Voltage Reference

[JaseG]

Hi,

I'm just wondering what would be the best solution for a voltage reference in my current situation.  I'm implementing a dummy load circuit (basicaly the one from the eevBlog) and I want to be able to change the input voltage without altering the maximum alowable current.  To do this I need to have a constant voltage to work with.  My first thought was a voltage regulator which I think would work fine, but it seems like overkill considering it will only be for an op amp input (not even the vcc on it).  Here's my current schem:

https://raw.githubusercontent.com/JasonGilholme/electronics/master/constant_current/schematic.jpg

Any thoughts would be appreciated.

Cheers,
Jase

[Seekonk]

That is a solution, but it does seem like overkill.  I have a stock of new LM431 as they are less than a nickel each.  Without any extra resistors they are a good 2.5V reference.  Close enough for your application.  They can be found in almost any old PC supply and some older wall warts so you don't need to go out and buy one.

[Vgkid]

LM431
http://www.eleccircuit.com/circuit-shunt-regulator-good-quality-by-tl431/
On the attached circuit below, those are 2 resistor values you can use. Values for R1/R2 doesn't matter much as long as the ratio is about 0.2 No caps are in my circuit, you will need them

[JaseG]

Interesting - I have some PC power supplies that I found in the bin at work so I may just have one waiting to be used.  Thanks!

[JaseG]

Based on this image, wouldn't Vref change based on Vin and in turn alter the output voltage from the LM431?

[Vgkid]

yes, but not much, a few mV/V. Can be ignored for your purpose.
Here is the data sheet.
http://www.ti.com/lit/ds/symlink/tl431.pdf

[JaseG]

Sorry, not sure if I was clear.  It doesn't mention it in the schematic but i'd like to be able to vary Vin from 5v up to about 12v. 

Maybe the regulator is a more appropriate option then?

[Vgkid]

Voltage from the TL431 will be fine.
attached a graph of the output, good enough for your purposes.

[JaseG]

Excellent.  My misunderstanding.

Out of interest, what did you use for this graph?

[Vgkid]

Multisim 11th edition(student version). I will easily get a lot of hate for not using a direct spice version...

[Seekonk]

I looked at that schematic again. Why would anyone use a 10K pot to feed a 1K voltage divider?  Use the 2.5V reference and skip the 330/660 voltage divider.  That will get you to 0-2.5A.  If you want to limit it to 2A, put a 2.2K resistor in series with the voltage going to the pot.Use a 1-2K resistor to feed the LM431 and put a cap across it. That will work fine for 5-20V.  Don't think that just because someone puts a pretty looking schematic on the web that they know anything.  For me a nice schematic is a warning signal.

[JaseG]

I looked at that schematic again. Why would anyone use a 10K pot to feed a 1K voltage divider?

I don't know. I'm a beginner posting in the beginner section perhaps?  Can you explain why it's a redundant choice?

Use the 2.5V reference and skip the 330/660 voltage divider.  That will get you to 0-2.5A.  If you want to limit it to 2A, put a 2.2K resistor in series with the voltage going to the pot.

Thanks! I'll breadboard this up and get an understanding of what's going on there.

Use a 1-2K resistor to feed the LM431 and put a cap across it. That will work fine for 5-20V.

What will these two components do?  Is it to limit the current going into the 431 to keep it withing it's working ranges?

Don't think that just because someone puts a pretty looking schematic on the web that they know anything.  For me a nice schematic is a warning signal.

I made the schematic haha and I certainly don't claim to know anything!    Hence the beginner forum posts. 

[JaseG]

As I mentioned, I'm just working from the eevblog schematic to learn how this all works.  In the schematic it has a 50k pot going through an op amp (as a voltage follow / buffer i think??) before going into a 10k voltage divider. 

Is the addition of the op amp a contributing factor to the choice of resistor values?

[Seekonk]

The LM431 acts just like a zener.  Just tie the reference pin to the cathode.  that makes it 2.5 volt.  Just like a zener, a voltage dropping resistor has to go from the +5 to 20V supply.  The LM 431 will have 2.5V across it.  That can feed your pot directly or put a 2K - 2.5K tn series with the power to the pot to obtain  the 2V you want.

But you probably don't want 2V.   If you want to test 5V and use that as the op amp supply, then your gate drive @ 2A will only be 3V.  That could be a problem with many FETs at that current unless the op amp is driven with a separate 12V source. Suggest lower resistance, but it is better to drop that heat into a resistor than a FET.  Consider adding resistance to + that feeds each FET.  Then they will share better.