Author Topic: Bobcat 6V power supply  (Read 5336 times)

Offline gatorengineer

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Re: Bobcat 6V power supply
« Reply #15 on: May 07, 2012, »
It's all in how you design it so if you are not a DIYer, then yes voltage dividers are not for you....but then again, what is the name of this website?

I'll say it again, you can do it, but you will be much happier buying a 6v supply.

Offline rm357

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Bobcat 6V power supply
« Reply #16 on: May 07, 2012, »
I hate to respond this, but I cannot allow incorrect information to pass.
Warning, this post contains geek-speak. You do not need to know or understand this material to have success with light animation.

Anyone who has taken more than a basic class in electronic circuits should be familiar with E=I*R, which is sometimes written as V=I*R for strictly DC circuits. Using this formula, it should be obvious that a resistor based voltage divider cannot provide voltage regulation for a variable load. The servo is a non-linear device that draws very little current when stopped, but can draw a significant amount of power when in motion or under load.

If you are thinking about using Thevenin's Theorem, it is only valid for linear circuits.

While you might be able to get away with using very small value high wattage resistors such that the servo current fluctuation would not cause a large voltage swing, it would be a very poor circuit design as you would be wasting a lot of power through the resistors in the divider network.

At a minimum, you will need a power transistor and it would be a good idea to use a Zener diode as a fixed voltage reference. For all of that, it would be easier to just purchase a voltage regulator.

This goes well beyond the technical depth that is needed to be active member of this forum and to create light animation masterpieces.

Robert
Warner Robins, Georgia, USA

Offline Night Owl

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Re: Bobcat 6V power supply
« Reply #17 on: May 07, 2012, »
You Shall Not Pass!  (Sorry, couldn't resist.)

Mike and RM are right.  Voltage dividers should not be used for servos.  The load will change when the servo cycles on and off.  When the servo is moving, current will flow through the servo motor.  This current will not flow through the resistor in parallel with the servo, however it will flow through the other resistor, so your voltage supplied to the servo will drop.  The problem will get worse as you increase the number of servos.  If multiple servos are active at the same time, the voltage will drop even further.  Your servo speed will be unpredictable.

If you are looking to build something, use a linear voltage regulator, like the LM317 (only good up to 1.5A) or LM350 (good up to 3A).  They compensate for changing load conditions and will keep the voltage constant.  You could go fancy with a switching regulator for more efficiency if you are using batteries as a power source. 

Offline holland lights

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Re: Bobcat 6V power supply
« Reply #18 on: August 07, 2012, »
I am currently developing a dual 5 amp variable voltage supply. I got tired of trying to find a supply with the right voltage. You supply it with 12 volts in and it supplys two 5 amp outputs.

Offline rm357

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Bobcat 6V power supply
« Reply #19 on: August 07, 2012, »
Cool!
What are you using for U2 and U4?
Are you planning to use the clip on heat sinks like some of RJ's stuff or do you have something heavier in mind?


Robert
Warner Robins, Georgia, USA

Offline holland lights

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Re: Bobcat 6V power supply
« Reply #20 on: August 07, 2012, »
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Cool!
What are you using for U2 and U4?
Are you planning to use the clip on heat sinks like some of RJ's stuff or do you have something heavier in mind?
Lm338T    They are 5 amp regulators
I have a prototype board etched, just have not ordered the parts yet. i have a bom for it set up to make it easy when i do.

Offline n1ist

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Re: Bobcat 6V power supply
« Reply #21 on: August 08, 2012, »
Unfortunately, physics will get in the way...  At 5A and 6V output, you will need to dissipate 30W in the regulator and 3.5W in the series diodes.  Even at 1A, that would be 6W dumped from the regulator.  You would need a good heat sink and likely fan cooling.

To get this kind of power, you should really be looking at a switching regulator.  A bit  harder to design, but it will run a lot cooler.  I'd look at something in the simple switcher line by National.
/mike

Offline holland lights

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Re: Bobcat 6V power supply
« Reply #22 on: August 08, 2012, »
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Unfortunately, physics will get in the way...  At 5A and 6V output, you will need to dissipate 30W in the regulator and 3.5W in the series diodes.  Even at 1A, that would be 6W dumped from the regulator.  You would need a good heat sink and likely fan cooling.

To get this kind of power, you should really be looking at a switching regulator.  A bit  harder to design, but it will run a lot cooler.  I'd look at something in the simple switcher line by National.
/mike
hmm so i either redesign a different one or put a big heatsink on it and one of the fan i have laying around. I mean 30 watts would mean only if i am running it at the full 5 amps. Because 6 volts X 5 amps = 30 watts; whitch would mean I would have to be using all of the servos at once. In the talking skull i am building most likely all four servos will not be running at the same time. So i am hoping it will never reach the 5 amp power draw. I found a design of one online tody while searching.
« Last Edit: August 08, 2012, by holland lights »

Offline mkozik1

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Re: Bobcat 6V power supply
« Reply #23 on: June 29, 2013, »
Quite some time since this thread has been addressed and I was wondering if anyone has come up with a good solution to take 12 vDC down to 6 vDC?

Thanks,
- Mark

Offline rm357

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Bobcat 6V power supply
« Reply #24 on: June 29, 2013, »
Most of the servos out there run just fine on 5v, so my thought was to just use the 5v from a PC power supply. The servo will be a little slower and have a little less torque than it would at 6v, but if you have a smartstring hub nearby you can kill two birds with one power supply...

Just be aware that a lot of modern PC power supplies generate the 5v from the 12v power rail, so the current you draw on the 5v circuit will count against the total amps available on the 12v circuit.

RM
Robert
Warner Robins, Georgia, USA

Offline mkozik1

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Re: Bobcat 6V power supply
« Reply #25 on: June 29, 2013, »
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Most of the servos out there run just fine on 5v, so my thought was to just use the 5v from a PC power supply. The servo will be a little slower and have a little less torque than it would at 6v, but if you have a smartstring hub nearby you can kill two birds with one power supply...

Just be aware that a lot of modern PC power supplies generate the 5v from the 12v power rail, so the current you draw on the 5v circuit will count against the total amps available on the 12v circuit.

RM

Thanks Robert,

I have a couple of 6 vDC wall warts that I have been keeping an eye on and they seem to be staying steady at 6 volts.  I only intend to drive a couple of mouths on my skulls with the others so the 5 volts ought to do just fine.  Will also use a one separate from my Smart String Hub.

Appreciate your reply - Take care,
- Mark