Show Posts
Pages: 1 ... 69 70 [71] 72 73 ... 94
1051  Using Arduino / General Electronics / Re: Analog meter, positive and negative input. on: May 04, 2011, 04:28:17 pm
Rather than using a separate battery to set an offset there are "neater" ways.

Set up an artificial ground reference at +2.5 volts by using a pair of resistors across the 5 volt arduino supply.  With equal value resistors the mid-point connection will read +2.5 volts relative to system ground.  Now when the PWM output pin is at 0% this output will be effectively at true ground and hence -2.5 volts relative to the artificial ground.   Similarly when the PWM is at 100%, the output pin will be at +5 volts and hence at +2.5 volts relative to the artificial ground.

Now, you say that the meter needs 1mA to read full scale, so this need to be taken into account when determining a suitable value for the resistor pair.   If you simply use resistors, to minimise linearity errors, the resistor chain should pass at least 50 times the 1mA demand.  So you need 50mA down through the chain, which means a total resistror chain value of R1 + R2 = 5 x 1000 / 50     ie 100ohms.   So each resistor should be around 47 ohms.

If you cannot tolerate this "waste" of current, you could use a pair of higher value resistors, say 1kohms, and feed this high impedance artificial reference ground into a unity gain op-amp, the output of which will hold at +2.5 volts irrespective of meter current.
1052  Using Arduino / Motors, Mechanics, and Power / Re: Controlling an AC motor speed with a arduino board on: May 03, 2011, 03:48:05 pm
What's the reason for the poll

You might as well have asked "Can I get a flight to the moon"   The logical answer is "yes" but reality is a totally different matter.  You may well get lots of "Yes" and "No" answers but will be no better informed whichever way the poll falls

If you want the answer to a perfectly valid technical question then your simple question suffices.

Single phase (squirrel cage type) AC motors may be considered analogous to a slipping mechanical clutch.  You can control speed by varying the supply voltage but the speed becomes very dependant upon load.  In short, it can be done but neither reliable nor recommended.   If however the motor is a universal type, with commutator and brushes then speed control is easily achieved since the motor may be considered as  "DC" unit and varying voltage will vary speed.  Such devices were once universally available for the likes of drill control.

If you really want to control speed of an AC motor the best way is to use a 3-phase motor with a variable frequency inverter drive powered from the single phase supply.  Some of these accept low level analogue DC control signals which your Arduino PWM can produce once you've filtered it to produce a 0-5 volt analogue signal.
1053  Using Arduino / Motors, Mechanics, and Power / Re: Control 230V dimmable bulbs on: May 03, 2011, 07:54:21 am
On the basis that the lamps are 230 volt AC you cannot use a transistor - they are DC only.  For this application you would need a triac.  However, considering that one is working with lethal voltage levels and the skill or knowledge levels are minimal (or non-existant) then using SSR devices as the interface is the safest way to go.  These are relatively inexpensive (compared to either funeral costs or house rebuilding)

See the attached as an example.

E-bay sellers have lots of different rated units but the above is neat and possibly suitable for your application

SSRs have relatively slow turn-on  and turn-off characteristics so rather than using the fast Arduino PWM output you may have to create your own PWM signal with relatively low switching speeds.  Anything around 25Hz will not be noticable to the average human eye. 
1054  Using Arduino / Motors, Mechanics, and Power / Re: Control 230V dimmable bulbs on: May 03, 2011, 03:13:32 am
3 off SSRs rated for logic level input.  Then drive them using PWM
1055  Using Arduino / General Electronics / Re: Calculating for a resistor on: May 03, 2011, 03:10:49 am
Are you measuring the voltage across the LED or across the resistor.  If the resistor then it means the LED is drawing less than 100mA.  The specified value of 100mA is the maximum that you should permit to flow but the device may actually draw less depending upon its individual characteristic; even the forward voltage (1.2 volts) will vary.  LED control using passive resistors isn't an exact science.  If you want accurate control of current then you need a constant current source device which will adjust its output voltage to maintain a constant output current.

LEDs may be classed as "current driven devices" so when setting up LED systems I'd recommend measuring the current flow and adjusting the limiting resistor accordingly.   
1056  Using Arduino / General Electronics / Re: if i accidentally drop my arduino on water.... on: May 01, 2011, 08:24:19 am
Your biggest problem won't be damage caused by water to chips and components, it'll be the damage that it causes to all the pin connectors if any water is left inside them.

If the water was of the "fresh" variety then drying off as quickly as possible using a strong blower (hair drier on "cold") should give good results.  If you use a warm or hot hair drier make sure you do not get too close or you'll cook the components.  Do NOT bake in an oven or a  microwave.  Oven thermostats are pretty useless and you may end up overheating the plastics.  Microwaves just love to eat metal components.

If the water was "salty" or dirty then flush extensively in fresh running water to remove all salt and contaminants then dry as above.

If you have deionised or distilled water available, a flush in that before drying would be beneficial.

If the circuit was left wet and the component wires start to show signs of going green then kiss it goodbye - you are too late.

A squirt with some dewatering fluid such as WD40 might help after the event but you will end up with a sticky circuit board.  I suppose you could then flush that off with a propriety degreaser.
1057  Using Arduino / General Electronics / Re: Problem capacitors on: May 01, 2011, 04:06:46 am
And using Babelfish translator, that comes out as :

Good afternoon, then my question of novice goes there. I have condensers and now not what is how to see if podeis help me. Of whatever he is each? Or better, as it is interpreted?    smiley-confuse smiley-confuse

1058  Using Arduino / General Electronics / Re: Problem capacitors on: May 01, 2011, 03:08:52 am
I  guess at what you are asking

100nF   is  0.1 microfarad   

10nK100 is probably 0.01 microfarad rated at 100 volts

u1k63 is probably 1000 microfarad rated at 63 volts

10nJ100 is also probably 0.01 microfarad rated at 100 volts

1059  Using Arduino / General Electronics / Re: changing 0 to 12 volts to 0 to 5 volts on: April 28, 2011, 02:48:31 am
"Waste of Current"

With a 12 volt supply and, say, a 25k divider chain (15k + 10k), the "wast of current" will be less than 0.5mA.  What kind of solar panel are you working with if a 0.5mA load is deemed detrimental to its output efficiency.
1060  Using Arduino / General Electronics / Re: High speep vlave on: April 22, 2011, 07:47:22 am
What pressure and what volume, how long open, how long closed - basic questions but important ones.

Car fuel injectors work at very high speed so if volume required is low you might be able to use those.  However lubrication (or rather lack off lubrication) of the spindle may be a problem

On air horns that played "music" the technique was to use a rotary valve arrangement rather than solenoids.  As the rotor spun it opened and closed various ports which then transferred the air between the tuned horns.

25hz = 1500 RPM on a single stage rotary valve

1061  Using Arduino / Motors, Mechanics, and Power / Re: Why do I need a diode over the coil contacts on a relay ? on: April 21, 2011, 03:32:11 pm

The diode is also termed a "flywheel" diode

The current flows from B to A under normal use
When you switch the relay off the back emf voltage drives a current which flows through the diode from A to B  which then circulates back through the coil from B to A.  Hence the term flywheel since the diode and coil form a circulating circuit which tries to maintain a current flowing through the coil.  However the energy of circulation is dissipated in the coil resistance so "gradually" decreases to zero.  I use the term "gradually" but could be exceedingly short.

This is the reason why the relay drop-out is slower with the diode in circuit; because the current through the coil continues to flow (via the diode) after the switch is opened

1062  Using Arduino / General Electronics / Re: can I use two 103 caps in a row if I havent any more 104s? on: April 14, 2011, 02:05:38 pm
Your drawing shows them in series.  To build a 0.1 from 10 off 0.01 you need to arrange them in parallel
1063  Using Arduino / General Electronics / Re: Phase change through transformer? on: April 11, 2011, 02:45:26 pm
Hi Lefty,

You're probably on the right track with your comments.  The original site problem ( some 30 years ago) was to bring a back-up LV control voltage into a large AC gas compressor fed from two separate sources from a common plant power system.  It was eventually established that there was 30 degrees of difference between the two LV supplies and a transformer count verified that each supply system involved a different number of transformers in the chain.   I am unaware if there was indeed connection variations between phases though we did use the same phase pair to "eliminate" the obvious problems of out-of-phase supplies.  Similarly I recall when working with on-line drawings it was always essential to ensure that there were the same number of transformers in each leg of back-up or shared supply lines.  Perhaps some of our power engineers can clarify the issue.

Either way it must be physically impossible (however minute - or large) for output and input voltages to be in phase irrespective of load.  Input current produces magnetic flux and this must lag drive voltage ( CIVIL) since flux is a reaction to an action.  The output voltage can only be induced by the core flux which is already lagging the drive voltage and the output current again lags the output voltage.  Hnece the output voltage and current must lag the input voltage and current.

1064  Using Arduino / General Electronics / Re: Phase change through transformer? on: April 11, 2011, 02:04:21 pm
Will the transformer's output be phase shifted with respect to its input?

And here was me thinking the question was what is quoted above

1065  Using Arduino / General Electronics / Re: Phase change through transformer? on: April 11, 2011, 03:12:28 am
Can't recall the reason why but I believe there is a 30 degree phase shift between input and output.  I experienced this in reality when trying to connect a pair of power circuits in parallel many years ago.

The 0 or 180 degrees mentioned previously refers to start and finish end of the secondary winding.  However this does not consider the shift from primary to secondary.

If you don't believe this then there is a very simple experiment that can be done to illustrate that there is indeed a phase shift.

Set up two transformer circuits

Circuit "a" contains only one transformer , say 240 to 12 volts

Circuit "b" contains 2 transformers  the first is 240 to 110  and the second is 110 to 12

If there was no phase shift through the transformers then the two 12 volts outlets would be in-phase

However you will find they are not in phase, in fact they will be out by 30 degrees

Each transformer has a 30 degree phase shift so circuit "a" is lagging the input by 30 degrees and circuit "b" lags the input by 60 degrees (due to having 2 transformers)

Pages: 1 ... 69 70 [71] 72 73 ... 94