# Wiring a three-wire Digital Voltmeter/Ammeter

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#### JaneOdessa

Joined May 18, 2015
3
Hi everyone, I'm new to this board so I hope this question is in the right place. I'm working on a power supply for a prototyping board, and I thought a digital voltmeter/ammeter would be a nice touch. So I ordered a $6 special from eBay from Hong Kong, and when it arrived, it's a three-wire digital display. Red, black, and yellow. However, I'm left scratching my head as to how in the world this is supposed to be both a voltmeter and ammeter with only three wires. Red and black cause the voltmeter to come on, and display the voltage as expected. However, I can't figure out how to connect the ammeter so it outputs anything that makes sense. I've tried connecting the 'load' to the yellow wire, both on the positive and negative terminals to no avail. Since there was no instruction sheet in the package, unless someone can read Chinese, the sticker on the device probably won't do you any good. Anyone have any ideas? Thanks in advance. #### MrChips Joined Oct 2, 2009 21,823 Post the model numbers as well as photos of the unit and any documentation you have, including Chinese instructions. AAC is an international forum with staff that speak Chinese. Thread Starter #### JaneOdessa Joined May 18, 2015 3 Okay, here's the sticker. There's literally no other model numbers visible on it or the envelope it came in. This image will be up for 7 days. http://www.picpaste.com/DSC02296-eIaT9nTj.JPG Under the tabs, there's also a label with "010202" on it (maybe a date?) and there are no other markings I can see. Sorry for the low image quality, but hopefully the Chinese characters will be somewhat readable - although I fear it may just be a reprint of the information already there in English. Thanks again for your help and quick reply. EDIT: I decided also to include this link to the eBay auction page where I bought it, since this has much higher res photos. I can't find any model numbers or manufacturer in the text, though. http://www.ebay.com/itm/161482317247 Last edited: #### Reloadron Joined Jan 15, 2015 5,646 My guess is the yellow wire is a mV input for current. The tiny label seems to reflect 50 Amp 75 mV. The 75 mV shunts are popular with these meters. If you have a mV source try applying 5 or 10 mV up to 75 mV to the yellow lead. I figure you will need the shunt. Ron Last edited: #### MrChips Joined Oct 2, 2009 21,823 Here is a better photo of a similar module: #### MrChips Joined Oct 2, 2009 21,823 #### debe Joined Sep 21, 2010 1,185 The meter was cheep because they don't supply the shunt. You will now need to purchase one. Thread Starter #### JaneOdessa Joined May 18, 2015 3 Damn. Well, not the answer I was hoping to hear, but thanks to all for your help. I'll just use it as a voltmeter until I can pick up a shunt. Grounding the yellow lead makes the meter display --.--, which is better than it oddly bouncing between 25 and 30 amps... lol. Thanks again. #### crutschow Joined Mar 14, 2008 25,389 Note that it would appear the shunt needs to have one lead common with the power supply common. That means the shunt needs to be in the ground return of the load. #### svkrishnan Joined Aug 16, 2020 4 Hi everyone, I'm new to this board so I hope this question is in the right place. I'm working on a power supply for a prototyping board, and I thought a digital voltmeter/ammeter would be a nice touch. So I ordered a$6 special from eBay from Hong Kong, and when it arrived, it's a three-wire digital display. Red, black, and yellow.

However, I'm left scratching my head as to how in the world this is supposed to be both a voltmeter and ammeter with only three wires. Red and black cause the voltmeter to come on, and display the voltage as expected. However, I can't figure out how to connect the ammeter so it outputs anything that makes sense. I've tried connecting the 'load' to the yellow wire, both on the positive and negative terminals to no avail. Since there was no instruction sheet in the package, unless someone can read Chinese, the sticker on the device probably won't do you any good. Anyone have any ideas? Thanks in advance.

#### svkrishnan

Joined Aug 16, 2020
4
I have faced the same problem. With all my efforts, I have no solution. It is like an unsolvable puzzle !

#### MrChips

Joined Oct 2, 2009
21,823