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Topic: Noob question: Understanding power (Read 731 times) previous topic - next topic

Bobu

Oct 19, 2012, 11:58 pm Last Edit: Oct 20, 2012, 12:02 am by Bobu Reason: 1
Greetings Arduino community!

Apologies ahead of time if this is the wrong section or if I say anything that sounds really "noob-ish", the fact is I am one when it comes to hardware as my background is mainly in software development, but I am trying to learn!  :)

The project background
I've written a sketch that moves 4 micro servos independently which seems to be working fine (it will be 6 servos at the end however).  They're all powered via the Arduino 5v pin (I know people keep saying to not use this pin for more than 1 servo, but I'm getting there) and everything is working fine as far as I can see when there is no load.  When there is load, the servos move, but you can hear them beginning to struggle, which is probably because the arduino is connected via USB meaning it can only deliver 500mA at most. 

The end goal is to remove the atmega328 chip once everything is ready (this then controls the signals to the servos), and solder it permanently into the board as a permanent project (along with the other parts it needs of course like the crystal).  This then frees up the development board to work on a new project without wasting it (I obviously buy a new chip to replace the one removed).

So, as I'm planning to remove the chip at some point anyway and I've now hit the limit of the servos when powering them off the board directly, I thought it's best to try and get everything working off a breadboard now with an external power source as I need to add the last 2.


  • If I connect 6 servos in parallel and each need 1A, would I need to produce 6A from the power source or would they share it like in voltage and 1A is all that is needed from the power source?

  • If I need to produce 6A, is that dangerous when the voltage is somewhere in the region of 9v-12v?  I only ask as this link and one on wikipedia claims that above 2,000mA (2A) can produce some quite nasty results, but I'm not sure if it's only when a certain voltage is present (my guess is that is the case, but wanted to be sure before frying myself): http://www.osha.gov/SLTC/etools/construction/electrical_incidents/eleccurrent.html


If I connect 4 AA batteries in series, my understanding is this will increase the voltage but the current will be the same as 1 AA battery.  So if 1 AA battery is 1.5v and 1800mA/hr, it would be 6volts and 1.8A maximum for 1 hour.  If you do it in parallel, it increases the max current but not the voltage.  So in this case it would be 1.5v and 7.2A maximum for 1 hour


  • Is that understanding correct or completely wrong?

  • Could I mix the 2 to give me more max amps and a higher voltage with the same number of batteries? 
    For instance: 3 AA batteries in parallel giving me 5.4A maximum and then connect this in series to the last battery, will that give me 5.4A max with 3v?



Basically this all boils back to me trying to power the arduino board + 6 servos off 1 power source.  Each servo needs 4.8v-6v (a quick google indicates each servo should have 1A so it can operate to it's maximum potential under load) so I'm trying to figure out exactly how I can power this without having to use non-standard batteries or without having to have a lot of batteries.

I hope that makes sense and the questions aren't too ridiculous.  I really appreciate any help or advice you could provide because, like I said, I'm a hardware noob!

Cheers!

retrolefty

Is that understanding correct or completely wrong?
Could I mix the 2 to give me more max amps and a higher voltage with the same number of batteries?  
For instance: 3 AA batteries in parallel giving me 5.4A maximum and then connect this in series to the last battery, will that give me 5.4A max with 3v?

First thing to try and completely understand is that the maximum current capacity or any power supply or battery is just it's rate maximum capacity to supply current. Just like you house power outlet can power a 7 watt night light or also a 700 watt vacuum cleaner, it's the load that determines how much current will flow from a fixed voltage source, it's an Ohm's law thing. So the actual amount of current that will flow from your voltage source is a function of the load resistance attached to it, or in your case the current demand that the servo(s) will try and draw. A servo at rest may only draw a few milliamps of current, but when commanded to move they will draw more current and if trying to supply maximum torque to a mechanical load, still draw a higher current. The 1 amp per servo is just a general rule of thumb as the true system current will depend on number of servos, the specific model of servo, if they will be commanded to move all at the same time and what maximum mechanical load they have to try and move. So there is no simple answer to how much current capacity your specific servos and application will require. The 1 amp per servo rule is a good starting point at least.


Basically this all boils back to me trying to power the arduino board + 6 servos off 1 power source.  Each servo needs 4.8v-6v (a quick google indicates each servo should have 1A so it can operate to it's maximum potential under load) so I'm trying to figure out exactly how I can power this without having to use non-standard batteries or without having to have a lot of batteries.

To add to the complexity of your question, batteries don't maintain a fixed voltage over the life of their discharge curve, but your arduino does require a regulated +5vdc voltage source (at least it has to never see higher then 5.5vdc or risk damage), while your servos can handle a range of 4.8 to 6 vdc. Also when selecting batteries you have to have a decent handle on the total current draw during normal operation and how long you require the batteries to power the project before having to stop and recharge them.

I hope that makes sense and the questions aren't too ridiculous.  I really appreciate any help or advice you could provide because, like I said, I'm a hardware noob!

Those are good questions for a beginner, but as I hope you see there is no simple answer but rather a series of trade-off decisions, and we haven't even talked about cost limits or volume size limits if to be mounted inside something, and lastly what the charger requirements are going to be. Non rechargeable batteries make no sense at all in a servo based project, just throwing money away with each use.

Lefty


MarkT

One observation:  powering the servos from the +5V Arduino rail could lead to damage to the chips on the Arduino - its a risk I certainly wouldn't take, unless I had a spare board and didn't care about the expense.  Servos and motors produce nasty voltage and current spikes and generally you never share motor power with logic if you want reliability.

Secondly you really should get a multimeter and measure the current consumption of one of your servos (while continuously moving) to get a better estimate of the power supply / batteries you will need.  For now I'd suggest 4xAA battery pack for the servos only, share a common ground and run Arduino from USB.  At a later point you'll need to work out how to power the Arduino from a separate rail to the servos (whether via a LDO voltage regulator and / or filter circuit, or DC->DC converter, or separate battery).

Quote
If I connect 6 servos in parallel and each need 1A, would I need to produce 6A from the power source or would they share it like in voltage and 1A is all that is needed from the power source?

If they are active at the same time, 6A, if only one is moving at once, perhaps 2 or 3A (they take some current when parked).
Quote

If I need to produce 6A, is that dangerous when the voltage is somewhere in the region of 9v-12v?  I only ask as this link and one on wikipedia claims that above 2,000mA (2A) can produce some quite nasty results, but I'm not sure if it's only when a certain voltage is present (my guess is that is the case, but wanted to be sure before frying myself)

Unless you put wires on your tongue, 9 or 12V is perfectly safe.  Large currents can lead to _heating_ issues, but not shocks.

Quote
If I connect 4 AA batteries in series, my understanding is this will increase the voltage but the current will be the same as 1 AA battery.  So if 1 AA battery is 1.5v and 1800mA/hr, it would be 6volts and 1.8A maximum for 1 hour.  If you do it in parallel, it increases the max current but not the voltage.  So in this case it would be 1.5v and 7.2A maximum for 1 hour


Firstly units. Current is measured in amps, charge (capacity) is measured in coulombs or ampere-hours (Ah).  amps/hour is a rate of change of current, completely wrong here.

A measure of capacity (1800mAh, not mA/h) says nothing about how much current can safely flow.  You need to know the maximum discharge rate for that - anyway lets assume these hypothetical batteries have a maximum discharge rate of 1.8A, then yes in series the rate is unchanged, in parallel it adds.

Different battery chemistries have different properties (for instance most NiMH rechargeables would only be happy discharging at the hour rate or so, whereas a LiPoly battery might be rated to fully discharge in 120 seconds!).

Lastly if you buy some 2Ah batteries, assume their capacity will be down to 1Ah before too long - in the real world rechargeable batteries perform quite poorly without careful management and manufacturers bend the truth to breaking point.  And over-discharging many kinds of rechargeables completely knackers(*) them, especially when in series as one cell may get reverse-charged.

(*) capacity reduced by large factors.
[ I won't respond to messages, use the forum please ]

Bobu

Great, thanks for the info guys.  Looks like I need to pick up a few new bits :)

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