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Offline masaTopic starter

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power supply
« on: March 16, 2009, 01:03:00 PM »
Hi, I am a newbie :D

I am thinking of getting a power adapter to supply power from power point to MCU and servos. The problem is I don't know the current I required.

the power required by servos is as followed:

micro servo:
idle current: 8mA
no load running current: 280mA
test voltage: 4.8V or 6V

standard servo:
at 4.8V
idle current: 8.8mA
running current:350mA
at 6v
idle current:9.1mA
running current:450mA

I am using 12 micro servos and 2 stand servos, so:

12 units of micro servo need 280mA x 12 = 3360mA (for no load running only, so actual current needed may >3360mA)
2 units of standard servo need 450mA x 2 = 900mA

minimum total current needed =  3360 +900 = 4260mA ?

so I need to find one power adapter which output is 6V DC, 8A(need to be >4260mA) ? but would the current too high? I worry it could damage the MCU or Servos.

and would it be a problem if the MCU and Servos are using the same power supply?

another thing I would like to ask, could I just cut the wire of the power adapter, test the polarity (using voltmeter), and then connect it MCU board? Because I don't need the pin of the power adapter, just need direct wire connection. Also, will the polarity of the power adapter change when I use it next time?

Thanks!





Offline want2learn

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Re: power supply
« Reply #1 on: March 16, 2009, 01:52:22 PM »
You should look at a different kind of power supply as wall adapters aren't a reliable voltage source: the more current you pull from them, the lower the voltage tends to go. So when most of your servos are idle the pack may be supplying say 9 volts to the remaining active servos and when all the servos are active they may be getting about 6 volts.

For the setup your looking at I'd buy a baterry pack with a charger or some battery holders and individaul cells and a simple charger for them, then make up a power pack of your own.

I know they're both more expensive than the power pack option but they are the better and cheaper of many solutions.

N.B. If you're able you can convert an old pc psu for a lab supply. It'll provide more than you need (5V@10Amp(ish)) and total cost of parts for conversion inculding screw type banana terminals will only be around £5. ;D
« Last Edit: March 16, 2009, 01:55:50 PM by want2learn »
The question that drives me hazy:

Am I, or the others crazy?

Offline ArcMan

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Re: power supply
« Reply #2 on: March 16, 2009, 02:25:55 PM »
I agree with the battery pack idea.  It is a simple and cheap way to get that much current.  A AA NiMH battery pack should meet your needs.

Offline masaTopic starter

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Re: power supply
« Reply #3 on: March 17, 2009, 02:13:46 AM »
You should look at a different kind of power supply as wall adapters aren't a reliable voltage source: the more current you pull from them, the lower the voltage tends to go. So when most of your servos are idle the pack may be supplying say 9 volts to the remaining active servos and when all the servos are active they may be getting about 6 volts.

For the setup your looking at I'd buy a baterry pack with a charger or some battery holders and individaul cells and a simple charger for them, then make up a power pack of your own.

I know they're both more expensive than the power pack option but they are the better and cheaper of many solutions.

N.B. If you're able you can convert an old pc psu for a lab supply. It'll provide more than you need (5V@10Amp(ish)) and total cost of parts for conversion inculding screw type banana terminals will only be around £5. ;D

So is there any other way to get power from power point other than using wall adapter?

I agree battery pack is easy to set up, but it will be used only after done with programming. During programming, I would like to use wall power supply, so dont have to re-charge battery so often. 

But now I don't know what is the 'safety' current I should use. :(


Offline SmAsH

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Re: power supply
« Reply #4 on: March 17, 2009, 02:18:19 AM »
you could use the mighty usb port and just use the power pins. if you use a regulator which you should it will let the right amount of current through to your mcu.
Howdy

Offline ArcMan

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Re: power supply
« Reply #5 on: March 17, 2009, 01:37:24 PM »
The USB port is only capable of supplying 500 mA.  Nowhere near what he needs.  Did you see his power requirements?

Offline Soeren

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Re: power supply
« Reply #6 on: March 17, 2009, 05:36:12 PM »
Hi,

you could use the mighty usb port and just use the power pins. if you use a regulator which you should it will let the right amount of current through to your mcu.
Why "should" he regulate the 5V out of a USB-port when he needs 4.8V to 6V?
A DC-DC converter sounds better in that respect.
Assuming you mean a voltage regulator, how will it "let the right amount of current through"?

Besides, as ArcMan mentions, USB hasn't got the power for the job.


masa <- Power Point usually takes power (and time).

The servos will only take what current they need (and leave the rest for the poor).
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline masaTopic starter

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Re: power supply
« Reply #7 on: March 18, 2009, 08:54:26 PM »


So I could just use any current more than 5A, says 50A, and it won't damage the servos?

Thanks for the response!

Offline SmAsH

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Re: power supply
« Reply #8 on: March 18, 2009, 11:25:33 PM »
i think 50A might be a tad over what you need?? are you sure its 50A not 50MA?
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Offline want2learn

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Re: power supply
« Reply #9 on: March 19, 2009, 12:10:32 PM »
A 50Amp power supply is a little overkill (moneywise) but as Soeren says, your circuits/motors will only take what they need
The question that drives me hazy:

Am I, or the others crazy?

Offline masaTopic starter

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Re: power supply
« Reply #10 on: March 19, 2009, 12:50:12 PM »
lol... Of course I won't use 50A..I just want to make sure if I can really use 'any amount' of current.

Just wonder, how does a servo 'take' the amount of current it needs? I mean what actually control the amount of current intake by the servo?




Offline chelmi

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Re: power supply
« Reply #11 on: March 19, 2009, 01:13:44 PM »
Just wonder, how does a servo 'take' the amount of current it needs? I mean what actually control the amount of current intake by the servo?

Laws of electricity :) If I remember correctly I = U/R in this case, I is current, U is voltage and R is resistance.

Chelmi

Offline SmAsH

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Re: power supply
« Reply #12 on: March 19, 2009, 02:21:26 PM »
ok so with most circuits they only draw the current needed. imagine it like a river with a pump leading to your house, you turn the tap on when you need it and depending on how much water you want. like you can have 5 amps available but if your circuit only needs 1.7A it will only draw 1.7A.
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Offline madchimp

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Re: power supply
« Reply #13 on: March 19, 2009, 09:23:46 PM »
The voltage is what drives the current. If you want to look at it like a water system the voltage would be the water pressure, the amps would be water flow and your load (in this case we will just use a single resistor) would be the pipe. If you have a small pipe (high resistance resistor) you will only get so much water to go through it at a given pressure. If you want more water to flow through it (amps) you either need to increase the pressure (voltage) or get a bigger pipe (lower the resistance). Ohm's law if you want to look it up. Lots of info out there.

Offline SmAsH

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Re: power supply
« Reply #14 on: March 20, 2009, 07:31:19 AM »
thats the exact thing my electronics teacher told me at the start of the year :o
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