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RE: Need an overview to replace fixed voltage reg with adjustable voltage reg,

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Ryelands

In Reply to: RE: Need an overview to replace fixed voltage reg with adjustable voltage reg, posted by Cleet Torres on March 19, 2014 at 12:17:19

100R is probably as low as I'd choose for R1. The formula is:

R2 = (V-out/1.25) - 1

Thus, for R1 = 100R and V-out = 15v, R2 = 1,320 ohms (1k3)
And, for R1 = 120R and V-out = 15v, R2 = 1,100 ohms (1k1)

Ah, the joy of spreadsheets.

Follow Ups:

RE: Need an overview to replace fixed voltage reg with adjustable voltage reg, - Cleet Torres 13:41:37 03/19/14 (4)
Thanks Ryelands

How did you derive the 1.25? Woul;d you mind sharing the spreadsheet with me?

I want to do a LT1033 for -15v as well ... and probably a boat load of other regs once I figure out how to calculate the R1 & R2 values.

Regards
Paul
- RE: Need an overview to replace fixed voltage reg with adjustable voltage reg, - Ugly 19:03:35 03/19/14 (3)
  Here you go man. No spreadsheets necessary!
  
  The thing you gotta watch with that 100ohm you have is power. At the kind of current that will flow with 100 ohms as R1 will require a (15-1.25)^2/100 = 1.89 + margin W of resistor. A bit over the top really.
  
  Probably makes more sense to use the 100 ohm for the r2 spot.
  - voltage divider calculator (Open in New Window)
  - RE: Need an overview to replace fixed voltage reg with adjustable voltage reg, - Cleet Torres 20:46:20 03/19/14 (2)
    Hi Ugly
    
    Ok, you've lost me.
    
    Why is a voltage divider calc relevant to calculating R values in a adjustable voltage reg? Input voltage is not relevant other than not going to below the dropout voltage or exceeding the max Vin right?
    
    Apologies for the questions, but I new at this.
    
    But to your point, np, I can substitute the 100ohm for the R2 spot.
    
    Regards
    Paul
    - RE: Need an overview to replace fixed voltage reg with adjustable voltage reg, - Ugly 21:34:26 03/19/14 (0)
      If your brave you might try LTspice. I noticed they have LT1083 models. Some of LT's models can be pretty good but I haven't used that one I don't think.
      
      I'm becoming a real LTSpice junky myself but I'll tell you the learning curve is steep on that one.
      LTspice (Open in New Window)
    - RE: Need an overview to replace fixed voltage reg with adjustable voltage reg, - Ugly 21:13:44 03/19/14 (0)
      Note that Vin, Vout, etc variables being discussed on voltage regulator and voltage divider calculator web pages are generic relative terms which may mean completely different things when used in different contexts.
      
      The voltage divider in your circuit is R1 and R2. In the case of the voltage divider web page you'd use the regulator Vout as the voltage divider calculator Vin. And the voltage divider Vout would be the regulator Vfeedback pin, ie 1.25V or whatever the case may be.
      
      Find a voltage divider that works for you using your 100 ohm party on the bottom side. The tricky part will be combining values you can really purchase to make a Vout you can live with while keeping in mind component operating limits.
      
      A hint might be that an extremely obsessive compulsive would likely look at parallel or possibly even series parallel resistor combos to get the voltage divider tweaked just so.It really depends on your app what makes sense.

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