DiyLightAnimation
Hardware => Bobcat DMX Servo Controller => Topic started by: CW on March 14, 2012,
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I am looking for a 6VCD power supply in the 2 amp range and they seam to be limited availability. I have found 12 VDC x 2 amp output 6 VDC x 4 amp output switcher type. From what I read about these, they automatically switch voltage, is that OK for the bobcat logic side?
How about a 5 VCD power supply, they seam to be everyware and would match up with 5 V LED's.
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It automatically switches between 12VDC and 6VDC output? How does it know what voltage you want?
I dug through my parts bin and found a 6V 2.1 amp power supply. But I also built one out of a computer power supply and a LM350 adjustable voltage regulator. Been meaning to build it for a while, but this finally got me off my butt and put it together. I haven't hooked it up to the servo controller, but I can adjust it to 6V and the regulator is rated up to 3A.
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I haven't tried it, by my plan is to use 5v.
I have plenty of that available from the pc supplies I'm using for my smartstrings.
RM
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It automatically switches between 12VDC and 6VDC output? How does it know what voltage you want?
I am not sure, I have read wikipedia about these few times and still do not understand.
If a 5 volt will work, I will get one. I wish I understood electronics more.
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Can you send a link or model number for the power supply you are talking about...
I'm thinking that what you are looking at is actually a 12vdc to 6vdc converter. You provide it with 12v at 2 amps and it outputs 6v at 4 amps...
When I look at the servos at servocity.com, most of the seem to be rated for anything from 4.8 to 6 vdc. A few can use 7.4 vdc... At 4.8v, they are a little slower and have less power than at 6v, so part of the equation is how much power and speed do you need...
RM
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I haven't tried it, by my plan is to use 5v.
I have plenty of that available from the pc supplies I'm using for my smartstrings.
RM
RM, I'll toss this idea at you. The main reason to use 6VDC instead of 5VDC would be rotational speed. It will have a bit higher torque at 6VDC. You mention using PC power supplies, and that is my plan, too.
What I will probably try is this: Remove the jumper that splits my power inputs and then put 5VDC on the Logic Side. That solves the "brown out" concern mentioned in the Wiki. To power the Servo side, you can use the 12VDC line as Positive and the 5VDC as the Negative (as if it were the ground/negative wire). The difference voltage will be 7VDC !! That one volt bump over 6VDC will improve torque and speed and, being only one volt, should not damage your servos. Many people run them at 7 to 9VDC anyway.
I have done this before and the servos survived just fine, but I haven't tried it with the Bobcat (yet). I just finished soldering up #1 of 4 tonight, so will be powering things up soon. I've got Halloween skull plans.
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I can't look at it right now, but I don't think that's going to work.
First, I think the two grounds are the same. If they are, when you hook 5v to the servo ground terminal, you will short out the power supply.
Second, I'm not crazy about using the difference between the 5 and 12 volt power rails to get 7 volts. The 5 volt rail is meant to source 5 volts, not serve as the sink for another power rail. For small currents, maybe, but you start pushing much power into the 5 volt output, I think you are going to have issues... If you've got a big enough load on the 5 volt rail to ensure that current never gets pushed into the output, that might work, but I'd consider it risky to run it that way.
RM
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The two grounds *are* the same, so that technique won't work.
/mike
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After sketching out the power, it would definitely be a problem if you split the power. It would work only if you connected the +12vdc to the Servo positive and the +5vdc to the Servo negative. Nothing on the Logic side, so the jumper stays in place.
While this would put 7vdc to the servos, it would also make the voltage regulator work harder.
So, I would probably just run the whole board from a 5vdc connection if I didn't have a good 6vdc power supply.
Sent from my DROIDX using Tapatalk 2
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Tested the 3 Servo controllers I've finished (one to go!) and all 3 worked fine.
Used a 6Volt Lantern battery for power and two Futaba S3003 servos.
I was a bit concerned at first when, with power applied, I got no green LED. Once I plugged in the Bobcat Tester, though, it started slowly flashing. I was wondering if I'd remembered to flash the PICs!
Plugged in the servos and immediately got motion! Using the Bobcat tester, I quickly found that #1 worked perfectly.
Then, #2 got connected and it worked perfectly. Then #3 also worked! I suppose I'll work on #4 soon, though its more channels than I have planned for Halloween.
I'll play with the servo config utility soon, though. I'm only getting 90 degrees of rotation from 0 to 255. I was hoping for 120, so I'll play with that.
All in all, I'm happy that I have some nice featured DMX Servo controllers and should definitely get some good feedback when my usually static skulls begin turning with the music and following people with their glowing eyes as they walk by.
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I may be making this way too easy but just use a 12 volt power supply and make a voltage divider. Depending on the resistors you put in depends on the voltage you get out.
http://en.wikipedia.org/wiki/Voltage_divider
The basics of a voltage divider are you take two equal resistors with one end of each resistor tied together. You then take your output from this same point and you will have 6 volts. Next take one resistor and tie that to your 12 volt supply and take the remaining end and attach that to the neutral (it shows ground in the picture but the neutral is essentially the same thing for what you are doing).
If you are using 12 volts and 2 amps that's 24 watts. Just make sure you resistors can handle that much power and your all good. Cost, probably $.20 for the parts and $4 for shipping.
This is the cheapest solution but not the easiest to change. If you want to change the voltage on the fly, use a potentiometer.
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That won't work.
It assumes that the same current will flow through both resistors. Hooking up the bobcat and servos violates that condition.
There is a way to do it, but you are going to need some additional circuitry...
RM
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It will get you close for a cheap amount. The real reason why it might not be exactly 6v is because of the resistance across the servos, not the current flowing through it. While it is true the current would be different, it is because of the resistance seen across the servo. Also, if you use very large resistors, the resistance in the servo would be so small that it would still give you 6v, so yes it will still work.
Personally, I would just buy a 6v power supply.
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(edited)
No, it will not get you close. There is also a danger that as you play with different values of resistors trying to make it work, you will end up frying a servo.
The current needed by the bobcat controller will vary depending on the brightness of the LEDs, whether or not the servos are in motion and the load on the servos. The voltage drop across the resistor is equal to the current multiplied by the resistance. If there are wide swings in the current, there will be wide swings in the voltage drop.
Other options include getting a dc-dc converter or building a 6v regulator.
RM
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resistor dividers are only usable when you are drawing a very small and non-changing current. They can't be used as a voltage regulator.
/mike
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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.
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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.
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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.
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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.
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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?
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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.
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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
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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.
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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,
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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
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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,