# analog panel meter polarity auto reverse circuit

#### smilem

Joined Jul 23, 2008
160
Hello, I have 3V DC analog panel meter.
The problem is that the meter can be connected with reversed polarity and should not overscale to the other side.

So the circuit should reverse polarity automatically.
The circuit should not impose voltage drop either as the meter will be showing wrong voltage.

#### crutschow

Joined Mar 14, 2008
25,686
You could use an opamp precision full-wave rectifier circuit.
LTspice simulation of example circuit below:

#### MisterBill2

Joined Jan 23, 2018
7,327
You can use a diode across the meter so that with the correct voltage polarity the diode will be reverse biased and have not much effect but with the polarity reversed the diode will take the current instead of the meter. Use a diode with a low forward drop, such as a schotky type.

#### smilem

Joined Jul 23, 2008
160
You could use an opamp precision full-wave rectifier circuit.
LTspice simulation of example circuit below:
It's no clear how to use this circuit. This is for one wire? I need two of these for each wire? Do i connect op amp power to output from the circuit itself?

The op amps need separate power source from 3V to 15V that is stable in terms like battery. In my case this is a problem as the analog meter needs no power to work I would like to keep it so.

The analog meter measure to 3V so without separate power source not possible to measure like lower voltages. Schottky diode voltage drop 0.2V so that means 0.2V is not measurable. Is there any other way?

#### Audioguru again

Joined Oct 21, 2019
2,090
If you cannot use the automatic polarity opamp circuit that must be powered then you must use a manual switch.

#### crutschow

Joined Mar 14, 2008
25,686
It's no clear how to use this circuit.
You need one circuit.
The input is from the voltage you are measuring and the output goes to the plus meter input.
The minus meter input is not affected.

But you do need at least a 3V supply for the opamp to get 3V out.
Could you use a battery if a micropower op amp is used so that the op amp could be left powered continuously and still have a long battery life?
For example, using two LPV511 micropower op amps with a CR2032 lithium coil cell gives a theoretical battery life of over 10 years (not including the meter current).
The circuit is shown below.
Note that it's a different circuit so that 3V out can be achieved with a 3V supply.
The first one can only supply about 2.5V max. Out for a negative input due to the diode drop at the first op amp output.

What is the meter input resistance?

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#### MisterBill2

Joined Jan 23, 2018
7,327
The TS needs to do some basic study of circuit theory to be able to understand the advice being offered. In addition there is a need for those offering advice to know more about the meter that the TS is wanting to protect.

I am making a GUESS that it is some sort of multimeter with a number of ranges. This is based on a previous post about a high voltage probe. If that is the case then advice on the use of a multimeter would be a benefit for the TS, and assist in preserving the meter in good condition.

#### smilem

Joined Jul 23, 2008
160
I have battery tester like this one

The resistance of meter is : 349.14ohm measured with agilent U1252B
10 Years from CR2032 lithium coil cell is very good

The meter will be hacked to measure 3V, 6V, 9V, 12V, 15V by means of adding resistors and a rotary swicth like

3V - > 1 division = 0.1V
6V -> 1 division = 0.2V
9V -> 1 division = 0.3V
12V -> 1 division = 0.4V
15V -> 1 division = 0.5V

https://aliexpress.ru/item/32811970...earchweb0_0,searchweb201602_,searchweb201603_

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#### smilem

Joined Jul 23, 2008
160
You can use a diode across the meter so that with the correct voltage polarity the diode will be reverse biased and have not much effect but with the polarity reversed the diode will take the current instead of the meter. Use a diode with a low forward drop, such as a schotky type.
That is not am option since 0.2V drop even best diode. In additional ranges of 12V 15V that is goind to throw voltage divisions way of

#### crutschow

Joined Mar 14, 2008
25,686
10 Years from CR2032 lithium coil cell is very good
Unfortunately that op amp output current is not sufficient to drive that low impedance meter, so a higher current op amp would be needed.
That is not am option since 0.2V drop even best diode.
The diode is not in series with the meter, it's across the meter terminals and only conducts for a reverse voltage application, so it has no effect on the normal meter operation.

Joined Jan 15, 2015
5,667
Hello, I have 3V DC analog panel meter.
The problem is that the meter can be connected with reversed polarity and should not overscale to the other side.

So the circuit should reverse polarity automatically.
The circuit should not impose voltage drop either as the meter will be showing wrong voltage.

I would just find an analog meter with center scale zero. Something like this meter. The actual meter movement is +/- 60 mV so you place a resistor in series with the movement making it whatever voltage you want. That done you recreate the scale. Actually I remember seeing some software for creating analog panel meter scales. Matter of fact the link will make you any scale plate you want. The idea being a center zero so now you have a +/- meter. Analog panel meters are not very expensive. The link is merely an example.

Ron

#### MisterBill2

Joined Jan 23, 2018
7,327
That is not am option since 0.2V drop even best diode. In additional ranges of 12V 15V that is goind to throw voltage divisions way of
I was NOT suggesting a diode in series with the meter, which would indeed have a voltage drop. I stated that the diode would be ACROSS the meter, wired so that a reversed polarity would forward bias the diode and prevent the meter deflecting very far in the wrong direction. A diode across a meter is a fairly standard means of protecting it.

#### smilem

Joined Jul 23, 2008
160
I was NOT suggesting a diode in series with the meter, which would indeed have a voltage drop. I stated that the diode would be ACROSS the meter, wired so that a reversed polarity would forward bias the diode and prevent the meter deflecting very far in the wrong direction. A diode across a meter is a fairly standard means of protecting it.
diode ACROSS the meter needs also a fuse that would blow in wrong connection how it that helpfull in a device like this, changing fuses all day is not fun

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#### crutschow

Joined Mar 14, 2008
25,686
What about the n-channel MOSFET protection?
That only works with an input voltage above the MOSFET Vgs threshold, which is typically over 1V.

#### MisterBill2

Joined Jan 23, 2018
7,327
Presently I am wondering about what sort of 3 volt full scale meter is involved, AND I have an idea to offer. Still put the diode across the meter to protect it from reverse voltages, BUT then also have a series resistor to limit the current, and if the polarity is correct, then a push button switch to bypass the resistor and get the correct reading.
A question, though, is if the meter is observed to start in the wrong direction with the shunt diode, is the person watching the meter smart enough to see that it is backward and then reverse the connections? OR, is this an application to be used by folks who really have no understanding of polarity? Why are those folks even allowed to touch a voltmeter?
I do know how to add a relay to reverse the connection if the polarity is reversed, but it is a bit complex in that it uses a polarity reversing switching function and a diode to allow the relay to operate only when the polarity is reversed. But if this is to be part of a system for checking batteries then it would increase the current draw on those batteries.
Probably a better description of the actual problem would help with getting a solution.