Panel Mount Digital Meters

[ko4nrbs]

Can anyone recommend an accurate mini panel mount amp and volt meter? I'm looking for the small ones you see on Ebay.

Since I will be measuring Lithium Ion battery charge voltage and current I'm thinking they should have a range of 0 to 5 vdc and 2 amps or so for the best accuracy. Make sense??

Bill

 

[hiuintahs]

For accuracy such as you would find in a handheld DVM, I like the panel meters that you find with a higher resolution of accuracy such as the 3 1/2 digit or 4 1/2 digit. A 3 1/2 digit meter has a max reading of 1999 and a 4 1/2 digit has a max reading of 19999. That is not volts but just the max reading on the display. Where you put the decimal point is optional based on which display you buy. The 1/2 signifies the "1" in the display. You can buy these in 200mV, 2V, or 20V max input. Any higher voltage would require a voltage divider and then you set the decimal point on the display as necessary.

Therefore a 0 to 5Vdc range would require a 20V display if you did not want to mess with voltage divider resistors. Thus a 3 1/2 digit display would require setting the decimal point such that you would get 2 decimal place accuracy with a max reading of 19.99 or 19.999 on a 4 1/2 digit display. These types of meters are more expensive.

The cheaper types utilize the on-board analog to digital converter in a microcontroller where the accuracy is a function of the number of bits of the A/D converter. They are cheap because you just need a microcontroller, some 7-segment LED digits and the firmware, which after it has been developed costs nothing. But they won't be as accurate as the analog chips that provide the LCD drive to a 3 1/2 digit or 4 1/2 digit LCD. You will notice that all digital voltmeters use 3 1/2 or 4 1/2 digit displays.

Search on Ebay for "3 1/2 digit panel meter 20v". Price looked fairly reasonable. The cheaper and less accurate meters will not bother with a "1" on the far left side of the display.........nor do they fall into a category of 200mV, 2V, or 20V. There are some current meters too.

I guess it depends on the accuracy you need. I did a lithium ion charger project where I display voltage and current and I just used the microcontroller route and three 7-segment displays. My A/D resolution was only 10 bits so accuracy was +/- 0.004 volts but that was good enough for me with just the 2 digits to the right of the decimal point.

 

[ko4nrbs]

Thank you for taking the time to provide a detailed explanation!!

Bill

 

[hiuintahs]

You're welcome......fun stuff! I don't know what your electronic capability is or what your project entails, but if wanting to build your own volt and current meter, I could help you. Another approach that I have done was with a 16 by 4 line character display. That way I could display more information. This is also going the microcontroller route. Rather than send the data to three 7 segment displays, I sent it to the proper location on the 16x4 character display. Again, I didn't need the accuracy from a 3 1/2 digit type of voltmeter.

 

[ko4nrbs]

I want to use them for testing my Lithium Ion Solar chargers. Wanted them to be as accurate as is reasonable.

How about Refresh Rate? I see it stated in Milliseconds and Hertz.
In general this is what I found on the web:
So if converting to HZ a Refresh Time of 2 to 5 HZ (500mS to 200mS) would be acceptable).

As far as electronics I have a basic skill level. If I build something a complete kit with instructions is better for me.

Bill

 

[hiuintahs]

Refresh rate......ie: how often the display is updated? That shouldn't matter. Sometimes a slow rate is desirable to avoid flicker on the least significant digit.

You know another idea to consider if you are doing solar and the charge current is in-line between the panels and the charging cradle, is to consider one of those in-line RC power meters. I have a "Watt's Up" meter between my charge controller and my battery on my solar setup. But there are some other pretty decent ones in the $15 area. You can see voltage and current at the same time on the display. It's on a 12v system but I think the voltage goes down to 0. And it seems pretty accurate from recollection......but I'd have to look at it to verify. Do a search for "RC power meters."

The only concern for one of those would be to know what the voltage drop is across the meter so that you will not be too far below 4.20v when finishing the CC part of the charge cycle. The other concern is the minimum voltage to power one of those meters........if that is possible with the voltages that are present in your system. That could be a problem when the charge power is also the power that powers up the in-line meter.