# Comparing two voltages

Discussion in 'General Electronics Chat' started by Avid Solderer, Jul 2, 2013.

1. ### Avid Solderer Thread Starter New Member

May 30, 2013
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This electronics beginner asks you to imagine two batteries of marginally unequal voltage, their negative poles connected together with the slider terminal of a potentiometer. One potentiometer terminal is connected to one battery positive and the other potentiometer terminal is connected to the other battery positive. Also connected between the battery positives, in parallel with the potentiometer, is a centre zero reading moving coil meter. Given an appropriate value for the potentiometer, will operating the slider null the meter? And, if it will, and the potentiometer is calibrated, will it indicate the same null point for any two batteries with exactly the same proportional difference of voltage, regardless of total voltage in the circuit? Also, is this the simplest circuit for establishing such comparisons? Or have I got this all terribly wrong?

2. ### WBahn Moderator

Mar 31, 2012
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This sounds a lot like homework. Whether it is or not, you need to make your best effort to solving it, or at least setting it up. Draw a circuit diagram and analyze the circuit to see what effect the position of the potentiometer has, if any. Also, you need to specify whether the batteries are ideal or if they have internal resistance.

That will give us a starting point for helping you figure it all out -- which is how you will learn the most from it.

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3. ### Avid Solderer Thread Starter New Member

May 30, 2013
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WBahn - Thanks for that but, no, it's not homework! It relates to an earlier post of mine wanting help with adapting a luxmeter circuit to a colour temperature meter. I enquired if it was possible to compare the outputs from two sensors by a nulling method, which brought no very helpful replies. Thinking about what I had asked, it seemed wiser, after reading some of the Wheatstone bridge pages here, to formulate this simpler question to see if I was completely barking up the wrong tree. Am I? I've not got access, at the moment, to equipment to try any of this out.

4. ### MrChips Moderator

Oct 2, 2009
12,636
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This is what you are looking for?

If you know that one voltage source is always equal to or greater than the other, you can simplify the circuit. Here V1 is equal to or greater than V2:

Last edited: Jul 2, 2013
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5. ### t_n_k AAC Fanatic!

Mar 6, 2009
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I thought this is what the OP was (rather curiously) describing in words ...

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6. ### MrChips Moderator

Oct 2, 2009
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That is what the OP describes but it will not work for what the OP wants.

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7. ### Avid Solderer Thread Starter New Member

May 30, 2013
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t n k - Thank you. Yes, that's exactly what I was describing ""rather curiously"(?) I have no method of drawing a circuit, so a description of such a simple one seemed the way to go.

MrChips - Thank you both for pointing out that my suggestion wouldn't achieve my objective and for giving two circuits that would. It's a matter of the utmost simplicity to ensure that one sensor output remains routinely higher than the other and, since I want the calibrated pot. to reflect the state of balance between the two sensors, thus indicating colour temperature, the second circuit is the one I'll go for. It's very rewarding to have such a to-the-point answer so, again, thank you.

8. ### studiot AAC Fanatic!

Nov 9, 2007
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I didn't see you original thread, which sounds interesting.

I recommend you PM (private messsage) a moderator to join your two threads together and move them to the projects forum.

The development of your new meter will make a jolly good project.

I haven't a lot of time tonight, but will post something tomorrow.

I'm gald to see that you have identified :

In your original EE article there is only one voltage source viz the battery.
the sensors are light dependent resistors, which respond by varying the current passing (ie their resistance) under illumination.

The proposed Vishay sensor is a light sensitive diode.
This produces an increase in current / voltage under illumination.

So the original circuit needs to be replaced. There is no need for a null method, since modern components can amplify, scale and buffer the output in a differential amplifier to directly drive a readout or meter. This is the Hamamatsu method, which is not difficult these days.

I take it you are intending to use the EE red/blue balanced filter and use the output to resolve colour temp?

Do you have all the other filters described for calibration?

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9. ### Avid Solderer Thread Starter New Member

May 30, 2013
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Studiot – Thank you for that. My thinking ran as follows. The EE article describes two light dependent resistors, one behind a red filter and the other behind a blue filter, that form two arms of a Wheatstone bridge. Nulling the centre reading meter via a calibrated pot. indicates the state of balance between these sensors and thus the colour temperature.

A more reliable and linear sensor, in the form of a photodiode, forms the basis of the Hamamatsu luxmeter. It struck me that if one had two such circuits, with a red filter over one photodiode and a blue filter over the other, then an easy way to turn them into a colour temperature meter would be to compare their outputs using a nulling method, as before. Since what's being compared this time are voltages, rather than resistances, I was looking for a suitable circuit to allow this.

My attachment to nulling is that it's a particularly sensitive technique, capable of allowing for both wide and narrow spreads of reading without too much switching of components and, most importantly, I have a particlarly nice, large centre zero reading meter sitting on a shelf doing nothing.

I don't, at the moment, have any filters for calibration, although I have filters for the sensors. I'll see what this thread produces for the moment, since it's streets ahead of my earlier attempt, before possibly taking up your suggestion of merging threads and making it a project.

For anyone interested, the other thread, titled “Wanted – Colour Temperature Meter Circuit from Lux Meter Circuit” can be found by clicking on my name and then going to “Find all posts by Avid Solderer”.

Nov 9, 2007
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