skaliwag

My Feedback: (1)

An flying buddy with an electronics bent gave me a schematic to regulate the 7.4V Li-poly.
A 250Mah Li-poly with a micro switch and harness weighs the same as a 50Mah Nicad.

skaliwag

My Feedback: (1)

Attachment for Li-poly circuit.

combatpigg

My Feedback: (3)

Thanks for sharing that! I guess the performance of the circuit depends on howw much the IC loads it down. I'm going to give it a try, obviously it should out perform a 50 MAH NICAD!

BMatthews

Be sure to use the larger version of the regulator with the heatsink tab on it. The little tiny TO-92 plastic cases current limit at around 200 ma and the receiver and two or three servos running simutaniously can pull more than that without too much trouble. When that happens you're in trouble real quick.

The regulator can be mounted at the on off switch of the usual power harness for the model and the pack can be disconnected to hook up to the charger. No real need for that extra switch.

William Robison

My Feedback: (3)

Sorry, this isn't a good circuit.

The 74xx must have an input voltage at least 3v higher than the output, or they tend to drop out.

Using a 7805 regulator with a three cell Li-Po series pack would work fine, as would a 7806. But even the 5v 7805 with two cell Li-Po is still 0.6v too low on the input for a reliable supply.

You can build a reliable regulator that works with a voltage differential down to a fraction of a volt, but not with a 7805 or 7806.

Bill.

William Robison

My Feedback: (3)

Bruce:

The case you're referring to is the TO-220, the 78xx regulators can also be had in a TO-3 case, but it's a fair bit heavier.

Bill.

PS: The 78xx still is not a good idea. wr.

ZAGNUT

My Feedback: (1)

i thought the diff needed was only 2.5 volts...still not good though. add up the weight of the TO-220 package, a pair of capacitors (good idea to use them) and all the soldered connections and you start to lose your weight advantage...nimh batteries might be a better choice and don't require a different charger than nicds.



dave

William Robison

My Feedback: (3)

Dave:

On the capacitors you are right, but not on the weight. A pair of tantalum caps will do nicely, and weigh next to nothing. The 7805 in the TO-220 case itself is only a couple grams, the problem is still the input voltage.

Bill.

mclintock

My Feedback: (3)

just string a few silicone diodes together till the sum of the .6 volt drops gets you to whatever voltage you want.. three or four shrink wrapped in series in place of the positive rx lead should do. Schottky may even be better, but I'm not sure of the drop for those-
the schottky are more linear I think, that is not linear I should say- the drop across stays more constant for a given change in current than common 1n4001 types...

ZAGNUT

My Feedback: (1)

sounds way too complicated to me...can you post the schematics?



dave

mclintock

My Feedback: (3)

shematic ahoy!
add switch as needed..

plain ol' common radio shack 1n4002 (or 3,or 4 as the last digit) will probably do. for up to an amp anyway..

My cad program (hand-cad) is sorta shaky..

I don't know for sure if this will work, so don't just build it and slap in a plane, as the lipo batts have more warnings than [link=http://www.happyfunball.com/hfb.html]happy fun ball[/link] . May smoke a receiver too.. But it will probably just sit there and work fine.
-clint

William Robison

My Feedback: (3)

Clint:

I find it hard to believe that you took Dave at his word on the complication of using diodes to drop the voltage.

If you do it that way use nothing less than a 1N4004 silicon diode, because of the current. and three in series will give the needed voltage drop from a 2s Li-Po battery.

Bill.

mclintock

My Feedback: (3)

I knew he was kidding, that's why my drawing is so silly.
Isn't the last digit of the 1n400x related to voltage, rather than current? I'd have to go look it up, but I thought a 1n4007 for example was 1A at 1000 peak inverse volts, while a 1n4004 was still just 1 amp but less inverse volts, around 300 or some such?

I agree using at least 1.5 amp or 2 amp diodes depending on the receiver load would be safer- some use of a meter with a couple of servos stalled should give the worst case current to expect (using a normal rx batt). I just couldn't remember any of the bigger common diode part numbers..

clint

William Robison

My Feedback: (3)

Clint:

As I remember, the '04 is 1Kv PRV and a one amp forward current rating.

Bill.

ZAGNUT

My Feedback: (1)

is hand-cad freeware? bill has a nice program called [link=http://www.rcuniverse.com/forum/m_1936504/anchors_1936504/mpage_1/key_ramp/anchor/tm.htm#1936504]manu-plot[/link]

iv'e been doing the diode thing for years with gell-cells and 3W ignitions. always made me feel cheap and dirty so i made sure they were hidden from view. feels great to finally come clean....


dave

mclintock

My Feedback: (3)

the diode thing is common practice in the old camera realm for making the meters work right on silver oxide batteries in circuits that were originally made for mercury cells, now contraband. gotta use a schottky for that tho..

Be proud of your diodes- good enough for Leica-philes (in the CL)!

William Robison

My Feedback: (3)

Dave:

My cheat is a length of shrink over the diode, then say I had to splice the wire.

Haw.

Bill.

Andrew

Yup -- the 1N400x series are rated at 1 amp avg. with 30 amp max non-repetitive surge. The x is tied to the PRRV. See the [link=http://www.jameco.com/Jameco/Products/ProdDS/35975.pdf]data sheet.[/link]

the "other" Andrew

DICKEYBIRD

Nope, all you dudes'r wrong. Everybody knows the only solution is a a small lump of purified dilithium crystals clamped between 2 thin discs of transparent aluminum inside a hydroformed shell of polymerized unobtaniuum. For extra crash protection, I put it all inside a sphere of spun nobendium.

Seriously, is there anything you guys on this forum don't know? I'm continually amazed!

Lynn S

Don't you have to post a 24hr security guard to monitor those things??

Dukester

My Feedback: (31)

Don't need a security guard, just lock it all in your beryllium sphere....

(couldn't help myself, I put the blame squarely on DB for starting all this)

On a more serious note, if you want to do this and don't want to cobble together a circuit, you can just get a GWS electronic speed control. An ICS-50 speed control only weighs 5 grams with wires (and you could remove the motor output wires for our purpose). Price is a little higher at about $10 to $12, but you get regulated 5v/1A BEC with them.

Duke

BMatthews

Sorry but stringing a series of diodes together does not make a regulated output. Far better to use a regulator.

Bill you're quite right about the overhead voltage. I didn't see that one. But there are low dropout regulators that will work fine in this capacity. The LM2940 is a 1 amp version with a drop out voltage of only 0.5V. But be warned that LDO regulators have very fussy output capacitor requirements. The internal series resistance needs to be the correct value. This could be a problem. The 2940 wants to see an equivalent series resistance (ESR) of pretty much 0.5 ohms. Less is bad and more is bad.

And what with the very restrictive charging issues with Li-po's and the equally restrictive discharge needs I'm not so sure this is a good idea anyway.

William Robison

My Feedback: (3)

Bruce:

There's something I'm missing here. If the equivalent series resistance is 0.5 ohm, doesn't that mean the circuit has to be drawing 10 amps at 5 volts?

Please clarify. I've never used the LDO regulators, so pardon my ignorance, please.

Bill.

mclintock

My Feedback: (3)

I was figurin' if the lipo battery voltage has gotten discharged enough to be unable to keep the voltage fed to the rx through the series diodes between 4 and 6 volts without being regulated, then it must be time for a re-charge.

Active regulator ICs can be wasteful for battery circuits, which are clean DC to start with, plus the regulator dissapates a bit of power even when the rx isn't driving servos. In my days as an electronics repairman, I saw way more smoked regulators than bad diodes, which makes sense as the regulator is an integrated circuit with scores of internal junctions, where a diode is just one simple silicone junction.

If my wattage esc wasn't brand new and under warranty, I'd peel it and see what they are doing in there, but I doubt I'd find anything on the teeny circuit board that I'd recognize, looks like just a few surface-mounted black specks.
I'm more comfortable with vacuum tubes, and at least components with leads and big enough to have writing on the cases, or at least some colored stripes! (not to 'dis modern smd electronics- 1/2a using vacuum tubes would bite)

I know it would be nuts for 1/2a sized planes, but I wonder if anyone has ever put an on-board charging system in an r/c engine powered plane. Most permanent-magnet motors double as generators if spun, so maybe putting a teeny motor, like a 180 or whatever is smaller size on a model that is powered by say a .15 engine with the little 180's prop behind the prop of the .15, would serve as a generator for charging the rx pack? Sorta like the prop (windmill?) driven pump on a crop duster, or the little prop on front of the messershmidt rocket plane used to drive the power for the instruments..

Or whutabout jb welding a couble of neobdmum (spelling??) magnets to the back of a prop or prop washer (near the hub) and putting a wee stationary coil nearby to form an alternator? Then you could use just enough of a light teeny battery to last long enough to get back down if the motor quits..
My luck I'd crash on the first launch and the teeny coil would hit the prop and permanently wind around the prop/crank etc..

doe! I should shut up and glue some balsa..

clint

William Robison

My Feedback: (3)

Clint:

Onboard generators already exist. Check the Sullivan "Genesis" system. It uses magnets in a disc mounted behind the prop, just as you said, and a cored coil positioned to make the bits into an AC generator. Then there is the control/regulator box back inside the fuselage. Basic unit is about $90, including the disc, magnets, coils, and the regulator box.

Bill.