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31  Using Arduino / General Electronics / Re: About BiPolar Power Supply on: June 18, 2014, 03:47:25 pm
Wow, a manic depressive power supply   smiley smiley

Sorry, it was too good to overlook.
32  Using Arduino / Motors, Mechanics, and Power / Re: Reading state of a 0-200VDC motor. on: June 17, 2014, 01:30:27 pm
Might I suggest a hall effect current transformer unit which monitors motor drive current.  These units are available with outputs of 5 volts DC at full range current.   This design also provides total isolation from the motor voltage.
33  Using Arduino / General Electronics / Re: making a potentiometer that has a low range of resistance?i on: June 16, 2014, 01:23:24 pm
Low resistance pots are normally called rheostats and are wire-wound (no use at RF).

Not so.

A rheostat is a variable resistor and generally needs only 2 terminals.  It provides no potential divider capability.  Certainly rheostats are generally of low resistance but that does not  mean low resistance potentiometers should be called rheostats.
Rheostats are generally linear devices but can be obtained with a nonlinear function.

A potentiometer (whatever its resistance value) has at least 3 terminals (some have a fourth fixed value terminal at mid scale 50%) and provide a potentiometric divider function.  Track resistance can be logarithmic, anti-logarithmic or linear.

Potentiometers can act as rheostats by using one end terminal and the slider terminal.  It is generally good practice to also connect the "unused" end resistance terminal to the slider.  This ensures there is always a resistance in circuit even if the slider contact goes open circuit.
34  Using Arduino / Motors, Mechanics, and Power / Re: Solenoid type on: June 13, 2014, 12:25:21 pm
Read my input  -  slug endeavours to reach a balanced position at the mid-point of the coil.
The position of the stop does affect the final pull value with it being zero at the point where the slug centre is at the coil centre.  In your illustration this will only occur if the slug is shorter than the coil
35  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.
36  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.
37  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.
38  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.
39  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.
40  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.
41  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)
42  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
43  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.
44  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)
45  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
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