# measure direction of dc

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Hello,

i need to know in what direction flows current of a bidirectional
connection (pullips with ~10k). I think there will be a current of max.
1mA and 1kHz.

Can you advice me a circuit that gives me the chance to see the
direction (diods with resistors??) ?

Thanks,

Heiko.

Re: measure direction of dc

What's a "pullip"?  A pull-up resistor whose grand-parents were
tulips?

In the subject you say "DC", now you state 1 kHz.  Something doesn't
match up here.

If it's actually DC, any multimeter that can measure current at all
should do just fine.  Since it seems to be a signl line, you'll
probably want to use a shunt resistor and an oscilloscipe instead (and
possibly a differential probe).

--
Hans-Bernhard Broeker ( snipped-for-privacy@physik.rwth-aachen.de)
Even if all the snow were burnt, ashes would remain.

Re: measure direction of dc
Sorry, you are right - i said it in a wrong way.

I have a bidirectional communication on one wire. The bits are sended
with 1kHz. With "DC" i wanted say that it is not AC (powernet or
so...). And sure, i have Pull-UP resistors ;-).

My goal is, to see who is sending the bits to debug the communication.

Any ideas?

Thanks.

Re: measure direction of dc

Strictly spoken, this *is* AC, as in "alternating current", even
though it's fixed polarity, i.e. the voltage on the line has the same
sign, all the time, but the direction of current flow changes, and that's
what you have to measure.

Which means my original answer stands: you just need any odd
amperemeter.  Given the details of the job, it may have to be slightly
more advanced than a plain dumb multimeter for good results, but even
one of the latter should be able to give you a first impression.  For
good measure, use an oscilloscope with a differential probe and a
shunt resistor (or abuse the signal cable itself as a resistor...)

--
Hans-Bernhard Broeker ( snipped-for-privacy@physik.rwth-aachen.de)
Even if all the snow were burnt, ashes would remain.

Re: measure direction of dc

If it's a pulled-up bus, with active pull-downs: if you put a little bit of
series resistance in the wire, watching one end with a 'scope should make
things obvious: the local end will pull to a lower voltage than the remote
end.

Steve
http://www.fivetrees.com

Re: measure direction of dc

Put a series resistor (10 or 100 ohms) in the data signal and connect
the inputs of a comparator (or opamp) to each end of the series resistor.
The output of the opamp will tell you which side of the bus is driving
the data signal.  You will probably need an opamp with rail-to-rail
inputs and a low offset voltage.

Connect the output of the opamp to one channel of an oscilloscope and
the data signal to another.

-jm

Re: measure direction of dc

Since you specified the 1 kHz frequency, it appears that this is AC.

What is wrong with a simple current transformer with one turn in the
primary and say 1000 turns in the secondary ?

Paul

Re: measure direction of dc

You add a LED in the circuit across the secondary
and you have a visual indicator.

Re: measure direction of dc
AC by definition means that the current flows first in one direction,
then the other.
This case could probably be described as pulsed DC.

Regards,
Ken Asbury

Re: measure direction of dc

Which, in this case, is exactly what happens.  Remember: the OP is
direction will indeed change depending on which end of the link is
sending data (by sinking current to ground on its pin), and which end
is receiving it.

--
Hans-Bernhard Broeker ( snipped-for-privacy@physik.rwth-aachen.de)
Even if all the snow were burnt, ashes would remain.

Re: measure direction of dc

Per Merriam-Webster Online:
"alternating current: an electric current that reverses its direction
at
regularly recurring intervals -- abbreviation AC"

I'll stick with pulsed DC

Ken Asbury

Re: measure direction of dc

I'll call BS on that one. Don't go to Merriam-Webster for electrical
terms, and quote the results to a room full of EEs.

Re: measure direction of dc
On 6 Jul 2005 15:09:26 GMT, Hans-Bernhard Broeker

If the current is flowing for a while in one direction and then for a
period in the opposite direction, a current transformer would still
produce a positive or negative going voltage peak, when the current
changes direction.

The advantage with a current transformer is that you do not have to
worry about the common mode voltage range and if the signal current is
reasonable, a simple Schmit trigger after the transformer should be
enough.

Paul

Re: measure direction of dc

Suggestion dismissed on the following grounds:

1.  Too simple
2.  Analog
3.  Thinking exceeds dimensions of box

Re: measure direction of dc

Did you ever use (or price) a Tektronix current probe for a scope?
One wire through a toroid, with mechanism for opening it up for
insertion, is the fundamental mechanism.

--
"If you want to post a followup via groups.google.com, don't use
the broken "Reply" link at the bottom of the article.  Click on
We've slightly trimmed the long signature. Click to see the full one.
Re: measure direction of dc

Yes, of course.  Also AC ammeters.  Brings back
memories of working on Fujitsu POS systems where
it used the same technique with toroid transformers
for each register to pick off and insert into the
signal path.

The point was that the idea scored well because it
was so  direct, simple, elegant, brilliant  rather
than the other intrusive & overblown suggestions.

Re: measure direction of dc

Response dismissed for lack of original question not
seen in this newsgroup at this site.

Re: measure direction of dc

I assume it's some sort of bidirectional data, and you want to see who
is transmitting.

One way is to add a resistor- say a few hundred ohms- in the line-
enough so you can see the voltage across it, low enough not to disrupt
the logic levels. Position it between the pullup (if it's just at one
end) and one of the devices. Look at the voltage across the resistor
with a scope - depending on the system and your scope you might need to
make a differential amplifier. The magnitude (if single pullup) and/ or
direction (if pullups each end) will tell you which device is transmitting.

Paul Burke

Re: measure direction of dc
[Takes digital hat off[
[Puts RF hat on]

If it's proper AC, I would imagine that you will need a Directional
Coupler, which makes things a bit more complicated.

Someone will be along in a few moments to show you a fearsomely simple
and effective circuit using 3 op-amps and a handful of passive
components (or at least that's what usually happens)... Or failing
that, post it to sci.electronics.design

Mike