LCD voltmeter issue when used with op-amp and ACS712 - Page 1
Forum section: electronics / projects

author: JessyMathault
author_rank: Newbie
timestamp: September 01, 2017, 09:50:04 pm
content: Hi all,I'm about to go nuts with this issue. Seems simple enough, yet I've spent way too much time trying to figure it out.I'm trying to do a PSU with current limiter. I'm using a hall effect sensor (ACS712) to measure the current. The current limiter using the ACS712 works fine (U3).I had a few voltmeter like this one :http://www.ebay.ca/itm/Mini-DC-0-1-30V-Red-LED-3-Digital-Display-Voltage-Voltmeter-Panel-for-Motor-Car-/142493564868?hash=item212d473fc4:g:YhAAAOxyEoFSWsC8So I figured, I'd use one to measure the voltage and another to display the current. The ACS712 has a 0A -> 2.5V output and then increases up to 3.5V->3A. So I used a LM4040 to substract 2.5V from the signal and I use (U2) and (U1) to amplify the resulting voltage to the corresponding current value. This works fine. The lcd voltmeter also works fine when measuring other voltages, but when connected to U1, or anything related to the output signal of the ACS712, it's completely off. It'll show 0.1V when I measure 1V with my fluke.I tried filtering for noise but it doesn't change anything.I've attached the problematic circuit (U1, U2) and part of the current limiter circuit that could be part of the problem (U3) (U3 has something connected on its positive input)Any help is appreciated!Thanks,Jessy

author: StillTrying
author_rank: Super Contributor
timestamp: September 01, 2017, 10:20:27 pm
content: Are U1,U2, single supply op-amps with outputs that work below 1V, if the input impedance of the meter is quite low it might pull the 1V down. Have you tried both the meter and fluke on U1 output at the same time.Is the meter's -ve connected to the circuits GND.

author: JessyMathault
author_rank: Newbie
timestamp: September 04, 2017, 03:41:04 pm
content: Thanks for your recommendation.It's a LM358 and it works down to 0V.I tried replacing the ACS712 with my function generator and played around with the offset (2.5V to 3.5V) of a 0.2V amplitude sinus. The meter and the circuit worked fine with the generator.The meter only has 3 wires, one gnd, which is connected to ground, one 5V and one for the measurement.I'll try to pull up (and down?) the ACS712 to see if it does anything.

author: nctnico
author_rank: Super Contributor
timestamp: September 04, 2017, 07:21:11 pm
content: The Allegro current sensors need a minimum load at their outputs. IIRC 4.7k Ohm to ground.

author: JessyMathault
author_rank: Newbie
timestamp: September 04, 2017, 08:44:33 pm
content: The 1k resistor in series with the diode does the trick for the minimum load.The pull-up/pull down did not change anything.BUT! I found the problem. I used a buck converter (cheap ebay ones ) to generate the 5V from the supply (9-15V).It was all because of the current path back to ground. The vin- and vout- of the buck is shorted, but the current flowing back with this wire caused the 5V to increase and mess up the rest of the measurements.So by connecting the load gnd directly on the source, everything works. (see image)I'll need to check the circuit of the buck converter to better understand this problem.

author: DBecker
author_rank: Frequent Contributor
timestamp: September 05, 2017, 03:34:06 pm
content: The AC712 output at zero current isn't nominally 2.5V. It's Vcc/2. The difference could be significant if your 5V suppy isn't exactly 5.000V.

author: StillTrying
author_rank: Super Contributor
timestamp: September 06, 2017, 10:56:50 am
content: Quote from: JessyMathault on September 04, 2017, 08:44:33 pmIt almost looks like the buck converter has its current sense in its gnd line.On 5V you won't get more than about +50mV to +3.5V at the output of a LM358.