2 cell lithium and 7805 regulator [Archive]

Bill Mixon

Aug 21, 2005, 02:18 PM

I have read about people being concerned with going too low on the voltage on a 2s pack. Really I don't see how it's possible to go below 7v. It starts at 8.4v and with the minimal load from a couple of servos I'm not seeing much below 8v. I guess if you were using cells at their max discharge rating there would be some possible issue.
Also I have tested this arrangement with a 6v battery which droped to 5.7v under load and it still worked just fine at 5.7v input.
Could you elaborate on this?

I know better regulators are available, but I have these and would like to use them.

Ric Duley

Aug 21, 2005, 03:05 PM

Also I have tested this arrangement with a 6v battery which droped to 5.7v under load and it still worked just fine at 5.7v input.You can be assured that your output voltage was not at 5V if the input was at 5.7v under load.
A 7805 will start to go out of regulation at 7.5 volts and below.

Whether it will work "just fine" is highly subjective - what works on the bench may not work at all in the air. A lot of that is dependant on what receiver you're using - some operate at lower voltages, some don't.
You can rest assured that servo response will be much slower at lower voltages.

You're not flying your bench - you're flying a plane. Is your plane/radio worth the small cost of buying a higher quality regulator? Or is it worth the risk just because you already have them?

Only you can decide that. I wouldn't fly anything with them.

BTW, I'd use the input & output bypass caps. They can help prevent the regulator from breaking into oscillation - an unlikely occurance in this application but a cheap safety net. Then again, I prefer to design conservatively.

Dan Baldwin

Aug 21, 2005, 03:37 PM

The spec sheet (http://www.fairchildsemi.com/ds/LM/LM7805.pdf) for an LM7805 shows that the drop out is 2 volts at 1 amp, so if you want to be sure that you have 5 volts available at all times, you have to make sure that you don't let your battery pack go below 7 volts. If you're not going to use very much of the capacity of the pack, and you know that the voltage will never go below 7 volts, then it should be fine. As mentioned by Ric Duley and Mr. RC-CAM, the MFG suggests putting bypass capacitors on the input and output of the regulator.

Dan

Bill Mixon

Aug 21, 2005, 04:43 PM

You can be assured that your output voltage was not at 5V if the input was at 5.7v under load.
A 7805 will start to go out of regulation at 7.5 volts and below.
Yes, I checked it and it was around 4.3 volts. No, I would not use it at that low of a voltage, but it didn't stop working (0 volts).
You're not flying your bench - you're flying a plane. Is your plane/radio worth the small cost of buying a higher quality regulator? Or is it worth the risk just because you already have them?
Bench testing can be far more brutal on the equip than flying ever could be.
It's not really worth it to buy a order a better regulator when I could just as easily and for only a few bucks more buy one of the switching regulators ready to use like the one from Dimension.
Only problem with it is the fact that it won't fit in the DLG that I want to convert to lipo RX battery so it would have to be mounted outside the pod.
I guess I'll toss this idea and keep using the 4 cell 100 mah nimh pack.

And if I wanted to avoid risk I would never put anything between a battery and a RX. All regulators can fail. I have seen the factory built for rc regulators fail which is a bit scary.
This leads me to a couple more questions.
What exactly does dropout voltage mean? How do you calculate it?
Who makes a ready to use regulator capable of handling a few amps that has a low failure rate, that doesn't weigh a whole lot? (this is for a large electric pulling 5 digitals)

thanks

AndyOne

Aug 21, 2005, 07:37 PM

Bill,

I would take note of what MR RC-Cam says on this subject. These voltage regulators can be far more fickle than you may imagine.

Drop out means exactly what it says, the regulator's output drops out to zero below a certain input voltage. Relying on it to do anything that the datasheet doesn't say it will puts you on very unsafe ground as it may suddenly stop when the temperature changes a bit or you get a transmitter near it or some other disturbance.

If you want to power 5 digital servos then to be safe you will need a regulator that can handle more than 1 Amp which is all the 78 series regulators in a TO220 package can handle.

I saw mentioned on this forum somewhere, a device which I think was called Uber BEC and I think it handles up to 3-5A, this may be more appropriate for your application.

Andy.

Bill Mixon

Aug 21, 2005, 08:06 PM

Bill,

Drop out means exactly what it says, the regulator's output drops out to zero below a certain input voltage. Relying on it to do anything that the datasheet doesn't say it will puts you on very unsafe ground as it may suddenly stop when the temperature changes a bit or you get a transmitter near it or some other disturbance.
Andy.
I trying my best to understand, but I still don't know what drop out voltage means. The dropout of the 7805 is listed at 2 correct? Does that mean that the input voltage can be a minimum of 2v? Clearly that isn't the case as everyone says you can't go below 7v. Yet it doesn't drop to 0 when in the mid 5v range input. :confused:
There has to be something I'm missing regarding dropout voltage.

What would I be doing that is beyond limit on the datasheet?
So it's possible for the regulator to totaly quit working with a change in weather or possitioning of other rc equip.? :eek: :confused:

If you want to power 5 digital servos then to be safe you will need a regulator that can handle more than 1 Amp which is all the 78 series regulators in a TO220 package can handle. Yes, that's another application, and as I mentioned needs to handle a few amps.
The BEC's (uber bec Ubec, etc) are confusing as they are made to work from the same battery that is powering the motor. I would use a separate 900 mah 2 cell battery for the RX.
I have been looking at the duralite and Jaccio for that application, but I don't know if I should trust them. I have seen them fail for unknown reasons. I guess something as simple as a temperature change could take them out as well.

Mr.RC-CAM

Aug 21, 2005, 10:38 PM

There has to be something I'm missing regarding dropout voltage.
Drop out is the highest Vin-Vout that still allows the IC Vreg to maintain regulation. With the 7805, the min input voltage is 7V.

Unlike LDO VRegs, which are designed for battery apps, the LM7805's behavior is not predictable once the dropout spec is breached. Some will continue to pass current (not at full spec though) and others will turn into brain dead paper weights. Some are in between. The exact behavior will depend on the mfg (the 7805 has been cloned by dozens of silicon vendors) and wafer lot variabilities.

Despite the reports of success, using the LM7085 on two Lipo cells is really not worth the risk (the voltage under full servo load can easily drop below 7V during the useful discharge curve of pack). Especially since there are IC Vreg's designed from the ground up to work decently in a battery sourced application. The LM2940T-5.0 is an LDO that as old as the hills and is just one of hundreds out there that would be better than the 7805.

Dan Baldwin

Aug 22, 2005, 12:41 AM

Drop out is the highest Vin-Vout that still allows the IC Vreg to maintain regulation. With the 7805, the min input voltage is 7V.

Well, close. I know that Mr.RC-CAM knows what it is, I just don't think he phrased it right. Drop out is the LOWEST Vin-Vout that still allows the IC Vreg to maintain regulation. When the input voltage drops below Vout (5 volts in this case) plus V dropout (2 volts app at 1 amp) the regulator drops out of regulation, and all the regs I've seen will do their best to follow the output voltage as closely as they can as your setup did. Never seen one "drop to zero". I believe that the 2 volts shown in the data sheet is worst case, so typically it will be less.

Dan

Dan Baldwin

Aug 22, 2005, 12:47 AM

The BEC's (uber bec Ubec, etc) are confusing as they are made to work from the same battery that is powering the motor. I would use a separate 900 mah 2 cell battery for the RX.
I have been looking at the duralite and Jaccio for that application, but I don't know if I should trust them. I have seen them fail for unknown reasons. I guess something as simple as a temperature change could take them out as well.

Ubecs don't have to be used from the same battery that is running the motor. They should work fine from your separate 900 MAH 2 cell pack. I don't have any idea how reliable they are.

Dan

HELModels

Aug 22, 2005, 01:04 AM

This is an intersting discussion on regulators. I have a related question as I have considered building a DLG and would like to use existing 2 cell lipos. Is it safe to use a very small speed controller as a BEC only - no motor hooked up?
I've done this before, but wondered if the open on the motor could cause some kind of overload. When I did it, it didnt seem to get warmer than usual.

Mr.RC-CAM

Aug 22, 2005, 02:21 AM

...and all the regs I've seen will do their best to follow the output voltage as closely as they can as your setup did. Sadly, they don't always do that. I've seen a huge assortment of behavoirs, including LM7805's that barely pass any current at all when the voltage goes too far below the recommended min. That is why I am not a fan of using the 7805 on low voltage battery operated applications.

Bill Mixon

Aug 22, 2005, 12:45 PM

This is an intersting discussion on regulators. I have a related question as I have considered building a DLG and would like to use existing 2 cell lipos. Is it safe to use a very small speed controller as a BEC only - no motor hooked up?
I'm thinking about doing that as well. I can't see how anything could be wrong with it.

Andy W

Aug 22, 2005, 01:19 PM

That would work fine. Some Aveox ESC's had the BEC on a seperate board. Most of mine were removed for high power applications, so I have some spares.. :)
..a

raymcm

Aug 22, 2005, 01:21 PM

The low dropout regs only cost a few pennies more than a 7805 so why argue over it !

Tristar500

Sep 01, 2005, 10:48 PM

Bench testing can be far more brutal on the equip than flying ever could be.
It's not really worth it to buy a order a better regulator when I could just as easily and for only a few bucks more buy one of the switching regulators ready to use like the one from Dimension.

If you are flying really lightweight stuff you might go with one of these cheap switching regulators. My concern is the folks who are using then on big models where the risk to others on the ground is a factor. Personally, I wouldn't put a Dimension regulator on anything bigger then a Slow Stick from my past experience with their products. Fortunately the failure we had was caught on the ground before launch and the aircraft and persons on the ground we safe. The unit did toast a expensive GPS unit, but hey like I said nobody got hurt.

If you are just powering your receiver and no motor, How about some of those tiny lithium button cells? They are reliable as hell and can't cost that much?

Acetronics

Sep 02, 2005, 05:26 AM

Hi, Tristar

I totally agree ... Current and temp limitation is also, most of time a double edge sword.
Note motors include servo motors ...for button cells ( High internal impedance )!!!

And I add that : Why take aboard electrons just to heat "mocking" birds ????

Basic question ... always forgotten.

Alain

Bill Mixon

Sep 03, 2005, 12:58 AM

Ok,
I now have a little better understanding of dropout voltage.
I have decided to use up my last nimh pack on the glider, and later will be using a 2s connected to one of my small esc's for brushed motors. I have the esc, it isn't being used, and is far smaller that any regulators I have seen so it should work out well.

Now for larger projects what are my options?
Could I parallel two or three of the TO220 case regs such as the LM2940T to get the amperage I would need without any problems?
Or would I need to look at a different size IC?
What manufacturer?
Also where is a good reputable place to buy from that doesn't have a high minimum order requirement?

Thanks

Ric Duley

Sep 03, 2005, 01:19 AM

Also where is a good reputable place to buy from that doesn't have a high minimum order requirement?I've bought parts from Mouser Electronics (http://www.mouser.com/index.cfm?&handler=data.listcategory&D=*LM2940T*&terms=LM2940T&Ntt=*LM2940T*&Dk=1&Ns=SField&N=0&crc=true) many times and been very happy with their service and quality of the parts. Waaay better than the junk that Radio Shack sells.

Mouser has a great on-line catalog and will send you their HUGE hard copy catalog free. :D

Acetronics

Sep 03, 2005, 03:19 AM

Hi, Bill

Parallelling regs is not so easy ... have a look to the Datasheets ( National Semiconductor or Micrel Semi. offer good design guiding )

Alain

AndyOne

Sep 03, 2005, 09:41 AM

There's nothing in the National Semi datasheet that I can see about boosting the output current of the LM2940 but it suggests that it will withstand peaks of just over 2A.

Andy.

Acetronics

Sep 03, 2005, 09:50 AM

Hi, Andy

Look at the LM 317/ 350 or 338 applications ...

or Micrel Design manual ...

Once more, the limit is given by power dissipation ....not max current !!!

Alain

Bill Mixon

Sep 03, 2005, 02:49 PM

almtmd

Sep 03, 2005, 08:33 PM

You can use cheap ESC to power your RX and servos with 2S LIPO.

Thanks

dj_spray

Sep 08, 2005, 11:55 AM

if you use a cheap ESC, will it be good enough for the Lipos (in terms of cutoff voltage "reliability"?). Also, will these small ESCs not cut at a low Amp also (limit to 4 servos only).

I would also be interested in putting lipos in my gliders (6servos and more, pretty big scales) for the weight gain but also mostly for being able to put a very high capacity (like 4amps for the weight of a 2amps pack).
any recommendations?

almtmd

Sep 11, 2005, 08:54 AM

Flown 30 size nitro plane with ESC to power Rx and 4 normal size servos with 2S LIPO.
It was a cheap plane for fun fly.
For your case 6 servos and more too much for single ESC.
Suggest you use 2 ESC and isolate the BEC ouput into two.

Thanks

pmackenzie

Sep 11, 2005, 09:32 AM

Thanks for the link, Ric
If I wanted a different voltage how could I determine which of the LDO IC's is suitable? For example that particular reg seems to be available in 5v,8v but not 6v. So if I wanted a 6v regulator what would I need to look for. Maybe a better question is how do I interpret the number of a regulator.

I'm a bit lost now about using these in parallel. The unit can handle 1 amp, so why would it not work to parallel 3 of them together to get 3 amp capability?
For 6 volts use an adjustable LDO regulator. You only need two more resistors to set the voltage.
The LM1084 (http://www.national.com/ds/LM/LM1084.pdf) has a current limit of 5A and a power dissipation of 30 watts if you keep the case below 70C. Digikey sells them.
If you want to use it with higher input voltages put two in series with the first one dropping half of the voltage drop.
This will split the heat up between the two devices.
Total heat to dissipate will be the same so you will might still need a heat sink.
Pat MacKenzie

Bill Mixon

Sep 13, 2005, 02:43 AM

Now that looks promising,
It looks as if I would only need the LM1084, a couple of capacitors, heat sink, and a couple of resistors to get 6v. Say a 6.8k and a 1.8k?

Dan Baldwin

Sep 13, 2005, 08:45 AM

Yup. That should work fine. One word of warning; although it's called a low drop out regulator, it has a dropout of about 1 volt at 1 amp, and 1.5 volts at 5 amps, so you still need to take the dropout into account.

Dan

Bill Mixon

Sep 13, 2005, 12:50 PM

Yes, I noticed that.
Going to 6v puts me back in a bad situation if the pack voltage drops too low (less than 7.5-7.0volts).
It looks risky to try getting more than 5.0-5.5v regulated from a 2 cell pack. I thought about going to 3 cells for higher Vout, but then you are more limited in amperage due to the high voltage (watts).

thanhTran

Sep 16, 2005, 06:50 PM

...Personally, I wouldn't put a Dimension regulator on anything bigger then a Slow Stick from my past experience with their products. Fortunately the failure we had was caught on the ground before launch and the aircraft and persons on the ground we safe. The unit did toast a expensive GPS unit, but hey like I said nobody got hurt.

...
What kind of problem did you get? is it a design issue or is it just a defect problem? I'm using a Dimention Engineering switching regulator in my electric helicopter (TREX), and I want to minize any electronics problem. I'm wondering if you have any info/hint that I can take a look at.

Thanks

thanh

Tristar500

Sep 17, 2005, 03:10 PM

What kind of problem did you get? is it a design issue or is it just a defect problem? I'm using a Dimention Engineering switching regulator in my electric helicopter (TREX), and I want to minize any electronics problem. I'm wondering if you have any info/hint that I can take a look at.

Thanks

thanh

The best way to mimimise any possible problems would be to use a BEC that is as simple as possible. (fewest parts).

I never got a satisfactory answer from the Dimension guys. They put the blame on me for looking the adjustment screw. ( a common practice to be sure it doens't move due to vibration) They they sugested I return the burnt up GPS to Garmin and blame it on them.., Long story short, We lost faith in the company and their products. I don't use them in anything I fly.

Just my $.02

Do illiterate people get the full effect of Alphabet soup? :)