Way to find max current rating of ammeter? - Page 1
Forum section: electronics / beginners

author: anvoice
author_rank: Frequent Contributor
timestamp: May 04, 2020, 12:18:19 am
content: 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.

author: Manul
author_rank: Super Contributor
timestamp: May 04, 2020, 12:36:41 am
content: 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.

author: OamSlaugh
author_rank: Newbie
timestamp: May 04, 2020, 12:48:21 am
content: 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 ofhttps://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#descriptionIs there an IC visible on the back side of the PCB that we could use to get a better idea of the capabilities?

author: anvoice
author_rank: Frequent Contributor
timestamp: May 04, 2020, 01:21:26 am
content: Quote from: Manul on May 04, 2020, 12:36:41 amQuote from: OamSlaugh on May 04, 2020, 12:48:21 amNo 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!

author: WattsThat
author_rank: Frequent Contributor
timestamp: May 04, 2020, 04:20:23 am
content: 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.

author: jackthomson43
author_rank: Contributor
timestamp: May 04, 2020, 04:52:13 am
content: Quote from: anvoice on May 04, 2020, 12:18:19 amYeah 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 beSchottky Diodewill help.

author: anvoice
author_rank: Frequent Contributor
timestamp: May 04, 2020, 05:00:37 am
content: Quote from: WattsThat on May 04, 2020, 04:20:23 amI'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?Quote from: jackthomson43 on May 04, 2020, 04:52:13 amAlready ordered, time for another 3 week wait!