Rescaling The 8145 Series 4-1/2 Digit Meter...

microcarl
Posts: 1
Joined: Mon Dec 12, 2011 10:58 am

Rescaling The 8145 Series 4-1/2 Digit Meter...

Postby microcarl » Mon Dec 12, 2011 2:37 pm

I recently designed and installed a solar power generator using off the shelf components. The system uses 24, fifteen watt solar panels for the renewable power source. The system also uses a 60 ampere PWM based charge controller, a 2000Watt true sine-wave 110-VAC inverter and a 900 ampere-hour battery bank.

The charge controller provides numeric data for charge current being delivered to the battery bank, current voltage across the battery bank, watt-hours delivered to the battery bank during the most recent charge and total watt-hours delivered to the battery bank since the last master reset of the charge controller. The 2,000 watt inverter provides current battery voltage at the terminals of the inverter and instantaneous Kilo-watts being consumed by the load connected to the inverter.

The numerical data provided by the charge controller is adequate. However, the inverter data lacks for a couple of reasons. The first reason the inverter data lacks is that the current consumption displayed is that of the load only and not that of the internal inverter current usage.

The second reason is that the inverter power usage does not have the resolution I require. The power display is only to 2 significant decimal places. That is, for a 2,000 watt load, the display shows 2.00 KW. While the inverter is capable of delivering full 2,000 watts (3,000, intermittent), the application I’m using the inverter for is to operate my pellet stove, independent of the power grid. The pellet stove uses around 100 watts, once it is lit and in full operation. This equates to 0.83 amperes, which shows that the pellet stove is consuming zero energy. As such, I am blind with respect to monitoring and collecting any data with respect to current usage.

This inverter requires about 220 amperes in order to deliver the required 2,000 watts of energy, if fully loaded. To externally monitor the load current being drawn from the battery bank a current shunt will be required. When using an external current shunt, it is recommended that shunt current not be more than about 30% of its full current rating. As there is the possibility of the inverter using in excess of 200 amperes if the inverter is fully loaded, I chose a 500 ampere shunt. So, 200 amperes is 40% capacity of the specified 500 amperes of the current shunt I’ve chosen for my system – close enough. The actual full-scale specification for the current shunt I’m using is 100mV @ 500 amperes. This is a standard current shunt specification.

The next challenge was finding a suitable digital display that won’t break the bank. The cost for a suitable digital meter that has the range and resolution I wanted started at nearly $100.00 and only increased from there.

I came across the LightObject line of meters via a web-search. I’d never heard of this company before. I did some looking around the web-site and found the 8145 series of 4-1/2 panel meters. I was a bit skeptical about using these meters because there wasn’t a meter that would directly fit my requirements. I needed a 100mV based current shunt and, the LoghtObject line of meters seem to be configured around 75 mV shunts.

The lack of direct compatibility was also compounded by the fact that the base range of the analog to digital converter is not specified. And, although the meter appeared to be configurable, there is not adequate information anywhere on the web-site that will allow a perspective designer to determine if a given meter will meet his/her requirements, without actually purchasing a meter and then determining its usefulness through experimentation and evaluation.

I figured that less than $20.00 for an experiment might be useful, so I purchased one EPM-8145R200A meter. In addition, as the EPM-8145B20V did directly meet my requirements, I purchased one of these, as well. So now you have my basic goal and my apprehension for using the meters sold by LightObject. Lets see what I’ve discovered.

My goal from here on will be to describe my experiment and provide a valid method of rescaling the EPM-8145R200A 4-1/2 digital panel meter to suite my need. In fact, the method I will describe will be useful for nearly every meter in 8145 series.

The EPM-8145R200A meter is specified to have a 75 mV input, producing a 199.99 ampere display. I need a 100 mV meter, displaying a value of 500.0 amperes. The first challenge was to determine how the user configuration worked. One help in the discovery was the fact that a chart of common input range configurations is silk-screened on the printed circuit board, but this is specific to input voltage range configuration and not current input range.

I got lucky in that I also purchased a EPM-8145B20V 4-1/2 digit meter, as well. I was able to use this meter to discover clues that would help me with the reconfiguration of the EPM-8145R200A current meter.

The chart for the 20.00 volt configuration specified that, R1 = 1 Meg Ohm, R3 = 10K Ohm and R4 = 10K Ohm. Using an Ohm meter, I did some continuity checks and determined that R1 and R3 form a voltage divider while, R4 is used to set gain for an amplifier that is located on the back side of the printed circuit board, along with the analog to digital converter.

Using the values that are installed on in the meter for the 20.00 volt range, I was able to determine the voltage at the R1-R3 node that feeds the amplifier stage that drives the A/D converter using the voltage divider rule:

Code: Select all

               R3
    Vo = Vi ---------
             R1 + R3


I calculated that the voltage at the R1-R3 node should be 200 mV. This was confirmed by physically driving the meter input to 20.000 volts and measuring the voltage at the R1-R3 node. I got 200 mV, almost exactly. This is also confirmed by the fact that the first configuration voltage in the chart is 200 mV and that R3 is not present in the chart for the 200 mV configuration, indicating that there is no on-board voltage attenuation.

We are close to having what I need to rescale the current meter from a range of 75 mV @ 199.99 amperes to 100 mV @ 500.0 amperes.

One consideration is that I am not using the full 19999 count capacity of the meter. Rather, I am only using the meter up to a 5000 count. In simple numeric terms, I am only using 25 % or ¼ of the range. Keep this in mind as we move forward.

A second consideration is that in the current meter I purchased, R3 is not installed. In addition, R1 is 10K. Because R4 is still 10K, we don't need to consider it for this specific meter rescaling.

From here we have to determine the next step in the rescaling process. That is, we have to figure out how to make the display read a count of 5000 with a 100 mV input to the meter. The first thing to understand is that, because R3 is not present in this meter when measuring a current of 199.99 amperes, the base range at the node of R1-R3 must be 75mV, as determined by the value of R4 and other scaling resisters biasing the pre-amp to the A/D converter. I have found this to be correct by physical measurement.

As we are only using ¼ of the numeric count of this current meter, we can simply divide 75 mV by 4 to get: 75 mV / 4 = 18.75. As a sanity check we can verify this concept using the ratio of the meter in its current 199.99 A configuration and the required reconfiguration, substituting the R1-R3 node voltage requirement with ‘X’:

Code: Select all

              75       X
           ------- = -----
            19999    5000

So, rearranging, we get:

Code: Select all

           75 * 5000
          ----------- = X = 18.75 mV
            19999

Next we must consider that the original 75 mV input is the maximum voltage that this current meter can read without over-ranging. If we drive this meter in it’s current configuration with a 100 mV source, not only will we exceed the 5000 count, the display will exceed the 19999 count and show EEEEE or –EEEEE, depending of the input polarity.

So this brings into question, what are we measuring when we measure current with a shunt? We are measuring voltage, albeit a small voltage. Because it is intended that this meter use a 75 mV = X amperes full scale shunt and, because this meter uses a base range of 75 mV @ 200.00 amperes, a voltage division is not required and therefore, R3 is not needed. But because the current shunt I want to use is a full range voltage of 100 mV, the input range of this meter is exceeded by 25 mV and so, we will have to add a resistor at the R3 location to prevent the meter from over-ranging. But the question is, what should the value of R3 be to get this current meter to meet my requirements?

We can determine exactly what value is need for R3 that will turn the EPM-8145R200A current meter into a 100 mV @ 500.00 (in fact, it becomes a 400 mV @ 1,999.9 ampere meter, full scale) amp meter for use in my solar power generator with the Voltage Divider rule:

Code: Select all

                          R3
              Vo = Vi ---------
                       R1 + R3

Re arranging for determining R3 we get:

Code: Select all

                    Vo * R1
              R3 = --------- = R3
                    Vi - Vo

So then:

Code: Select all

                      18.75 mV * 10K
               R3 = ------------------- = 2.308K Ohms
                     100 mV - 18.75 mV

The above calculations and resistor value was confirmed with physical measurement.

The nearest value resistor that you can purchase is 2.32K Ohms. Digi-Key part number RNF14FTD2K32CT-ND. Digikey.com

As it turns out, the first three scale configurations in the configuration chart all use R4 = 10K Ohm so the above concept can be used for any configuration with a 200 mV base voltage at the node of R1-R3.

The final step in this rescaling process was moving the decimal point selection from D3 to D4, providing 1 tenth ampere resolution for the 500 ampere rescaling.

I have attempted to provide a clear example of real world problem and a reasonable design solution. I hope this work proves to be beneficial to those wanting to rescale the majority of the 8145 series meters.

In closing, good documentation is essential to the success of any company. LightObject should not make the assumption that engineers are willing to invest time for discovery of information that should be clearly documented. In fact, this is the first meter module that I've ever purchased that did not have some sort of documentation accompany it. The fact that it is indicated that these meters can be configured for other ranges is reason enough to provide clear instructions so engineers don't spend valuable time researching something that should be well documented with examples.

Happy metering!!!

tonka1020
Posts: 6
Joined: Sat Mar 17, 2012 6:39 am

Re: Rescaling The 8145 Series 4-1/2 Digit Meter...

Postby tonka1020 » Tue Mar 27, 2012 5:13 pm

I am not an electrical engineer but I will agree with the end of your post! I am trying to re-scale from 20.00v to 2.000v, and it says it's as simple as a flip of D5, but it's not! There is no documentation with the meter nor a very good example on the internet.

Buried in your post may be the answer to my question if you have time to pass it on in my direction!

Thanks!
T

Tech_Marco
Posts: 4311
Joined: Mon Jun 15, 2009 3:00 pm

Re: Rescaling The 8145 Series 4-1/2 Digit Meter...

Postby Tech_Marco » Tue Mar 27, 2012 5:49 pm

To microcarl , we're not the manufactuere. We're just a reselller and we have no way to provide such information for a $19 meter modification other use than it's lable to. Our job is too sell what provide a product that works the way it should. If it's a 20V meter, then we make sure you have a meter that capable to display 20V or 19.999V when input is 20V or within the limit of accuracy. Or, a 200A meter can display 199.99A used with a 200A 75mV shunt with 200A input. It's all intend for DIY project, nothing else. By the way, the manufacturer didn't provide us NOTHING. For most manual and info you found on our site, it's my own work. If you find anyone who is selling the same meter with official document, may be you can past to us for sharing.

Marco


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