4-20mA Loop powered application? - Page 1
Forum section: electronics / beginners

author: FlimFlam89
author_rank: Newbie
timestamp: November 09, 2018, 08:56:39 am
content: Hello everyone!I’m new here so please bear with meI’m working on a project for school and I though that it was done, but oy boy was I wrong.The project is to interface with a 4-20mA pressure sensor and read out the data to a screen. All of this should be loop powered.The micro controller and screen draw 3.5mA - 4mA at 3.3V (~0.5mA at 24V) so it’s not too much.The sensor I’m interfacing with is rated down to 8V.When I hook everything up individually it all works, I can read the mA signal from the sensor both with the multi meter and the ADC of the micro controller but as soon as I hook them up together only the microcontroller works. And the voltage at the sensor is at ~2.5V. What is going on?The schematic below is of the DC-DC and the measuring of the current.Thanks in advance for any help, its greatly appreciated!*EDIT*Added attached image of schematic

author: Zero999
author_rank: Super Contributor
timestamp: November 09, 2018, 11:38:03 am
content: Quote from: FlimFlam89 on November 09, 2018, 08:56:39 amI can't see the schematic. Please upload it here as an attachment, rather than on an external hosting site.

author: Sudo_apt-get_install_yum
author_rank: Regular Contributor
timestamp: November 09, 2018, 01:13:29 pm
content: Here you go!You can right click it and, check source image and save/open it!Oops, forgot to add my commentHave you checked the grounding between the measuring circuit and the regulator?

author: Zero999
author_rank: Super Contributor
timestamp: November 09, 2018, 09:26:50 pm
content: It isn't very clear. Where's the MCU in the circuit?Draw the whole circuit, connected together, including the MCU and sensor. Ensure you make it crystal clear which part is which.It sounds like a grounding problem.

author: FlimFlam89
author_rank: Newbie
timestamp: November 12, 2018, 07:35:06 am
content: Quote from: Hero999 on November 09, 2018, 09:26:50 pmYeah it might be a grounding problem.Could it be that the DC-DC converter in the sensor and the DC-DC converter in my design are "fighting" over the power and resulting in a voltage drop? Like why solar power circuits use a MPPT to regulate the power while not dropping the voltage like it does?

author: EPTech
author_rank: Regular Contributor
timestamp: November 12, 2018, 03:22:56 pm
content: Hi there,If I am interpreting your schematic correctly, and the two symbols for GND are actually a different GND, them the GND of your converter is floating (with the pressure sensor) and the GND of the MCU is at input GND reference. Try referencing everything but the sensor of course to input GND. Place the sensor (with the current sensor) in parallel. You only need to make sure that both your own circuit and the sensor together do not consume more than 4mA on 24V, as to not influence the current loop to the point where you enter the band of measurement.Happy metering...

author: FlimFlam89
author_rank: Newbie
timestamp: November 13, 2018, 09:52:39 am
content: Quote from: EPTech on November 12, 2018, 03:22:56 pmI updated the schematic to be clearer. I’ve read that loop powered things should always be in series of each other and drop the voltage down to not influence the current signal. They should never be in parallel since this would cause both electronics to add to one another and would result in an offset of the signal. I could be mistaken since I’m just a studentI kind of solved this...So the micro controller ground is grounded to the DC-DC ground with is floating, sorry for the confusion in the previous schematic. The op amp is still referenced to "main" ground because I’m not sure how ill ground it.How I kind of solved it:I noticed that when powering up the system with nothing attached it worked fine and when adding my electronics to the already powered on system it worked (If I used a resistor in parallel with my electronics). However when turning them on at the same time nothing worked. I have no idea what is causing this but after testing it a few times this seems to be the case. I added a push button, resistor and a transistorIn the update schematic, the button has to be pressed during power on. However I want to replace the push button and make it do the switching automatically, I’m not sure how I’ll do this since I can’t use the micro controller to do the switching since it won’t be power on during this startup phase.Because I’m using a resistor to drop the voltage I’m wasting a lot of energy and when the sensor puts out <4.7mA my electronics start to flicker and when the sensor outputs >16mA my electronics consume so much power that the sensor shuts down. I need a better DC-DC converter...The remaining problems:Powering on the electronics after the sensor has powered on ~1sNot suing a resistor to drop the voltageI really appreciate all the help!