Cheap panel Volt/Ammeter gives wrong current reading

[PinheadBE]

Hi there,

I have this exact model of cheap Volt/Ammeter for a power supply.
https://www.amazon.nl/gp/product/B09TP2MNS1

The supplied shunt is 1.5 m    

When a current of 10 A passes through it, I measure 15 mV at its connectors.
But the display shows 2.00 A
The fine tuning trimpot can adjust a bit the reading, but not to a factor of 5

So, I'd need a shunt that could support 50 A, but gives 75 mV for a current of 10 A, thus, with a value of 7.5 m

On Ali* and Amazon, most shunts are mentionned to be "75 mV".   Does this mean it gives 75 mV at their rated max current ?   Or at another specific current ?

In my power supply, I'd need a shunt capable of supporting a continuous current of 30 A.   
The meter itself is capable of displaying up to 50 A (given the correct shunt value)
(Note that the voltmeter part is ok)

==> How can I find a shunt of 7.5 m , and capable of supporting 50 A continuous ?   How would it be labelled on Ali ?

Thanks !

[Whales]

> On Ali* and Amazon, most shunts are mentionned to be "75 mV".   Does this mean it gives 75 mV at their rated max current ?   Or at another specific current ?

Yes.  Typically a shunt will be labelled with only its max current and voltage, from that you calculate the resistance.

> How would it be labelled on Ali ?

The seller probably doesn't understand the product.  Maybe once upon a time the meter was correctly matched to the shunt, but new stock of the meter is different (and looks identical from the outside).

I'd modify the meter, not replace the expensive shunt.  It will have small resistors inside that you can change the value of.  The amps part of the meter is just a voltmeter with a different internal software multiplier, some resistors and the letter 'A' instead of 'V' on the screen.

[Jwillis]

A 75mV 10A shunt will drop 75mV at 10A.  A 75mV meter has full deflection at 75mV. The 75mV is put on the shunt so it can be matched with a 75mV meter. Normally you won't find a shunt that has the resistor value noted.Because the resistance values are so low, it's more accurate and more efficient to calibrated by a voltage drop at a set amperage and not by resistance measurement.

Depending on the rating of those meters there could be just a small power resistor or a piece of manganin for a shunt. If your meter is rated for above 20A there should be no on board shunt. There may have been a mix up and the onboard shunt hasn't been omitted for external shunt. Worth a check.

[bdunham7]

==> How can I find a shunt of 7.5 m , and capable of supporting 50 A continuous ?   How would it be labelled on Ali ?

You might want to figure out the voltage drop and power dissipation for a 7m5 shunt and 50A, then reconsider.  If you have a 50A meter, it sounds like the shunt is correct but the meter is not set up properly somehow.

[golden_labels]

Don’t expect intermediaries, who are engaging in mass import/export of cheap items, to have even basic knowledge about the items they sell. Be happy if the description makes any sense.

One option is to demand a refund. If that’s a local Dutch seller, they fall under EU-wide law. That streamlines resolving this kind of disputes and most sane vendors don’t object, but either refund or replace the item. If they’re from China, returning the item is not feasible, so you may receive a refund while keeping the item.

As for resolving the situation “electonically”: this seems to be a 3-digit display in “0.00” format. You can scale down measured voltage by 5, so for 10 A it reports “0.10”. Suboptimal and you lose one digit, but at least readable. For 50 A range this would be 2% resolution.

[alligatorblues]

With those types of packages, V/A meter with shunt, the meter and shunt are made to go together. Truthfully, I would just put the set you have in a box, and buy one that's matched with the shunt. They're about 15.00 on eBay. I use them all the time on custom low-voltage/high-current buses used in home solarelectric systems. I use the same connection technology, scaled down, that is used in magnetic crane pickups that consume 10,000A.

Or, you can take the one you have apart, and spend 6 hours matching it to the shunt.   

[kosine]

I bought some of these a while back and also had problems with measuring the current. For whatever reason, it only seemed to work if the current measurement was done in the circuit return path. So maybe the shunt needs a proper ground for the meter to work properly.

[CaptDon]

I was surprised to see 75mv shunts although I have not been shunt shopping in about 25 years. All of the shunts in the labs I worked in were 50mv at the rated current although there were just a couple that were 100mv and I think they were special order so the DMM readings were 'direct' after moving the decimal multiplier such as 100mv @ 10 amps.

[shapirus]

Please show us how you are wiring it.

[PinheadBE]

I found this schematics online: https://github.com/Upcycle-Electronics/DSN-VC288/blob/master/PDF's/V2.0%20DSN-VC288.pdf

It seems, as pointed out by Whales (see above), that I could change the value of some resistances in Ammeter sense amplifier, rather than buy another meter.
I have not yet verifies the values of those resistances in the meter I have (needs a microscope  ), but I do not assume that the values shown on the schematics above match the values of my module

I will collect the values of those resistances, and then adapt them so that the gain is 5x the present one.
To simplify things, I intend to replace the less resistances possibe (they are really small).   Which one (reference on the schematics) would you suggest I change ?

[Jwillis]

I found this schematics online: https://github.com/Upcycle-Electronics/DSN-VC288/blob/master/PDF's/V2.0%20DSN-VC288.pdf

It seems, as pointed out by Whales (see above), that I could change the value of some resistances in Ammeter sense amplifier, rather than buy another meter.
I have not yet verifies the values of those resistances in the meter I have (needs a microscope  ), but I do not assume that the values shown on the schematics above match the values of my module

I will collect the values of those resistances, and then adapt them so that the gain is 5x the present one.
To simplify things, I intend to replace the less resistances possibe (they are really small).   Which one (reference on the schematics) would you suggest I change ?

If you meter is the same as the circuit diagram, the gain of IC3B is between 23 and 25 depending on the Value of R3 and the position of RV2. An additional 5X gain would be around 115 to 125.  Using standard resistor values, A 1M at R3 would give a gain of around 123 to 125. The higher the gain increase the less effect RV2 will have in calibrating.

[PinheadBE]

As of now, the total feedback resistance goes from 180 k to 200 k.   If I want to stay in the same range of adjustement, it should now go from 900 k to 1 M.    So, with a fixed 1 M resistor, I could be stucked....

And, (good remark !), to get a better adjustement range on high gain, I will replace the trimpot by a 100 k .

And so, I'll try to replace R3 (on the schematics, but is R4 in my module), by a 1 M and a 10 M on top of each other (in parallel), giving approx 900 k .   Problem is I only have those in a 1206 format (0805 is required).
Otherwise, a 330 k and a 560 k in series, placed as two facing tombstones....    I'll be creative 
If nothing else does, through hole resistors with short leads, but I am afraid of the possible noise.....

Thanks for your help.   
I'll post my results here, so that it could be of help for others

[PinheadBE]

The deed is done: 100 k trimpot and 330 k + 560 k in series, mounted as tombstones for feedback.

I verified the total feedback resistance, and it varies between 910 k and 1.02 M, (the trimpot I used is a cheap one from Ali*, explaining the tolerance; my 0805 resistances are from a renowned German reseller, ELV.de).

So, did I win the chicken dinner ?

Nope.

I measured still off the scale results:

For a 1.00 A current: depending the trimpot, it displays between 2.66 and 3.06 A
For a 5.00 A current: between 6.20 and 6.95 A
For a null current, it still displays 0.34 A ....

So I guess there's another resistance to adapt ?
(Hopefuly, it's not a firmware value to modify.....)
 
(Sorry if those questions seem dump, but opamp theory and analogue in general are not my forte ...)

[Jwillis]

The deed is done: 100 k trimpot and 330 k + 560 k in series, mounted as tombstones for feedback.

I verified the total feedback resistance, and it varies between 910 k and 1.02 M, (the trimpot I used is a cheap one from Ali*, explaining the tolerance; my 0805 resistances are from a renowned German reseller, ELV.de).

So, did I win the chicken dinner ?

Nope.

I measured still off the scale results:

For a 1.00 A current: depending the trimpot, it displays between 2.66 and 3.06 A
For a 5.00 A current: between 6.20 and 6.95 A
For a null current, it still displays 0.34 A ....

So I guess there's another resistance to adapt ?
(Hopefuly, it's not a firmware value to modify.....)
 
(Sorry if those questions seem dump, but opamp theory and analogue in general are not my forte ...)

Try backing off on the gain. I've got a meter like yours but it has an onboard shunt and i don't have any 50A shunts. Was thinking on dissecting it. 

[PinheadBE]

I'll do that, but what seems strange is that the final result seems to be not linear

I measured the voltage at the shunt (on the Kelvin connections, of course), and what I measured is consistent with a 1.5 m resistance, whatever the current.   Temperature has no effect: at 10 A it does not notably warm (of course)

[PinheadBE]

I may have found something (rather obvious, I must say  )

I simulate the circuit, and the op-amp saturates at a shunt voltage of approx 42 mV....   The gain being higher.... 

So, I guess R4 and/or R5 are next in line to be adapted, aren't they ?

You can see the simulation here (use the cursor at the right to make the shunt voltage vary):

https://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3BWc0EDYAsAmVBmMAOfdA-MAdkxFSRshAQFMBaMMAKAHMQBOVETNDz45I6KFDYB3cN0qZI+GXMGQ2AQyWacOPmFk8QOaDnFh48KLEzcy3BJlYEHs6khiQE3dKkjVue9DxFM1VpbV19cP4VNgAnTQFhHWi+Om5zVXjeFISVEHx5UM1WSj1SslS2AHlDZLKQdDhiiXi6EqFwMEo6bUz+TDF2kUGu8Uwycyl+kdL9dtVySiihok7ukDJDYy2TOj2UEAA1AGFjqeG12t1RhYpTdDE6eUU6TfcED8tIMjdxGCRDgB7AA2ABc1BwGCBAQA7EAAZQAFgBXGGgqZtB6mG5sAAeICY9n4IioT20DXAiiRqNBAB0AM5AsEQhhTbLterzNgAYw6XUU2USpmYb1gnm8vk8ASCEmk7MwAuu3TYgI2VEeDUg3GC+3MNmoqAEpjYQA

[PinheadBE]

I give up  :'(

I have bought a new module on Ali*, configured (at least I hope so) to work with the 50 A shunt I have.
I have also bought a 10 A shunt to use with the module I have (I will rebuild it with the original resistors and trimpot)   

In 3 weeks time, I will be able to go on with my project.....

Thanks all for your hep.   Any comments are still welcome, anyway !