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1066  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

1067  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
1068  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.

1069  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

1070  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)

1071  Using Arduino / General Electronics / Re: EXTREMELY Simple Questions on: April 07, 2011, 01:14:47 pm
Hi Mike,

I can't blame my calculator,   it 119 and I also added on 20mA plus a bit for tolerance so rounded up to 150mA but somehow put it in as 250mA  (0.25)   Doh!

1072  Using Arduino / General Electronics / Re: EXTREMELY Simple Questions on: April 07, 2011, 01:04:09 pm
Assuming your output is 3 volts and your LEDs forward voltage drop is a nominal 2.2 volts then your series resistor controlling LED current is (3-2.2)/0.017   or 0.8/0.017  =  47ohms

You want to control 7 LEDs but you have only 6 outputs.   You therefore need to do some multiplexing if you want independent control of each of the LEDs

If you can tolerate 2 of the LEDs in parallel then this can be done but each LED must have its own 47ohms control resistor.

Your batteries will be in series to produce 3 volts

AAA are limited in current capability and if all 7 LEDs are lit at 17mA your load current will be at least 0.25 amps (inclusive of the lillypad)
If size is an issue then these batteries might work, but don't expect any more than a few 10s of minutes or so of battery life.   Obviously if only 1 LED is illuminated at a time then battery life will be greatly extended.  I'd suggest at least AA cells for better life and more stable operation.
1073  Using Arduino / Motors, Mechanics, and Power / Re: PWM current on motor jam ? on: April 06, 2011, 01:16:25 pm
The peak jamming current remains at 1.7amps but the mean current is 1 amp as you calculate.   Because the peak is occurring repeatedly your H bridge must be rated capable for the locked rotor stall current.
1074  Using Arduino / Motors, Mechanics, and Power / Re: Stepper pulling too much current on: April 05, 2011, 03:45:41 am
Could be the multimeter but a hot finger is still a hot finger
1075  Using Arduino / Motors, Mechanics, and Power / Re: Fluid motion with servo, is it possible? on: April 03, 2011, 04:10:05 am
Only an opinion but might be worth consideration.
Ideally you need to use a stepper motor rather than a servo.  With the stepper you can define EXACTLY what the motor does with respect to position and speed - two essentials for fluid movement.  A servo, unfortunately, is a compromise between position and speed, try to move it too fast or just a little and you will get both overshoot and /or jerkiness.  In control system terms, a servo's performance is optimised under one set of conditions, whereas for fluid movement you require optimal performance under many different conditions.
1076  Using Arduino / General Electronics / Re: What size capacitor needed to power solenoid? on: April 02, 2011, 04:31:05 pm
Turn-on time for a DC SSR will be considerable slower than a FET.  This might have a bearing on how quickly you can get the required energy into your solenoid, the inductance of which will itself tend to slow down the current in-rush.
1077  Using Arduino / General Electronics / Re: Substitute for a reed switch? on: March 26, 2011, 03:15:09 am
The obvious question -

Why don't you want to use a reed switch ?   They are simple, cheap and utterly reliable
1078  Using Arduino / Motors, Mechanics, and Power / Re: Need help with simple problem on: March 23, 2011, 05:39:01 pm
Are your servos powered from that small pack of 4 AA cells.  If so I don't think it'll run 5 servos every 10 seconds for very long before it becomes depleted - certainly not "ad infinitum".  In fact if all 5 servos are commanded to move at the same they may pull the voltage too low to operate satisfactorily.
1079  Using Arduino / Motors, Mechanics, and Power / Re: will this SSR work for Arduino? on: March 23, 2011, 05:31:22 pm
I know it's obvious but remember to use a digout, not a PWM.
1080  Using Arduino / General Electronics / Re: Battery pack to power a camera on: March 23, 2011, 11:28:22 am
Forget AAs.  If you are building an external pack then use a pair of D sized cells.  If you use decent ones you should get many hours of photography from them.  Using cells in parallel is no substitute for using larger cells.
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