Digital Vdc Panel meters with independent power source

Jump to Latest
T

thehoj

Member
Joined 2009
#1
I'm trying to find a little digital voltage panel meter like these ones from Adafruit

https://www.adafruit.com/product/705
https://www.adafruit.com/product/460

But I need one that has a completely independent power source. The 2 wire one draws current from the thing it's measuring to power itself, and this won't work for my application.
The 3 wire one draws power from another source, but there's still a common ground shared between the thing that is being measured and the meter.
For the life of me I cannot seem to find some sort of 4-wire version of this, that uses an independent +5V or +12V source connected via dedicated 2 wires, and then a separate 2 wires that connect to whatever I'm measuring voltage on.

There must be something like this available somewhere.. I'm thinking along the lines of a multimeter, it's got its own power source, and then a pair of leads that measure voltage. I need something that is dedicated to measuring voltage, and could be panel mounted though.

Has anyone seen something like this? I've been scouring Amazon, digikey, mouser, ebay.. Cannot seem to find what I'm after. Any direction on this would be appreciated.
 
Last edited:
#7
thehoj said:
Oh, I don't have to use a battery supply though. These could be wired into a power source,
they just can't be powered by the things I'm measuring.
Click to expand...

The voltage isolation of these meters is very low. They also cannot be used for floating measurements
in general, unless their power supply is also floating and isolated, like with a battery,
 
Last edited:
#9
rayma said:
The voltage isolation of these meters is very low. They also cannot be used for floating measurements
in general, unless their power supply is also floating and isolated, like with a battery,
Click to expand...

If I could find it, I'm just thinking that the meter would be connected to an isolated +5V source to actually power the LED's, and then there would be a pair of leads (which are also isolated) that actually measure the voltage. So there would be no current draw on the circuit I'm measuring.
Maybe LCD is my only option though.
 
#12
thehoj said:
If I could find it, I'm just thinking that the meter would be connected to an isolated +5V source to actually power the LED's, and then there would be a pair of leads (which are also isolated) that actually measure the voltage. So there would be no current draw on the circuit I'm measuring.
Maybe LCD is my only option though.
Click to expand...

Don't forget the generally, both the metering circuit and the supply circuit of these panelmeters have a common ground. They can not be independently isolated.

Jan
 
#13
You don't need to isolate voltmetters input ,it has 30k input resistance or even more ,and draws very little current. Common ground is a problem ,if you need measure negative polarity voltages , so then you need isolated converter , which must have seperate pins for input and output . Not buck and no step-up converter suitable for this ,they have common ground . Also some voltmetters with ampermeter ,2 in 1 ,have seperate power supply pins ,but ground is still common , also ampermetter measures negative wire ,so ground is even more common . Only isolated converter is your solution .
 
#14
chrisng said:
Something like this?

YB5145B LED Digital DC voltimetro 4 1/2 medidor de voltaje de voltios de CC 200mV 2 V 20V 100V 200V 300V 500V 600V de 4,5 voltios medidor de Panel|volt panel meter|panel meterdigital dc voltmeter - AliExpress
Click to expand...

I feel like this is what I've been looking for! Thanks.


jan.didden said:
Don't forget the generally, both the metering circuit and the supply circuit of these panelmeters have a common ground. They can not be independently isolated.

Jan
Click to expand...
I'm wondering about this though.. Why is this? I'm going to be measuring 2 different things.. One of them is a rod coleman regulator output, so it's completely floating. There is no ground reference.

The other thing I want to measure is a stepped down output of a high voltage B+ supply in an amp.. So I was going to use a 5M (multiple resistors) / 50.5K voltage divider on a 1KV supply, so a 1KV supply I would measure 10V over the 50.5K resistance.
 
Last edited:
#17
I'm not sure I understand how to apply that to my circuits.
It isolates input from output it looks like (attached an image from datasheet).

I've also attached a schematic showing the two things I'm hoping to do.

I could actually probably use one of the 2 wire meters for the coleman regulator measurement, because they could easily handle the extra 30-50mA current draw.
I couldn't do that for the voltage divider measurement though, as an extra 30-50mA draw through that would definitely not work, it would be like 50W of power dissipation through the 5M resistors.
 

Attachments

  • isolateddc.png
    isolateddc.png
    36.1 KB · Views: 87
  • measuring.png
    measuring.png
    44.8 KB · Views: 92
Last edited:
#18
thehoj said:
Hm. Actually that Aliexpress one probably wouldn't work.. It says "The negative of power supply and the testing side are connected".
Maybe I just need to use analog meters..
Click to expand...

You answered your own question. That is what I was saying.

On your above schematic: if you use two meters, sure that will work. If you want to switch between metering the two units with a single meter, then you need to switch all three meter wires.
If you do that you don't need the isolated power brick.

But I am wondering what that 5V is ultimately connected to; it will somehow have to be connected to the other circuit somewhere. Where is that?

Jan
 
Last edited:
#19
And the reason a multimeter can do these measurements without drawing current from what is being measured is because it's using a battery?

If I used a power brick like this Blocked with one of the 3 wire meters, would that effectively be the same as using a battery? These kind of power bricks don't have some sort of ground reference do they?
 
#20
jan.didden said:
You answered your own question. That is what I was saying.

On your above schematic: if you use two meters, sure that will work. If you want to switch between metering the two units with a single meter, then you need to switch all three meter wires.
If you do that you don't need the isolated power brick.

But I am wondering what that 5V is ultimately connected to; it will somehow have to be connected to the other circuit somewhere. Where is that?

Jan
Click to expand...

Oh maybe I wasn't clear enough in my original post. I want to use separate meters to measure the two different things. But I don't want either of the things I'm measuring to be ground referenced to something else.

So for the coleman supply that is completely floating, if I use a power brick that is just powered by AC directly, and connect the + and - to the 3 wire meter, and then that shared - is connected to the negative terminal of the floating supply I'm measuring, and the + is measured to the positive terminal of the floating supply, I wouldn't be somehow ground referencing the coleman supply?

The meters are going to be in a box external to the amp chassis (which contains the things I'm measuring). The external box will be connected via some sort of umbilical cable to the amp chassis. The circuits I'm measuring will be fed through that umbilical back to the box that has the meters. The meters in the external box will be connected to their own power source via power bricks. Maybe this is a dumb way to do this, but I just want to have a box sitting beside the amp that I can quickly reference to make sure these voltages are where they should be.

Currently I have test points on the amp that I can connect a multimeter to, to measure.. But I want a permanently affixed box that has constant readouts of these things.
 
Last edited:
You must log in or register to reply here.
Top Bottom