Cheap aneng multimeters null current issue

[gabriele]

Hello everyone,
Last year I brought an ANENG AN8009 that has always had a problem on current measurements when no current flow throught it. I managed to get a partial refund.
Thinking that is faulty, recently i brought another one, an AN8008 from another place and it has the same issue but a bit smaller.
The problem is present only in DC current, mA and uA, the others measurements are fine.
Is there anyone who has this problem with this multimeters? Is there any solutions? Thanks.

[SilverSolder]

Exactly what is the issue you are seeing?

[gabriele]

Sorry, I didn't upload the photo correctly.
When there is no current flow, in DC, both the multimeters don't show 0.0mA or 0.0uA, instead they show a small negative value like an offset.

[SilverSolder]

Does this also happen if you put a short circuit between + and - on the meters?

[gabriele]

Does this also happen if you put a short circuit between + and - on the meters?

Yes, it does.

[Kilrah]

My 8008 shows 0 on both mA and uA, somttimes goes to +/-1 LSD but that's it.

[SilverSolder]

The offset is too much, even for an inexpensive meter.  I just turned on three different modest cost meters here and they basically have no offset:





Have you ruled out EMI from lamps or other sources nearby?  (e.g. take the meters outside...)

There might be some potentiometers inside your meters that can be tweaked? 

[gabriele]

The offset is too much, even for an inexpensive meter.  I just turned on three different modest cost meters here and they basically have no offset:


(Attachment Link)


Have you ruled out EMI from lamps or other sources nearby?  (e.g. take the meters outside...)

There might be some potentiometers inside your meters that can be tweaked?

I moved away the two meters from any electronics thing, outside, but nothing changed.
I attach a photo showing the inside, there is no potentiometer.
I found this topic: https://www.eevblog.com/forum/testgear/cheap-aneng-meter-calibrationreset-procedure/

[SilverSolder]

Is the "offset" constant -  e.g. if you push 10mA through it, is the offset still added the same as if there is no current?

If the calibration includes a zero calibration, it's worth a try...

[floobydust]

There is no offset cal for the DMM (unless capacitance function), only a span value to set full-scale for each range.
You can put the DMM in CAL mode and it will show raw A/D counts, if you suspect the A/D is bad. I've never seen offset  problems measuring current, the shunt is low resistance not much can upset that.

But I always have hassles with the rotary switch making poor connections due to oxidation. Try give it a few whirls back and forth.

Make sure your batteries are good and consider trying the capacitor mod where you add a large ceramic cap across the big fat electrolytic. More here but that 1,000uF cap seems to be a bit overboard. The MCU/LCD charge pump makes a lot of high frequency noise on the 3V power (worse with weak batteries and extra cheap electrolytic cap).
You could have purchased DMM's with the low-resolution IC like 4000 or 6000 count that doesn't work well at 9999 counts, I hear it gets noisy and not accurate at full scale.

[gabriele]

Is the "offset" constant -  e.g. if you push 10mA through it, is the offset still added the same as if there is no current?

If the calibration includes a zero calibration, it's worth a try...

Ok, i tested them, and i used another cheap blue multimeter for helping.
I set the output of power supply to 5V and i measured it with each meter, the DC voltage function is fine, all of them show almost the same, I have attached the photo.
I took an 1kΩ resistor, and I did the same, same story, resistance function is fine. They have measured: Orange=978Ω Red=980Ω Blue=976Ω
I put red and orange multimeters in series and I measured the voltage accross the resistance with the blue, in order to calculate the real current.
Conclusion:
The calculated current is 5.1mA, and every multimeter shows its own..
In the photo where the red has a blur segment, it is changing form 4.5mA to 4.3mA.
Yes, it seems there is an constant offset added to the measure. Sorry for the long reply.

[gabriele]

There is no offset cal for the DMM (unless capacitance function), only a span value to set full-scale for each range.
You can put the DMM in CAL mode and it will show raw A/D counts, if you suspect the A/D is bad. I've never seen offset  problems measuring current, the shunt is low resistance not much can upset that.

But I always have hassles with the rotary switch making poor connections due to oxidation. Try give it a few whirls back and forth.

Make sure your batteries are good and consider trying the capacitor mod where you add a large ceramic cap across the big fat electrolytic. More here but that 1,000uF cap seems to be a bit overboard. The MCU/LCD charge pump makes a lot of high frequency noise on the 3V power (worse with weak batteries and extra cheap electrolytic cap).
You could have purchased DMM's with the low-resolution IC like 4000 or 6000 count that doesn't work well at 9999 counts, I hear it gets noisy and not accurate at full scale.

In the first photo I attached, I shot on both the first value they show.
Ok, I'm going to take look on the rotatory switch.
If the multimeters work, surely it is worth to mod this multimeter, but i don't think that it could solve the problem, it should work also without it like other multimeters I think.
I don't think the batteries are bad, other functions work well.
Both the multimeters, in CALIBRATION MODE, after they have made the beeps, they are showing ErrE, I have not be able to find the meaning.

Thanks all for the replies

[SilverSolder]

Very interesting.  And very crazy!   

The fact that it works on the Volts and other ranges, indicates that there is probably nothing wrong with the ADC...

Wonder if there is a circuit diagram for these meters somewhere on the Internet...

[gabriele]

There is no offset cal for the DMM (unless capacitance function), only a span value to set full-scale for each range.
You can put the DMM in CAL mode and it will show raw A/D counts, if you suspect the A/D is bad. I've never seen offset  problems measuring current, the shunt is low resistance not much can upset that.

But I always have hassles with the rotary switch making poor connections due to oxidation. Try give it a few whirls back and forth.

Make sure your batteries are good and consider trying the capacitor mod where you add a large ceramic cap across the big fat electrolytic. More here but that 1,000uF cap seems to be a bit overboard. The MCU/LCD charge pump makes a lot of high frequency noise on the 3V power (worse with weak batteries and extra cheap electrolytic cap).
You could have purchased DMM's with the low-resolution IC like 4000 or 6000 count that doesn't work well at 9999 counts, I hear it gets noisy and not accurate at full scale.

Ok probably ADC values are on this photo shooted on current measurements. (CALIBRATION MODE)
I took a photo of the the rotatory switch, the contacts are protected by a layer of grease.

[mikerj]

The offset is too much, even for an inexpensive meter.  I just turned on three different modest cost meters here and they basically have no offset:

Your Aneng meters are set to the uA range and are showing 40-60 nanoamps of offset.  Two of your other meters are set to a mA range so can not be compared.  The other meter is set to a combined 20A/20uA range and presumably the range is selected by the position of the probes but you have no probes inserted.

What is the AN8008 meter's accuracy specification for the uA range?

[gabriele]

The offset is too much, even for an inexpensive meter.  I just turned on three different modest cost meters here and they basically have no offset:

Your Aneng meters are set to the uA range and are showing 40-60 nanoamps of offset.  Two of your other meters are set to a mA range so can not be compared.  The other meter is set to a combined 20A/20uA range and presumably the range is selected by the position of the probes but you have no probes inserted.

What is the AN8008 meter's accuracy specification for the uA range?

The meters in the first photo are the same that are in the second. They have both the problems in mA and uA range.
In the range 99.99uA accuracy is ±(0.8%+3). Even if they were within accuracy specification it would not be normal in my opinion, to show a value llike this without any current flowing throught them. In many youtube videos I could not see this problem.

[floobydust]

I'm confused, you're showing oddball readings with the rotary switch on the ohms function?

[gabriele]

Are

I'm confused, you're showing oddball readings with the rotary switch on the ohms function?

Aren't they the ADC values that you mentioned?
I don't really know how to view them, I put the multimeter in CAL mode, and took a photo of the first value that could be the more plausible.
I uploaded on the next post a photo with the current.
Sorry for the confusion

[SilverSolder]

 Reply #2 shows the problem...  the meters "read" a phantom current.

[floobydust]

If DCmV works fine, then the A/D etc. is OK and the only hardware difference between measuring voltage and current is the two switches and different CAL factor in EEPROM (span, not offset).
Could it be unintentional thermal EMF (thermocouple junction) from dissimilar metals in the current-shunt circuit? That could be up to a couple mV generated by say the R33 10A shunt. 
The multimeter is a crappy PCB design, they have solder, copper-constantan, solder, gold, quite a soup and improper Kelvin connections, so the current measurement is never stellar.

You could try with a good multimeter, measure DCmV at the (bad) multimeter's current-shunt points, COM jack and R24. It should ideally be 0mV.

[SilverSolder]

Thank you for finding a schematic @Floobydust.

It is possible that the DTM0660 chip is defective and leaking a tiny current out (or in) through pin PB1 and/or PB2.  That tiny current would cause a small voltage drop across R9, which would cause a small offset to be displayed by the meter no matter which current range you set the selector switch to.   

Such leakage current through R9 would also add to the reading of any real current being measured, which is consistent with what you tested earlier.

To check this, you could try unsoldering R9 and replace with a 10K resistor, and see if the displayed offset changes to something 10 times smaller...   (You would lose some ESD protection if you did this permanently, but it might make the meter usable).

[MLXXXp]

FWIW, the ANENG AN8009 that I purchased recently does the same thing. However, mine seems to mostly show -2 in the LSD, switching frequently to -1 and occasionally up to -3

This is for auto-ranging or a manual range with a display of -000.2 mA or -00.02 µA. Selecting the alternative manual range shows 0.000 A or 000.0 µA.

The above is for DC mode. All current ranges in AC mode show 0000 (with the appropriate decimal point).

[Electro Detective]

Before going any further and making unnecessary work and more headbanging  you don't need

Take out the batteries and work the selector switch back and forth to 'exercise it'

Go back and forth from A mA to ua,  and from OFF to the far side and back

I reckon there may be some crap and or cheap lube in there that needs a ~shake up~ 

Put back the batteries > after testing they are ok (!) and test meter again

If still drama, clean the meter sockets and lead probe plugs and tips 

If there's a fuse in there, test it and check it's not loose or wonky and or the terminals and fuse ends are not crusty,
adjust/tighten and lube. 

Otherwise it's the  "buying too cheap a meter"  penalty you and many others have to endure   
and don't trust it EVER near CAT fatal voltages

Good luck 

[gabriele]

If DCmV works fine, then the A/D etc. is OK and the only hardware difference between measuring voltage and current is the two switches and different CAL factor in EEPROM (span, not offset).
Could it be unintentional thermal EMF (thermocouple junction) from dissimilar metals in the current-shunt circuit? That could be up to a couple mV generated by say the R33 10A shunt. 
The multimeter is a crappy PCB design, they have solder, copper-constantan, solder, gold, quite a soup and improper Kelvin connections, so the current measurement is never stellar.

You could try with a good multimeter, measure DCmV at the (bad) multimeter's current-shunt points, COM jack and R24. It should ideally be 0mV.

Probably I have not the right instrument to do these precise tests, but i tried, and while I couldn't notice any voltage between the ground terminal and the mA/A terminal, heating the bridge with the soldering iron, caused to create a potential difference, that grew over 100uV. The display shows over +2mA while is cooling down.
Also heating the chip and the area around it cause to show some milliAmps in the display.
This can be a possible problem but without temperature difference, the voltage seems to be null.
Probably it is a dc voltage like this that cause the problem, because in AC it doesn't happen.
I attached the two photos.

Thank you for finding a schematic @Floobydust.

It is possible that the DTM0660 chip is defective and leaking a tiny current out (or in) through pin PB1 and/or PB2.  That tiny current would cause a small voltage drop across R9, which would cause a small offset to be displayed by the meter no matter which current range you set the selector switch to.   

Such leakage current through R9 would also add to the reading of any real current being measured, which is consistent with what you tested earlier.

To check this, you could try unsoldering R9 and replace with a 10K resistor, and see if the displayed offset changes to something 10 times smaller...   (You would lose some ESD protection if you did this permanently, but it might make the meter usable).

That probably is a generic schematic, in the meter PCB, R9 is the current limiter resistor that drives a transistor that drives a buzzer (the multimeter becomes silent), due to my bad I destroyed this area and rebuilt (in some other way) with another resistor and a bridge.
The resistor that we are looking for is R6, also here did some tests. Put a 10kΩ resistor in parallel with it did not change anything. (third photo)
How I know that it is R6? Removing it, break completely the uA, mA/A functions (ADC is floating, and the current shown in the display has grown to a couple of Amps), and this interesting, because it leaves completely working the V functions. Two ADCs? Or more simply the rotatory switch, switches it between the resistor and the IC, and consequently R6 is not used in V measurements?

I also tried to measure the voltage across R6 but it is null. To measure leakage current I connected the blue meter in uA between R6 (IC side) and COM terminal did not measure anything again.
Could it be a real not compensated opamp offset?

FWIW, the ANENG AN8009 that I purchased recently does the same thing. However, mine seems to mostly show -2 in the LSD, switching frequently to -1 and occasionally up to -3

This is for auto-ranging or a manual range with a display of -000.2 mA or -00.02 µA. Selecting the alternative manual range shows 0.000 A or 000.0 µA.

The above is for DC mode. All current ranges in AC mode show 0000 (with the appropriate decimal point).

Yes, practically my meters do it in the same way.

Thanks all for the replies again.

[SilverSolder]

What happens if you short PB1 and/or PB2 to ground with a short jumper wire?

That should force the meter to display zero amps, and eliminate any other circuitry in the meter as a suspect.  If it doesn't display zero in this condition, then there is something wrong inside the chip.