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Topic: Regulator gets hot with 7.4v and 11.1v li-po (Read 2 times) previous topic - next topic

Piton

I have an arduino mega 1280 and i have an 8 segment led attacched, and an LCD screen.

If i plug the USB power all works fine without heathing or something...

If i plug the 7.4 on the power plug, or the VIN the regulator just start to get hotter and hotter, till i cant even put my fingher on it. If i plugh the 11.1 it gets instant-burning!

How can i solve this? Why the usb ( i think is 12v ) is not having this same behaviour?

danb35

USB isn't 12v, it's 5v.  The regulator is getting hot because it's dissipating quite a bit of power as heat.

DuaneB

8 segment - do you mean 8*8 = 64 individual LEDs ? if so how much current are you passing through them ?

Duane B

rcarduino.blogspot.com

Piton


doughboy

search ebay for dc-dc buck converter. costs under $2.

kf2qd

When connected to the USB port the onboard 5V regulator doesn't do anything as the computer is supplying the 5V.

When using an external supply through the power jack the 5V regulator is then working. As it is just a linear device (cheap and supplies a nice 5V ) it is also somewhat inefficient. As your supply voltage goes higher the device wastes some of the energy passing through it as heat. The higher the voltage, the higher the heat.  The more current you are drawing from the regulator, the higher the heat. This is normal operation for this type of regulator.

How much current are the attached devices drawing?

To reduce the amount of heat generated you can power the devices attached with a different supply ( and it will also generate some heat...), just tie the grounds together.

JoeN

#6
Sep 19, 2012, 09:19 pm Last Edit: Sep 19, 2012, 09:20 pm by JoeN Reason: 1

This is an  8 segment...

https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcTvwpptbcHLJTuY6Xw26Dlhf_zLACgBBDED2lnn8dEx7q6bh3z7cA


So how can i solve this issue? I would like to use 7.4v batteries


Though there are obviously 8 LEDs, those are almost always referred to as 7 segments.  14 and 16 segments the same way, though they are actually 15 and 17.

http://en.wikipedia.org/wiki/7_segment_display
http://en.wikipedia.org/wiki/14-segment_display
http://en.wikipedia.org/wiki/16-segment_display
http://en.wikipedia.org/wiki/8_segment_display   XD
I have only come here seeking knowledge. Things they would not teach me of in college.

Piton

If i supply 3.7v runs just fine, but i'm afraid it wont last long... i cant supply lcd's with different voltage since they have a max 5v ratings.

MarkT

If the regulator overheats it should shutdown - so I'd say you can run at 7.4V, the regulator will be hot, not ideal but unless it shuts itself down this can be tolerated.  11.1V sounds right out (it will be causing about 2 1/2 times more heating in the regulator).

If you'd like those 11.1V for some other reason then an external 5V regulator with a proper heatsink, or a little switchmode DC->DC converter are the options.  The latter is more efficient / cooler, but will put a little noise on the 5V rail in my experience (only as issue for analog sensing).
[ I won't respond to messages, use the forum please ]

jrmcferren

Remember that at a full charge these battery packs will be putting out a higher voltage. The 7.4 volt pack will be running at 8.4 on full charge and around 6.6 when fully discharged and actually not recommended as it will drop below 7. Your 11.1 volt pack will be about 12.6 volts (like a Car battery that has been sitting unloaded) at full charge and 9.9 volts at full discharge. Since the 7.4 volt pack could cause problems with the regulator dropping out and not providing the full 5.0 volts, you should go with the 11.1 volt pack, but you run into the problem of heat on the boards regulator.

The fix for this is to dissipate that heat elsewhere, but how? This again is simple, use two regulators in tandem to disipate part of the heat on board (internal regulator) and some elsewhere (external regulator). All you need to do is build the external regulator circuit (very basic you can find plans online) with a LM7808 regulator. The 7808 will drop the output of the three cell pack to 8 volts which is above the minimum for the Arduino board and the internal regulator will drop it down to 5.0 volts to power the microcontroller. There is very little efficency penalty in doing this, it simply move some of the heat away from the on-board regulator. You will also want to make sure that your power cuts off when the battery voltage drops below 9.9 (ideally 10.0) as going lower will over-discharge your battery and that is around the drop out voltage of the 7808 regulator anyway.

MarkT

Since the 7.4 volt pack could cause problems with the regulator dropping out and not providing the full 5.0 volts, you should go with the 11.1 volt pack, but you run into the problem of heat on the boards regulator.


Unless the circuit has any ADC measurements going on have the regulator drop a little below 5V on the output isn't an issue (Arduino at 16MHz will actually run as low as 3V in practice, running at 4.5V is actually within spec for most 5V chips!), and frankly it will probably still hold 5V with 6.6V going in (remember datasheet drop-out values are often worst-case across temperature range at full rated current - typical drop-out voltages may be much better).  And going for a 2S rather than 3S pack is a big saving in cost and thermal dissipation issues.  Besides its not a great idea to fully discharge LiPo packs in the first place, any imbalance in the cells and you shorten the working life.

If you can measure the drop-out (bench power supply) then my theory can be tested of course.
[ I won't respond to messages, use the forum please ]

Piton

BTW i really dont get the point of making arduinos not supporting higher voltages. 5v is too much for standard batteryes too low for li-po packs. 

I have a project with a myAVR board that i have purchased year ago, and this is working with different voltages, from 7v to 12v , you should just set the battery on the menu.
And there is no heating, no regulator theory etc.

KeithRB

And not much current draw!

Hook up the same load and you will have the same problem.

Note, that any heat you can feel is just wasted amp-hours from the battery. Using the switching convertor will make your battery last *much* longer.

zoomkat

When powering the arduino from an external power source thru the external power connector, be sure to check the actual voltage on the arduino 5v bus. I tried using an external 12v battery thru the external power connector and the arduino 5v pin voltage went to 8v+. Had to power the arduino via an 7805 chip connected to the arduino 5v pin. Don't trust the external power supply setup without checking.
Consider the daffodil. And while you're doing that, I'll be over here, looking through your stuff.   8)

MarkT


BTW i really dont get the point of making arduinos not supporting higher voltages. 5v is too much for standard batteryes too low for li-po packs. 

I have a project with a myAVR board that i have purchased year ago, and this is working with different voltages, from 7v to 12v , you should just set the battery on the menu.
And there is no heating, no regulator theory etc.


Its a BoM issue - start to add more sophisticated regulation and the price goes up.  If you want that option, you can add it yourself.  You can go and make your own board design and sell it if its popular - this is the whole point of open-source hardware.  Someone may be doing this already.  (there's the lithium backpack for instance)
[ I won't respond to messages, use the forum please ]

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