired in parallel and not in series. So you are saying a 12v battery at 3ah. While i don't quite fully understand the amp-hours yet, do i really need 3? It seems that all of the batteries that are rated that high are very large. Also all of the components i use don't require that much.

The ping says it needs 20mA
The motor drives says 1A
The servo is at 160mA
I couldn't find the rating for the Arduino itself, but do i really need a battery at 3ah or would i be able to use at slightly less for the sake of size?

If i'm still way off base here feel more then free to bring on the onslaught of verbal abuse lol as long as there is some help mixed in with it.

Thanks again


Sep 02, 2010, 04:00 pm Last Edit: Sep 02, 2010, 04:23 pm by Grumpy_Mike Reason: 1
The ping says it needs 20mA
The motor drives says 1A
The servo is at 160mA

So add them up and get 1.18 Amps, the arduino needs about 40mA so that brings it to 1.22 Amps.

So if your battery will provide 3 Ah it will provide 3 amps for 1 hour.
Or 1 amp for 3 hours.
As you are only taking 1.22 amps you will get 3 / 1.22 = 2.46 hours.

Well that's the theory in practice you get under this figure but it is a start.
As you can see your motor dominates the consumption.
Lower battery capacity the shorter it will run.

It seems that all of the batteries that are rated that high are very large.

Yes they are, that is quite a battery.


Your motor is rated at 10 volts so you are going to need a 12 volt battery to get the full power from it.  Also you'll be using the pWM output of the arduino if you want variable speed.  If it's simply on/off then you can use a digital output.  Either way you WILL need some form of interface to get the 5 volts from the arduino up to the motor voltage.  If it's PWM then you can use a FET.  If it's on/off then use either a FET or a relay.

If the 10 volt maximum is critical and you are using a digital drive then 2 diodes in series in the motor feed will drop the 12 volts by around 1.5 volts to give you around 10.5.   If you are using PWM then the voltage control is taken care of by limiting the pWM output to around 90%.

How often do you intend to run the motor  (10% of time, 50%, 100% etc)   This info is important since it has a material effect upon the battery size.  You also need to work out how much the motor is loaded since this affects how much current it demands.

Say you run the motor at 70% load for 50% of the time.
Motor mean current is 1x.7 =.7amps
Motor duty = 50%

So in 1 hour you consume 0.7 x 0.5 = 0.35 ampere-hours or 0.35AH

Using a 3AH battery, if all things were ideal you would get 3/0.35 = 8.57hours of run time.

Ideally you should look at sizing your battery at a minimum of 5 times the load AH, so in this case the minimum battery size will be 0.35x5 or 1.75AH.   In this case you might expect 4 hours run time.

Note that I've ignored the several low current devices in the above calculations since the intent was to illustrate the high demand motor.  As an exercise how about going through all of the above taking into account the whole system.



Alright so seeing as how my apparent stupidity knows no bounds, i have hopefully one last question.

Would something like this work?

While it is .9v less then what was suggested everything would still be able to run off that, and it is 4ah which is more then suggested, but if my research is correct that will just make the battery last longer...I think.

Thanks sooooo much.


Would something like this work?

Looks fine to me.   :)


Alright so if anyone is still here i was curious if this lithium battery is rechargeable, it is much cheaper then the one i found earlier and is much smaller. I understand that this battery will not last as long, but for my application it will not need to. My main concern is if it is rechargeable the fact that i could not find that information on the site leads me to believe that it is not, but for some reason i thought that all lithium-ion batteries are.


If it is not does anyone know of one that would be?

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