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16  Using Arduino / General Electronics / Re: Help with math on circuit on: June 13, 2014, 12:21:13 pm
Quote
For a relay taking 30mA I'd drive it with a small transistor like a 2N2222. 

That 30mA is likely the holding current, the energizing (pull-down, activation( current could be much higher.

Best practice is simply to always use a driver (transistor, FET).  And, of course, that flyback diode!


Ray

If the relay was an AC device the pull in current would indeed be much higher than the holding current since Xl depends upon coil inductance which changes with armature position.  However as the relay is a DC device the coil current may be simply calculated from coil resistance.  If anything the inrush current will be less than the steady state value due to the coils self-inductance tending to inhibit current change.
17  Using Arduino / Motors, Mechanics, and Power / Re: Solenoid type on: June 12, 2014, 03:43:56 am
Type 1 has a problem in that the plunger (or slug) is not within the main magnetic field so the pull force is relatively low.  The nearer the slug gets to the coil armature the greater the force experienced

Type 2 plunger is the armature and is contained within the magnetic field so the pull action is much higher.  The plunger will attempt to "balance" itself  to the centre of the magnetic field.  Providing the plunger is longer than the coil you can get it to extend beyond the coil end.
18  Using Arduino / Motors, Mechanics, and Power / Re: Power a big solenoid on: June 10, 2014, 10:10:29 am
That should work OK.
Ensure the diode is placed as close as possible to the solenoid and that the connection leads are capable of carrying the current.  You might even want to consider glueing the diode to the solenoid body and connecting directly to the solenoid wires.
19  Using Arduino / Motors, Mechanics, and Power / Re: Power a big solenoid on: June 10, 2014, 02:59:30 am
Draw us a picture of how you intend to connect your circuit.  Then we will be able to advise.
20  Using Arduino / Motors, Mechanics, and Power / Re: Power a big solenoid on: June 10, 2014, 02:53:44 am
With a 7A solenoid coil you will get one mighty fat spark when you de-energise the coil so the diode needs to be rated at least 15A and of the schotty type.  General rule is that the diode is required to pass the same current as the coil draws when powered.  A simple 1n400x  type diode will not suffice.
21  Using Arduino / General Electronics / Re: HElp with pH meter, need fast help on: June 10, 2014, 02:49:42 am
Are you using both +5v and -5V with the common 0v from each supply connected as the ground reference.  When you measure between the +5v and the -5v rails do you get 10 volts.  If not then you have your supplies incorrectly connected.
22  Using Arduino / General Electronics / Re: Custom PCB Oxidation on: May 28, 2014, 03:25:27 pm
Rather than steel wool - which will conduct if you leave bits behind - try a typewriter rubber  (that's an eraser if you live in the USA otherwise you might not get what you want  smiley-grin)
23  Using Arduino / General Electronics / Re: For slow digital circuits, do I need ground between each wire? on: May 24, 2014, 10:39:22 am
Whilst I admit I was wrong, I didn't actually state which type or size of ribbon cable I was referring to.    smiley-grin
24  Using Arduino / General Electronics / Re: Power supply phenomenon:Can't go beyond a certain voltage even when knob turned on: May 23, 2014, 08:36:24 am
Your PSU is current limiting - that's what the second LED is showing.
25  Using Arduino / General Electronics / Re: For slow digital circuits, do I need ground between each wire? on: May 23, 2014, 07:23:43 am
Damn, wrong again  smiley-confuse

Basically you need to consider two problems

a) Cross-talk pick-up which can be minimised by inter-core grounding or low impedance loading of the cores
b) degradation of the high frequency components on the leading and trailing edge of the pulse due to cable capacitance

It really is irrelevant whether you are transmitting 1 pulse per day or 10,000 per second; the above problems still exist (or not, if you aren't too bothered about  cross-talk or pulse shape)
26  Using Arduino / General Electronics / Re: For slow digital circuits, do I need ground between each wire? on: May 22, 2014, 02:37:05 am
Think about all those ribbon cables that are used in computers carrying fairly high signal rates.  They certainly do not have inter-core grounds
27  Using Arduino / Programming Questions / Re: Multiplexing multiple analog values on: May 20, 2014, 09:18:19 am
It really boils down to how much i/o you wish to use and how it's configured
5 digout + 1 anin by my method or 3 digout + 4 anin by Paul's method
28  Using Arduino / Programming Questions / Re: Multiplexing multiple analog values on: May 20, 2014, 02:40:28 am
Alternatively you could use 4 mux units to address your 30 inputs  read by 3 address lines.  You then need one more mux unit to address the 4 mux outputs so's the system reads only one output.  This last mux needs only 2 address lines.  That way you use a total of 6 lines (5 address lines and 1 input line ) to read all your 30 inputs.
29  Using Arduino / General Electronics / Re: Continuous Bourns Pot on: May 19, 2014, 12:08:44 pm
The electrical range is roughly 340 degrees of rotation so there is a "dead" area of about 20 degrees.
30  Using Arduino / General Electronics / Re: Circuit for AC measure? on: May 18, 2014, 04:33:11 pm
Why do you not wish to use transformers ?

There is NO safe way to connect an arduino to an AC power system with the intent of accurate voltage measurement without using a means of galvanic isolation.

Opto-couplers will give you isolation but the ability to accurately monitor the AC system will be impaired

Transformers are the way to go
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