Powering an FRC-like system from a wall outlet

Technical Technical Discussion
#1

We’re building a simple linear motion mechanism that we’d like to be able to plug into a wall to use. I don’t have much experience with electric systems, so I was wondering what might be a good way to go about that. The requirements we have are listed below:

  1. Powering a mini-CIM with amperage needs consistent with moving a load of about 20 lbs along a horizontal track at ~20in/sec (25:1 Versaplanetary reduction) with little friction.

  2. Using a Spark motor controller

  3. Needs to plug into a standard American wall outlet

Thanks!

#2

Depending on your current requirements, something as cheap/simple as a used PC Power Supply might be everything you need.

#3

+1 on PC Power Supplies. Large 12v supplies are also available from places like Ebay for cheap. I got a 12v 15a power supply with several outlets for only $30, and the bigger you go the cheaper it gets per amp (to a point).

#4

Another fun bit about using PC Power supplies: You can use the 5v rail for something like an Arduino, which keeps the control system cost down, and removes the need for extra regulating circuits.

#5

Hi Eric,

I’ve inputted the numbers into the Linear Motion page of the JVN calculator, and it seems that the Mini-CIM isn’t going to draw a huge amount of current. The calculator gave an answer of around 8-10A( this is lifting the 20lb vertically not horizontally). As long as you don’t intend intend to stall the mechanism or increase the load or speed, a 12v 20A power supply such as this type should work nicely for you:

You should also be able to use an old computer power supply if you have one around. A 350watt PS( if you have a more powerful one use that) should be able to give out 16A on one of the 12v power rail( just look on google to find a chart of the power rail colours). If you need to, you’ll also be able to power any control system parts of the other rails of this computer power supply as well.

The Computer PS will be able to power the C-rio, Robo-rio, RPI or Arduino etc. that you use for generating the PWM signal for the SPARK, which should simplify things for you( again look up the PS on google to find a chart of rail colours and voltages)

#6

Thanks, this was a lot of help. Just to check, looking at a power supply like this: https://www.amazon.com/eTopxizu-Universal-Regulated-Switching-Computer/dp/B00D7CWSCG

Is it possible with this unit to have a line going out to the motor controller and then convert another line to 5v to power the control system?

#7

I would say that should work, but if after looking at that power supply, I noticed two things; one, it doesn’t have a built in port for a power cord, just terminals. This is linked below it on Amazon, and would allow you to use a standard computer power supply cord. The other thing I noticed is that the terminals on the power supply are almost completely exposed. Yes, they do appear to have a plastic cover, but it probably comes off so you can access the screws. Not too much of a problem for the 12 volt DC outputs unless something conductive falls on them, but the 120 volt AC input terminals are a different story… If you decide to use it, make sure the high voltage terminals are protected and that it’s unplugged before doing any work on it. ::safety::

#8

We saw the switch and are buying it, but good catch on the exposed terminals. In the end, all of the electrics will be pretty heavily protected, but we’ll print or otherwise make a cover for those terminals before we’re through.

#9

Yes, that should work for you. In regards to turning 12v into 5v to power your control system. You should be able to disassemble a car 12v to 5v USB charger in order to obtain your 5v ( just make sure it can supply enough current to power your controls).

The AC terminals are exposed, please use a great deal of caution when working with the AC.

Stay safe!

#10

If you’re running off a switching supply, I would make sure to ramp the control to prevent doing 12V stall of the motor.

#11

Something like this should work as well.

Edit: There could be capacitors inside of the power supply that will keep power going to the high voltage terminals even when unplugged, though they shouldn’t if everything is wired properly.

#12

Beware of using a PC power supply. Many required balanced 5V and 12V loads to get advertised current outputs, meaning you’ll have to throw some big resistors on your 5V rail.

#13

Get yourself a couple server PSUs. http://www.rcgroups.com/forums/showthread.php?t=1292514

I use a HP DPS-600PB to power a 2200 W (peak) brushless spindle for my CNC router. Works great!

#14

Good advice. These can be picked up cheap-ish or free from PC recycling places.

#15

A quick word of advice that is hopefully timely:

The max current your power supply will be asked to supply is not the motor’s steady-state current draw, it’s the transient levels that occur when the motor speed (or direction) changes, or the load changes - And - These transients won’t last very long, but they will be much larger than the steady-state currents.

It’s a safe bet that your biggest transients will occur at startup, when the motor is stalled (or during a quick velocity change) (or at any time an unplanned stall might occur).

I’m not sure wheher some of the supplies folks have suggested (like PC power supplies) will do well when asked to supply startup current to your motor. They might, or they might not.

Bottom Line: You will want both your power supply, and and any fusing (built-in, or added-by-you) that protects your equipment (and the flammable stuff around it) to behave well when a transient occurs. Check their specs.

Have fun,
Blake
PS: It’s a safe bet that for one reason or another, at some point in the life of the system, it will be stalled long enough to fry itslef, and perhaps its surroundings, if it doesn’t have an auto-shutdown (a fuse) built into it. Detecting stalls with sensors and software in order to avoid frying things is useful, but it isn’t a substitute for a fuse.