Quote:
Originally Posted by Julesomega
The most accurate reference is at the top of the chain. You divide this down with fixed resistors and there is no increase in battery loading.
|
Honestly, I've think you've missed the points I made about the potential divider. Obviously, I don't know your expertise level, so forgive me if any of this is stating the obvious, but from what you're saying it's clear that I do need to add a bit more detail.
Let's not worry about accurately selecting the resistors for your potential divider for the time being.
Instead, what value are you going to choose for these resistors? Or more pertinently, what's the total value of your resistor chain?
Let's say 10k. And to keep the numbers simple, let's say your input voltage is 10V rather than 12.5V.
This of course means you've got 1mA flowing in these resistors. Not a big deal, perhaps...
On the other side of the coin, what is the source impedance of your voltage source?
Obviously that will vary with the output voltage you select, and reaches a maximum at 5V, where it will be 2.5k.
This is a big problem for something that is supposed to be a
voltage source.
I'm assuming you've read my deliberations around the protection part of the circuit, where I calculated that a 820 ohm resistor (in conjunction with that chunky Zener) would do the job of protecting the LM4040s? But I wasn't happy with such a high output impedance, and discussed how that would introduce errors - eventually ending up with the simple JFET solution...
Clearly, a source impedance of 2,500 ohms is considerably worse than 820, or indeed the 135 that I ended up with. Of course, it's your project, so you must run the numbers and decide what you can accept, but if you invest heavily in making a very accurate voltage source and throw away that accuracy in a potential divider that results in different output voltages depending on what meter you connect to it, then that is not a
well balanced design - if doing that, you might as well save money/design effort on the voltage source. Indeed, if this was a commercial project, you would be obliged to do that (or do something else to balance the design).
OK, so to improve the performance of the potential divider, let's divide the resistances by a factor of 10. This gives a total of 1k, and a maximum source impedance of 250 ohms. This is much better, but hopefully you can now see the problem with current consumption that I mentioned: your divider is now drawing 10mA. Not good for something that runs on PP3s.
As I mentioned, the only way to resolve this conflict is to buffer the output of the potential divider. Not impossible, but definitely moves us out of KISS territory.
Quote:
Originally Posted by Julesomega
I appreciate the KISS principle behind your design: my point is that you dropped the statistical benefit of multiple V-refs for no compelling reason.
|
If you've understood everything above, you now have your compelling reason!
What I've done ensures a consistently low output impedance at all settings of the voltage control, without having to use an op-amp to buffer a potential divider. That is excellent, though I prefer your choice of word. Compelling indeed
Quote:
Originally Posted by Julesomega
You were able to select a) V-refs and b) resistors, to bystep the statistical approach. I may not be so fortunate, that's why I will go back to (your) first principles I will also add fine adjustment with a pot at the 12.5V point so that /all ranges can be set precisely with the one preset when I am next passing an obliging Calibration House.
|
I take your point about selection, but look at the context - I was only aiming for 0.1% initially, which is what the A version of the LM4040 does by default. The fact I was able to select to get much better than that is nice, but effectively "outside the scope of the project". My article includes showing the results of random sampling to show what can be achieved by someone without the means to select them, and the results are pretty good on the whole.
Because the LM4040 is not a high-precision voltage source, I would expect some drift with time. I'm quite up-front about this in the article - there are plenty of voltage references that are higher precision (and higher cost!) than the LM4040, and they should have much better long-term accuracy. But the point is simple - I had a few dozen LM4040s sitting around...
BTW, I didn't select resistors. I didn't need to
A word of caution: if you feel you must include trimmers, do remember that the resistive elements are usually not as good as fixed metal film resistors, so ensure that they exercise the absolute minimum influence over the final result.
I mention this when discussing the divide by 10 section - I put a 50 ohm preset in series with a 1,100 ohm resistor, which gives a plus/minus 2% adjustment range. So if the preset resistor changes by 10% over the years, say from 25 ohms (if it's in the mechanical centre) to 27.5 ohms, the total change seen in the lower limb of the potential divider is from 1125 to 1127.5 ohms - about a quarter of a percent... Imagine if I'd used a 2k preset there, with no resistor. It actually takes a lot of effort to arrive at the optimum amount of range for a pre-set; you need to cover the range caused by the known tolerances plus a bit for safety, but no more. Some test gear - like the old Thurlby PL-series power supplies - are nearly impossible to set up because their presets have far, far too much range.
Again, I apologise if any of this is stating the obvious - I don't know your level of expertise and experience, and can only respond based on what you've written. But I honestly hope this helps. Good luck with your design!
All the best,
Mark