Way to find max current rating of ammeter?

[anvoice]

Hello, I've got a panel ammeter that I would like to use in a project. However, I bought it years ago and can't remember the rated current cap. No spec sheet either. I need about 20A, but browsing online, one with a similar-looking shunt seems to be a 0-5A range. Is there any way to check/estimate what the maximum current for this meter is?

Attached is a picture of the device. I've tried measuring the shunt's resistance but it's too low for my multimeter.

[Manul]

Hello. Why not power it up and try? You will see where is decimal separator. If you see that it has just one decimal place, there is a good chance, that it will go to 20A.

[OamSlaugh]

Hi anvoice,
Since this is using an external shunt I would expect that you are more likely to saturate the meter's ADC reading than to cause physical damage.  I.E. a 5A rating would be the maximum current it can resolve rather than a power dissipation limit, provided that the traces between the input wires and shunt are also similarly wide.  For a rule of thumb you could try to measure the gauge of the shunt and get the maximum recommended current from that (take a look at the chassis wiring rating of https://www.powerstream.com/Wire_Size.htm).  The Mark I eyeball says the insulation on those wires will smoke before the meter though...

Manul beat me to it, but you could do a sanity check by applying a 1A load to the device.  If it reads 1.00 then it is going to max out at 9.99A unless it gets fancy and uses that second decimal point to scale up.  However this one from Adafruit has two decimals in the display and still maxes out at 9.99A:
https://www.adafruit.com/product/574#description

Is there an IC visible on the back side of the PCB that we could use to get a better idea of the capabilities?

[anvoice]

Hello. Why not power it up and try? You will see where is decimal separator. If you see that it has just one decimal place, there is a good chance, that it will go to 20A.

Hi anvoice,
Since this is using an external shunt I would expect that you are more likely to saturate the meter's ADC reading than to cause physical damage.  I.E. a 5A rating would be the maximum current it can resolve rather than a power dissipation limit, provided that the traces between the input wires and shunt are also similarly wide.  For a rule of thumb you could try to measure the gauge of the shunt and get the maximum recommended current from that (take a look at the chassis wiring rating of https://www.powerstream.com/Wire_Size.htm).  The Mark I eyeball says the insulation on those wires will smoke before the meter though...

Manul beat me to it, but you could do a sanity check by applying a 1A load to the device.  If it reads 1.00 then it is going to max out at 9.99A unless it gets fancy and uses that second decimal point to scale up.  However this one from Adafruit has two decimals in the display and still maxes out at 9.99A:
https://www.adafruit.com/product/574#description

Is there an IC visible on the back side of the PCB that we could use to get a better idea of the capabilities?

No IC unfortunately, that PCB is totally empty on the back.

I tried applying a steadily increasing load to the device, and it maxes out at 0.99A apparently... It doesn't seem like it got damaged when I exceeded that (the screen started going wonky but showed the current properly again once I went below 1A), but for my application that's unusable. It does seem like OamSlaugh was right in that for this particular type of meter, exceeding the max current doesn't break the meter at least.

Thanks for much for the help!

[WattsThat]

Based on the size of the shunt on the meter, that’s looks to be the 10 amp version sold all over eBay. Not sure why it would go wonky at an amp, are you sure your supply could deliver the current? Sounds like ac ripple or current limiting kicking in.

Also, be careful how you supply those Chinese ammeters, they are not isolated in any way, the shunt is connected to the negative supply.

[jackthomson43]

Hello, I've got a panel ammeter that I would like to use in a project. However, I bought it years ago and can't remember the rated current cap. No spec sheet either. I need about 20A, but browsing online, one with a similar-looking shunt seems to be a 0-5A range. Is there any way to check/estimate what the maximum current for this meter is?

Attached is a picture of the device. I've tried measuring the shunt's resistance but it's too low for my multimeter.

Yeah it can bear max 5A, for 20A you should buy a new one as 20 is quite high load and from the looks of it, this board can't hold 20A. May be Schottky Diode will help.

[anvoice]

Based on the size of the shunt on the meter, that’s looks to be the 10 amp version sold all over eBay. Not sure why it would go wonky at an amp, are you sure your supply could deliver the current? Sounds like ac ripple or current limiting kicking in.

Also, be careful how you supply those Chinese ammeters, they are not isolated in any way, the shunt is connected to the negative supply.

I'm certain my PSU can deliver about 10A, and definitely more than an ampere. The display acts up as soon as it crosses 0.99A. I do get that 0-1A is not a standard range for these meters, perhaps this one is old enough to have gone bad?

Yeah it can bear max 5A, for 20A you should buy a new one as 20 is quite high load and from the looks of it, this board can't hold 20A.

Already ordered, time for another 3 week wait!