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  #16   Spotlight this post!  
Unread 21-07-2014, 12:30
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Re: Power From The Wall

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Originally Posted by tuXguy15 View Post
There is an electrical trainer and a pneumatic trainer. The electrical trainer's highest load comes from the mini-cims without any load on them so the current draw is minimal. The pneumatic trainer's largest load is going to come from the compressor. I dont think current will be an issue with the electrical board, but I do think it might be an issue with the compressor since that does occasionally draw a lot of current.
I would swap the motors for lower draw ones to make this easier.

From there, you can probably get away with surplus server power supplies.

Similar suggestions to PC ones above, but higher power, cheaper and easier to modify.

http://www.tjinguytech.com/my-projects/HP47A

is a good resource.
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Unread 21-07-2014, 12:39
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Re: Power From The Wall

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Originally Posted by adciv View Post
I disagree that it could damage the battery, although I understand why people say that. But damaging the power distribution board? How?
The manufacturer's recommended rated is ~5.5 amps. Charging the battery at 6 amps is pushing it, but allowed because virtually all chargers that are rated for 6 amps only put out 6 amps for a very limited time. Charging a battery at a constant 20 amps is asking for trouble. ([1])

As for the PDB, the short answer is that all chargers are not created equal. Some chargers will cause more damage to the PDB power supplies than others, since the supplies are more sensitive to voltage variances than batteries. ([1],[2])
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Unread 23-07-2014, 07:55
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Re: Power From The Wall

Here's what you're not taking into account. With a 20A charger in the system, you're not charging the battery at 20A. You're using the 20A charger to supply the base load of the robot. When the power demand is for greater than 20A, the battery picks up the load. This will discharge it slightly, and once the load has been reduced, it will recharge at a low rate. The only way you exceed the 5.5A rate is if you have started to drain the battery. This could also be solved by using a motorcycle battery in place of the standard FRC battery.

As to the voltage spikes, that's an engineering problem which can be solved a few different ways. One way would be making sure you don't use that kind of charger, another would be to increase the capacitance in the system to dampen the spikes. I'd like to know the load on those chargers when the spikes were recorded.
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Unread 23-07-2014, 08:12
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Re: Power From The Wall

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Originally Posted by evanperryg View Post
There are ways to modify computer power supplies for custom use... I have done this in the past with a couple of different OEM supplies if you are interested in how to do it.
I'm interested. Have you got something written up that you could post or link to?

(Not sure what to do with all the wires; see attached pic)


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Unread 23-07-2014, 08:21
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Re: Power From The Wall

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Originally Posted by Ether View Post
I'm interested. Have you got something written up that you could post or link to?
Here's a couple:


I've found one that is variable and uses a PWM circuit vs. the typical LM317 adjustable regulator. Here's another one.

Now that I've been reminded, I have an old 480W PSU to repair (solder in some new 2200 uF caps) and convert to a bench power supply. It's a little more wallet-friendly than this
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Unread 23-07-2014, 13:39
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Re: Power From The Wall

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Originally Posted by Ether View Post
I'm interested. Have you got something written up that you could post or link to?
There are a number of instructional videos on how to convert ATX supplies to benchtop supplies, but this is the one I used: https://www.youtube.com/watch?v=z2oSFpKh_Uw

I added LEDS to the "power good" and "5v standby lines" as status indicators. Not necessary, but it's helpful at times.

SIDE NOTE: I suggest you avoid Dell OEM supplies. They sometimes have different color codes.
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Last edited by evanperryg : 23-07-2014 at 13:43. Reason: added side note
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Unread 23-07-2014, 13:39
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Re: Power From The Wall

Quote:
Originally Posted by Ether View Post
I'm interested. Have you got something written up that you could post or link to?

(Not sure what to do with all the wires; see attached pic)


I would say that unless you need the 5V rail, the use of server power supplies I linked are much more cost effective.
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Unread 23-07-2014, 13:41
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Re: Power From The Wall

Quote:
Originally Posted by stinglikeabee View Post
I've found one that is variable and uses a PWM circuit vs. the typical LM317 adjustable regulator.
I doubt the LM317 would be capable of putting out the amount of current required for OP's test bed. The cRio alone is rated for 800mA, which is half the LM317's nominal current output. Add a sidedar and some motors, and you're well over the 317's 1.5A rating.

Here's some pics of my conversion. I've done a couple others, and they all work very well.
https://drive.google.com/file/d/0B4P...it?usp=sharing
https://drive.google.com/file/d/0B4P...it?usp=sharing
https://drive.google.com/file/d/0B4P...it?usp=sharing
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Last edited by evanperryg : 23-07-2014 at 13:53.
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Unread 23-07-2014, 14:07
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Quote:
Originally Posted by evanperryg View Post
I doubt the LM317 would be capable of putting out the amount of current required for OP's test bed. The cRio alone is rated for 800mA, which is half the LM317's nominal current output. Add a sidedar and some motors, and you're well over the 317's 1.5A rating.

Here's some pics of my conversion. I've done a couple others, and they all work very well.
https://drive.google.com/file/d/0B4P...it?usp=sharing
https://drive.google.com/file/d/0B4P...it?usp=sharing
https://drive.google.com/file/d/0B4P...it?usp=sharing
Ether didn't ask for one capable of powering a robot

That's a good point in case someone wanted to use it though.
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Unread 23-07-2014, 14:15
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Re: Power From The Wall

Quote:
Originally Posted by stinglikeabee View Post
Ether didn't ask for one capable of powering a robot That's a good point in case someone wanted to use it though.
True If you're doing something smaller, a LM317 should work fine, just throw a heatsink on it (and, in a cinch, mayonnaise makes an effective thermal paste). The supply pictured is a 270W OEM supply from an early 2000s IBM desktop. I have also converted an off-the-shelf Antec 350W supply, and another OEM supply I had sitting around. All of them work, and I prepared all three in the exact same way, except for the dummy load resistors, which vary due to supply size/what I had sitting around.
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Unread 23-07-2014, 21:21
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Re: Power From The Wall

The CNC Lathe at my work has a 20hp vector drive amplifier that powers a DC brushless spindle servo. The problem is that it takes 3 phase 240 volt ac power and cost $5,000.

In realistic terms, maybe try getting as big of a battery as possible? maybe a car battery or a really big lipo (very expensive)?

The thing is, batteries running out of juice isn't necessarily a bad thing. The motors get really hot (especially because they are hauling a 150lbs robot around), the compressor needs to catch up (it only has like a 20% ish duty cylce) and it can give you a way to take turns.
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Unread 24-07-2014, 11:00
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Re: Power From The Wall

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Originally Posted by AdamHeard View Post
I would say that unless you need the 5V rail, the use of server power supplies I linked are much more cost effective.
It's just that I already have a half dozen PC desktop power supplies that I harvested and have been wanting to experiment with them, and possibly find a use for them (other than as a door stop).


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Unread 24-07-2014, 11:04
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Re: Power From The Wall

Quote:
In realistic terms, maybe try getting as big of a battery as possible? maybe a car battery or a really big lipo (very expensive)?

The thing is, batteries running out of juice isn't necessarily a bad thing. The motors get really hot (especially because they are hauling a 150lbs robot around), the compressor needs to catch up (it only has like a 20% ish duty cylce) and it can give you a way to take turns.
Converted ATX supplies are very realistic. They're designed to provide very consistent power for very sensitive components. The motor will only get severely hot if there is a significant difference between the forward voltage and back EMF. At free speed, the back EMF should match the forward voltage, meaning the motor won't heat up significantly. In other words, OP, don't put any force on the motor, let it spin freely.
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Last edited by evanperryg : 24-07-2014 at 15:31.
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Unread 24-07-2014, 13:22
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Re: Power From The Wall

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Originally Posted by evanperryg View Post
SIDE NOTE: I suggest you avoid Dell OEM supplies. They sometimes have different color codes.
Bummer. I was going to use the Dell (pictured in earlier post).

Do you know of a resource anywhere for decoding the Dell color scheme?

The motherboard is long gone so I can't trace where the wires were going.


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Unread 24-07-2014, 13:43
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Re: Power From The Wall

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Originally Posted by Ether View Post
Bummer. I was going to use the Dell (pictured in earlier post).

Do you know of a resource anywhere for decoding the Dell color scheme?

The motherboard is long gone so I can't trace where the wires were going.


I would imagine the pinouts are still the same since it will conform to the ATX standard. if you have the molex connector on there, check out http://en.wikipedia.org/wiki/ATX#Power_supply

Just beware that ATX supplies can be rather noisy (electrically speaking)
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