# Subtract two dc voltages.

1. ## Subtract two dc voltages.

Hi,

I need to design a passive circuit that subtract two voltages. Can you gve some suggestions?

Thank you

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2. ## Re: Subtract two dc voltages.

Hi,

What's the problem?
Mathematics? Just subtract the values: Example: 9V - 1.5V = 7.5V

Hardware? Reverse connect the second one: example: A 9V battery and a 1.5V battery. Connect the MINUS of the 1.5V battery to the PLUS of the 9V battery. Use a voltmeter, connect the VM minus to 9V battery minus, and the VM plus to the 1.5V battery minus.

EDIT: sorry, just recognized you clearly talk about DC voltages:
Thus the following lines become invalid:
(Or are you talking about sinewave, or any other waveform?
Or RMS values?)

Klaus

3. ## Re: Subtract two dc voltages.

Install a resistor between the two sources. One source becomes ground for the other source. Measure voltage across the resistor to read differential or subtractive value.

As an alternate method, install 2 anti-parallel led's between the two sources. (Safety resistor is necessary in real life.) The lit led (and its brightness) tells you differential or subtractive value.

4. ## Re: Subtract two dc voltages.

Connect the MINUS of the 1.5V battery to the PLUS of the 9V battery. Use a voltmeter, connect the VM minus to 9V battery minus, and the VM plus to the 1.5V battery minus.
I am confused: how it is going to work in real life?

You are connecting to batteries in series but measuring the voltage across the 9V battery.

Am I missing something?

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5. ## Re: Subtract two dc voltages.

Originally Posted by c_mitra
I am confused: how it is going to work in real life?

You are connecting to batteries in series but measuring the voltage across the 9V battery.

Am I missing something?
Although this, I believe, is not what the OP wants, I believe it is an oversight.

The V probe of the meter should be connected to the +ve of the 9V battery and the COM probe of the meter to the +ve of the 1.5V battery. Both -ve terminals of the two batteries should be tied together.

The OP wants a passive circuit equivalent to the opamp subtracter.

6. ## Re: Subtract two dc voltages.

Hi,

I am confused: how it is going to work in real life?

You are connecting to batteries in series but measuring the voltage across the 9V battery.
Thank you for the feedback. I made a mistake. I should be:

A 9V battery and a 1.5V battery. Connect the PLUS of the 1.5V battery to the PLUS of the 9V battery. Use a voltmeter, connect the VM minus to 9V battery minus, and the VM plus to the 1.5V battery minus.

Klaus

7. ## Re: Subtract two dc voltages.

Install a resistor between the two sources. One source becomes ground for the other source. Measure voltage across the resistor to read differential or subtractive value.

As an alternate method, install 2 anti-parallel led's between the two sources. (Safety resistor is necessary in real life.) The lit led (and its brightness) tells you differential or subtractive value.
Hi,

Thank you for your reply. I am designing a phase detector for which I need to convert differential output in single one by subtractng them. I need difference of two voltages with respect to ground.

8. ## Re: Subtract two dc voltages.

By installing a capacitor between the two sources, rapid zero-crossings are detectable. The capacitor produces spikes.
However it may also produce spikes from abrupt transitions which are not zero-crossings. Therefore it depends on abruptness of your source waveforms.

If you wanted to see gradual changes in the subtracted voltage. Then you might disconnect the capacitor when it is fully charged. Then connect one lead of the capacitor to 0V ground. At the other lead is the differential voltage. That's the theory. An H-bridge may be able to do the same thing electronically.

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9. ## Re: Subtract two dc voltages.

Hi,

Maybe it's time to start to give useful and complete informations.
It's neither clear what you need , nor what voltages and currents you are talking about.

Klaus

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