Volts Amps digital meters.

[davelectronic]

I think I have posted in the correct topics section. It is a technical question, I have a digital Volts and Current panel meter. It one of the cheap Chinese two colours display of red Volts and blue Amps meters. This meter has a 10 Amp current shunt built in to the meter. I wanted to find out if I can put this meter through an external 50 Amp current shunt, as I need current readings of up to 15 Amps from this meter. I'm asking as the meter has this built in 10 Amp current shunt. Thanks for reading, any help appreciated.

[tru3533]

To measure current, the meter measures the voltage over the shunt resistor.

So first question will be: are the instrument able to display 15A? Test this by applying voltage to the shunt wires.
Measure the voltage over the resistor, when running 1A load.
remove the 10A shunt resistor and apply voltage on the connection points on the pcb for the current resistor, in the same voltage range you measured.

Second the 50A shunt resistor need to have the same ohm to be able to give you the correct reading, if you have no possibilities to recalibrate the instrument for the new range.

[davelectronic]

Thank you for replying, the meters current shunt is a metal bar type shunt, and not a resistor. I will try to test the other parameters you have suggested. I think it has reading ability to three decimal places for the current led numbers.

[Vovk_Z]

50/50 if it can show numbers more or much more than 10.00 A.

[tru3533]

Even a metal bar is to be considered as an resistor.
A metal bar also have a resistance, maybe yours have 0.1 ohm?
 

[glentek]

I traced out some of the circuit for one of these modules, it may be of help.
https://www.eevblog.com/forum/projects/volts-amps-digital-meters/?action=dlattach;attach=1268740

[xavier60]

I measured the shunt resistance on a 200V/10A model at 4.5 milliohms.

[davelectronic]

Some of this might be a bit beyond my ability, I can use solder wick to remove the shunt and measure it's resistance. But I don't know if the meter can display upwards above 10 Amps DC. I can buy a meter in the same style with the external 75mA shunt for £16 or so. I only though I could use a meter with a maximum range of 10 Amps with an external 75mA shunt, as I have these sitting around already. I was hoping with an extra resistor I could convert the 10 Amp shunt to use the external 75mA shunt and read higher than 10 Amps DC. But it's not that straight forward from what you've said, I can see this.

[tru3533]

First of all, OHMs Law applies here.
The only thing that don't change in a shunt resistor is the ohm.
So "75mA shunt resistor" make no meaning. A shunt resistor is measured in OHM.

Put a load on the meter, so you can read a Amp value on the LCD
Measure over the shunt resistor with your multimeter set to Volt DC.
Lets say you measure 0.5VDC
Remove the 10A shunt resistor and apply voltage from your power supply
on the connection points on the pcb for the shunt resistor,
in the same voltage range you measured. In this example 0.5VDC

Now by varying the voltage from your power supply, you will see
The LCD screen changing the Amp value.
By increasing the voltage, you will see if the display is able to show more than 10A
Be careful, the voltage range is very narrow.

By doing this, you will see if the LCD on the meter is able to display 15A
If not you will need a converting table written down on a piece of paper
If you see 10A on the LCD then you actually measure 15A

[xavier60]

Not "75mA" but "75mV" is a common spec for shunt resistors.
In this example, the shunt drops 75mV at 100A so needs to be connected to a meter that reads "100" with 75mV applied.
https://handsontec.com/index.php/product/fl-2-100a-75mv-current-shunt-resistor-for-ammeter/

[davelectronic]

Thank you for your help, I'm just going to order the correct meter. It's to hit and miss, knowing my luck I will end up destroying the meter. I thought it might be as easy as adding a resistance to the built in shunt. As the test voltage to the removed shut is tight, it's not worth the risk.
Thanks again.

[xavier60]

I traced out some of the circuit for one of these modules, it may be of help.
https://www.eevblog.com/forum/projects/volts-amps-digital-meters/?action=dlattach;attach=1268740

That CMOS switch IC explains why the current reading is so accurate and stable despite having an LM358 amplifying a 4.5milliohm shunt.
The MCU auto-zeros the current range 125 times per second.
The most annoying aspect to these meters is that the refresh is always much slower than the usually advertised 300mS.
Just moments ago, I received this nonsense from an ebay seller,
"The voltage and current are 500ms each, and the total is 1000ms. We will correct our description as soon as possible."
The refresh for the meter is actually 1000mS for each display, annoyingly and unnecessarily slow.