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Author Topic: Measuring current, 12v, 5v, 7805, Arduino  (Read 1760 times)
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I'm building a battery powered RGB LED display (for a bike).  

It consists of 4 strands of 12v RGB LEDs, a 7805 voltage regulator an atmega328 running the UNO bootloader and some transistors.

It currently drains 8 AA's (1.5v*8=12v) in about 2 hours and I want it to last at least 8, or ideally 12 hours.  

Each alkaline AA should be 2700mah or 21 Ah for the 8 ones.

So, now I'm learning about power.  I've never tried measuring power and so I think I'm doing something wrong.

I placed my multimeter between the battery and my circuit board and with the lights plugged in I'm getting a .6 reading when the meter is set to 10A and a 2.8 reading when it's set to 20m.  Does that mean that I should multiply .6 * 10A to get 6 Amps or is at .6 A or 600 mAh.  

.600 A makes some sense to me since the light should use about 115 mA per color per meter and I have about four meters and one color is on at a time (on average).  If that were true then I my lights should last for days instead of hours.

To move things further along when I measured without my lights the circuit (the 7805, 16 mhz atmega and a bunch of transistors switching nothing frenetically) is using .25 * 10A).  I thought that an arduino should use something like 25 ma while it's running so I'm off by a factor of 10 or 100 depending on how my meter is reading.

That leads me to think that I should remove the 7805 and give the Arduino it's own power source.  Is that recommended for minimizing battery drain.

Summary:
* 4 meters of 12 RGB LED strip @ 114 mA / meter (http://tinyurl.com/12vrgbled)
* 16 mhz 'breadboard arduino' (on a protoboard)
* 7805 giving the arduino 5v (and the bypass to get the lights 12v)
* 8 AA batteries

Power reading
@ 10A .6 with lights, .25 without lights
@ 20m 2.8 with lights 1.15 w/o lights

Battery life
* 1.5 -> 2 hours with 8 alkaline AA's

Questions:
1. How do I read my ammeter (is .6 @ 10A setting .6 amps or 6 amps)
2. How long would you predict the circuit would last?
3. Should I avoid the 7805 and power the arduino from a separate power source
4. How much would I gain if I did that.

Thanks again, your struggling friend,

Zeke


« Last Edit: August 16, 2011, 11:28:45 am by zekekoch » Logged

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1. How do I read my ammeter (is .6 @ 10A setting .6 amps or 6 amps)

Digital meters read the current directly, so a reading of 0.6 on the display means 0.6Amps. The 10Amp range means that the maximum current that can be measured is 10Amps, and will read out as 10.0 on the display.  What other current ranges do you have? Is there a 1Amp range? Or is the next lower range 200mA or something like that?
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That's what's confusing me.  When the multimeter is set at 20ma I'm getting a reading of "2.8", but when I set it at 10A I get ".6".

Maybe my multimeter is broken...  I'm using a GB-220 (Gardner Bender)
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When the meter is set to 20mA mode, surely the current drawn by your circuit will be way out of range?  So is the meter just trying to indicate that?

Also, when you use the 20mA range, does the circuit work properly?  And are you connecting the red meter probe lead to the correct terminal for the current ranges?
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When the multimeter is set at 20ma I'm getting a reading of "2.8", but when I set it at 10A I get ".6".
Back when I was in school, I remember my teacher telling me "do part of the math in your head, to check your work" while using a calculator.  Just because the calculator says an answer, doesn't mean it is correct.   (Of course, I was always the kid who said the answer is "ERROR" or "5318008.")

Here's a good example of checking your work.

As explained, the range selections on your Multimeter represent the maximum that range can measure.  You already know approximately what your LEDs draw (~100mA ea) and approximately what the Arduino draws (~25mA).  So how meaningful is it when you use the maximum 20mA scale range and you get a value of 2.8mA, knowing your circuit draws a minimum of 25mA?

Quote
Maybe my multimeter is broken...
Again, check your work.  Measure the current running through just resistors.  A 1kOhm resistor connected to 5V should draw how much?  100ohm?  Put some in parallel to increase the current flow so you can test the 10A range.

Quote
Each alkaline AA should be 2700mah or 21 Ah for the 8 ones.
How are your batteries configured?  I'm guessing in series to get up to 12V.  Batteries in series have the same current capacity as a single cell.  (Batteries in parallel give you more current capacity.)

Assuming your batteries are in series,
The simple formula of 2700ma-h / 600ma = 4.5 hours. 

However, 2700mA is estimated by the battery manufacturer at low current draw conditions.  600mA probably isn't considered a low-current draw situation so 4.5hours is the absolute best you could get with the real life being much much l.ess
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Maybe my multimeter is broken...  I'm using a GB-220 (Gardner Bender)

Making proper current reading with a multimeter is very specific to the meter being used. Most use a different measurement lead plug-in for the postive meter lead when doing current readings. Also some have both a high current plug position and a low current plug position depending on what current range you have selected. Also many meters have internal fuse protection on the current plug-ins that will blow open if you improperly try to measure a current but instead probe across a voltage source. I tried to search for your meter model at the manufacture site but no mention or manual shown for that model.

Lefty

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Hi,

I am working on project in which i have use 7805 to supply my arduino board,

but my input supply is 12V and 10-12 amp,

does this burn my arduino?

can anyone tell me is this safe or not,

and my second question is how to convert 12v 10amp supply  to 12v 2amp supply.
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Hi,

I am working on project in which i have use 7805 to supply my arduino board,

but my input supply is 12V and 10-12 amp,

does this burn my arduino?

can anyone tell me is this safe or not,

and my second question is how to convert 12v 10amp supply  to 12v 2amp supply.

posting the same question twice in separate threads is not smart
hijacking someone else's thread is not smart
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them that understands binary
and them that doesn't

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