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1  Using Arduino / General Electronics / Re: Help selecting battery please? on: April 23, 2014, 03:38:49 pm
On the basis that you use a switching regulator, the 12v 4AH battery will provide a longer duration than the 6v 4AH battery.   

This is because the switching regulator "transfers" watts rather than a linear regulator which simply "transfers" volts.  Note that the word "transfers" is a simple metaphor to illustrate the point.

The 6v battery stores 24WH of energy and the 12v battery stores 48WH  (volts x AH)

Battery capacity is usually related to a discharge period of 10 hours, so your 4AH battery discharged at C/10 should deliver 0.4 amps for a period of 10 hours.   If you reduce the discharge rate to, say C/20 you could expect to get around an extra 20% of capacity.  This 20% gain factor tends to repeat for each doubling of the C factor. 

Your quoted discharge of 200mW out of an 80% efficient regulator implies an input wattage of 200 / 0.8 =  250mW.  For a 12 volt battery, this is a load current of around 0.02 amps.  Compared to the rated battery C/10 current of 0.4 amps, this implies the battery load is C/(10 x .4 / .02) = C/200 which is very conservative.   At such a low C loading you might expect the battery capacity to rise substantially, based on 4AH x 1.2 (C/10 to C20) x 1.2 (C/20 to C/40) x 1.2 (C40 to C80) x 1.2 (C80 to C160).  That all works out at around 8AH.

That's the good news, now the bad :

If you are using lead-acid type batteries, these shouldn't be discharged lower than 50% capacity and ideally not below 80%.  Depth of discharge defines battery life.  Deep discharging reduces battery life drastically.   So, assuming you limit depth of discharge to 80%  or 1.6AH and assuming a load current of .02 amps, your battery duration will be 1.6 / 0.02 = 80 hours.
2  Using Arduino / General Electronics / Re: Wire reverse polarity indicator led to handle voltage above breakdown? on: April 23, 2014, 09:52:13 am
Perhaps because 'clever' people always try and make thing complicated.
The golden phrase is KIS   Keep It Simple
3  Using Arduino / General Electronics / Re: Wire reverse polarity indicator led to handle voltage above breakdown? on: April 23, 2014, 09:47:38 am
Nope, the reverse voltage across the non-conducting LED is Vf of the conducting LED  
Vin-Vf is the voltage dropped across the resistor.
4  Using Arduino / General Electronics / Re: how to work out a capacity of a battery on: April 23, 2014, 09:45:11 am
For all you know there could be a lump of lead inside your battery (for feel-good weight) and a low capacity battery to give an output.  One way to determine capacity is to fully charge it then conduct a controlled discharge, measuring current and voltage at specific periods of time.  From that data you draw a curve of watts versus time then totalise your readings to get watt-hours, divide the results by average voltage to establish a rough idea of AH capacity.   It would be even better if you could discharge at constant current but that is a little more complicated to do.
5  Using Arduino / General Electronics / Re: Wire reverse polarity indicator led to handle voltage above breakdown? on: April 23, 2014, 08:23:53 am
Why not use a single resistor connected to the two LEDs wired back-to-back with each other.  Whichever LED is correctly biased will illuminate and limit the reverse voltage across the other LED to around 2 to 3 volts.
6  Using Arduino / General Electronics / Re: Help selecting battery please? on: April 23, 2014, 07:43:23 am
Basically yes (or maybe)

A switched mode regulator will be around 80 to 90% efficient, but that depends to some extent on the drive voltage.  Input current depends upon input voltage whilst output current depends upon the specific load demand.  Best to look at output wattage to determine input wattage.

Linear regulators are much less efficient.  For example if you only want 3 volts and your battery is 12 volts the maximum efficiency will be 25% since 9 volts are being "lost" across the regulator and the input current is slightly greater than the output current.

If possible you'd be better to select a 3volt battery to drive the laser device directly
7  Using Arduino / General Electronics / Re: Help selecting battery please? on: April 23, 2014, 05:19:46 am
Basically that's correct.  If you use the battery until it's dead then you have indeed shortened its life.  the exception to this is NiCad which will tolerate running completely flat.
8  Using Arduino / General Electronics / Re: Help selecting battery please? on: April 22, 2014, 02:50:13 pm
If you are concerned about 4.85 volts from a 6 volt battery not being enough then you need to reconsider your understanding about battery abuse.  At that voltage the battery is well on its way to that great battery store in the sky.

What current does the laser draw and what capacity of battery are you using.  In other words battery duration is based upon AH capacity and load current.

I take it you appreciate that a 12V 5AH battery will last no longer than a 6V 5AH battery in your application.
9  Using Arduino / General Electronics / Re: Basic question: Arduino wires on: April 18, 2014, 04:05:07 pm
I think unobtanium is often used in electro-magnetic systems
10  Using Arduino / General Electronics / Re: can I use duct tape on this project? on: April 18, 2014, 02:33:39 am
Mike, if you've never used the aluminium type you don't know what you're missing.  Aluminium foil (no fabric) with an adhesive backing, extremely thin.  It's wonderful stuff for sealing all sorts of gaps.  It moulds in well and sticks like **** on a blanket, and it's relatively heatproof.
http://www.tlc-direct.co.uk/Products/TLAT50.html?source=msn&kw=16327816894
11  Using Arduino / General Electronics / Re: what resister values to use with a transister NOT gate? on: April 17, 2014, 03:51:40 pm
You need a collector resistor to control the current through the transistor.  Do not run it at 100% (20ma) so suggest you use a 1k in the collector circuit.  The second resistor is to control base current.  I suggest you use a 10k in that location.  This should feed enough base current to ensure full saturation of the transistor (ie turn it fully on).
12  Using Arduino / Motors, Mechanics, and Power / Re: It's faster a motor or a solenoid? on: April 16, 2014, 03:52:17 pm
A loudspeaker is effectively an "inside out"  solenoid and can oscillate at around 20kHz
A DC electric motor can run in excess of 20,000RPM
They can both do 20,000 "somethings" but there ends any comparison.
One is linear motion and the other is rotary motion, so what exactly does your question relate to.
13  Using Arduino / General Electronics / Re: valley peak detector on: April 15, 2014, 01:37:01 pm
A zener diode won't do as it'll fail to conduct until the zener point is reached.  You can use any old diode providing you rig it up as a "precision rectifier".  see the following circuit  http://en.wikipedia.org/wiki/Precision_rectifier  which also shows it used as a peak detector, further down the page.
14  Using Arduino / General Electronics / Re: valley peak detector on: April 15, 2014, 02:31:40 am
What do you mean by "valleys"  Do you mean the maximum negative peak or the lowest voltage that the "rectified" signal fall to.
15  Using Arduino / Motors, Mechanics, and Power / Re: Failed to drive a CD Rom sled motor via Hall effect sensor. on: April 14, 2014, 12:24:23 pm
and then rotate it because it's also back to front
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