Coldfusion 4.5 digit LED 2V panel meter

[silicon_ghost]

I recently purchased an inexpensive, high accuracy panel meter.  This particular model was USD19.50 plus shipping.  They are sold from both Lightobject.com (http://www.lightobject.com/Search.aspx?k=4.5%20digital%20meter) and eBay (http://stores.ebay.com/coldfusionxelectronics).

Most panel meters in this price range are either LCD (bleh), or only have 1 or 2 digits of precision.  It is a 4 wire design using 5V power.  I've included a snapshot of the advert on eBay and I took a few action pictures after doing a minor bit of calibrating the unit.  The measurement is darn near spot-on with typical error of less than 1 or 2 least-sig digits.  I checked it against my precision voltage references from http://voltagestandard.com (my thanks to whomever talked about these cheap voltage references).

[T4P]

What IC is it based on? I have my bets it's a ICL7135

[silicon_ghost]

What IC is it based on? I have my bets it's a ICL7135

You may be right.  It is a 28 pin chip.  TI lists only DIP but their website indicates a number of second source manufacturers.  The IC has been sanded down.  I don't see any indication that there is recoverable info.  A quick glance through gaps between the LED panel and the PCB shows that all of the components are on the top layer of the board.  Here are a couple photos.

[T4P]

It isn't the ICL7135 at first glance
ICL7135 pin 1 is V+
I think it's a uC of some sort, can't really see anything beyond all the awful flux remains

[jusanother]

Many thanks for taking the time to review the eBay find, nice pictures. Can I ask is the -ve input is tied to the supply -ve ? The reason I ask is I have one of these:

http://www.ebay.co.uk/itm/4-1-2-Digital-DC2V-Voltage-Panel-Meter-Voltmeter-/320684620851 (ICL7135 based)
seller: mib_instruments pic: attached

On this model the two -ve's are tied together on the PCB, and I suspect it's causing me problems when measuring low (>10mV) voltages that are derived from the same source as the meter is powered.

Any observations gratefully received.

Cam.

[silicon_ghost]

Can I ask is the -ve input is tied to the supply -ve ?

It appears that they are tied together.  I got less than 1 Ohm when I checked resistance between the two leads.

--
Matt

[jusanother]

Thanks for checking that, looks like I'm either using two supplies, or building my own meter, ah well. Shame they are wired this way, true differential would be better. My module draws 100mA ish, the ground lead supplying it measures @ 58mohms, giving a drop of 5.8mV from the real ground potential. Guess this is my prime suspect.

[SeanB]

Get one of those 1W modules Dave was testing, should be perfect for this application.

[jusanother]

Well one bodge wire and a little pcb ground plane hacking later and I have separated the -IN from GND, the meter is much better in my application now. Not pretty but beats a PCB redo for this one off. If anyone is interested the project was a redo of the old EDN battery internal resistance meter, updated the original to give it 3 ranges, works pretty good.

SeanB: What is this "1W modules Dave was testing" you speak of ?

[SeanB]

Look at his recent videos and you will get the DC-Dc converter testing.



EEVblog episode 324.

[jusanother]

Ah yes got you now. Cheers.

[ViewPoint]

I recently purchased an inexpensive, high accuracy panel meter.  This particular model was USD19.50 plus shipping.  They are sold from both Lightobject.com (http://www.lightobject.com/Search.aspx?k=4.5%20digital%20meter) and eBay (http://stores.ebay.com/coldfusionxelectronics).

Most panel meters in this price range are either LCD (bleh), or only have 1 or 2 digits of precision.  It is a 4 wire design using 5V power.  I've included a snapshot of the advert on eBay and I took a few action pictures after doing a minor bit of calibrating the unit.  The measurement is darn near spot-on with typical error of less than 1 or 2 least-sig digits.  I checked it against my precision voltage references from http://voltagestandard.com (my thanks to whomever talked about these cheap voltage references).

Hi,
I am planing to buy a PentaRef  from
voltagestandard.com

Is the board in the specification?

Best

[silicon_ghost]

I recently purchased an inexpensive, high accuracy panel meter.  This particular model was USD19.50 plus shipping.  They are sold from both Lightobject.com (http://www.lightobject.com/Search.aspx?k=4.5%20digital%20meter) and eBay (http://stores.ebay.com/coldfusionxelectronics).

Most panel meters in this price range are either LCD (bleh), or only have 1 or 2 digits of precision.  It is a 4 wire design using 5V power.  I've included a snapshot of the advert on eBay and I took a few action pictures after doing a minor bit of calibrating the unit.  The measurement is darn near spot-on with typical error of less than 1 or 2 least-sig digits.  I checked it against my precision voltage references from http://voltagestandard.com (my thanks to whomever talked about these cheap voltage references).

Hi,
I am planing to buy a PentaRef  from
voltagestandard.com

Is the board in the specification?

Best

Do you mean is the PentaRef within its specs?  Voltage Standard does provide the accuracy to be expected from the unit.  All of my voltage standards from them have performed well when I've cross checked them against other devices.