efish

My Feedback: (5)

So I was having a chat with a friend who repairs electronic appliances and I asked him if there was a cheap and easy way to use my lipo packs to power my receiver/servos. After hearing about multiple resistors or using ESCs, he chuckles and pulls out a little IC chip. "This is what you need!", he says and smiles broadly. He then hooks up his power supply set at 7.4 v and demonstrates that the output voltage is indeed 5v.

Has anyone tried this yet? Is it reliable? After further discussion, my friend even gave me a matching heat sink and a 1000 mju farad capacitor when I told him current loads may go up to 1A on my larger planes with 5 full size servos. In fact, I doubt they would exceed 200mA on the 1/2A planes, so one or even both may not be required.

If he's right, then this would sure beat having to solder a bunch of resistors together! The IC chips themselves apparently cost less than a buck!

Anyone?

supagloo

I am using a regulator in my smaller planes. It saves a lot of weight!

Be careful with the type of regulator you choose. Make sure it is a low drop type.
If the right type of regulator is used, theoretically the battery can be discharged to its lower limit so its full capacity (in mAh) can be used. At this point the voltage for the RX is still 4.9V so it works well.

This subject was discussed before. http://www.rcuniverse.com/forum/m_3859102/tm.htm

These are the parts I use. The regulator weighs 0.13 gram, the capacitors are the tantalic type.
It is easy to see that the biggest part of the total weight are the wires and the connectors.
The total built up and ready to use weight (with thin copper heathsink) is 5 grams.

supagloo

B.T.W. regulators are advertised on this forum. On my monitor right now!

http://www.smart-fly.com/Products/Re.../regulator.htm

efish

My Feedback: (5)

Hi Supergloo, thanks! I knew someone must have thought of this before.

My friend gave me a 7805CT. I gather from the other thread that this is not a good idea for 2S lipo because there's not enough of a voltage drop.

Could you tell me what regulator and capacitors you're using? And also if they've posed any problems? I've got no background in electronics, but I'm always amazed at all the components in the parts shop; this does seem like an easy project to get started on.

Thanks.

supagloo

I just looked at the data of the 7805CT. I found a dropout voltage of 2V.
This means for a 5V regulator that the lowest input voltage has to be 7V to work well. As long as your battery voltage is higher than 7V you can use this type of regulator.

If the battery voltage drops under 7V there is a possibility that the 7805CT cuts the voltage in the output. Try this before using it in any R/C power supply.
The regulator I use doesn't have this problems. I will look up the type for you as soon as I get home.



There is a little thing that you allways have to remember when you are using a regulator to adapt the voltage from a battery: Most (if not all) types of regulators are still drawing a little current if the output is not connected to the receiver.
Allways disconnect the battery - or put a switch between the battery and the regulator - when you don't use it. If you put the switch/ disconnect between the regulator and the receiver, the battery gets empty without using it.

Andrew

Look at a LM1086CT-5.0 or LM1085CT-5.0 Both are LDO V-regs rated at 1.5A and 3.0A respectively. You can also pick up LDO adjustable regulators in the same series, but it will require the addition of a couple of resistors to fix the voltage level.

ptulmer

My Feedback: (5)

What size lipo's are you guys using? I can't find anything that would make this something worth pursuing. The nimh flight packs are about the same weight when you break them in half and only cost a few bucks. Compare that to more than twenty bucks each for the lipos and the question keeps rising in my mind, why? I'm always looking for a way to lighten a "Blink"!
I have been wondering if the Aero Ace size batteries would be good for this? Something like this: http://www2.towerhobbies.com/cgi-bin...?&I=LXJCR0&P=0

indoorff

My Feedback: (2)

I have been using a 110mah nicad pack for my models. I have two 145 mah lipoly cells that are MUCH lighter than the nicad pack, but do not have a regulator for receiver use. Plus they only costed about 5 bucks if I remeber. Definately weight savings to be gained, or capacity increase for the same weight.

Chris

ptulmer

My Feedback: (5)

Oh, nicds make terrible receiver batteries. The 300mah nimh packs I have weigh barely over an ounce and the 220's come in about .8oz. Take this battery for example, http://www2.towerhobbies.com/cgi-bin...?&I=LXGHD9&P=0
It's .9oz plus the weight of the needed electronics and leads. The 220mah nimh pack weighs less and serves the same function. Now, the 150mah lipo might just be the ticket, but where do you get them for five bucks!?

indoorff

My Feedback: (2)

oh of course, you said nimh didnt you, my mistake. Yes, nimh are quite good. a 2 cell lipoly 140-150 (kokam cant seem to decide exactly, you see both) will weigh about 8g without connector, less than 1/3 ounce. My nicad pack weighs 30g, and is only 110mah. It is high enough capacity for plenty of flights despite its fairly small capacity.

The place I got my Lipoly cells for about 5 bucks seems to no longer carry them, you can get the cells now for 6.50 from http://www.bsdmicrorc.com/productDet...23&catID=10001

Chris

indoorff

My Feedback: (2)

see, for a receiver battery application, you only need 4.8V, and the liploy cells are 3.7 (~4.2 fully charged) - you cant quite do it with one cell, and two is much more than you need. Hence, there is not much of a weight saving when you compare it to a NiMH battery for receiver application. There are some low voltage receivers that can run off 1 lipoly cell - that is where you can get a huge advantage.

for power applications in an electric craft, this is where LiPolys REALLY shine because two LiPoly cells are much lighter than the 7-8 nimh cells needed to equal their voltage.

Chris

BMatthews

Efish, just for your info you can't use a resistor to do the job. The widely varying current makes this idea a non starter. The current variations would make the voltage across the resistor vary so at one point the Rx would be seeing it's proper 4.8 but the next, when a couple of servos move, the rise in current would "starve" the Rx and servos for voltage. The only way to do it is with a regulator as we are seeing above.

And I second the need for the low dropout voltage option. The regular regulators won't work with the 2 cells over the entire voltage discharge curve and the output will tend to drop off. The low dropout ones require far less voltage "overhead" so they will provide proper regulation with the 2 cells. The dropout voltage is the required minimum input to output difference. For the 78xx series this is around 2 to 2.2 volts. For the typical low dropout regulators this is down around .5'ish volts.

efish

My Feedback: (5)

Thanks for the info guys. Will go to the components shop and look out for the LDOs later this week. Will I need capacitors? If so, any suggestions on size?

PT, as it happens, one of the reasons I use LiPos (apart from the fact that I have them around already) is because they're smaller than NiMHs. Coincidentally, I was asking about the reulators to install one into one of your Blinks (sport flying version).

Here are some pics, its a 400mAH 2S Lipo batt that cost me just over about 8USD. As you can see, it slides nicely into the body of the Blink.

There's also a picture of the same battery as used on my PBF - you can see the cheap GWS ESC I'm using as a regulator. Cost me about the same as the batt.

supagloo

You can find the correct size of the capacitors on the datasheet of the regulator. Manufacturers have datasheets available online. Just try a search for the regulator type and "datasheet".

10uF (10V or more) between input and gnd and 100uF (6V or more) between output and gnd will work fine in this application.
If you are using 3 servos in a small plane most of the time you don't need a heathsink for the regular (bolt-on-to-the-heathsink-) types of regulators.

Andrew

Here's a [link=http://www.jameco.com/wcsstore/Jameco/Products/ProdDS/192313.pdf]datasheet[/link] on the LM1086 series.

supagloo

Just found the specs of the regulator I use. It is the LM2940. It was the combination of the low drop and the low voltage behavior that made me decide to choose this one.

As the battery voltage (input) drops under 5,5V the output will be the input voltage lowered by 0,5V. The RC gear is still working although a LiPo battery of 2 cells is considered to be fully discharged. This might save a plane one day.
I hope the picture I stole from the data sheet will explain.


The datasheet can be found here: http://www.national.com/ds/LM/LM2940.pdf#page=1




I heard that the same type is used in ESC s as a BEC feature.

BMatthews

Yes supagloo, that's right. The lipos will likely be toast from the excess discharge but the plane will be saved.

supagloo

I agree with that!
But...... I experienced that LiPos survive this kind of disabuse. I forgot to disconnect the battery a few times.

......After that you have to tell your charger that there is only 1 cell connected until the voltage is high enough to be recognized as a 2 cell battery. Now it can be charged as you are used to. (KOKAM had this procedure on their internet site 2 years ago)
I discharged (5xC) and charged the battery 2 times to check for capacity loss and couldn't find any difference......

I guess you shouldn'd do this too often but I had no problems so far.

efish

My Feedback: (5)

Hi supagloo, thanks for the info! Not just on the regulator, but also on the LiPo - that's interesting. I have a 3S cell pack home that doesn't seem to work anymore, probably from over discharging it. Will try it your way and see if it can be brought back to life!

efish

My Feedback: (5)

Hi guys, this project kinda stalled for me for a while. Finally managed to get hold of the LM2940.

Hooked it up and tested the output - with no load on, it does seem to vary quite significantly - anywhere from 5.2v to 6.8v.

Will this damage my receiver/servos? should I be thinking about putting in a capacitor or something?

Another question would be: how many servos (and the receiver) can I expect to drive with the 1A capacity? Is 5 (on a throttled flaperon setup) ok?

Any help/guidance/suggestions would be greatly appreciated.

Thanks!

supagloo

Do you use any capacitors?
It is possible that the regulator is oscillating without a capacitor on the output. You have to put this one on at least!


In some situations the input capacitor can be omitted (look at the data sheet).

The maximum current that is drawn by the RX and servos can be found by stalling one servo (and measuring the current of course)Multiply the current you find by the number of servos you use. This is an indication of the maximum current the system will use if all servos are delivering maximum power. In regular use this is not likely to happen.

efish

My Feedback: (5)

Thanks Supagloo. Please bear with my ignorance, but I'm pretty lost when it comes to electronics. [:-]

Just relooked at the datasheet - I see now that there is a Cout indicated; that must be the output capacitor? 22mjufarads huh? Any advice on whether to increase or decrease for RC purposes?

Also, do I need the input capacitor? I know the little note says that this is "Required if regulator is located far from power supply filter.", but what does that mean in an RC plane? I'm planning to hook up the battery, through a switch to the IC, total wire length is probably about 4-5 inches?

Thanks so much.

supagloo

My thought was: safety first! For weight reasons I would like to get rid of the input capacitor (C1) but I also like the electronics to be stable. So I put a capacitor on the input as well.
The job of the input capacitor is to block high frequency components that are in the input voltage. In my opinion you don't want these in the low-frequency part of your RX equipment.



This is the battery and the receiver I currently use. The regulator set up is the item below the receiver.

efish

My Feedback: (5)

Hi Supagloo, thanks. Safety is of paramount importance for me too. Apart from the cost of the plane, I'd really hate for one of my screaming creations to come crashing down on someone for any reason. Will put capacitors on both the input and the output ciruits.

Really wished I had paid a little more attention during those long high school physics lessons. Even remember telling myself that I would never need to memorise the symbol for a capacitor.... how wrong I was!

supagloo

Nobody is perfect. Sometimes I don't memorise my mother in law