# 555 timer voltage and current analysis

Discussion in 'Homework Help' started by majawahar, Jan 28, 2016.

1. ### majawahar Thread Starter New Member

Jan 28, 2016
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Can any one let me know how to measure the current and voltage across r1 and r2 of a timer circuit mathematically.
I want mathematically calculate the current and voltage in a 555 timer circuit and compare it with the simulation circuit

2. ### sailorjoe Member

Jun 4, 2013
361
63
You need a scope, since the voltages and current are changing in time.
You can measure the resistances to get an accurate value. Then measure voltages with the scope. Then compute current with Ohm's Law.
Does that make sense?

3. ### MrAl Distinguished Member

Jun 17, 2014
2,562
518
Hi,

Do you know how to calculate the quantities in question?

4. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
First off, it's hard to tell which resistors you are talking about since you haven't provided a schematic to tell us what "r1" and "r2" even are. Instead, you are forcing us to read your mind -- and engineering is NOT about mind reading.

Nor have you given any clue what time of 555 timer circuit you are talking about. Monostable? Astable?

Nor have you told us what it is you want to know about the current through and the voltage across these resistors. Average? Instantaneous? RMS?

So, reading your mind, I'm assuming (another thing that engineering is not about) that you are wanting to measure the average current and voltage for the two timing resistors in a stereotypical 555 astable multivibrator circuit. Is that correct?

All you have to do is look at each phase of such a circuit and derive the equation for the voltage and current in each resistor as a function of time and built up those functions for an entire cycle. Then calculate the averages the same way you would for any function of time.

Jan 28, 2016
15
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6. ### majawahar Thread Starter New Member

Jan 28, 2016
15
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Dear Mr.WBahn

Thanks for your reply.I am enclosing as attachment the circuit.I want to measure the instant current and voltage accross the resistors of this astable circuit and analysis the circuit.Which method should i apply for example whethet it should be ohms law or network law or thevinin law ....etc

Regards'

M.A.Jawahar

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7. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
Can you determine the voltage and current in R in the following circuit, assuming that at t=0 the capacitor is completely discharged?

Mar 31, 2012
18,087
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9. ### majawahar Thread Starter New Member

Jan 28, 2016
15
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Dear Bahn,
I am giving below the answer, if i am answer is stupid please correct me.I am just doing my 1st year electronics and am looking forward for your guidance

Now when the capacitor is fully discharged there will no further current flow through the resistor "R" at that INSTANT, so the v{t} will be vcc and i(t) will be v)t) / R

10. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
You are claiming two contradictory things.

First you are saying that there will be no current flow through the resistor, and then you say that the current will be vcc/R. Since vcc and R are both finite and non-zero, that will be a non-zero current.

When fully discharged, all you know about the capacitor is that it has zero voltage across it -- you have no idea what the current may or may not be in it. That is determined by the rest of the circuit.

So with zero voltage across it, all of the voltage appears across the resistor and, hence, it's current is Vcc/R.

But that is just at that one instant in time. What is it for all other instants in time?

11. ### hp1729 Well-Known Member

Nov 23, 2015
2,103
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I'm sorry. I didn't realize this was the homework forum. I got here through one of the quick links and it doesn't tell me I'm in Homework.

Last edited: Jan 29, 2016
12. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
It happens -- it's certainly happened to me more than once.

13. ### majawahar Thread Starter New Member

Jan 28, 2016
15
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post: 958713, member: 161605"]You are claiming two contradictory things.

First you are saying that there will be no current flow through the resistor, and then you say that the current will be vcc/R. Since vcc and R are both finite and non-zero, that will be a non-zero current.

When fully discharged, all you know about the capacitor is that it has zero voltage across it -- you have no idea what the current may or may not be in it. That is determined by the rest of the circuit.

So with zero voltage across it, all of the voltage appears across the resistor and, hence, it's current is Vcc/R.

But that is just at that one instant in time. What is it for all other instants in time?[/QUOTE]

Sorry, i agree when the capacitor is fully discharged there will be no voltage across it and the full vcc will appear against the resistor.
Now when the capacitor is fully charged there will be no current flow and the same vcc source voltage will appear against it.
At other instances i am really not sure how to calculate.I will appreciate if can explain it in a simple way

14. ### hp1729 Well-Known Member

Nov 23, 2015
2,103
233

Sorry, i agree when the capacitor is fully discharged there will be no voltage across it and the full vcc will appear against the resistor.
Now when the capacitor is fully charged there will be no current flow and the same vcc source voltage will appear against it.
At other instances i am really not sure how to calculate.I will appreciate if can explain it in a simple way[/QUOTE]

Series circuit? One representing the voltage across the resistor(s). One representing the voltage across the capacitor.
The tricky part is in knowing how much voltage is across the capacitor - when. understanding how the IC works is important.

15. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
So you are being asked to find the average current in a first-order RC circuit but you don't know how to find the instantaneous current as a function of time in a first-order RC circuit? It sounds like whatever course you are in that is assigning you this problem is too advanced for you (meaning that either you have not had the prerequisite course(s) or you didn't learn what you needed to learn from them). You need to take a step back and deal with those deficiencies now, before trying to tackle this problem. You can either try to come up to speed quickly on the transient response first-order circuits and see if you can salvage things or you can drop and take this course (or possibly the prior course) again in order to learn the material.