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Topic: 7805 voltage regulator help (Read 869 times) previous topic - next topic

jboncek

Hey folks,

I have a 36v battery that i need to use to power my arduino fio.  I'm trying to use a 7805 voltage regulator to bring the voltage down to an acceptable voltage for the arduino to regulate (<12).

I've tried to read up and figure this out with some success, but not without problems.

I've added the 2 capacitors, as suggested for a clean signal, and I do get a 5v output BUT as soon as I connect the BAT+ to power and BAT - to ground, the LED lights dimly and the "available voltage reads 1.8v!?!?  I don't get it!

Can anyone help or tell me what I'm missing?

Thanks in advance.

James

dhenry

You probably want to figure out how much current your load takes. 36v Vin is close to the limit and if your load takes just a little bit current, it can cause a thermal shutdown.

Your option is to use a pnp to expand the current capabilities of your 7805 (less efficient but lower ripple), or to configure the 7805 into a switching mode regulator (+ a coil + diode. more efficient but also more ripple).

Osgeld

#2
Nov 12, 2012, 01:26 am Last Edit: Nov 12, 2012, 01:29 am by Osgeld Reason: 1
7805 regulates to 5 volts, the regulator on board requires about 2 volts above 5 volts to give 5 volts. verify that the 7805 is actually putting out 5 volts and attach it to the 5v pins bypassing the regulator

even then you are still going to have problems (not even counting that 36 volts is beyond the capibilities of the part)
(vin-vout)*current = watts

(36-5) * 0.1 = 3.1 watts

50 degrees per watt * 3.1 watts + ambient temperature (lets say 27) = 182 degrees c (359f) is quite a bit higher than the absolute max of 125 just at a 100mA load
http://arduino.cc/forum/index.php?action=unread;boards=2,3,4,5,67,6,7,8,9,10,11,66,12,13,15,14,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,86,87,89,1;ALL

oric_dan

Quote
(36-5) * 0.1 = 3.1 watts


I second this. Using Vin = 36V is a really bad idea for linear v.regs like the 7805. If you tried running
the device at it's so-called 1A current rating in this case, it'd dissipate 30+ watts, and probably catch
on fire/whatever. [I've had the solder joining a TO-220 chip to the metal tab actually melt and flow
out on occasion]. Even worse for a tiny smt v.reg.

Better to toss it, and use a dc-dc converter, or interpose a dc-dc converter [eg, 9V] between the 36V
and 7805.

jwatte

As others have said, there are two problems here:

1) The Arduino has an inefficient linear regulator on it that requires about 6.5V in to generate 5V out. Thus, you should regulate to >= 6.5V, not to 5V. (7.2V, 9V, and 12V are all easy to find parts for and would work well.)

2) Linear regulators are really inefficient when regulating high voltage drops. They have to pass all the current that the load is drawing, and burn it off as heat. Switching power converters are a lot more efficient in these cases.

You can buy a suitable power converter either at distributors like Digi-Key ("dc dc power converters") or perhaps at hobby shops (where they will be called "UBECs" -- just make sure you get one that can deal with 36V input.)
Also, Pololu are selling a few, which are in the same family as the ones you'll find at Digi-Key, but I don't think any of them go as high as 36V.

Finally, you can build your own step-down ("buck") converter with a switching controller IC, and a few discrete parts; typically a diode, an inductor, a few resistors, and a capacitor or two. For an easy-to-use through-hole part, try the MC34063, which is not particularly efficient, but it can take high voltage and is robust even on breadboards.


MarkT


Hey folks,

I have a 36v battery that i need to use to power my arduino fio.  I'm trying to use a 7805 voltage regulator to bring the voltage down to an acceptable voltage for the arduino to regulate (<12).


Well that could be an expensive mistake - the absolute maximum rating for a 7805 input voltage is 35V.   You would do much better to use a DC->DC converter - far less power wasted / heat generated too.
[ I won't respond to messages, use the forum please ]

jboncek

Thanks for all the input folks.  I missed a day here cause I forgot to hit notify for replies.

1 important detail for some of the responses:  I'm using a Fio, which runs on 3.3v.  Primarily for its size and ease if use with an xbee.

But that aside, I still need a 5v power source for the rest of the system. 

From the responses, however, I would imagine the 7805 is the going into a thermal shutdown state, when current is applied.  Without load it puts out a steady 4.95vdc.

Right?

I guess I'll look into a Dc to DC converter as jwatte and dan mentions.

Thanks.

jboncek

For those who find this thread after me, I just ordered this dc-dc switching regulator. We'll see how it performs.

http://www.digikey.com/product-detail/en/R-78C5.0-1.0/945-1395-5-ND/2652148

And in truth, it's not really a 36v supply, it's 39 fully charged.

zoomkat

A UBEC like below might work.

http://www.ebay.com/itm/GX-Series-7A-9-35V-3-8S-Input-Switch-Mode-UBEC-/140704647239?pt=US_Character_Radio_Control_Toys&hash=item20c2a68847
Consider the daffodil. And while you're doing that, I'll be over here, looking through your stuff.   8)

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