# RC Circuit

#### SteveYTI

Joined Feb 9, 2004
23
I am currently trying to delay the output of my circuit. I can do that after the output logic with a time delay relay, or I can put passives in before the output transistors. I am leaning towards the output transistors, so leading to my question... Does anyone have a source for known time delays and the accompanying resistor and capacitor values. I am currently looking for this do all chart before I break out a book and use the equations. Thanks for anyone who has a good source for this stuff!!

#### mozikluv

Joined Jan 22, 2004
1,435
hi,

if you are familiar with the RC timing circuit of the 555 timer, it is basically the same if you apply it to a transistor. except for the current limiting resistor connected to the base.

here is some formula you can use:

Time = 1.0986 * R * C

Resistance = Time / (1.0986 * C)

Capacitance = Time / (1.0986 * R)

my answer is on the assumption that you already know the correct biasing of what ever transistor you will use.

#### Dave

Joined Nov 17, 2003
6,969
I have a copy of the universal time constants curves charts for LR and CR circuits, which relates the number of time constants to the percentage of max voltage or max current. If used with the appropriate mathematics, you should be able to deduce the relevant time delays that you require for your RC circuit.

If this might help, I can try and upload the picture onto the web for you to have a look at.

#### SteveYTI

Joined Feb 9, 2004
23
The time delay doesnt need to be exact. 6-8 seconds is all I need. I will be taking the output of a logic gate and dividing down to about .7 volts, output to the base of a transistor. This transistor is going to bring neutral to the coil of a relay. There isnt anything more to drive in this circuit, I think it is going to work for what I want it to do. Thanks to everyone for your help, if I run into problem I will surely post about it

#### Battousai

Joined Nov 14, 2003
141
Wow what is a universal time constant chart? Never heard of that before...

#### SteveYTI

Joined Feb 9, 2004
23
The answer depends on how deep you want of a response. Take a look at the responses above but on a basic level the time constant relates to a resistor-capacitor network. A DC source through a resistor will charge up a capacitor and depending on the values of C and R will determine a "time constant" this time constant represents how long it will take to charge the capacitor about 63% of its remaining capacity. Every following time constant charges up to 63% of whats left over the period of time RC. If you are putting in 10 volts and have an RC constant of say about 4 Seconds you will see 6.63 volts in 4 seconds followed by approx 1.20volts over the second 4. I knew for this application how much time I wanted, I was interested in finding the values from time vs the other way around. Check it out on the web, its the analog way to put a delay into your system.

#### Dave

Joined Nov 17, 2003
6,969
Steves explained it reallt well there. If you want to know more Battousai, here is a link to a particular favourite site of mine, tpub.com, it gives a more detailed explaination showing a lot of the mathematics behind it.

I will try and upload the chart if I can get my scanner working.

#### Battousai

Joined Nov 14, 2003
141
OH!!! that chart... lol I didn't know it was called that... :lol:

#### SteveYTI

Joined Feb 9, 2004
23
New news on that RC circuit!! You see I was trying to delay an output from some digital logic gates. I am thinking that my mix of analog and digital circuitry was not as thought out as it should have been since I am fairly certain that the RC network at the end of my logic definitely is not able to run based on what the chip(s) will give. I am using just an AND and NOT gate. basically I will have supply voltage 3.3 currently but whatever I need at the input some immense amount of current capacity however I am looking more towards the output being able to push a voltage divider 3.3 -> .7 and then the delay with enough current left to bias a transistor that will bring neutral to a relay 200Ohm coil. i will toil at this hoping that sometime in the future it works or someone has done this before.

EDIT--
I had accidentally switched the two ICs on my protoboard, using a hexidecimal inverter as an AND and the AND as a hex inverter. :huh: Needless to say I should not have assumed upon finding this out that these ICs would have been "OK" after this mix up. Everything seems to work fine, Ill see how the addition of transistors go B)

#### Dave

Joined Nov 17, 2003
6,969
Is this a practical work-in-progress? If so what are the chances of uploading some pictures so we can have a look?

#### SteveYTI

Joined Feb 9, 2004
23
This is a very practical work in progress. Basically this is the input logic and analog output to the automatic mode of a valve controller based on two pressure TCs. The point is not to misvalve the system causing a loss of substrates, damage to pumping machinery and a very detailed cleanup. What kinds of pictures are you interested in? I have roughly finished the entire schematic including this section which will be on its own board. I would be happy to upload anything I have done on this project as it is not very proprietary. The logic is simplified from 2 inputs, uses an RC network and 2N3904 transistors. These transistors fire relays that control valve solenoids(air). This is all to control a High Vacuum Chamber so basically the output logic is based on good vacuum fundamentals. Let me know what it is that you are looking for and Ill see what i can upload

#### Dave

Joined Nov 17, 2003
6,969
It would be interesting to see some of the schematics you've come up with.

I see in your signature it says BSEE 2002, is that an Electrical Engineeing degree? I'm unfamiliar with it, probably because I'm English!!

#### SteveYTI

Joined Feb 9, 2004
23
Yes Bachelors of Science in Electrical Engineering from Worcester Polytechnic Institute in Worcester Massachusettes. I may be headed to Chicago for a service adventure on something I have never worked on but that is simply in the works. When things settle a bit I will see what i can do, worst case ill save these schemes as pictures and upload them. It is a very interesting design since it incorporates very old technology with semiconductors. Relays and ICs who woulda thought...

#### Dave

Joined Nov 17, 2003
6,969
It is a very interesting design since it incorporates very old technology with semiconductors. Relays and ICs who woulda thought...

Just what I was thinking, its one of the reasons I am interested in what your doing, because the approach you appear to be taking is quite novel.

#### SteveYTI

Joined Feb 9, 2004
23
I as an engineer cannot justify spending a large amount of money developing a microprocessor based design and the surrounding circuitry (costs of chips, programmer, labor) in an application where I can hold the schematic and "program' in my head. This was a fundamental idea that I believe everyone who becomes an electrical engineer should have, one that was instilled during my education. As an engineer you should be able to confidently defend every decision made. Whether sitting at a work bench or sitting in court defending a design... These chips cost 25 cents a pop MAX. They arent going to fail really and if they do my design is fail safe that is to say when something goes wrong every valve closes which is a very safe way to fail in this application. My superiors will ask me why my design came in 300 - 500 dollars less than my predecessors did, Ill be able to tell them why and how and give my device a very easy troubleshoot at the same time. I will send some schematics your way. Please note though that I am not well versed in the use of Transistors etc. so I think I have found the right values and what not but I will let you see what I am up to maybe you can tell me if theres something very wrong. I will let you know when I have a bit of time. Thanks for the inquiry

#### Dave

Joined Nov 17, 2003
6,969
I can fully agree with what you are saying Steve. Sadly it was the case when I worked for my previous employer, a large Britsh engineering company whose name I shall not mention, that this attitude was not adopted. It was a case of throw money at everything and it'll somehow work out. As a result most things were inefficent for the job and over-engineered. Heck could I tell you about some of the disasters I had the mis-fortune to work on.

Luckily I'm back in the student mode and just trying to sit back and look at engineering from a more enjoyable perspective.

I am looking forward to seeing your schematics, and I will have a check over your use of transistors (this is my field) and help out if I can.

Keep us posted

#### SteveYTI

Joined Feb 9, 2004
23
Quick question for an analog guru. I have set points that will fire a relay that will bring a voltage of 3.3 volts as a logical high into my circuit. until that 3.3 volts comes back I need my chip to stay at 0 volts. I have thought about a latch and a clock but I am unsure that will work for what i want it to do I am still floating a value. basically I need to find a way to keep this pin at or close to ground (logical zero) until i raise it to a high value by tossing in 3.3 volts. a quick suggestion would be great

edit-- solved it using a 1N34A diode to put forth 3.3 volts when ground isnt there (setpoint relay in the n.c. position)

#### Dave

Joined Nov 17, 2003
6,969
The simplest method I have seen for your above problem, Steve, is the basic voltage divider with two latches either side of the output node which is situated on the middle of the voltage divider. The resistor values can be taylored to ensure you have a suitable undefined region between your logic '0' and logic '1'. Your latches can then be simply controlled using a logic circuit or relays, whatever your choice really.

An example of where you could see this is in the PIC18F452 microcontroller which utilises two different methods, one like I explained above where current is drawn as required, and one where the circuit always draws current. The first design tends to be used for output circuits and the second one tends to be for input circuits.

Its quite a good model because its very simple to impliment, but can very easily be adapted to suit a wide range of voltage requirements. But most importantly you can taylor your undefined regions as you see fit to obtain specific logic '0' and '1' voltages.

Its just one of the many, many ways you could do it really.

#### SteveYTI

Joined Feb 9, 2004
23
Seems like this topic has had quite a bit of throughput. Very exciting times for this forum... and I have only posted here a handful of times

#### Dave

Joined Nov 17, 2003
6,969
Originally posted by SteveYTI@Feb 18 2004, 08:15 PM
Seems like this topic has had quite a bit of throughput. Very exciting times for this forum... and I have only posted here a handful of times
Very true Steve, its good to see that we are beginning to get some regular members who have a genuine interest in electronics. I think this topic has have a good response because its quite a unique challenge for you and it has posed a few interesting problems. Keep it going