Re: Digital meter feedback?
- From: "Chris" <cfoley1064@xxxxxxxxx>
- Date: 26 Mar 2006 17:06:53 -0800
Ken C wrote:
I bought a Chinese digital ammeter on eBay like this one:
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
Your vendor is deserving of negative feedback if he told you a
7107-based Digital Panel Meter (DPM) can handle what's called
"high-side measurements". He didn't know what he was talking about.
Actually, it's only because of your caution and using a current-limited
supply that your DPM is even working.
Now, there are two ways to do this. First, you can use an isolated
DC-to_DC converter which will work over the automotive voltage range
(11V to 16V, with load dump protection) for each DPM (you can double up
if both shunts have the battery voltage as a common, but you may have
to disable the negative segment of the first LED -- they'll both read
negative amps).
The second involves some cobbling. You can use an op amp configured as
a diff amp, along with some precision resistors (and probably a tweaker
pot to offset null) to do the level translation. Also, with a single
supply op amp, you may have a bit of trouble getting dowm to below
100uV (one count of your DPM is 100uV). From your prior post, this
sounds like it might be a little too much, but it's your call. If
you're interested, and are willing to cobble together an op amp circuit
to help, post back.
Of course, you could still go retro, and use your 1 milliohm shunts
with a couple of 0-50 amp analog panel meters (All Electronics P/N CAT#
PMD-50A, $12 each in single quantities). Then you don't have to worry
about power supplies.
http://www.allelectronics.com/index.html
Good luck
Chris
.
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