To regulate or to buy a wierd battery, that is the question...

[Qbranch]

Hello all, q here. I'd like to enlist the help of the CD collective concious for a problem I'm having.

I find myself on a tractor pull team out here in Illinois. The rules stipulate that we can use as many 16hp briggs engines we want (provided we don't go over weight limit), and can use only one battery (there are many other rules of course but these two are pertinant).

Currently, our design has five engines. As you can guess, making a throttle cable system to pull all five throttles EXACTLY the same amount at the same time would be a lot of trouble... so I suggested using airplane servos to move each throttle individually.

Unfortunately, the 'only one battery' rule is the problem. Airplane servos run, maximum, 6v... and our starters and everything else runs 12v. With the number of servos I need to run... they can draw up to 4A@6V.

So here's my two questions:

A) Can anyone help me get started (or show me a design) on a regulator circuit that can provide the 6V@4A from a 12v dc source?

B) Do any of you know of a battery that provides both 12v and 6v? Something with a tap connected midway between the cells?

Thanks for any assistance,

-q

[dtengineering]

Presumably you could use an LM7806 regulator for each servo, however given the voltage drop, and your peak current demand estimates some kind of switching power supply might be more efficient. It occurs to me that the Victor speed controllers are effectively a switching voltage supply... at least if you put a nice big capacitor on the output end. Then you could run a little feedback loop to increase the "speed" of the Victor if the voltage dropped a little low, or "slow" it if the voltage got a little high. I'd probably try to avoid reversing it, though.

That is a bit of a hack job... surely there is a more elegant solution available... there has to be a 12v-6v DC to DC switching power supply solution commercially available.

Jason

[Greg Needel]

could you use something like this? http://www.robotcombat.com/products/0-BEC5.html



the other thought I had was to get a custom battery pack made for you with multiple taps on it. Email sales@battlepack.com I am sure that he could come up with something for you. Also any batteries+ should be able to help you aswell.

[Kevin Sevcik]

Swapping batteries or getting a custom made one seems like a bad idea. Especially as those battlepack.com guys appear to do NiCAD and LiIon, and car electronics typically expect lead-acid batteries.

But digikey comes to the rescue with some open-frame DC-DC converters:
A very flexible TI one
A cheaper, less flexible one

The TI one looks like it has better transient responses and a wider input voltage range. Which you'll need on if you're running this with an alternator as well, since the electrical system's voltages can vary quite depending on loads. At any rate, for either of those you'll need to come up with a modest enclosure to protect them and read through the datasheet to make sure you're giving them everything they need to be happy. Specifically, they need a particular resistor to set the output voltage and capacitors on the inputs and outputs.

[Tom Line]

http://www.newcircuits.com/circuit.php?id=pos002

With a fan on a large heat sink, 6A shouldn't do any harm to the circuit.

However, I think Kevin's might be the better choice.

[Qbranch]

Thank you all so much for the suggestions and solutions!

dtengineering/Tom: I know a linear regulator would probably work in this situation, but it would produce a large amount of heat (a linear regulator is the only kind i know how to build (zener diode and a transistor), and because of the need for a cooling fan or something of the sort, isn't the most attractive solution for me since this module will be in VERY harsh environments (dust/dirt/vibration/possible external high temperatures (may be mounted near the rear axle's differential)). But, thank you for the vote of confidence on that being a viable solution and for your thoughts.

dtengineering: It would be interesting to try and pull it off with a big 1F capacitor or something and a victor speed controller, but I'd be worried the victor wouldn't update quick enough to respond to the rapid changes in loading.

Greg: I like that these are partially encapsulated, and they'll definitely do the trick, so this may be what I end up using for now... but the price is a little prohibitive (since I'd need two).

Kevin: I think this is a winner. I'll have to look at some efficiency curves to figure heat output, but for now I think I'm going to run with these boards and encapsulate them myself (provided the thermals work out).

Thank you all!

-q

[Al Skierkiewicz]

Alex,
You can easily run servos from the main battery with a handful of 7805 regulators and some 1N4000 diodes. Both are available at Radio Shack. Each 7805 in a TO-220 case can handle 1.5 amps. If you put a diode in series with the common lead on the 7805 it will output 5.6 volts. If you put two in series, the regulator will output 6.2 volts or three will make 6.8. Wire the anode to the regulator and the cathode (the banded end) to the power common. (In the series string, wire the cathode of one diode to the anode of the next in the string.) You could easily make up four of these combos, mount the 7805 on a piece of aluminum to act as a heatsink and distribute as needed. You didn't state what else you may be using the 12 volt battery for, but should it fall below 8 volts, the output of the regulators will drop as well. The last time I bought 7805s at RS they had a schematic on the back. I would put a capacitor near the power input of your regulator group. A 1000mfd@25volts should do nicely also available at RS.

[MrForbes]

Originally Posted by Qbranch

As you can guess, making a throttle cable system to pull all five throttles EXACTLY the same amount at the same time would be a lot of trouble.

Then why bother? does it really matter if one engine puts out a bit more power than another?

(I'm just looking at it from the KISS perspective...electronics and servos and tractor pulls seems like a tough mix to me)

[vivek16]

Originally Posted by squirrel

Then why bother? does it really matter if one engine puts out a bit more power than another?

(I'm just looking at it from the KISS perspective...electronics and servos and tractor pulls seems like a tough mix to me)

Thats exactly what I was thinking, it may be different for gas motors but under any significant load, dc motors help in the end. I would probably just go with the KISS perspective too.

[Qbranch]

I think I should have described the throttle situation a little differently... the system used previously to pull all the throttle cables together caused gross throttle variances accross the tractor, causing some engines to pull hard and others to backfire against the torque. I could also put this a different way... the team tried the kiss method before, and found it wanting.

Unfortunately, unlike DC motors, these engines don't like a power mis-match too much. Plus, in this game every horsepower counts when your whole tractors at around 80hp...

-q

[MrForbes]

Are the throttles governed like they are on most little industrial engines?

[VEI Dude]

My 2 cents, on the original question...

Buying a wierd/custom battery is not good, due to the uneven drain.

Regulating is the correct answer, but I would avoid linear regulators. If you lose 6V in the linear regulator (12V in - 6V out), and doing this at 4A, you're losing 24Watts in heat! That is significant! Also, most 78xx regulators are rated for 1A (some at 1.5A), but that is with a lower drop voltage. Dropping 6V would probably cut you back to well under 1A, so you'd need more than 4 regulators. And to dissipate 24W, you'd need *significant* heatsinking, plus some type of air movement. And also make sure that the specific brand of regulators allow paralleling outputs.

The DC-DC converter is the correct answer. That Lineage Power dc-dc converter that Kevin pointed to is really nice for the price. Another option is to build your own, with the circuit designed for you automatically by one of the online switcher tools. One of my favourites is National's Webench. Enter your requirements and it will select appropriate switching regulator IC's for you, and then design the circuit for you. You can then tweak component values and see how the circuit responds (regulation, efficiency, etc). You can usually even order a kit with all the selected components, but then it gets costly. There are other manufacturers that do this too, btw.

For your requirements, I'd think one of the ready-made dc-dc converters would be best, but look through the datasheets to see if they require any external capacitors, as some do.

Cheers,
-Neil.

[MrForbes]

Does each engine have a generator that can be used to recharge the battery? if so then 24 watts being wasted probably isn't a big deal. Do the servos actually draw that much current when they're just holding a position, or is the rating for maximum power draw at maximum slew? Can you just use one 7805 or 7806 per servo?

again, trying to keep it simple....

[Qbranch]

squirrel: I think they are governed to avoid redlining... but I'm not entirely sure... this is the first project I've ever worked on that involves internal combustion (other than changing the oil on my hybrid).

VEI dude: Thanks for the suggestions.

squirrel: We've tried using six regulators for six servos... but this needed heatsinks on each regulator and a cooling fan. Trouble is, the very dusty environment didn't jive very well with the fan, and (mainly due to bad placement on the part of last year's design team... they put it on top of the nice n hot rear diff) still occasionally went out.

For now, we're going to order up one of those TI regulator boards Kevin mentioned. It looks like it should work, and we should be able to use a passive heatsink then encapsulate the rest of it.

-q