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1066  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)

1067  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!

1068  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.
1069  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.
jack
1070  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
1071  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.
1072  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.
1073  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
1074  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.
1075  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.
jack
1076  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.
1077  Using Arduino / General Electronics / Re: Finding reed switches? on: March 22, 2011, 03:56:24 am
Bicycle speed/distance "computers"
Domestic water/shower booster pumps (flow sensing)
Central heating boiler water flow sensor
Miniature DIL relays
Wind direction indicators (the cheaper ones use 8 or 16 reeds)

1078  Using Arduino / General Electronics / Re: Measuring current on: March 22, 2011, 03:48:48 am
Think low tech
Put a black bag over it
1079  Using Arduino / Motors, Mechanics, and Power / Re: Stopping stepper if an obstacle is detected ? on: March 20, 2011, 11:26:46 am
Correct me if I'm wrong but isn't the current in a stepper motor independent of the load on the shaft.  After all, all you are doing is providing a rotating magnetic field which is being followed by a permanent magnet. The strength of the field and the magnet is what determines the output torque and all you are effectively doing is switching field coils on or off to produce a fixed field "rotational effect"  

Edit :  I suppose at high speed (pulse rate) there is some back emf induced into the coils by the rotor which will tend to reduce the coil current but at low rotation speeds or when simply jogging this effect must be minimal so coil resistance (or rather reactance) will determine current irrespective of whether the rotor is free to rotate or jammed.  There may however be some "signal" induced into the coil (perturbation on the pulse shape) as the rotor moves which might be "seen" by by an intelligent monitoring system.

jack
1080  Using Arduino / General Electronics / Re: Measuring current on: March 20, 2011, 11:20:15 am
I think you might find a hall effect sensor meets your needs.  These can measure both AC and DC  current and give a DC voltage output which is proportional to the current.  In effect your arduino would be reading a simple DC analogue input as frequently (or infrequently) as you desire.

http://sensing.honeywell.com/index.cfm?Ntt=csla&x=9&y=11&Ntk=si_all_products&ci_id=154286&la_id=1
gives you a starting point.  Generally these require around 12 volts DC to excite them and give an output signal of around 50mV per amp.

jack
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