how to avoid my digital ammeter from shorting in my power supply? - Page 1
Forum section: electronics / beginners

author: d4n13l
author_rank: Regular Contributor
timestamp: May 03, 2017, 01:20:27 am
content: Hello everyone,I'm working on a power supply, but I'm having problems connecting the ammeter. It has 4 lines, two to power the meter and two for the measuring. In my power supply I used a lm317 to get 5v to power the meter, so common ground.The problem is that when I connect the ammeter's measuring lines in series with the + of the power supply, the voltages drops and the fan that I'm using as a load for testing stops working, although the ammeter says there is current. So I checked the meter and the ground line powering the meter has continuity somehow to both measuring lines (that are labeled + and -, which I'm not sure why that matters since the meter goes in series with the load) so I figure they are shorting the circuit.I also found that if I don't use the negative labeled measuring line and I only connect the positive measuring line to the negative of the load the circuit works and meter work, however the current it measures is about 20mA lower that when I use and external power supply to power the meter.Would the only option for the meter to work properly be to power it from a secondary power source? any ideas as if it is possible to connect the meter powering from the lm317?https://www.dropbox.com/s/wzu9k4c7ax8bigd/diagram3.jpg?dl=0

author: sleemanj
author_rank: Super Contributor
timestamp: May 03, 2017, 02:10:35 am
content: It must be used low-side, not high-side like you are trying.The ammeter's negatives are (effectively) commoned.

author: Brumby
author_rank: Unknown
timestamp: May 03, 2017, 02:47:53 am
content: Your image isn't visible.It seems it is not publicly accessible.

author: d4n13l
author_rank: Regular Contributor
timestamp: May 03, 2017, 04:00:17 am
content: Sorry about the image, I put a link that should work now

author: d4n13l
author_rank: Regular Contributor
timestamp: May 03, 2017, 04:08:06 am
content: sleemanj: I'm not sure how would I connect it low-sided, if you mean as I did in the image section 2 but using the negative measuring line instead of the positive the circuit works but the meter measures a low negative current.

author: sleemanj
author_rank: Super Contributor
timestamp: May 03, 2017, 06:35:55 am
content: Connect the ammeter's load input and output (+/- side of the shunt/resistor) in series with the low side of the load, usually "thick" white or red and black wire.Power the ammeter to the +/- wires of the ammeter, usually the "thin" red and black wires.Notice that the thick and thin black wires are commoned togethor on the PCB usually.Often the accuracy does suffer in this instance of powering the ammeter from the same supply as is being used to power to the load (fan), you might find it better to not connect the - for power (thin black wire), or preferably use an isolated supply (isolated DC/DC converter powered from main supply, or a battery, or totally separate supply) to power the ammeter (thin wires).

author: d4n13l
author_rank: Regular Contributor
timestamp: May 04, 2017, 03:04:08 am
content: Thanks man, that worked. And actually it makes sense now. I'm pretty new to electronics but I'm having fun learning.I do get about 10mA difference in this configuration but it's good enough for now.