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I rolled my own LCD digital panel meters using the 7106 chip. I suspect this chip is what is used in all the small inexpensive meters and in the topology described for the digital panel meter on this website.
I am using two Murata DC-DC isolated converters (one for each meter) to produce the isolated 9 VDC needed but when I hook everything up, I get different readings when compared to my digital multimeter. Here's the mystery, when I use a standard 9 V battery, the meters read accurately - when I put in the isolated supply, the readings are not accurate.
I talked to Murata and they were stumped as well. The converters need a minimum load (the meters take 1 mA so I have ballast resistors to add another 10 mA of current. Plus used a 1 uF tantalum cap for noise filtering. Nothing worked.
Have used other panel meters as well, same result.
Any thoughts?
Thanks in advance for your help.
Gerry
gwshand@runningdogcapitalists.comEdited by GerryS 2013-09-19
isochronic Guru Joined: 21/01/2012 Location: AustraliaPosts: 689
Posted: 07:34pm 17 Sep 2013
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just wondering, are you powering both from the same 9v battery,
(ie in parallel with a common ground) ?
BobD Guru Joined: 07/12/2011 Location: AustraliaPosts: 935
Posted: 08:39pm 17 Sep 2013
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Have you checked the converter output under load, with an oscilloscope, for noise and ripple?
What speed is the 7106 oscillator running?
Have you discussed this problem with the makers of the 7106? Is that Intersil?
The only application notes I saw all show a battery supply for the 7106. My readings were limited.
Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333
Posted: 10:33pm 17 Sep 2013
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I think its a simple problem of ground loops and not a common ground as Chronic had pointed towards.
I think if you connect both grounds (negative) of each circuit together (common ground) than your problem might be solved.
Measuring voltage with one and amperage via a shunt through another.
isochronic Guru Joined: 21/01/2012 Location: AustraliaPosts: 689
Posted: 03:24pm 18 Sep 2013
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The meter power source ground, and the measurement negative
reference connection, are not necessarily the same.
IMHO the critical result would be from using two separate
batteries to power the two 7106's. One meter measures the
voltage from power+ to power-, the other one measures the
potential difference across the shunt resistor. Apart from the
measurement connections, the meters should be isolated at this stage.
Then, use your multimeter to take measurements across the same
points, and compare the live results. They should be the same.
If they are different it means there is an unaccounted offset
to start off with.
Then narrow it down by swapping in the modules.
Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904
Posted: 03:19am 19 Sep 2013
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I had similar problems with a shunt reading Amp meter on my sailboat. The problem went away when I located the shunt in the negative battery lead which allows one to connect the common input & common power together on the Amp reading digital panel meter.Edited by Tinker 2013-09-20Klaus
Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me.
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