Howdy,

As a minor feature in a gamma scintillator I'm building, I would like to provide a battery check function. The values may seem funky but they are a function of certain design points. I've muddled together a theoretical circuit to do this, but if people could look it over and let me know a couple things, I would appreciate it. I need to know if:

1) Will this work?
2) Is there an easier/better way of doing this?
3) It would be nice to do this without a negative ref...

Basically, three different key battery voltage levels provide three distinct output currents to a 50uA wiggle stick, which sports a pre-existing scale which makes sense what I'm doing.

thanks,

Attached Images

vbat.PNG (21.9 KB, 34 views)

__________________
Corey

Rapp's Law of Inanimate Reproduction:
"If you take something apart and put it back together enough times, you'll eventually have two of them."

 

I would have used 3 terminal adjustable voltage references for the 2.7 and 3, unless you don't need much accuracy or your power supply voltages are precise and well-regulated.

 

Right.

How I drew the references at this point was just for clarity sake. My supplies are indeed well regulated, but I'm not adverse to providing more precise refs, if needed. (After all, it's just a battery check, so I'm not TOO geeked out about adding more precision than required...)

Any way to make it simpler/smaller?

Thanks for your input!

__________________
Corey

Rapp's Law of Inanimate Reproduction:
"If you take something apart and put it back together enough times, you'll eventually have two of them."

 

Your offset voltages don't do what you want because the 1k resistor value is much too low and the 2M resistor and pot values are much too high.

Attached Images

offset voltage adjust.PNG (21.7 KB, 10 views)

__________________
Uncle $crooge

 

Cool, that's useful. So how do I fix it?

(I was scrounging through books trying to figure out how to do offsets with unity gain, guess I didn't get what they were saying...)

__________________
Corey

Rapp's Law of Inanimate Reproduction:
"If you take something apart and put it back together enough times, you'll eventually have two of them."

 

AG,

So, how's this? (The referenced app note indicated a slight change to my previous circuit for non-inverting offset was in order.)

Is there a fundamentally better approach for this than what I'm trying?

thanks,

Attached Images

vbat2.PNG (27.8 KB, 12 views)

__________________
Corey

Rapp's Law of Inanimate Reproduction:
"If you take something apart and put it back together enough times, you'll eventually have two of them."

 

The value of the 1k feedback resistor is so low that your offset parts don't do anything.
The values of your offset parts are so high that they don't do anything.

Your first and third opamps are simple followers of the voltage at their (+) input.

You don't want and don't need a negative supply.

__________________
Uncle $crooge

 

So how do I make this work? With the first and third opamps I was trying to get offset without gain. Is my approach flawed?

What's the best way to translate the three Vbat levels to their respective, indicated output currents? (With a single resistor between Vbat, say 155k, and the 50uA meter, I can satisfy the first of the three. But then I don't get the spread to fullscale with the highest indicated Vbat...)


thanks,

__________________
Corey

Rapp's Law of Inanimate Reproduction:
"If you take something apart and put it back together enough times, you'll eventually have two of them."

 

If my above approach doesn't work, what would? I'm looking for a circuit to give me the following:

INPUT OUTPUT
5.4V --> 35uA
...... --> .......
6.4V --> 50uA

thanks,

__________________
Corey

Rapp's Law of Inanimate Reproduction:
"If you take something apart and put it back together enough times, you'll eventually have two of them."

 

Hi Corey,

Check this out.

on1aag.

Attached Images

Battery Indicator.GIF (6.9 KB, 12 views)

 

Thanks for your response. Unfortunately, the 50uA meter is single-ended (grounded) for its main function in the circuit, and I don't have sufficient switch contacts to "unground" it for use as you have it wired.

I'll look at your approach more thoroughly shortly, however.

thanks,

__________________
Corey

Rapp's Law of Inanimate Reproduction:
"If you take something apart and put it back together enough times, you'll eventually have two of them."

 

Okay, I'm back from the drawing board, here's my third try. Please let me know whether you think this will work, or if there a fundamentally better way to do this:

Attached Images

vbat3.PNG (25.3 KB, 8 views)

__________________
Corey

Rapp's Law of Inanimate Reproduction:
"If you take something apart and put it back together enough times, you'll eventually have two of them."

 

So you have the 5.4 V source in series with a 154 KΩ resistor in series with the 50 µA meter, and you get 35 µA.

Now, you shunt this 154 kΩ resistor with a two terminal or three terminal Zener that fires at 5.4 V in series with "a resistor".

When the voltage is 6.4 V the meter is required to pass 50 µA.

It gets 6.4 V/154 KΩ = 42 µA from the existing circuitry so it only needs 8 µA from "the resistor".

1 V/8 µA = 1/8 MΩ.

Classical piece-wise-linear analysis.

For extra credit, what is the square root of 69?
If you've already heard this one don't post the answer.

 

I like all that, and I understand it, but I'm really hoping for all the values in between, too. That is, I'm looking for a circuit to translate any voltage between 5.4V and 6.4V to a current output of between 35uA and 50uA, respectively (even below and above that a bit.) Which means a circuit that carries off the math consistently; at least that's how I see it.

Thanks everyone for your help! Perhaps it's not do-able :-)

__________________
Corey

Rapp's Law of Inanimate Reproduction:
"If you take something apart and put it back together enough times, you'll eventually have two of them."

 

I also had a go at it but it didn't work.

Your latest circuit has an input to the first opamp of 2.4V when the battery is 5.4V. The first opamp has a gain of 3.5 which changes a little with the setting of the pot.
The output of the first opamp is 3.6V.

The second opamp attenuates and level-shifts the output of the first opamp then multiplies it by 2. Its gain is not unity it is a little more.

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Uncle $crooge