LED Ampere panel meter from ebay - common ground? (Dummy constant current load) - Page 1
Forum section: electronics / projects

author: delmadord
author_rank: Regular Contributor
timestamp: January 14, 2014, 05:33:25 pm
content: Greetings folks, I have bought one EbayLED Ampere panel metersome time ago. It is exactly like this although not from that exact link. I am going to use it in circuit very similar to Dave's #102 Dummy constant current load.1) I am wondering how to find out, if this particular model can be used in that circuit - if it can have shared common ground. And then, if it cant, what are my options, if I still want to implement it, aside from buyingIsolated power module, like probably needed inthis one. For examplehereis one that along with chinglish text has some pictures about usage, and on the second one it is used in common ground configuration. There are a lot of others showing the common ground. However, however this one not the same model as mine, I couldn't find any pictures regarding my model.2) Another thing is the shunt. Obviously I am not using a recommended 75mV shunt here, but rather 1ohm one (consisting of 10 parallel 10ohm power resistors, like in Dave's project). What did he use to scale 0 - 2V drop on his shunt to 0 - 75mV (or whatever values his LED display accepts)? In the video, he shows some voltage dividers, are they capable of doing this alone? Or did he use some of the channels from LM324 along with that voltage dividers?Any help appreciated. Thanks.

author: rdl
author_rank: Super Contributor
timestamp: January 14, 2014, 05:36:52 pm
content: When you measure the voltage drop across a 1 ohm resistor it is the same as the amps. This is what Dave did. You don't need an amp meter.

author: delmadord
author_rank: Regular Contributor
timestamp: January 14, 2014, 05:40:47 pm
content: Quote from: rdl on January 14, 2014, 05:36:52 pmBut he said, he do not want to connect multimeter there all the times, so he then put there a display. Or did I miss something importatnt?

author: Excavatoree
author_rank: Frequent Contributor
timestamp: January 14, 2014, 05:44:40 pm
content: Many of those inexpensive panel meters require a separate, isolated supply.I have a dual current source, simply made from two 317 regulators, and I purchased two panel meters much like that one to measure the currents. I knew the supply had to be separate and isolated, but I was surprised to find that I can't use the same 9v battery to power both. Because the current each is measuring are common, I need to use two 9v batteries.

author: rdl
author_rank: Super Contributor
timestamp: January 14, 2014, 05:47:39 pm
content: QuoteCorrect, he wanted a full time meter as part of the device instead of having to connect a multimeter every time he wanted to use it. He used a pretty much standard LCD digital panel meter set to read 0-2 volts. Ones capable of being used with common ground are easy to find. You can still use the amp meter you have, but you'll need to use the current shunt it was intended for.

author: delmadord
author_rank: Regular Contributor
timestamp: January 14, 2014, 06:28:21 pm
content: Quote from: rdl on January 14, 2014, 05:47:39 pmIt is really impossible/impractical to scale 0 - 50V into 0 - 0.075V with op-amp to make it work without another shunt? :-O

author: delmadord
author_rank: Regular Contributor
timestamp: January 14, 2014, 06:36:50 pm
content: Anyway, maybe the easier solution is just use proper panel meter. Probablythis onewill work without a lot of hassle, am I right?

author: rdl
author_rank: Super Contributor
timestamp: January 14, 2014, 07:36:29 pm
content: I'm not sure exactly what is meant by "75mV shunt", but with a bit of work you can probably figure out how to scale a input to read correctly with your ammeter, or just buy a voltmeter. Either way can work.

author: delmadord
author_rank: Regular Contributor
timestamp: January 14, 2014, 08:13:13 pm
content: Quote from: rdl on January 14, 2014, 07:36:29 pmhttp://en.wikipedia.org/wiki/Shunt_(electrical)#Use_in_current_measuringIt is a resistor that will make a voltage drop of 75mV when maximum current for which is designed is flowing through it, in case of my panel amp meter it is 50A.75mV shunt is, from what I have observerved, very common one.Now I am thinking it will be far less complicated to use standard panel volt meter instead of amp meter, as Dave used and you rdl pointed out for me.I am wondering, which one will work better, theoneI linked few posts before or this cheaperone. These are probably the cheapest suitable ones I have found on Ebay. Did anyone use any of them in the past? If so, please tell me so I can choose the better. Or are they both crap? Thanks for all the help so far.

author: rdl
author_rank: Super Contributor
timestamp: January 14, 2014, 08:48:06 pm
content: That one should be okay. You don't get a fancy bezel for mounting it in a box though. I bought three of these a couple of months ago. They work pretty good considering the price. None were off more than .02 volts compared to my Fluke 87-III.http://www.ebay.com/itm/Mini-LED-DC2-5-30V-Red-Volt-Voltage-Meter-Display-Digital-Voltmeter-Self-Powered-/360732171649

author: delmadord
author_rank: Regular Contributor
timestamp: January 14, 2014, 11:07:38 pm
content: Quote from: rdl on January 14, 2014, 08:48:06 pmYeah I had exact the same one but used display of it to another project (urgent needed, you almost cant beat the price anyway). I was wondering how precise they are, since you are saying it is pretty good, I am happy with it. Thanks for info!

author: Kevin.D
author_rank: Frequent Contributor
timestamp: January 14, 2014, 11:46:52 pm
content: So your going to use a panel meter with a minimum input of 2.5V to measure a 1v shunt. ? You can use any panel meter with a lower voltage input range than what you want to measure ,then just use a simple voltage divider to scale input to appropriate range .Also if your doing battery powered projects then keep in mind the power consumtion of some of these Led display panel meters .The people who sell these things on ebay and even the eastern suppliers often never include any info on if their various panel meters can measure common supply inputs ,which is an important bit of info . Pains

author: delmadord
author_rank: Regular Contributor
timestamp: January 15, 2014, 12:13:10 am
content: Quote from: Kevin.D on January 14, 2014, 11:46:52 pmNo, I am going to use its 4 digit,3 wireversion.That one with supply shared with sense, which neds 2.5V will ofc not work well.

author: rdl
author_rank: Super Contributor
timestamp: January 15, 2014, 12:23:48 am
content: Yeah, sorry if that was confusing. I just used that link because it was handy. Those little meters also come in a 3 wire (separate sense line, common ground) and 4 wire versions (completely separate measure inputs). Also in several different LED colors