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Aubrey Kagan

Excel-Optimizing Resistor Ratios, Part 1

Aubrey Kagan
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vasanjk
vasanjk
7/15/2014 1:56:01 PM
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Re: Free Download
sachin

 

It is very much suitable. It is not a shift register. It is an octal buffer. I regularly use it with my 3.3v mcu for interfacing with other 5v peripherals.

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vasanjk
vasanjk
6/30/2014 10:38:40 PM
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Master
Re: Free Download
AK

That's a nice article.

By the way, I made a level shifter with smd resistors and transistors as I needed multiple such shifters in a design. The circuit was a PCB with a minimum of two back to back converters. I could stack as many as I wanted.

 

JK

P.S- This is my screen name henceforth.

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SachinEE
SachinEE
6/30/2014 8:57:27 PM
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Master
Re: Simple Web App
I did not understand your point..

@amrutah, my point was with respect to the web app link that he has uploaded. If you visit that link then you will see that the voltage values doesn't have any impact on the outcome. 


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SachinEE
SachinEE
6/30/2014 8:54:51 PM
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Master
Re: resistor calculator spreadsheet
I do think about the factor of 10 when looking at the result table. So if you want values in k-Ohms, then keep that in mind when looking at values.

@RedDerek, thanks for the keynote. I will keep it in mind when using your work sheet.


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amrutah
amrutah
6/30/2014 8:46:45 PM
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Master
Re: Simple Web App
@Aerohoff: You need to consider either the ratio or the absolute voltages (you can disable one of the user inputs).  Best you can do is to ask the user to input, Vin, Vout, Total current from Vin (To decide the total resistance) and come up with the ratio.  You can then add complexity like Watt, Multiple Vout taps, resistor network reduction and so on.

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amrutah
amrutah
6/30/2014 8:46:19 PM
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Master
Re: Simple Web App
@SachinEE: I am sorry, I saw the web app after posting the previous question.  I understand your point and the weakness in the webapp.

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amrutah
amrutah
6/30/2014 8:37:06 PM
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Master
Re: Simple Web App
@SachinEE:  "what is the need to take voltage input and voltage output values from the user because I didn't find the output resistor values are dependent on voltage."

   In a voltage divider, Say Vhead is the input voltage and vtap is some output voltage which is from the resistor divider then,

    Vtap= Vhead*(Rb/(Rt+Rb)).  To calculate the resistors, we need atleast 2 of the voltages and then assuming the total resistance RT=Rt+Rb, can calculate the Rb.

  I did not understand your point..

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RedDerek
RedDerek
6/30/2014 8:24:33 PM
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Master
Re: resistor calculator spreadsheet
@SachinEE - changing the value in B8 will change the number of visible results in the "Usable set" table area. Essentially, the results listed in the Usable Set table is only shown if the error is better than what is entered in B8.

I look at the Usable Set table to see what set best meets my design needs. For example, if you want a set that provides a certain total series resistance, you can add the two numbers and then use that set for the circuit. In the case of a power supply you may want the top and bottom resistors to add up to 100k equivalent.

One key note: I do think about the factor of 10 when looking at the result table. So if you want values in k-Ohms, then keep that in mind when looking at values.

Each tab operates independently of the others.

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aerohoff
aerohoff
6/30/2014 7:47:13 PM
User Rank
Newbie
Re: Simple Web App
SachinEE, the app allows the user to enter either the ratio, or the input and output voltages, which are used to calculate the ratio. No need to enter all three. Perhaps that is not clear. Frequently you know you have 12v and want 2.5v out, but you might not have bothered to divide the two.

An improvement I should make would be to consider power disipation. My app would currently recommend the same resistors for a 12V to 2.5V divider as it would for a 1200V to 250V divider (115Ω and 437Ω). The latter divider would disipate 2600W (not ideal).

Also, 437Ω is not stocked on digikey, so I wouldn't want to use it in a design. I'll need to address that too.

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SachinEE
SachinEE
6/30/2014 6:46:50 PM
User Rank
Master
Re: resistor calculator spreadsheet
@RedDerek, thanks for sharing that excel with us. I am curious to know if we would like to change the % error where should we change it ? I tried changing B8 value in Ratio tab, do i need to change the value in some other place as well?

 

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