Richard Aston's:............
Resistor Selector!

1. What is this rubbish?
2. What kind of fruitcake uses something like this?
3. How do I use it (assuming for the moment I am a fruitcake)?
4. Great. Like, how often am I going to need that?

5. Disclaimer
6. Download

This little application is intended to make selecting resistors FUN!!

How many times have you been there, trying to find two resistors from the range that would give the closest result to the ratio you need?  Pain in the a*se, isn't it.

This program tests them all to find the best fit.

The kind of fruitcake who uses this is me.  


You have an inverting amplifier used to sense the current through two parallel resistors.  The current is 15mA and the output you want is 7.5V at pin 6 of the op-amp.  
Let's not ask why you want this or why you chose 47K and 33K.  You did, that's all.  

What do you do?  Well, let's say you have the RS catalogue in your left hand and your calculator in your right.  You divide 7.5V by 33K and store the result.  You multiply it by 47K to get the voltage at the input (junction R1, R2, R3).  Subtract the stored current from the 15mA and then divide the calulated voltage by the current to get the perfect parallel resistance of R1 and R2.  Then, choose a larger value from the catalogue.  Then, use a parallel subtract to find the other resistor and do a look-up for the closest to that.

Then, calculate R1, R2, R3 in parallel and multiply by 15mA.  Multiply by 33/47 and get answer.  Calculate error from 7.5V and go back and choose another R1.  Do this until bored.

This program is stack-based.  The answer is left on the stack.  It chooses the nearest above (normal) and below (mirror).  The best is selected.  

It does the same steps you would do.

1. Replace "Target Value" with 7.5.
2. Start from lower minimum value - 100R is too low but it doesn't matter.  Put 100 into the "Minimum Value" box.
3. Copy the rest of the example.
4. "Step" through it to test it.
5. Click "Process" to zip through all the possibilities from Minimum to Maximum.

Note: "STEP" on the left calls the next value from the E12, E24... series and is the same value until after the "LOOP" instruction.  "TABLE SELECT" looks up the closest value.

It's so bloody obvious what it does that it isn't worth explaining any further.

You will need it rarely but when you do use it, you will be happy!

p.s. You can see who had a Jupiter Ace computer many years ago!

Legal disclaimer:
Oh, bugger off and stop whinging. It's free, isn't it?  That ought to be enough but some people...  Sheesh!
They are your calculations, not mine and if it all goes wrong, then you should have
checked it more thoroughly.
So ner.

Yeah, yeah.  All that.  Where can I get it?
You can download it here.