Cheap panel ammeter in positive line ? - Page 1
Forum section: electronics / projects

author: outremer
author_rank: Contributor
timestamp: August 31, 2016, 11:05:31 pm
content: Hi all,I am planning to install digital ammeters on my transmitter and several power supplies. I found these cheap volt-ammeter from the Empire of the Middle (see pictures).These models are to be inserted in the negative line with a proper shunt resistor. Amongst the multiple models I did'nt find a more classical one which would come into the plus line. Unfortunately ALL the rigs PSUs and accessories are grounded to the same point in my shack thus putting the small voltage to be checked at the shunt resistor to ground... Even if I cut these groundings I would have DC zero potential on all cabinets thru the coax connections.Does some know of a positive ammeter for my purpose ? I did'nt find on fleabay or elsewhere.Regards

author: Zero999
author_rank: Super Contributor
timestamp: September 01, 2016, 08:53:37 am
content: What's the current range of these meters? Presumably it depends on the shunt resistor?What current do you need to measure?What's the voltage of the power supply?One solution could be to use a current mirror to convert the high current on the positive side to a smaller current on the negative side and measure that.https://www.maximintegrated.com/en/app-notes/index.mvp/id/3669In your case ROis the shunt resistor in your ammeter. To save power, you can make the value much higher than it would otherwise be.The op-amp's common mode range must include the positive supply. The TL071 should do the job. The MOSFET can be the BS250, as it doesn't carry much current or dissipate a lot of power.

author: ebclr
author_rank: Super Contributor
timestamp: September 01, 2016, 09:18:53 am
content: This may helpshttp://www.aliexpress.com/item/Free-Shipping-Hot-Sale-ACS712-30A-Range-Hall-Current-Sensor-Module-ACS712-Module-For-Arduino-30A/32515954572.html?spm=2114.30010308.3.21.nRjOAk&ws_ab_test=searchweb201556_0,searchweb201602_3_10057_10056_10065_10037_10055_10054_10069_301_10059_10033_10058_10032_10017_10070_10060_10061_10052_10062_10053_10050_10051,searchweb201603_2&btsid=5cb82d3c-3639-456b-b31f-13d39b11652a

author: outremer
author_rank: Contributor
timestamp: September 01, 2016, 10:46:17 pm
content: Hi all,First I thank you very much for help and good ideas.Tohero999: The panel meter is intented to measure an up to 50 A 13.8 V PSU. For a 50 A current the shunt resistor voltage must be 75 mV. So the provided shunt resistor is about 1.5 milliohm. The shunt resistor must be inserted in the negative return line because the ADC in the meter will work from 0 to 75 mV. If I insert the same shunt in the positive line the ADC will have to deal with 13.8 to 13.875 V which will most probably blow it. I considered the use an op amp to "shift" the voltage down to the correct value. But the need for a +/- VDC power supply drew me back.Toebclr: So i had a look to the power sensors and found several interesting parts which will be much easier to operate : they provide a linear voltage output from 0 to X mV. I plan to adjust the voltage with a precision potentiometer to suit the meter's ADC high impedance input and give an accurate reading on the ammeter. I'm planning to use the Allegro ACS758LCB-050U.Do you have any comment about this ? Shall it work ?Best regards.

author: Zero999
author_rank: Super Contributor
timestamp: September 02, 2016, 08:53:26 am
content: Quote from: outremer on September 01, 2016, 10:46:17 pmWell the idea was to change the sense resistor inside your meter to something much higher, say 15Ohm, then use a current mirror with the 1.5 milliOhm resistor on the high side to convert the 50A current into 5mA flowing on the low side.QuoteA Hall effect sensor will work but bear in mind that one needs a 5V power supply.