110V AC to 12V DC suggestion
 

We're building a control system test board (power distribution board, cRIO II, DSC, radio, Jags, et al) and I looking for a team with experience using an AC-to-DC transformer as a substitute for the 12V battery. It'll be easier than lugging around a battery while doing testing.

I see several sites that sell transformers but I don't know enough about the output of the 12V cell to know I'm getting the right thing.

Any help will be greatly appreciated!

Re: 110V AC to 12V DC suggestion
 

I've been using old Xbox 360 power supplies for 12v high current needs (chargers, etc.) for a while, I've found 3 for around 5$ each at goodwill. As long as you're not running any motors off of it, I would guess you will be fine.

I will say I have never made a controls test board to run off anything other than the kop battery though, YMMV.

Re: 110V AC to 12V DC suggestion
 

A power supply with enough current to run a robot system (with motor) will be very expensive.

A battery is the best cheap option.

Re: 110V AC to 12V DC suggestion
 

We have had good results with desktop power supplies out of no longer needed computers. Most have over current protection so it just shuts down if a motor is overloaded

Re: 110V AC to 12V DC suggestion
 

M,
You have some good suggestions here but if you don't know your way around 110 VAC circuitry it could be lethal. If you are not using motors, most any CB radio power supply should work. Several are available from most stores. Simply look for 12-13.6 volt power supplies. Those made for radios will be able to put out 2-5 amps and should run $50 or less. I have two high current power supplies I use for ham equipment and both are rated for greater than 30 amps surge and 25 amps continuous. However, both will trip out trying to run even small FRC motors. If you are just demonstrating you can substitute 12 volt lamps for the motors connected to the output of the speed controllers. Be sure to use small car lamps like taillights or interior lighting and you should be ok.

Re: 110V AC to 12V DC suggestion
 

That is quite true if you really need or want to run a whole robot. But if you really need or want a non battery system you may be able to get one to work with a stripped down version of the robot. This can also come in handy for training members of the electrical team. Our team is building a development station right now that we intend to run from a DC power supply. (Although we were particularly lucky to get an unusually large supply donated, if you are careful a mor modest supply might work.) We have had some luck previously with some scaled down sub components. If you build up a system with just the CRio, a power distribution board, digital side car, and modem, but not much else, you will have much better luck powering it without a battery. Just do not expect to run a compressor or any CIM motors from an average size DC power supply. Even small DC motors under load are probably a bad idea for this approach.

In place of motors and motor controllers you can use some servos with little arrows on the actuator pointing to a home-made gauge marked with Forward, OFF, Reverse. This only simulates the motors and does not really replace the need at some point to test the code on a real robot. But it can give the software team a lot of debug capability without access to a full robot. Just an idea.

You can possibly add some Jaguars, victors, and spikes without taking a big hit on additional power requirements if you use small toy motors that are not under much load. It all depends on what your intent really is. There are times when it is really great to let the software team or electrical team have access to something that does not need a battery, and is safer than a full robot even if it is not fully representative of the final product. It is also nice to have options if you do not always have access to a place where it is safe to operate a real robot, or if you have a hard time keeping the software team from killing your batteries.

A while back I built a small sub-portion of this that you might consider as a way to try out this idea on a much smaller scale. I mounted a toy motor, victor, and encoder together on small block of wood with an old 12 volt laptop battery charger as the power supply. Plug in the supply and this setup is ready for stand alone training in PWM, encoders, PID, etc. You can hook it up to a PWM output or PWM signal generator to demonstrate how the motor controllers work. Or you can hook the encoder up to a scope or digital input of the CRio to show how encoders work. Or you can hook both to a digital side car for an exercise in PID control. It is not a good representation of the real world torque, rotation inertia, or time constants you will encounter in a real robot. But it is simple and easy to use, and gets the basic concepts across. It worked for me. Your mileage may vary.

Re: 110V AC to 12V DC suggestion
 

Thanks to all for the ideas. Our team has done some research and found the battery puts out around 120A/1440W of power! If you were going to try to drive CIMs off of that with the rest of the control system, it looks like the cost is over $1K. Suddenly, lugging the battery around looks a lot more reasonable.

I really like the servo motor idea mentioned in the thread. That gives the programming team what they need without adding the extra expense.

Thanks again to all!

Re: 110V AC to 12V DC suggestion
 

We have a desktop training system mounted on a 2' x 4' piece of plywood.

It has the cRio, the full electrical distribution system, pneumatics, window motors connected to the Jaguars' etc.

It is powered by an Astron RS-12A power supply. It works find but I'd suggest something like an RS-35 or so.

As Al pointed out, do NOT screw around with electricity if you are not sure what you are doing.

The Astron supply is a rugged supply, UL approved and provides a reliable supply for what you are doing.

Go here and select 'DC Power' on the product type pulldown selector on the left menu bar.

Re: 110V AC to 12V DC suggestion
 

M,
The battery is capable of much more than that for short periods of time. Short circuit currents run above 600 amps for full charge.

Re: 110V AC to 12V DC suggestion
 

Thanks to all for the great ideas. In the end I found converters capable of the output of the 12V battery to be VERY expensive. Suddenly, lugging around a 12V battery seems reasonable. I think we'll wire the system up for a 12V DC cell and convert over to a converter source if the programmers can use it without the more powerful DC motors. The servo idea is particularly interesting. Thanks again, Michael

Re: 110V AC to 12V DC suggestion
 

If simulating the power supply of the battery is a concern with a power supply please keep in mind that you'd have to simulate the battery's internal resistance characteristics.

Even if you could get a power supply that could withstand the surge currents that battery is cable of (I have some really big power supplies and battery chargers with high current engine start functions) chances are high that the output of the power supply will have different impedance than the battery meaning that the motors under various conditions might behave differently.

On a slightly different note if the issue is just constantly swapping batteries and since you don't mind the cord perhaps you could rig up a charger for the battery with a switch / relay / contactor / MOSFET to disconnect and reconnect the charger more quickly or automatically in between using the robot.

Then the battery is no different than during competition, but the charging of that battery will be more consistent. Unfortunately then you're tethered to the wall socket.

Re: 110V AC to 12V DC suggestion
 

I'm reluctant to share this actually...

...but a blade server power supply could potentially be used for this.

They show up on eBay for well under $100 quite regularly, and push out over 175A at 12V.

David Edwards, a mentor for 1310, and I have thrown around the idea of modifying one of these to power a functional FRC robot.

But truthfully, I have never pursued it, because I simply don't trust myself enough to modify something that puts out that much power.

If you have an experienced power electronics engineer at your disposal, then maybe you could safely give this a shot.

If not, please do not try this at home!

Re: 110V AC to 12V DC suggestion
 

Using a single blade server power supply won't work for a stangard FRC robot since A CIM can pull 132 AMPs at stall (which is where motors start at) and there are typically 4 of those plus other motors.

Sistering (pairing) these power supplies may or may not be possible depending on their overcurrent protection. Most overcurrent protection detects a high current (low output resistance) and shuts down the output, then will periodically turn back on the output and re-check for overcurrent. This results in a periodic pulse (normally called hiccup mode) on the outputs until the overcurrent is removed.

If there is no overcurrent protection on these supplies then pairing is possible using "or-ing" diodes capable of more than the supplies output (typicallly 50% or more). These work by having the power supply with the highest output voltage supply all the current until its output voltage sags to the level of the next highest supply, at that point both supplies share the load until they both sag to the level of the next supply (et cetera).

Re: 110V AC to 12V DC suggestion
 

If a CIM is pulling 132amps wouldn't PD 40amp breakers throw? And there is a 120amp breaker, so the robot should never pull more then 120 amps, and if it tires to wouldn't the breaker just kill power?

Those power supplies look like the deal of the decade for high power applications, I might get one to run my battery charging array...

Re: 110V AC to 12V DC suggestion
 

The breakers are thermal-limited to 40a or 120a.

This does not mean that any current greater than the trip point will instantly trip the breaker.

Current transients can heat the breakers for a short time without tripping them. For the 120a main breaker, this number is several hundred amps for many seconds - I seem to remember 200a for a minute, but I don't have the spec sheet on hand. The 40a breakers can also take several seconds to trip under CIM stall load (which IS ~130a).

All motors will pull their stall current when starting (assuming supply voltage = spec voltage). This is because the current draw is proportional to torque, which is inversely proportional to speed. Since the speed is 0 when starting, torque is stall torque, so current is stall current. As the motor applies a force to the output, the output beings to move and the motor's output torque and input current go down. The motor will still draw ~130a for a short period of time.

Assuming 4 drive CIM's, then a forward/reverse launch would instantaneously draw ~520a.

Server power supplies are not designed for high current transient loads like motors. They are primarily designed for steady-state computing and cooling loads, which do not produce transient current draws anywhere near that of CIM motors. Unless I had data or specs to say otherwise, I would assume the spec output current is the peak output current, and a single power supply would not be able to handle the load of multiple CIM motors starting.