how does a DMM take a differential voltage measurement

P

panfilero

Guest
I've been trying to find some info on this on google... but I just
keep finding sites on DMM and how to measure voltage with them... but
my question is... how does a DMM manage to read a voltage without
access to the common point of the circuit your measuring?

If anyone knows of a good website that explains that could you please
point me towards that... I can't find one.

Thanks!
 
"panfilero" <panfilero@gmail.com> wrote in message
news:d58cb611-1764-4791-b3aa-347b89f90a47@e21g2000yqb.googlegroups.com...
I've been trying to find some info on this on google... but I just
keep finding sites on DMM and how to measure voltage with them... but
my question is... how does a DMM manage to read a voltage without
access to the common point of the circuit your measuring?

If anyone knows of a good website that explains that could you please
point me towards that... I can't find one.
Click to expand...
A voltage is a potential difference between two points.
All voltage measurements are from point A to point B.
On a DMM the voltage is from probe tip to probe tip. It helps to have good
probes.
 
On Wed, 1 Apr 2009 13:31:56 -0700 (PDT), panfilero
<panfilero@gmail.com> wrote:

I've been trying to find some info on this on google... but I just
keep finding sites on DMM and how to measure voltage with them... but
my question is... how does a DMM manage to read a voltage without
access to the common point of the circuit your measuring?

If anyone knows of a good website that explains that could you please
point me towards that... I can't find one.

Thanks!
Click to expand...
It's like a tape measure; it measures the distance (voltage) between
any two points. And like a tape measure, there's no available "zero"
point to make absolute measurements from. We sometimes use a chassis,
or the earth itself as "ground", but that's still arbitrary.

John
 
"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:3rp7t4hk7hhqbat5413n2870pi8hs2a8vb@4ax.com...
On Wed, 1 Apr 2009 13:31:56 -0700 (PDT), panfilero
panfilero@gmail.com> wrote:

I've been trying to find some info on this on google... but I just
keep finding sites on DMM and how to measure voltage with them... but
my question is... how does a DMM manage to read a voltage without
access to the common point of the circuit your measuring?

If anyone knows of a good website that explains that could you please
point me towards that... I can't find one.

Thanks!

It's like a tape measure; it measures the distance (voltage) between
any two points. And like a tape measure, there's no available "zero"
point to make absolute measurements from. We sometimes use a chassis,
or the earth itself as "ground", but that's still arbitrary.
Click to expand...
It's hardly arbitrary when measuring a voltage drop across a component.

Generally, it's seldom arbitrary.
 
On Wed, 1 Apr 2009 18:26:40 -0400, "Charles"
<charlesschuler@comcast.net> wrote:

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:3rp7t4hk7hhqbat5413n2870pi8hs2a8vb@4ax.com...
On Wed, 1 Apr 2009 13:31:56 -0700 (PDT), panfilero
panfilero@gmail.com> wrote:

I've been trying to find some info on this on google... but I just
keep finding sites on DMM and how to measure voltage with them... but
my question is... how does a DMM manage to read a voltage without
access to the common point of the circuit your measuring?

If anyone knows of a good website that explains that could you please
point me towards that... I can't find one.

Thanks!

It's like a tape measure; it measures the distance (voltage) between
any two points. And like a tape measure, there's no available "zero"
point to make absolute measurements from. We sometimes use a chassis,
or the earth itself as "ground", but that's still arbitrary.

It's hardly arbitrary when measuring a voltage drop across a component.

Generally, it's seldom arbitrary.
Click to expand...
So, what is the absolute voltage of Planet Earth?

John
 
"panfilero"
I've been trying to find some info on this on google... but I just
keep finding sites on DMM and how to measure voltage with them... but
my question is... how does a DMM manage to read a voltage without
access to the common point of the circuit your measuring?

If anyone knows of a good website that explains that could you please
point me towards that... I can't find one.
Click to expand...

** Try: www.aprilfool.com




...... Phil
 
"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:95u7t4lu5umelf96tclj4gdpcip78d1rl1@4ax.com...
On Wed, 1 Apr 2009 18:26:40 -0400, "Charles"
charlesschuler@comcast.net> wrote:


"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in
message
news:3rp7t4hk7hhqbat5413n2870pi8hs2a8vb@4ax.com...
On Wed, 1 Apr 2009 13:31:56 -0700 (PDT), panfilero
panfilero@gmail.com> wrote:

I've been trying to find some info on this on google... but I just
keep finding sites on DMM and how to measure voltage with them... but
my question is... how does a DMM manage to read a voltage without
access to the common point of the circuit your measuring?

If anyone knows of a good website that explains that could you please
point me towards that... I can't find one.

Thanks!

It's like a tape measure; it measures the distance (voltage) between
any two points. And like a tape measure, there's no available "zero"
point to make absolute measurements from. We sometimes use a chassis,
or the earth itself as "ground", but that's still arbitrary.

It's hardly arbitrary when measuring a voltage drop across a component.

Generally, it's seldom arbitrary.


So, what is the absolute voltage of Planet Earth?
Click to expand...
Look here:
http://en.wikipedia.org/wiki/How_many_angels_can_stand_on_the_head_of_a_pin%3F
 
On Wed, 1 Apr 2009 20:14:15 -0400, "Charles"
<charlesschuler@comcast.net> wrote:

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:95u7t4lu5umelf96tclj4gdpcip78d1rl1@4ax.com...
On Wed, 1 Apr 2009 18:26:40 -0400, "Charles"
charlesschuler@comcast.net> wrote:


"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in
message
news:3rp7t4hk7hhqbat5413n2870pi8hs2a8vb@4ax.com...
On Wed, 1 Apr 2009 13:31:56 -0700 (PDT), panfilero
panfilero@gmail.com> wrote:

I've been trying to find some info on this on google... but I just
keep finding sites on DMM and how to measure voltage with them... but
my question is... how does a DMM manage to read a voltage without
access to the common point of the circuit your measuring?

If anyone knows of a good website that explains that could you please
point me towards that... I can't find one.

Thanks!

It's like a tape measure; it measures the distance (voltage) between
any two points. And like a tape measure, there's no available "zero"
point to make absolute measurements from. We sometimes use a chassis,
or the earth itself as "ground", but that's still arbitrary.

It's hardly arbitrary when measuring a voltage drop across a component.

Generally, it's seldom arbitrary.


So, what is the absolute voltage of Planet Earth?

Look here:
http://en.wikipedia.org/wiki/How_many_angels_can_stand_on_the_head_of_a_pin%3F
Click to expand...
Never give up hope!

Merely count the number of protons in the earth, and subtract the
number of electrons.

Multiply that by 1.6e-19 to get the net charge on the planet, in
colombs.

Divide by 708e-6, the capacitance of Earth in farads.

And there you are.

ftp://jjlarkin.lmi.net/Exit201.jpg

John
 
panfilero wrote:

I've been trying to find some info on this on google... but I just
keep finding sites on DMM and how to measure voltage with them... but
my question is... how does a DMM manage to read a voltage without
access to the common point of the circuit your measuring?
Click to expand...
Don't use the circuit common.

DMMs are differential measuring instruments by design.

Graham
 
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