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61  Using Arduino / General Electronics / Re: 24VAC - 5VDC Power Supply Design on: April 25, 2014, 07:40:13 am
The purpose of the smoothing capacitors is to "fill in" the missing portions and so eliminate ripple.  With a "perfect" capacitor and no load on the circuit the smoothing capacitor would completely eliminate the ripple and the output voltage would sit at the peak value.  In the case of your 28vac system this would be (28x1.414) - 1.4 = 39.6 - 1.4 = 38.2v  (the 1.4 being the voltage loss across the bridge diode series pair)

You do not require the 4004 for your stated purpose since there is no way the completed circuit can "see" a reverse polarity on the input to the regulator
62  Using Arduino / General Electronics / Re: 24VAC - 5VDC Power Supply Design on: April 25, 2014, 06:46:01 am
The peak voltage from your 24vAC is 34 volts which is just about the limit for your intended regulator unit.  I suggest you measure the off-load 24vAC so ensure it doesn't actually exceed 24 otherwise you might seriously shorten the life of the regulator module.

The transformer is rated to 30VA and I suspect the off-load voltage may well be around 30 volts which will take the rectified output to 42 volts - well in excess of the module rating.

That being the case, might I suggest going for half-wave rectification and a suitable smoothing capacitor, say 200 microfarad  or there abouts.  That will drastically reduce the rectified voltage and your regulator module will be much happier.
63  Using Arduino / General Electronics / Re: 24VAC - 5VDC Power Supply Design on: April 24, 2014, 01:14:07 pm
You MUST lose the resistor divider, whatever type of regulator you use.
Assuming your load is 200mA and you are using a linear regulator.
Current into regulator equals at least 200mA.  This current must come via the 1k7 resistor and at 200mA the voltage drop across the resistor will be I x R = 0.2 x 1k7  = 340 volts !!!   I think not.
Even without the resistor the regulator will be dissipating a fair amount of heat.  Say output of your bridge is around 30v, the regulator voltage drop will be 25v and heat dissipation will be V x I = 25 x 0.2 = 5 watts
Using a switch-mode regulator will reduce the input current and heat dissipation.
64  Using Arduino / General Electronics / Re: Analog voltmeter vs ampmeter on: April 24, 2014, 07:25:43 am
It depends on what you want to measure and how much work you want to do, you mentioned neither in your enquiry.  
All meters are ammeters (it's the current in the coil that causes meter deflection).  
The voltmeter will already have a series resistor built into it such that it takes 5 volts to cause full scale deflection.  
The 1mA ammeter is the best option if you want to measure current flow as its self-resistance will be relatively low and hence have minimal effect on circuit current flow.  
If you wish to measure more than 1mA using the current meter you will need a suitable shunt resistor and if you wish to measure voltage using the same meter you will need a suitable series resistor.
65  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 no more than 20% of capacity rating.  Depth of discharge defines battery life.  Deep discharging reduces battery life drastically.   So, assuming you limit consumption to no more than 20% of capacity (1.6AH  based on the increase to 8AH above) and assuming a load current of .02 amps, your battery duration will be 1.6 / 0.02 = 80 hours.

Edit : Last paragraph edited to clarify discharge limit
66  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
67  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.
68  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.
69  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.
70  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
71  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.
72  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.
73  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
74  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
75  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).
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