Re: Digital meter feedback?
- From: Jamie <jamie_ka1lpa_not_valid_after_ka1lpa_@xxxxxxxxxxx>
- Date: Sun, 26 Mar 2006 22:24:01 -0800
Ken C wrote:
I bought a Chinese digital ammeter on eBay like this one:to be used as a amp meter thus using the local supply the
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=7592942796&sspagename=ADME:B:AAQ:US:1
It requires a 5VDC power supply. The vendor initially suggested you
could pull the power from the leads the ammeter was monitoring:
http://www.lightobject.com/support/PMeter.htm
I am using the meter with a 13.8V system. Without the shunt leads
connected, the meter draws 40ma. As soon as the shunt leads are
connected (with no actual current imposed on the shunt), the meter
current draw went through the roof. Fortunately, I was testing with a
current-limited lab supply.
The vendor then said that his drawing was wrong and that the shunt had
to be grounded if the monitored lead was used to power the meter.
Obviously, an ammeter that must ground its shunt is of limited
utility.
Moreover, the method did not work. The meter showed "-00.4" when
repositioned on a lead to ground when the load was open and there was
no current.
Finally, I switched to a separate power source -- a 9V battery into a
7805. Now the meter works OK, but runs the battery down in ten hours.
I had hoped to put two meters in an enclosure, which means a too-short
five-hour run time.
Apparently, there is an unnecessary and undesirable connection between
the power side of the meter and the sensing side.
Is there any way to isolate the 7805 circuit and still use one of the
13.8V leads?
Ken C
inputs to the meter must not be common to the supply.
in other words, you need like a differential or separate
common input that is not part of the common of the supply.
maybe you should check a little closer to see if the
has a low, common or - input along with the + input ..
normally you simply connect the path through these two
inputs with the shunt across the inputs.
if you don't think the meter has - and + input then
you could construct a simple op-amp using the - and + inputs
in the same fashion and the output to drive the meter as a
voltage level meter instead.
but in my opinion, if the meter is already designed to be
an amp meter then it should have an isolated common and
+ input or a - and + input..
you don't use the common to the supply to connect any
current path..
--
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5
.
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