Author Topic: Bobcat 6V power supply  (Read 5274 times)

Offline CW

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Bobcat 6V power supply
« on: March 14, 2012, »
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.

Offline Night Owl

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Re: Bobcat 6V power supply
« Reply #1 on: March 15, 2012, »
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. 

Offline rm357

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Bobcat 6V power supply
« Reply #2 on: March 15, 2012, »
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
Robert
Warner Robins, Georgia, USA

Offline CW

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Re: Bobcat 6V power supply
« Reply #3 on: March 15, 2012, »
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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.

Offline rm357

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Bobcat 6V power supply
« Reply #4 on: March 16, 2012, »
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
Robert
Warner Robins, Georgia, USA

Offline JonB256

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Re: Bobcat 6V power supply
« Reply #5 on: April 15, 2012, »
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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.

Offline rm357

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Bobcat 6V power supply
« Reply #6 on: April 15, 2012, »
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

Robert
Warner Robins, Georgia, USA

Offline n1ist

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Re: Bobcat 6V power supply
« Reply #7 on: April 16, 2012, »
The two grounds *are* the same, so that technique won't work.
/mike

Offline JonB256

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Re: Bobcat 6V power supply
« Reply #8 on: April 16, 2012, »
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.

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Offline JonB256

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Re: Bobcat 6V power supply
« Reply #9 on: May 04, 2012, »
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.

Offline gatorengineer

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Re: Bobcat 6V power supply
« Reply #10 on: May 05, 2012, »
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. 

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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. 

Offline rm357

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Bobcat 6V power supply
« Reply #11 on: May 05, 2012, »
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
Robert
Warner Robins, Georgia, USA

Offline gatorengineer

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Re: Bobcat 6V power supply
« Reply #12 on: May 05, 2012, »
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.

Offline rm357

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Bobcat 6V power supply
« Reply #13 on: May 06, 2012, »
(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
« Last Edit: May 06, 2012, by rm357 »
Robert
Warner Robins, Georgia, USA

Offline n1ist

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Re: Bobcat 6V power supply
« Reply #14 on: May 07, 2012, »
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