Re: Ammeter internal current draw


Ken C wrote:
On Thu, 02 Mar 2006 14:11:54 -0600, John Fields
<jfields@xxxxxxxxxxxxxxxxxxxxx> wrote:
The readings are going to stay pretty much the same no matter what
you do, DC wise, but as the length of wire between the shunt and the
meter increases you may wind up with an antenna... :-(

Will I minimize the antenna effect if I use coax for the signal leads,
like RG-174? There will be RF radiation (1.8-432 mHz) in the
immediate vicinity. RF currents in conductors is a mystery to me and
your experience would be invaluable.

Ken C

Hi, Ken. The input impedance of your ICL7107-based digital panel meter
is almost certainly 10 megohms. Mr. Fields is correct -- a longer line
might act like an antenna, picking up electrical noise. It sounds like
he's been there before. I have, too.

The ICL7107 is very good at cancelling out 60Hz noise, but less good at
other frequencies. If you've got a longer wiring length from the shunt
resistor to the DPM (say, over 6 feet or so), you've got a close source
of RF energy, or if your wiring is going through wiring duct with high
current lines (could cause inductive pickup), you'll want to provide
yourself some protection for your DPM input.

Note here that you will need a separate, floating power supply for the
DPM -- they work that way. Unless special circuitry is included on
board, the 7107 can't measure its own power supply.

Omitting the power supply, here's what your basic circuit should look
like (view in fixed font or M$ Notepad):

|
| ___
| .---o-o---|___|---o-o----.
| | |1 milliohm | |
| | | | |
| | | _ | |
| +| | + / \ | .-.
| --- '---(DPM)---' | |
| - \_/ | |1 ohm
| 12VDC| '-'
| | |
| '------------------------'
|
(created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)

Your "30 amp" shunt is almost certainly a 1 milliohm resistor, set so
the DPM will read 29.9 for 29.9 amps. One of the easiest ways to
reduce the effect of electrical noise is to provide a load resistor
across the DPM to directly provide a lower input impedance:

|
| ___
| .---o-o---|___|---o-o----.
| | |1 milliohm | |
| | | | |
| | | | |
| | | | |
| | | ___ | |
| | o---|___|---o |
| | | 220 ohm | |
| | | _ | |
| +| | + / \ | .-.
| --- '---(DPM)---' | |
| - \_/ | |1 ohm
| 12VDC| '-'
| | |
| '------------------------'
|
(created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)


This simple step helps most problems. But for higher energy
interference, you can either use protection diodes or a load capacitor
here.

| ___
| .------o-o---|___|---o-o----------.
| | |1 milliohm | |
| | | | |
| | 100 ohm| | |
| | .-. | |
| | | | | |
| | | | | |
| | '-' | |
| | | 2 X | |
| | | 1N4001 | |
| | | | |
| | o-----|<----o |
| | | | |
| | o----->|----o |
| | | _ | |
| +| | / \ | .-.
| --- '---(m V)---' | |
| - \_/ | |1 ohm
| 12VDC| '-'
| | |
| | |
| '---------------------------------'
(created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)

The diodes here are valuable, because they limit any voltage excursion
to 600mV or so, which is overrange for the DPM, but won't cause damage.
This setup is good for inductively coupled noise, which could be
higher energy.

Here's another setup, which is probably better for "antenna-type"
noise:

| ___
| .------o-o---|___|---o-o----------.
| | |1 milliohm | |
| | .-. | |
| |220 ohm| | | |
| | | | | |
| | '-' 0.1uF | |
| | | || | |
| | o----||-----o |
| | | || | |
| | | _ | |
| +| | / \ | .-.
| --- '---(m V)---' | |
| - \_/ | |1 ohm
| 12VDC| '-'
| | |
| '---------------------------------'
|
(created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)

Make sure you use a good ceramic cap which is made to be low impedance
at your frequencies of interest. And attach the resistor right at the
DPM.

Or use a combination of these. Component selection kind of depends on
additional information, like the length of your wire run, other
potential sources of noise, &c.

By the way, coax won't hurt, but you should be able to use plain
twisted pair for just about any application.

From the perspective of a "contriver of contrivances", you probably
don't have the equipment to track down problems here. It might be best
just to build in protection, test it well, and then test it better. If
it doesn't cause problems, it's probably good.

Many times, people forget that there's another solution to this type of
problem. If you have problems with other sensitive electronics in this
installation, it might be better to go with a time-tested solution
that's impervious to RF energy and doesn't require an external power
supply -- namely, an analog meter with external shunt. Your total
installed cost will be competitive, even if you didn't have any
problems with your DPM. If you have to spend hours of fiddle-factor
time on your DPM, the analog meter comes out way ahead. Assuming
everyone who's using the meter can actually read analog meters, you can
just install it, enjoy a quick win, and move on to the next problem.

Since your DPM is on the low end of its range (you're only going to be
using 299 counts of the 2000 count full scale range), a good analog
meter would probably be just about as accurate as your DPM, too.

Good luck
Chris

.

Relevant Pages

  • Re: Digital meter feedback?
    ... supply that your DPM is even working. ... How is this meter supposed to be used? ... showed "-00.4" amps with no current imposed on the shunt and supply ... There's separate connections for power and the measurement. ...
    (sci.electronics.basics)
  • Re: Current measurement
    ... troubleshooting I would say the roll your own shunt and a cheap meter ... A copper shunt looks like a good idea until you look at copper's ... used to make inexpensive RTDs (resistance temperature devices). ... I keep a 100 amp and a 500 amp shunt in my tool box all the time. ...
    (rec.outdoors.rv-travel)
  • Re: tubes Vs fets, ohmmeter schematics
    ... voltage of say 9V across the DUT, ... 100MegOhm FS = 100K Ohms shunt ... The 1M shunt would make the meter have about 800k of input resistance, ...
    (rec.audio.tubes)
  • Re: Current measurement
    ... A copper shunt looks like a good idea until you look at copper's ... used to make inexpensive RTDs (resistance temperature devices). ... I keep a 100 amp and a 500 amp shunt in my tool box all the time. ... also have a DC clamp-on meter or two but when I really need the right ...
    (rec.outdoors.rv-travel)
  • Re: what size shunt do I need?
    ... I have a 70A alternator and 220AH house battery capacity. ... "you need a 50mv, 70A shunt", where do I get one? ... If you have a meter calibrated for full scale at 50A to use with a 50A, ... Navy, can do this calibration, and does, even on matched 50mv units. ...
    (rec.boats.electronics)