Turns out these $5 999ma Ammeters are low-side only - Page 1
Forum section: electronics / testgear

author: cellularmitosis
author_rank: Unknown
timestamp: March 20, 2014, 03:02:44 am
content: So I was tempted by the "too good to be true" price ($5 shipped) of these "0-999ma" ammeters from China on ebay. I snatched one up to use in a simple LM317 project I'm working on.example:http://www.ebay.com/itm/0-36-Blue-LED-Digital-DC-Ammeter-AMP-Mini-Current-Panel-Meter-DC-0-999mA-/321241201719?pt=LH_DefaultDomain_0&hash=item4acb77bc37There are many listings for what appear to be the exact same ammeter, and some of them show them only in a low-side configuration, but one of them showed it in a high-side current sensing position.Well, it turns out they are low-side onlyI wired it up in this schematic:https://github.com/pepaslabs/DualLM317BenchSupply/raw/master/DualLM317BenchSupply_schematic.pdfand stupidly hooked it up directly to a wall wart, instead of a current limited bench supply. It let out just a little bit of magic smoke before I unplugged it, and displayed "999".Amazingly, it still worked when I rewired it into a low-side configuration.So, buyer beware, these are low-side only.

author: FrankBuss
author_rank: Unknown
timestamp: March 20, 2014, 05:04:22 am
content: ±0.1% measurement accuracy sounds good, can you verify it for different currents? Or maybe Dave could buy one and expose the real accuracyI guess it could have such a good precision, but for this accuracy I would expect an expensive resistor which alone would cost more (at Digikey there are 5 mOhm/0.1% resistors for $7, even for larger quantities), or it needs calibration. And the resistor looks strange: 050 looks more like 50 mOhm, not 5 mOhm.Could you post a picture of the rest of the circuit (I assume on the back) and what parts they are using? I hope it's not the usual black epoxy blob so that we can learn a bit from it how it works.

author: Monkeh
author_rank: Super Contributor
timestamp: March 20, 2014, 06:03:54 am
content: Quote from: FrankBuss on March 20, 2014, 05:04:22 am050 ought to be 5R..

author: sleemanj
author_rank: Super Contributor
timestamp: March 20, 2014, 06:25:33 am
content: Quote from: Monkeh on March 20, 2014, 06:03:54 amNo you you are thinking of 5R0. Where shown the R substitutes the decimal point and there is no multiplier.R050 is 50 mOhmEdit: sorry just noticed what you did there, yeah, the R is shown on the image in the eBay but you must have only looked at the post previous to yours which omitted it.

author: Monkeh
author_rank: Super Contributor
timestamp: March 20, 2014, 06:28:17 am
content: Quote from: sleemanj on March 20, 2014, 06:25:33 amI hadn't actually looked at the photo. '050' without a preceeding R would, I would imagine, indicate 5R.

author: FrankBuss
author_rank: Unknown
timestamp: March 20, 2014, 09:05:35 am
content: Thanks, I didn't know the "R" is important. But with the R it looks like it is indeed 50 mOhm:http://www.digikey.com/product-detail/en/LVK12R050FER/LVK12R050FERTR-ND/1632729But I bought one from eBay, too, just for fun. Found one in Germany, no shipping costs, even a bit cheaper, nearly the same product photo:http://cgi.ebay.de/ws/eBayISAPI.dll?ViewItem&item=141208282939In this auction the description doesn't say 0.1% accuracyBut even 5% would be still useful to add to some projects. There are versions up to 100A for the same price.

author: Harvs
author_rank: Super Contributor
timestamp: March 20, 2014, 09:40:25 am
content: You could just add a $2 SIP isolated DC-DC converter to get over the low side issue. Then you can stick it wherever it's needed.In fact I'd go as far as staying that would make it quite useful.edit: This is the sort of thing I was refering to:http://www.ebay.com.au/itm/260988729339I always buy them by the x10 because they're so useful.

author: FrankBuss
author_rank: Unknown
timestamp: March 20, 2014, 09:51:46 am
content: Quote from: Monkeh on March 20, 2014, 06:28:17 amRight, looks like 050 without a "R" would be 5 ohm:http://www.marsport.org.uk/smd/res.htmFirst two digits are the value, third digit is the multiplier. If there is a "R", it is the position of the decimal point. If there are a letter at the third position, it could be the EIA-96 marking method (I've never seen this).

author: sleemanj
author_rank: Super Contributor
timestamp: March 20, 2014, 02:04:19 pm
content: Quote from: FrankBuss on March 20, 2014, 09:05:35 amCareful there, I think you'll find that the higher current devices require an external current shunt, they won't have an onboard one.That's of course obvious from looking at the size, a pcb trace to take 100A of current would be rather large

author: rdl
author_rank: Super Contributor
timestamp: March 20, 2014, 02:26:29 pm
content: Somebody please explain this "low side high side". The meter is measuring a voltage drop across a 5 mOhm resistor in a current loop. What difference does it make here? Seems like there current and voltage drop would be the same at any point in the circuit being metered.

author: sleemanj
author_rank: Super Contributor
timestamp: March 20, 2014, 02:32:41 pm
content: Quote from: rdl on March 20, 2014, 02:26:29 pmThe device has 4 wires1. VCC2. GND3. Current In4. Current OutCurrent Out is I expect connected to GND on the PCB by it's design (they are not differentially sensing, just sensing the voltage at the top of the shunt and referencing that to GND).So if you put it high side, you have supply current going into Current In, which now shorts straight to GND though the PCB bypassing the load which you connected to Current Out,

author: FrankBuss
author_rank: Unknown
timestamp: March 20, 2014, 02:39:24 pm
content: Quote from: sleemanj on March 20, 2014, 02:04:19 pmThis 100A amperemeterlooks exactly the same as tha 1A version, but without the SMD resistor. Maybe it uses the PCB traces. But of course, the wires and traces would vaporize, if used with 100A.

author: cellularmitosis
author_rank: Unknown
timestamp: March 20, 2014, 02:49:00 pm
content: Frank, mine is about 10% off, but that's after letting a little bit of magic smoke out, so I hope that's not representative.I think I'll order another and sacrifice this one for a tear down (the circuit is hidden under the 7 seg display)

author: sleemanj
author_rank: Super Contributor
timestamp: March 20, 2014, 03:02:24 pm
content: Quote from: FrankBuss on March 20, 2014, 02:39:24 pmHere's one that shows the back of the board, there's no trace there between the SMD pads. I'm pretty sure that the only way their photo showing it working is real is if they have an external shunt that the "load" wires (now only sense wires) are connected to off-camera and the load is going through that external shunt. That would explain why1. it isn't a ball of flame2. why the on board resistor is removed (because the shunt is offboard)Maybe they include an external shunt in the package (but surely they'd show it), but as pictured, yeah, that can't work.

author: iloveelectronics
author_rank: Frequent Contributor
timestamp: March 20, 2014, 03:25:18 pm
content: Quote from: sleemanj on March 20, 2014, 03:02:24 pmAll these Chinese ammeters require an external current shunt for their 5A or above variants, and I'm pretty sure it's always a separate purchase.

author: cellularmitosis
author_rank: Unknown
timestamp: March 27, 2014, 01:12:37 am
content: Ok, got a replacement ammeter in from china. The results aren't much better. It is mostly about 6% off.I soldered the ammeter into an LM317 supply I've been working on, and used an adjustable dummy load to dial in various loads, and measured the load using three meters: an HP 3478A, a UNI-T UT61E, and the $5 eBay ammeter.Here's a chart of the absolute values, HP vs the other two:Here's the same data, but formatted as % error vs the HP 3478A:I'll tear one of these down and see what the circuit looks like. Too bad I don't see a trim pot, it looks like the error is just a scaling issue.

author: Bored@Work
author_rank: Super Contributor
timestamp: March 27, 2014, 01:30:18 am
content: Quote from: cellularmitosis on March 27, 2014, 01:12:37 amWhy? If this is what you indeed boughtQuote from: cellularmitosis on March 20, 2014, 03:02:44 amthen the shunt is right in front of your eyes:http://i.ebayimg.com/00/s/MTIwMFgxMjAw/z/DhwAAOxy3NBSdKgg/$%28KGrHqV,!mEFJtkgC7VuBSdKgeONew~~60_57.JPGYour readings are 6% too high, so add a 1 Ohm resistor and a 10 Ohm trimpot in parallel and adjust to reduce the overall shunt resistance by ~6%.

author: cellularmitosis
author_rank: Unknown
timestamp: March 27, 2014, 01:35:47 am
content: Quote from: Bored@Work on March 27, 2014, 01:30:18 amGood point! Thanks

author: cellularmitosis
author_rank: Unknown
timestamp: March 27, 2014, 01:42:49 am
content: Teardown photos:

author: cellularmitosis
author_rank: Unknown
timestamp: March 27, 2014, 03:05:12 am
content: Following Bored's advice, I corrected it with four 1 ohm resistors (1 ohm in parallel with 3 ohms).Here's the results:And here's a link to the spreadsheet with all of the data:https://docs.google.com/spreadsheet/ccc?key=0Aps_jTQV0tKndEdDQjlsdnpEQUxyNGZqcVBrY1VfTnc&usp=sharing

author: sleemanj
author_rank: Super Contributor
timestamp: March 27, 2014, 03:46:13 am
content: Neat photo capturing a segment half-lit :-)

author: FrankBuss
author_rank: Unknown
timestamp: March 27, 2014, 05:55:42 am
content: That's interesting, more chips than expected under the LED. This is the microcontroller:http://www.digikey.com/product-detail/en/STM8S003F3P6/497-11946-5-ND/2765682Not a cheap 1-bit CPU epoxy blob.I guess the 7130H IC is a voltage regulator. And maybe the 3rd 5-pin IC would be an OpAmp.

author: cellularmitosis
author_rank: Unknown
timestamp: April 28, 2014, 05:48:31 am
content: As a follow up to this, using a 10 ohm mutli-turn trimmer is the perfect thing for dialing in these ammeters.

author: sleemanj
author_rank: Super Contributor
timestamp: September 04, 2014, 02:18:16 pm
content: Bit of a follow up to this, thread dredge I know, but anyway.I picked up some "0-10A" versions of these, they are identical to the one posted in this thread, except for using an on board shunt link rather than the smd resistor.Unfortunately, a brief play around with them reveals they have some interesting properties.If you power it from it's own supply separate to the supply powering the load (of course remember that the negatives will be connected on board), everything is hunky dory, the accuracy seems reasonable.If you power the meter from the same supply as is powering the load, and leave the negative supply of the meter disconnected (remembering that the bottom of the shunt is connected to the meter's supply negative on the PCB), then it seems to read consistently about 80mA low - not a percentage of the load just always seems to be 80mA low.If you then connect the negative supply for the meter to the power supply, remembering mind you, that they are already connected together on the PCB, things get really crazy.There is supposedly a "calibration" procedure, but I've only ever seen it mentioned as existing, not what it actually is.The unpopulated 6 pin header on the left side of the board (see previous images in this thread) I have buzzed out as (numbered from bottom to top)...1 = Pin 20 of the mcu, "PD1(HS)/SWIM"2 = Pin 2 of the mcu, UART1_TX/AIN5/(HS) PD5 (this is marked +++ on the board)3 = Pin 3 of the mcu, UART1_RX/AIN6/(HS PD6 (this is marked ---)4 = Pin 4 of the mcu, "NRST"5 = VCC6 = GNDIf I get enthusiastic I might see if there is anything useful to be had by way of serial.

author: sleemanj
author_rank: Super Contributor
timestamp: September 24, 2014, 09:54:07 am
content: With experimentation I've stumbled upon some sort of calibration for these.Put a constant 1 Amp through it, then touch the pad marked +++ to ground it, should show 1.00 (at least for this 10A one I have), touch it again I think to save, or the one below it.The bottom most pin when grounded goes into some other mode which could possibly be some sort of adjustment, I haven't made sense of it.The third down from the top is reset.This seems to work with the "supply" negative (small black wire, as opposed to load negative large black wire) disconnected if you are powering from the same supply. If you connect the supply negative, it goes bonkers.The pin marked --- seems work the same way for calibration to 5A, but I can't try it out at the moment.

author: cellularmitosis
author_rank: Unknown
timestamp: March 24, 2015, 01:27:31 pm
content: Fantastic! I wonder what current the 999mA version should be calibrated with?