No, as in ALL your posts, you don't have a resistor - an inductor isn't a resistor.

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So it is a RLC circuit, right? I thought an inductor acts as a resistor in DC?
As I know RLC circuits do not have any time constant, right?

 

It certainly doesn't have a simple one, you need to select the values experimentally.

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But why it does not have a simple one??? I am getting confused!

 

T = C x R

But you don't have R - you also need to consider exactly what T is anyway (and I suspect it's not what you hope).

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Hello there,


If you are looking for something that is highly predictable then try using a
comparator and driver transistor to drive the relay, or skip the relay and
just use a transistor. That would be more reliable too.

At the very least use a transistor with the relay or just use two transistors.
That kind of circuit will be much more predictable too and will not depend on
the type of bulb used as long as it matches the supply voltage.

The reason why your original circuits are not that easy to analyze is because
they contain elements that are not that well defined...you may switch bulbs,
different kind of relays, etc. Different bulbs will cause oscillation at different
frequencies, and so will different relays. That's two elements that are very
hard to define without having the actual components right in front of you.
When you switch to transistors and possibly comparators, you can then work
with components that have characteristics that are very well known and so
they are very well defined and a lot of calculations with these kinds of elements
will be very close to what happens in real life circuits using these elements.
A big advantage too is that you can design the circuit in a way so that the
main element that is harder to define (the bulb) does not change the way
the circuit works any more...the circuit operation will not depend on that
any more and so you can use a wider range of bulb types without changing
anything else and still get the very same operation.

 

So a question which arises to me is that how designers design circuits? The above is a simple one but seems We have to go with trial and error!.

Another question is do you know any source or book to learn us how circuits are working? I mean a book with circuits and explanation to how they are working (I.e what a practical Capacitor, transistor, resistor... does in the said circuits??

 

While this is a DC circuit, it's not operating in a "steady state"... your interested in what happens in it's transition from one state (off) to another (on). Look at a RLC's response to a "step function"... then you will understand why there isn't a "simple solution" to your question.

 

Designers design circuits primarily by using mathematics and learning the theory. If they know the electronic theory, they know where to apply parts. Then they use math to find the right values for those parts.

 

Designers design circuits by trial and error. How do you think the Chinese designed circuits 50 years ago, when computers and books were not available?
If you think you can design a circuit using a "simulation program," you are fooling yourself.
Getting a circuit to work is much more complex than any simulation software package can provide and even simulation packages for microcontroller programs do not take into account any of the complexities of the input and output devices.
The only way to design a circuit is to build it and test it and then give it to someone else to use. That way you will find the problems and limitations, as they try to destroy it.
When it passes the "idiot test," you can market it.

 

I'm sure your not serious when you say... trial and error, and I'd bet there were books in China 50 years ago!!!


Electronic design is all about math.

 

I attend an inventors association.
Why does it take an electronics inventor years to "perfect" his design?
When you ask for a quote for a particular design, why does it cost thousands of dollars and 3 months?
If all it takes is "maths" to design a project, why does the printed circuit board company get prototype after prototype before the final design?
Why is it that the final design is identified as rev 1.6?
Tell this to the 70 inventors at our meetings . . . . all it takes is “maths” to design a project!!!!!!!
You are fooling yourself . . . you obviously haven't designed a project in your life.

 

Hi again,


That's not entirely true. The reason why this circuit is a little more difficult
to nail down is because you have given us so many variables and i dont think
you realized the consequences of doing that.
When a designer goes to design a circuit like this, they often start with
parts that they are already familiar with from past projects, and also have
more well defined objectives and also have data sheets on actual part numbers
that they can work with.
Here, you say you want it to work with different bulbs at least, so that
presents a different problem than if you knew exactly what bulb you
wanted to use from the start. However, given that same objective
a designer would have to handle that the same way: by designing a circuit
that doesnt depend on the bulbs characteristics, and this may lead to
a slightly more complicated circuit than if the bulb characteristics are
already known and will always be the same.
It's sort of like anything else...if you say you want to carry some water to
some other location we cant tell you how big of a bucket you need...we
would also need to know how much water you intend to carry. It's that
simple really.
A general rule might be that the more well defined the problem is the less
complex the circuit can be, but the less well defined (part values can change)
the more complex it usually becomes so that it can automatically adjust to
the changes that come later.
Maybe a good case in point is a solar array collector, where the user wants it
to track the sun position and also track the maximum power point of the
array. If the sun didnt move relative to the earth it would be cake, but it
does move and so the circuit and entire system becomes more complex.
The solar array may also become a little dirty and change the amount of
power available.

These kinds of problems have been dealt with in the past many times for
many kinds of circuits, and there is even some nomenclature that has come
up for these kinds of situations. Generally the change in parameter is viewed
as a 'disturbance' to the system, and the system being designed is designed
ahead of time to be able to automatically adjust itself to make up for that
disturbance, usually using some kind of feedback system to measure the
output and compare to some reference and induce changes that will keep
the output following the reference as well as needed.
The whole of these techniques are addressed in what has become known
as "Control Theory", and this theory can make itself useful even in simple
applications like the one you are suggesting here.

So it is not that the circuit here can not be done more simply, it's just that
when you want it to do more than just work with specific parts it's going
to take a little more circuitry to do the job correctly.
This happens with many many circuits so dont get too alarmed over it.

 

Hello and thanks to you and to all,
You explained it completely to me and cleared me so now I noticed several advices by that.

I was amazed when I noticed that my RC circuit is not an RC circuit but is an RLC circuit, so I tried to remove several components specially the bulbs and the discharging resistor, then I posted it here hopping that somebody will tell that I am dealing with a first order circuit and can use the known formula for RC time constant circuits and can use Vc=Vs(1-e^(-t/RC) to find the time constant for the circuit, then see how can solve the next problem I.e when bulbs and discharge resistors come in, but I told that it is not possible to simply find the Time constant for the circuit because it is an RLC circuit, while I do not know any formula to find the time constant for an RLC circuit!?
No one asked me to give some clearness about the components I used even no one asked me that for the charging circuit containing just the Cap and Relay's wound.

I wish there was a book with practical circuits explaining how they are designed? When no one respond me for a such book I thought engineers use softwares to do so, so they are not able to good define the ciruits

 

Hello again,


Actually there is a sort of time constant for the RLC circuit too, which is:
R/(2*L)
but instead of that indicating how the current decays it instead indicates how
the envelope of the current decays. It's almost the same unless you end up
with an oscillator, which your circuit is not in most cases.
Notice also that R/(2*L) doesnt help that much here anyway, because we dont
know what your coil inductance is, and also that inductance is very nonlinear
which varies as the armature pulls in. Also, for circuits like this it would be
a little rare to consider the inductance anyway and rather go with the known
resistance and nominal coil voltage and take it from there.

If you want to learn how to design circuits then you need to learn first how to
analyze circuits using various circuit analysis techniques. You then start to
analyze all the circuits you can find that are the type you are most interested
in. Doing this gives you a great sense of what works and what doesnt, and
how to proceed with a design of something. Sometimes it's like inventing but
other times it simply means following someone else's design procedure where
almost everything is mapped out ahead of time. It's always good to be able to
analyze circuits though so that you can test the design on paper before you
start building it up.

You might start by telling us what you have learned in the past such as
algebra, trig, geometry, calculus, differential equations, etc., etc.
I think people here will be able to help you from there.