Isolating a Digital Panel Meter (No transformer)

Thread Starter

RichardO

Joined May 4, 2013
2,271
I hope some of you find this interesting. :)

I want to use a cheap digital voltmeter for a project that needs to measure a DC voltage (less than 1 Hz of bandwidth).
Inexpensive (i.e. cheap) meters cannot measure their own power or ground.

For example one of these:
http://www.mpja.com/3-1_2-Digit-LCD-Panel-Meter-with-Scaling-Resistors-for-20VDC/productinfo/27537 ME/
A note for some of these meters says: "(Cannot monitor own power source or ground)".

The "free" Harbor Freight DVM has the same limitation.
Fortunately, only a small amount of power is needed to power the DVM. I measured the battery current at 0.25 ma for the "free" Harbor Freight DVM.

The small power requirement allows a 74HC gate or microcontroller output to drive an isolated power circuit.

I am using capacitors to get the isolation needed instead of a transformer.
To power the DVM, I am using a pair of voltage doublers with an extra cap in the ground leg for DC isolation.

Here is the simulation of the circuit:

Isolated_DVM.png

How it works
------------
A square wave at a few hundred kilohertz drives the voltage doublers.
The top doubler creates the positive isolated voltage, Iso+. This is about +3.2 volts. The bottom doubler supplies the Iso- which is about -3.2 volts. This gives about 7.2 volts total to power the meter.
There will be an offset in these voltages relative to circuit ground as a DVM power rides up and down on the input voltage to the DVM.

C1 through C4 were chosen to have a low impedance at the square wave frequency and a high impedance at the frequency of the voltage being measured.
C5 isolates the ground from the DVM power at low frequencies. It was chosen to be large relative to C1 through C4.
C6 adds additional filtering on the DVM power.

R1 represents the power supply load of the DVM.
R2 is used to verify that there really is isolation. Note that after the capacitors charge up there is no DC current through R2.

I have breadboarded the circuit using a real DVM (actually, a "free" Harbor Freight DVM) and it seems to work. :D
 

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debe

Joined Sep 21, 2010
1,156
Theres plenty of DC LCD Volt meters on Ebay that read there own voltage 7V upto 30V & they are quite cheep. Have used a few of them myself.
 

Thread Starter

RichardO

Joined May 4, 2013
2,271
Theres plenty of DC LCD Volt meters on Ebay that read there own voltage 7V upto 30V & they are quite cheep. Have used a few of them myself.
I have never used that type of panel meter. Nice to know that they work well.

There are some meters like that on the Marlin P Jones site as well. And the extra few dollars is quite reasonable.
But... I already have some of the "free" Harbor Freight meters. :D
 

WBahn

Joined Mar 31, 2012
25,256
Doesn't this rely on an assumption regarding the power supply level? What if the supply you are trying to read is 0.1 V? What is the minimum voltage for which this will work? What about maximum voltage?
 

benta

Joined Dec 7, 2015
101
Nice circuit, I've done similar supplies myself.
However, be careful with terminology.
This is in no way "isolated" as in galvanic isolation.
Rather, it will adjust to different DC levels input/output over time. "Floating" is be the right term.
Your use of the word "isolated" could lead people to use the circuit for mains connected appliances, which would be lethal.

Cheers,

Benta.
 

Thread Starter

RichardO

Joined May 4, 2013
2,271
This is in no way "isolated" as in galvanic isolation.
Rather, it will adjust to different DC levels input/output over time. "Floating" is be the right term.
Your use of the word "isolated" could lead people to use the circuit for mains connected appliances, which would be lethal.
I agree. I was sloppy in my terminolgy.
 

Thread Starter

RichardO

Joined May 4, 2013
2,271
Doesn't this rely on an assumption regarding the power supply level? What if the supply you are trying to read is 0.1 V? What is the minimum voltage for which this will work? What about maximum voltage?
It has been awhile since I played with this circuit. I will have to think about your questions a bit.
 

hp1729

Joined Nov 23, 2015
2,304
I hope some of you find this interesting. :)

I want to use a cheap digital voltmeter for a project that needs to measure a DC voltage (less than 1 Hz of bandwidth).
Inexpensive (i.e. cheap) meters cannot measure their own power or ground.

For example one of these:
http://www.mpja.com/3-1_2-Digit-LCD-Panel-Meter-with-Scaling-Resistors-for-20VDC/productinfo/27537 ME/
A note for some of these meters says: "(Cannot monitor own power source or ground)".

The "free" Harbor Freight DVM has the same limitation.
Fortunately, only a small amount of power is needed to power the DVM. I measured the battery current at 0.25 ma for the "free" Harbor Freight DVM.

The small power requirement allows a 74HC gate or microcontroller output to drive an isolated power circuit.

I am using capacitors to get the isolation needed instead of a transformer.
To power the DVM, I am using a pair of voltage doublers with an extra cap in the ground leg for DC isolation.

Here is the simulation of the circuit:

View attachment 95809

How it works
------------
A square wave at a few hundred kilohertz drives the voltage doublers.
The top doubler creates the positive isolated voltage, Iso+. This is about +3.2 volts. The bottom doubler supplies the Iso- which is about -3.2 volts. This gives about 7.2 volts total to power the meter.
There will be an offset in these voltages relative to circuit ground as a DVM power rides up and down on the input voltage to the DVM.

C1 through C4 were chosen to have a low impedance at the square wave frequency and a high impedance at the frequency of the voltage being measured.
C5 isolates the ground from the DVM power at low frequencies. It was chosen to be large relative to C1 through C4.
C6 adds additional filtering on the DVM power.

R1 represents the power supply load of the DVM.
R2 is used to verify that there really is isolation. Note that after the capacitors charge up there is no DC current through R2.

I have breadboarded the circuit using a real DVM (actually, a "free" Harbor Freight DVM) and it seems to work. :D
I don't know if it will fit your needs but G21310 from Electronic Goldmine is a three digit voltmeter at a price of $5.00. It cam measure the voltage it runs on. Down sides? Can only measure positive voltages. Can only measure voltages referenced to ground.
For a picture of it see "What's on your breadboard?"
 
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