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HELP!: Making a circuit so that a 7.2v batt produces 6v

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Old 04-09-2010, 08:51 AM
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brendonv
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Default HELP!: Making a circuit so that a 7.2v batt produces 6v

Hey im making an rc car and need to power the servos. I got 4 4600mah 7.2v batts but no 6v batts n dont really want to fork out 60 odd bucks for a 6v battery so im going to make a little circuit board to bring the voltage from 7.2v down to 6v.

Now my question is, what do i need to achieve this. Its been a while since year 11 physics so i forgot the majority of how to work this stuff out.

I worked it out that i need a .24 ohm resistor but im not sure if this is right.
I used ohms law and made the potential difference as 1.2v (7.2-6v) but i dont no if this is right. Then 5A is constant so
R= V/I
Therefore R= 1.2/5
= 0.24 ohms.

Can someone help me out here. Thanks.
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Old 04-09-2010, 10:07 AM
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EloyM
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Default RE: HELP!: Making a circuit so that a 7.2v batt produces 6v

A dropping resistor works only when the current is constant, and servos draw a few mils at rest and much more when moving and working.
What you need is an IC voltage regulator - MC7806 - which will supply 6V regardless of load. It is a simple three pin device: In, Out, Ground. Luck mate, Eloy
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Old 04-09-2010, 11:04 AM
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Default RE: HELP!: Making a circuit so that a 7.2v batt produces 6v

You shouldn't have to do anything. The ESC that controls the motor should provide an output of around 5V to the receiver and servos. If the ESc doesn't have a built in BEC, then an external BEC device will do the job of providing regulation from your main battery pack
Pete[8D]
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Old 04-09-2010, 08:19 PM
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brendonv
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Default RE: HELP!: Making a circuit so that a 7.2v batt produces 6v


ORIGINAL: pilotpete2

You shouldn't have to do anything. The ESC that controls the motor should provide an output of around 5V to the receiver and servos. If the ESc doesn't have a built in BEC, then an external BEC device will do the job of providing regulation from your main battery pack
Pete[8D]
Its petrol powered, no esc, no electric motor.
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Old 04-09-2010, 08:28 PM
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Default RE: HELP!: Making a circuit so that a 7.2v batt produces 6v


ORIGINAL: EloyM

A dropping resistor works only when the current is constant, and servos draw a few mils at rest and much more when moving and working.
What you need is an IC voltage regulator - MC7806 - which will supply 6V regardless of load. It is a simple three pin device: In, Out, Ground. Luck mate, Eloy
Regarding the Voltage regulator i see it only puts out 1A. Is that enough to power two 15kg servos and one 5kg servo? If not, which voltage regulator will i need?

Will one of these do? http://www.jaycar.com.au/productView...r&form=KEYWORD
http://www.jaycar.com.au/productView...r&form=KEYWORD
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Old 04-09-2010, 11:53 PM
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Default RE: HELP!: Making a circuit so that a 7.2v batt produces 6v

better yet - something like the 10A BEC here:

http://www.castlecreations.com/products/ccbec.html

or any of a multitude of similar products. Being a switching regulator, it won't get nearly as hot as a linear regulator under load.

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Old 04-10-2010, 07:35 AM
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Default RE: HELP!: Making a circuit so that a 7.2v batt produces 6v

Iuse these:

http://hobbyking.com/hobbyking/store...idProduct=3735
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Old 04-10-2010, 08:01 AM
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Default RE: HELP!: Making a circuit so that a 7.2v batt produces 6v

Or put a 50 cent 10 amp diode in the line. .7 volt drop right there, might be enough to do the job.
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Old 04-10-2010, 08:29 AM
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Default RE: HELP!: Making a circuit so that a 7.2v batt produces 6v

How many amps will i need to power two MG995 15kg servos and a 5kg Tamiya servo TP-S3003?
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Old 04-11-2010, 07:56 AM
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Default RE: HELP!: Making a circuit so that a 7.2v batt produces 6v

Hey guys thanks for the help, ive decided its easier, better and cheaper to buy a 5-10 amp 6v SLA battery. I need some weight on the front so it will do the job and give more than enough power to the servos and wont run out for ages.
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Old 04-11-2010, 10:25 PM
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Default RE: HELP!: Making a circuit so that a 7.2v batt produces 6v

If you don't have much invested in those batteries just yank a cell and resolder.
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Old 04-12-2010, 06:55 AM
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Default RE: HELP!: Making a circuit so that a 7.2v batt produces 6v


ORIGINAL: jester_s1

If you don't have much invested in those batteries just yank a cell and resolder.
That sounds like a winner to me.-BW
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Old 04-15-2010, 07:41 AM
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Default RE: HELP!: Making a circuit so that a 7.2v batt produces 6v


ORIGINAL: Bonified Wingnut


ORIGINAL: jester_s1

If you don't have much invested in those batteries just yank a cell and resolder.
That sounds like a winner to me.-BW

Agreed..
And to the OP - where the heck are you paying $60 for a simple 6v rx battery??? Is it gold plated or something?


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Old 05-12-2010, 03:11 PM
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Default RE: HELP!: Making a circuit so that a 7.2v batt produces 6v

I never used a dedicated 6 or 5V battery pack to power the receiver and the servos. When not powering my boats with a 6V battery, I usually use my homemade BEC: a 7805 5V regulator, rated at 1A (more then needed). It's not recommended to use the 7806, 6V regulator because you need at least 2V more at the input.

Better use the LM2940 which requires a lower voltage difference at the input. I usually solder it to the ESC positive and negative terminals (homemade electronic ESC's).

Regards
Robert
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