Ampmeter Feedback issue on power supply

[bradix14]

I've attached a video of my problem since it's too much to type.

https://www.dropbox.com/s/h28z3hy0yuejdy1/2014-11-27%2016.45.01.mp4?dl=0

Basically, I have a mini amp meter that I want to measure the output of my power supply. It's built into the power supply and powered internally. After frying one circuit, I learned that these amp meters need to be powered from a source separate from what they are measuring. I now have an isolated power module to power the meter and then I'm running my output through the meter as well. Whenever I turn that output on, the amp meter reads around 6 amps. I haven't left it on for more than a second or two cuz I don't want to fry more things.

I gave more details and images in the video. Let me know if more info is needed or what ideas you have.

Thanks tons

[pebe]

Hi there,

Nice video and nice construction.

I am currently building a replacement for my aging 30V 2A power supply and I am going the same route as you - using a buck converter. I see you are using 4 of them so presumably you have made 2 variable + 2 fixed voltage supplies,

Can I assume that both ammeters are powered by a common isolated supply? A simple block diagram of the connections between the various units would help.
Can I also assume you purchased the buck converters from Ebay. If so, it would also help to know the ebay identification numbers.

[bradix14]

Thanks for the response.

My design evolved as I was building it. I ended up doing one variable output, one fixed 5v, one fixed 12v, and then one buck converter just to power peripherals like the volt and amp meters.

Here is the amp meter in question - http://www.ebay.com/itm/0-36-Blue-LED-D ... 1284554966

Buck converter - http://www.ebay.com/itm/DC-DC-3A-Buck-C ... 2a47f02c53

Also, I've attached a schematic.

[pebe]

Digital ammeters work by passing the test current through a sensing resistor and measuring the voltage across it. The one you are using has a built-in shunt across the sensor to give an increased current range. The LEDs and chip can be powered by an input of 4V – 30V so the display must contain an internal voltage stabiliser to keep the LEDs supply at ~4V

I think you have it wired as in the RH circuit shown in the ebay advert, ie. the white wire of the sensing resistor is connected to the +ve of the variable power, and the black wire of the sensor is connected to the +ve of the load, with the –ve of the load being taken to the –ve of the variable power.

I think your problem is the use of a 12v buck converter for powering the ammeter. Although not stated in the advert, none of the buck converters I have seen are isolated; the –ve input is the same as the –ve output (check the underside of the PCB for verification). I cannot be certain how it is connected because you have not shown the colours of the meter’s wires, but I think that loop currents between the various –ve connections are flowing through the sensing resistor and giving a reading, because on the PCB of the ammeter the two black leads will almost certainly be common.

You can check if that is your problem by disconnecting the 12V buck converter and temporarily powering the ammeter with a 6V battery. If that is the problem, then there are two ways round it.
1. Wire it up so that the variable supply powers the ammeter as in the LH supplied circuit (but that way has the limitation that its negative terminal is at a different voltage than the other –ve terminals on your panel), or
2. Power the ammeter with a separate 5VDC supply removed from a mains ‘wall wart’ 5V charger (the better option).

I would be interested in your findings.

[bradix14]

I currently have the ampmeter powered through an isolated power module - godsend vb1212ls-1w.

I've been working on it and turns out that the meter is just fried... I ordered some new ones and we'll see how it goes...

[ShaunRay]

Ofcourse you need the different power for the Ampmeter. These are simply works on 3 to 5V. You don't worry while measuring current using different power for your Ampmeter. It will not burn your Ampmeter. But you must take care of the range of the Ampmeter while you are measuring current in the circuit.

assembly board