Measuring Isolated DC Voltage of Unknown Polarity

F

Frank

Guest
Hi Everyone,

What's the safest way to determine the magnitude and polarity of a DC
voltage?

Think along the lines of powering your DMM from your car battery.
Then trying to use it to measure the polarity and amplitude of a
cigarette lighter adapter. Also assume that the polarity of the
cigarette lighter socket could be either tip-positive or tip-negative.
Also assume the DMM's ground input is NOT isolated from the car's
ground.

(The above description is for illustration purposes, and was the best
way I could think of to describe my problem).

How do you not smoke things in the case of the adapter's polarity
dumping the full power into the DMMs ground (i.e. the car's ground)?

OK, I know I can optically isolate the inputs, but I'm not sure of the
best type of optoisolator to use. I'd be interested in measuring
voltages in the 1.5-15v range.

I had thought of using a bridge rectifier to insure positive to
positive and neg to neg, but with the low 1.5v requirement, the
voltage drop across the two diodes would be almost the entire minimum
voltage (1.5v).

I've seen some op-amp absolute magnitude and polarity circuits, but I
don't understand them enough to see how they'd work.
(http://www-micrel.deis.unibo.it/~benini/ELEI/Reading/AN-31.pdf, see
page 13).

Any insight would be appreciated!


Thanks,

-Frank
 
<Frank> wrote in message
news:lpbeb0h76jrdnju4sp0ramnan9jeu025n7@4ax.com...
Hi Everyone,

What's the safest way to determine the magnitude and
polarity of a DC
voltage?

Think along the lines of powering your DMM from your car
battery.
Then trying to use it to measure the polarity and
amplitude of a
cigarette lighter adapter. Also assume that the polarity
of the
cigarette lighter socket could be either tip-positive or
tip-negative.
Also assume the DMM's ground input is NOT isolated from
the car's
ground.

(The above description is for illustration purposes, and
was the best
way I could think of to describe my problem).

How do you not smoke things in the case of the adapter's
polarity
dumping the full power into the DMMs ground (i.e. the
car's ground)?

OK, I know I can optically isolate the inputs, but I'm not
sure of the
best type of optoisolator to use. I'd be interested in
measuring
voltages in the 1.5-15v range.

I had thought of using a bridge rectifier to insure
positive to
positive and neg to neg, but with the low 1.5v
requirement, the
voltage drop across the two diodes would be almost the
entire minimum
voltage (1.5v).

I've seen some op-amp absolute magnitude and polarity
circuits, but I
don't understand them enough to see how they'd work.

(http://www-micrel.deis.unibo.it/~benini/ELEI/Reading/AN-31.
Click to expand...
pdf, see
page 13).

Any insight would be appreciated!


Thanks,

-Frank
Click to expand...
Why would any sane person power their DVM from the unknown
source?
Use a portable DVM powered from its own internal and
ISOLATED battery.
Most modern DVMs are Auto Ranging and Auto Polarity Sensing.

--
John G

Wot's Your Real Problem?
 
<Frank> wrote in message news:lpbeb0h76jrdnju4sp0ramnan9jeu025n7@4ax.com...
Hi Everyone,

What's the safest way to determine the magnitude and polarity of a DC
voltage?

Think along the lines of powering your DMM from your car battery.
Then trying to use it to measure the polarity and amplitude of a
cigarette lighter adapter. Also assume that the polarity of the
cigarette lighter socket could be either tip-positive or tip-negative.
Also assume the DMM's ground input is NOT isolated from the car's
ground.

(The above description is for illustration purposes, and was the best
way I could think of to describe my problem).

How do you not smoke things in the case of the adapter's polarity
dumping the full power into the DMMs ground (i.e. the car's ground)?

OK, I know I can optically isolate the inputs, but I'm not sure of the
best type of optoisolator to use. I'd be interested in measuring
voltages in the 1.5-15v range.

I had thought of using a bridge rectifier to insure positive to
positive and neg to neg, but with the low 1.5v requirement, the
voltage drop across the two diodes would be almost the entire minimum
voltage (1.5v).

I've seen some op-amp absolute magnitude and polarity circuits, but I
don't understand them enough to see how they'd work.
(http://www-micrel.deis.unibo.it/~benini/ELEI/Reading/AN-31.pdf, see
page 13).

Any insight would be appreciated!


Thanks,

-Frank
Click to expand...

Hi, Frank -

If it were my project, I would be more inclined to build or buy an isolated
power converter rather than try to isolate the inputs. Dealing with the
power supply should be much less critical than dealing with the measuring
circuit.

Good luck.

John
 
<Frank> wrote in message news:lpbeb0h76jrdnju4sp0ramnan9jeu025n7@4ax.com...
Hi Everyone,

What's the safest way to determine the magnitude and polarity of a DC
voltage?

Think along the lines of powering your DMM from your car battery.
[...]
Click to expand...

You don't give us enough details about your problem to explain why the issue
exists; so, you're liable to get a lot of answers that aren't very useful to
you. How about if you give us more detail on the problem?

Based on what you've said so far, the answer is "use a DMM." But maybe you
mean "measure" in some other way, e.g., output a signal to a
microcontroller, etc. If so, then what?

Never underestimate the usefulness of a micropower circuit and a lithium
battery. For many applications, changing a battery every 10 years is
perfectly acceptable.
 
<Frank> wrote in message
news:lpbeb0h76jrdnju4sp0ramnan9jeu025n7@4ax.com...
Hi Everyone,

What's the safest way to determine the magnitude and polarity of a
DC
voltage?

Think along the lines of powering your DMM from your car battery.
Then trying to use it to measure the polarity and amplitude of a
cigarette lighter adapter. Also assume that the polarity of the
cigarette lighter socket could be either tip-positive or
tip-negative.
Also assume the DMM's ground input is NOT isolated from the car's
ground.
Click to expand...
The best way to do that is to measure the voltage at common, and the
voltage at hot, and subtract the 2.

Norm Strong
 
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