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JUAN DELA CRUZ
Mad Scientist
Philippines
476 Posts |
 Posted - Aug 09 2008 : 07:28:33 AM
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Hi everyone...
A undergraduate student is seeking some 'tips' in calculating parts value in designing a Window comparator.
I've searched in google and found this comparator ckt. with 'HYSTERYSIS':
Download Attachment:  Adding Hysterysis to Comparator.GIF
13 KB
I am trying to design a Window Comparator ckt. as the High/ Low detection ckt... but I was confuse in the above stated formula.
I found in different thread that used 741/ LM10 op-amp as the Low-batt. cut-off ckt...
.. but I will use a Quad op-amp (like LM324/ LM348) instead and
I want to design by my self to learn such basic things. Unfortunately, my know-how is really limited 'coz I am an undergraduate.
So, could anyone tell me the other formula needed (in a comprehensible way) in calculating parts value, voltage ref.,etc. ..but in this case using Zener with pull-ups resistor as the reference voltage and also the formula in calculating 'hysterysis'..
Any help would be greatly appreciated.
Thank you.
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juan dela cruz
Penniless INVENTOR |
Edited by - JUAN DELA CRUZ on Aug 09 2008 07:40:03 AM |
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pebe
Nobel Prize Winner
United Kingdom
1078 Posts |
Posted - Aug 09 2008 : 08:28:43 AM
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| Juan, I may be able to give you an explanation. Are you familiar with the term 'source impedance'? |
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JUAN DELA CRUZ
Mad Scientist
Philippines
476 Posts |
Posted - Aug 10 2008 : 01:04:39 AM
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quote:
Originally posted by pebe
Juan, I may be able to give you an explanation. Are you familiar with the term 'source impedance'?
Greetings,
Impedance through which output current is taken from a source-- source impedance? ..confused 
BTW.. Mr. Pebe I think if the Math Formulae in calculating value in designing a Window Comparator is stated I think I can catch-up ..
Thank you. |
juan dela cruz
Penniless INVENTOR |
Edited by - JUAN DELA CRUZ on Aug 10 2008 01:05:28 AM |
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pebe
Nobel Prize Winner
United Kingdom
1078 Posts |
Posted - Aug 10 2008 : 02:45:21 AM
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The problem with the formula shown is that there was no explanation given. You could try to remember the formula but it is much better if you can break it down to see how the answer is arrived at.
It can be broken down into:
1. What is the effective resistance of R1 and R2 in the circuit?
2. How to calculate the feedback
For the first part, do a Google for 'Thevenin's theorum'. Or look at this simple explanation I gave in an earlier posting:
http://www.aaroncake.net/forum/topic.asp?TOPIC_ID=6737&SearchTerms=thevenin
When you have got that, I can give you hints on how to calculate RH. |
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JUAN DELA CRUZ
Mad Scientist
Philippines
476 Posts |
Posted - Aug 10 2008 : 03:29:14 AM
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quote:
Originally posted by pebe
The problem with the formula shown is that there was no explanation given. You could try to remember the formula but it is much better if you can break it down to see how the answer is arrived at.
It can be broken down into:
1. What is the effective resistance of R1 and R2 in the circuit?
2. How to calculate the feedback
For the first part, do a Google for 'Thevenin's theorum'. Or look at this simple explanation I gave in an earlier posting:
http://www.aaroncake.net/forum/topic.asp?TOPIC_ID=6737&SearchTerms=thevenin
When you have got that, I can give you hints on how to calculate RH.
Greetings..
quote:
Imagine a potential divider fed from a 20V supply. It has 100K in the top arm to V+, and 27K in the bottom arm to 0V. If we measure the voltage at the resistors’ junction with a digital (high resistance) meter we would get 20V x 27/127 = 4.25V
Regarding your post here Sir.. it implies that the voltage in the junction of potential divider will be equals to the voltage drop in the smallest resistance (27K). ..is that right ??
So, if I will make a voltage divider..
That is:
a 10k in the top arm to V+ (12V), and a 6V Zener in the bottom arm to the ground.. btw what is the resistance of the Zener ??
I really want to extend chatting with you but my pocket is really limited
I am very sorry Sir.. honestly my internet access is Limited
(I don't have my own PC.)
Here in the Philippines, majority of people didn't have their own PC (like me) so we are just 'renting a PC'
I really want to learn but you know living in a first world country is different.. I hope you will understand my situation..
Can you explain IT briefly with the fomulae..
Thank you.
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juan dela cruz
Penniless INVENTOR |
Edited by - JUAN DELA CRUZ on Aug 10 2008 03:35:12 AM |
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JUAN DELA CRUZ
Mad Scientist
Philippines
476 Posts |
Posted - Aug 10 2008 : 06:35:44 AM
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Greetings..
I've search in google about the THEVENIN's THEOREM and found this interesting site:
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/Thevenin.html#c4
But I have a question regarding the formula..
What will be the basis of the resistance value of R2/ R3 ???
... if I will use a ZENER diode in place of R3 ??.. R3 will be the reversed resistance of the Zener Diode?
... R2 is related with the op-amp as a comparator ?..what do you call that resistance??.. or it is ignored ?
What if a feedback is added in the comparator ckt.??
Thank you.
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juan dela cruz
Penniless INVENTOR |
Edited by - JUAN DELA CRUZ on Aug 10 2008 06:57:13 AM |
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pebe
Nobel Prize Winner
United Kingdom
1078 Posts |
Posted - Aug 10 2008 : 07:04:25 AM
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quote:
Originally posted by JUAN DELA CRUZ
I want to design by my self to learn such basic things. Unfortunately, my know-how is really limited 'coz I am an undergraduate...............So, could anyone tell me the other formula needed (in a comprehensible way) in calculating parts value, voltage ref.,etc. ..but in this case using Zener with pull-ups resistor as the reference voltage and also the formula in calculating 'hysterysis'........Any help would be greatly appreciated.
So I'm trying to help you.
quote:
Can you explain IT briefly with the fomulae..
But there are no short cuts.
quote:
Regarding your post here Sir.. it implies that the voltage in the junction of potential divider will be equals to the voltage drop in the smallest resistance (27K). ..is that right ??
Yes
quote:
So, if I will make a voltage divider..
That is:
a 10k in the top arm to V+ (12V), and a 6V Zener in the bottom arm to the ground.. btw what is the resistance of the Zener ??
If you make a voltage divider using a zener as the bottom element, the voltage at the cathode of the zener will be the zener voltage. For most purposes, its resistance is zero.
That has answered your two questions, but if you keep asking questions about little parts of a circuit, you will get specific answers about that part of the circuit but you will learn nothing, and you will be back again asking similar questions.
If you take the trouble to learn basic principles you will be able to solve problems and design for yourself. Doing that will save you computer time. If you are too impatient to learn basics, you will never understand op-amps with hysteresis.
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JUAN DELA CRUZ
Mad Scientist
Philippines
476 Posts |
Posted - Aug 10 2008 : 07:07:48 AM
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I've noticed if I will use a Zener diode in place of R3,
R1 will be a 'bleeder resistor' that will limit current to the zener diode. Am I right ? |
juan dela cruz
Penniless INVENTOR |
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JUAN DELA CRUZ
Mad Scientist
Philippines
476 Posts |
Posted - Aug 10 2008 : 07:29:48 AM
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quote:
Originally posted by pebe.
quote:
So I'm trying to help you.
I greatly appreciate it Sir.
quote:
If you make a voltage divider using a zener as the bottom element, the voltage at the cathode of the zener will be the zener voltage. For most purposes, its resistance is zero.
So, the Thevenin's Voltage e will be V1/ R1... if R1 = 10k, V1 =12V, R3 = 0 therefore e = 0/ 10,000 V = 0V ?? or the Thevenin's voltage is negligible?
quote:
That has answered your two questions, but if you keep asking questions about little parts of a circuit, you will get specific answers about that part of the circuit but you will learn nothing, and you will be back again asking similar questions.
If you take the trouble to learn basic principles you will be able to solve problems and design for yourself. Doing that will save you computer time. If you are too impatient to learn basics, you will never understand op-amps with hysteresis.
Sorry about that Sir.. but you know I really want to extend my computer time
(if I've enough budget)
'coz I really learn a lot of things here that I have never learned in College 'coz Unluckily I didn't finish my study.
Inspite of that, I am eager to learn.
Thank you.
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juan dela cruz
Penniless INVENTOR |
Edited by - JUAN DELA CRUZ on Aug 10 2008 07:31:50 AM |
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pebe
Nobel Prize Winner
United Kingdom
1078 Posts |
Posted - Aug 10 2008 : 08:36:05 AM
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I meant the equivalent source resistance (when working out the potential divider from Thevenin) is zero.
Let me try to explain. When a current flows through a zener its volts drop will be the zener voltage, whatever the resistance is that is feeding current to it. The voltage is not dependant on current so increasing the current by 1amp produces zero change of voltage.
The source resistance (Rs) = change of voltage/change of current. So Rs = 0/1 = zero |
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mrgone
Nobel Prize Winner
USA
1176 Posts |
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JUAN DELA CRUZ
Mad Scientist
Philippines
476 Posts |
Posted - Aug 10 2008 : 11:24:11 PM
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Hi everyone...
How about calculation of feedback?
Does anyone can explain the effect of hysteresis?
Thank you.
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juan dela cruz
Penniless INVENTOR |
Edited by - JUAN DELA CRUZ on Aug 10 2008 11:31:18 PM |
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audioguru
Nobel Prize Winner
Canada
4218 Posts |
Posted - Aug 11 2008 : 12:02:29 AM
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| Hysteresis is positive feedback that creates a "snap-action" of the switching. It is calculated with simple arithmatic. |
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JUAN DELA CRUZ
Mad Scientist
Philippines
476 Posts |
Posted - Aug 11 2008 : 12:30:02 AM
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quote:
Originally posted by audioguru
Hysteresis is positive feedback that creates a "snap-action" of the switching. It is calculated with simple arithmatic.
Hi Audioguru
What is that simple arithmetic that you are talking about ?
Thank you. |
juan dela cruz
Penniless INVENTOR |
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audioguru
Nobel Prize Winner
Canada
4218 Posts |
Posted - Aug 11 2008 : 11:31:58 AM
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quote:
Originally posted by JUAN DELA CRUZHi Audioguru
What is that simple arithmetic that you are talking about ?
The resistor values are calculated with simple arithmatic and Ohm's Law.
A little common sense helps. Then the values won't be too low or too high. |
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JUAN DELA CRUZ
Mad Scientist
Philippines
476 Posts |
Posted - Aug 11 2008 : 10:18:42 PM
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quote:
Originally posted by audioguru
The resistor values are calculated with simple arithmatic and Ohm's Law.
A little common sense helps. Then the values won't be too low or too high.
Hi Audioguru..
I didn't know that ' simple arithmetic ' that you are talking about.
I want to activate the HIGH voltage detection ckt. for about 14V
& the LOW ckt. for about 11V.
Can you show the formula in calculating that ' positive feedback '
Thank you |
juan dela cruz
Penniless INVENTOR |
Edited by - JUAN DELA CRUZ on Aug 11 2008 11:45:44 PM |
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Topic |
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COMPARATOR: Math Formula / Tips
ComparatorWindow.GIF