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1  Using Arduino / General Electronics / Re: Metalworking Advice on: September 27, 2014, 12:33:50 pm
I'd suggest a zinc diecast box as they are less prone to distortion, are relatively easy to cut and drill and do not tend to clog drills or jewellers saws as aluminium will.  Add to that, they can come ready painted, have nicely rounded off corners and edges and most come with cast in slots to hold circuit boards in place.
2  Using Arduino / General Electronics / Re: Measuring resistance on separate circuits. on: September 20, 2014, 03:17:49 pm
Bourns (amongst others) do 100 ohm wire-wound dual pots but they will no doubt be expensive.

However there is a cheap way.

Simply get hold of a couple of 100ohm linear slider pots, mount them side by side and couple their knob levers together.
3  Using Arduino / General Electronics / Re: Measuring resistance on separate circuits. on: September 20, 2014, 01:09:03 pm
The simplest way of doing this is to use a double gang pot (used in stereo systems) and use one for control and the other for display.

Something like one of these
4  Using Arduino / General Electronics / Re: precision rectifier on: September 18, 2014, 12:55:57 pm
The feedback voltage appears at the cathode of the diode, not at the anode (opamp output).  So it doesn't really matter what appears at the opamp output provided that the voltage at the diode's cathode matches what is fed into the positive terminal of the opamp.  ie unity gain.  You have to consider the diode is part of the opamp system, rather than as a separate discrete component.   
5  Using Arduino / General Electronics / Re: precision rectifier on: September 18, 2014, 03:15:45 am
Nothing happens instantaneously - everything takes time - as the output of the amplifier rises the diode starts to conduct and feeds back the signal to produce a unity gain output.
6  Using Arduino / General Electronics / Re: 0 Voltage from power supply when connected to peltier on: September 16, 2014, 07:16:09 am
If the fan uses a brushless motor (and many of them do) then extra volts will not materially increase speed.  The 3phase module that converts the DC to an AC generally operates at fixed frequency ior at a frequency determined by an external temperature sensor.
7  Using Arduino / General Electronics / Re: 0 Voltage from power supply when connected to peltier on: September 16, 2014, 05:25:40 am
It's a bit like beer drinking.

If you have a barrel of beer and you know that you go bonkers after only 2 pints, you'll still try and get four or five down your gullet.
8  Using Arduino / General Electronics / Re: 3.3V simple vs zener on: September 16, 2014, 03:11:34 am
Rather than using 3 diodes in series, you could instead use a single zener rated at 1.7 volts.  Simply place the zener in series with your 5 volt supply and you will "lose" 1.7 volts across the zener.
9  Using Arduino / General Electronics / Re: 0 Voltage from power supply when connected to peltier on: September 15, 2014, 12:03:09 pm
What is the power rating of the peltier.  If it draws in excess of the power supply rating, the power supply's protection circuit will be cutting off the output (crowbar protection) and won't reset until you remove the load.   
Typically a 50mm square peltier will pull 5 amps at 12 volts, so at 19 volts you could be talking about 7.5A which exceeds the PSU rating.
10  Using Arduino / General Electronics / Re: Measuring battery voltage to Arduino on: September 15, 2014, 02:44:21 am
Using a pair of 10k resistors in your divider chain gives a load of only 0.45mA which is less than 2% of what the arduino itself will draw.   So no problem there !
11  Using Arduino / Motors, Mechanics, and Power / Re: Electric Braking a scooter motor... on: September 14, 2014, 05:02:02 pm
Might I suggest you look at the 4QD website where you'll get loads of technical help with regards to regen braking
12  Using Arduino / General Electronics / Re: discharging a battery with a transistor on: September 14, 2014, 04:57:04 pm
On the basis that battery capacity is normally specified at 20, 10 or 5 hour rating you need to design your system based on how you wish to specify your battery rating.  I presume you wish to evaluate the Ah capacity of a battery

On the basis that it is 5 hours, you should base your maximum current on Ah/5, so for a C5 cell rated at 6.5Ah, the discharge current should be limited to 6.5/5 = 1.25A

Ideally you should employ a constant current discharge regime, whilst keeping a close eye on voltage.  This then makes calculation and determination of battery capacity relatively simple (only one variable  -  time; since Ah capacity is Amps x Duration. (voltage doesn't come into the equation)    You discharge the battery until the voltage has fallen to the minimum value specified by the manufacturer (see their discharge data curve)

In the past I've used such techniques to evaluate the Ah capacity of large wet acid batteries.  Monitor voltage whilst discharging at constant current.   Achieving constant current is relatively easy using something like the 2N3055 you suggest by fitting the load resistor as an emitter follower whereby the gain is near enough unity and by feeding the base with a constant voltage the voltage across the load resistor is kept constant, hence the load current is constant.  The variable "resistor" is the transistor itself and it of course disposes of the voltage as heat.   By correct selection of the emitter load (say 80% of the theoretical load value), the transistor does 20% of the work.  
13  Using Arduino / General Electronics / Re: The important of calibration of oscilloscopes. Or not? on: September 04, 2014, 04:58:21 pm

In practice quartz crystals are very reliable and timebase drift isn't really
going to happen in a modern scope.

(*) Digital

No but the crt scan is analogue and subject to external influence.
Leading to non linearity across the screen.

I have seen a scope display a diffetent waveform just by moving it 90 deg to show someone the measured signal.
They can be worse in the field depending what is around.

That was a high end tek scope as well.

Decent ones had mumetal screening covering the length of the CRT to prevent external magnetic fields influencing the display.
14  Using Arduino / Motors, Mechanics, and Power / Re: Linear motion design on: September 04, 2014, 05:46:46 am
Or you could go even lower-tech and use a capstan wheel on the servo motor to pull a cord around a pulley.    Provided you have at least twice the number of wraps around the capstan as actually needed for the required linear travel, you can fix the centre of the wraps to the capstan and hence eliminate any slippage problems.  Much like was used on old style chart recorders and radio dials.

Lots of images of typical arrangements here :
15  Using Arduino / General Electronics / Re: ACS712 sensor on: September 02, 2014, 05:40:57 am
Since the input terminals (2 of them) are current terminals, there is no need (within limits) to concern yourself with the measured current supply voltage.  The rated input limit Viso is 184, 354, 1500 or 2100, depending upon which standard of test you wish to consider.

The supply voltage Vcc (5 to 8 volts) is the voltage applied to the output side of the device and is used to drive the isolated DC output.
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