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# RC

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#### Electronman

##### New Member
Hi

Can somebody direct me how to calculate the RC constant for the bellow circuit? I want to determine the rate of flashing of the light but I am not sure if I can use The RC time constant formula or Have to consider the Inductance of the relay's wires?
My relay has a 400 ohms wound.

Thanks a bunch

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It's a very complex situation because you have the filament of the globe in series with a resistor and the actual voltage across the globe is unknown.
Try a prototype and see.

I think you can easily assume that RC time constant is much longer than the RL time constant of the relay.

The resistance of the light bulb will vary from a few ohms (cold) to about 25 ohms (hot). You also need the resistance of the relay because it is part of the RC circuit.

Can we determine the RC constant or light blinking time (speed)??

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its even worst than that. You also have to account for the mass of the relay armature, and the non-linear I/V curve of the lamp. Analysis of this circuit would be a good senior project for an EE student.

Just build it, and play with the capacitor until it works for you....

oOPS!

I would like to analys it. Can we change it somehow to could calculate RC constant?

What do you mean by the mas of the relay armature?

Can we change the circuit so that the lamps do not make any interference in the RC constant?

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The coil resistance is 400 ohms and the cap capacity is 3300uF.
T=RC, so can we simply calculate time constant for the bellow circuit so that detect when the the relay contacts react?

Can I consider the relay as a simple resistor?

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Can we simply calculate the RC constant in the circuir??

More like a "RLC" circuit. Your looking at a second order differential equation... that's not exactly "simple".

But As I know RLC circuits do not have any Time constant?!

Ok, Can you guys help me to find the RC constant for the bellow circuit?
I am in doubt if I can consider the following as a Just RC circuit and use the formula 'T= 5 (RC)'?

THANKS for any help.

P.s. because This circuit is a simple one So I would like to learn how to design it by myself.

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I have changed the circuit as simple as possible so that somebody could solve my problem!?

I would like to know how do you guys design your circuits while you think the last one circuit is complicated????! Do you use any special device/ program rather that using any formula..??

I have changed the circuit as simple as possible so that somebody could solve my problem!?

I would like to know how do you guys design your circuits while you think the last one circuit is complicated????! Do you use any special device/ program rather that using any formula..??

The circuit isn't complicated, but it's VERY complicated to try and calculate it's exact operation, as there are far too many unknown parameters.

For such a crude circuit, you would just build it, and then select the capacitor to give the timing you want by altering it's value.

Are you talking about my last pic?? is that a RC circuit now or no?
What are some of does unknown parameters while the last pic has just an inductor (resistor?) and a capacitor??

How can I select the capacitor? Can I do it by RC time constant formula?

the last pic is not a R C circuit, even by knowing the winding resistance, you still have an inductive reactance to take into acount, the moment current begins to flow through the coil a countervoltage is set up that causes the current to lag the voltage between 0 and 90 degrees, as well as a capacitive reactance to add to that, so a time constant is not easily recognizable, what you really have is a series resonant circuit.

But will in actuallity function as a RC timing flasher.

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Are you talking about my last pic?? is that a RC circuit now or no?
What are some of does unknown parameters while the last pic has just an inductor (resistor?) and a capacitor??

How can I select the capacitor? Can I do it by RC time constant formula?

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

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.

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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