current meter - Page 1
Forum section: electronics / projects

author: timelessbeing
author_rank: Frequent Contributor
timestamp: February 22, 2014, 06:08:04 am
content: Hi everyone. I bought an inexpensive 20A hall effect sensor board on eBay, and then decided to build a simple meter out of it. I breadboarded the circuit, and it seems to work. What do you experienced guys think about it? Is the design "sound"?The voltage regulator runs a bit hot, but I measured 60mA on the output which is within spec. I chose such low resistor values to overcome the current flowing through the voltmeter, which only has 20kohm.I'd also like rectify the sense voltage with a diode so that AC signals don't drive the needle in the meter backwards. To do that without affecting the reading, I would have to put it on the opamp output, but I would connect the non-inv inputafterthe diode, correct? And if I wanted to read the AC signal with a scope, I could simply probe the V(o) since it has much higher impedance right?Thanks for reading.

author: ohmineer
author_rank: Contributor
timestamp: February 22, 2014, 09:33:55 am
content: About the temperature, have you take into account the recommendations about input voltage and temperature from LM2936 datasheet? (I have attached a screenshot)That might be the reason of the hot temperature of the regulator.

author: nickm
author_rank: Regular Contributor
timestamp: February 22, 2014, 06:51:52 pm
content: What are the two 47ohm resistors for? That's about 50mA right there. If its to hold one end of your meter halfway between the rails there are better ways. One would be to buffer the point in between your two 47ohm resistors and change them to 100kohm. The output of the opamp would drive the one end of the meter.A better way is to just tie one end of your meter to ground which will only let your meter be driven in one direction by the opamp. You would still have to change your opamp to an active full wave rectifier if you want to have it respond to current in the other direction.

author: timelessbeing
author_rank: Frequent Contributor
timestamp: February 22, 2014, 07:15:35 pm
content: Quote from: ohmineer on February 22, 2014, 09:33:55 amI was only driving it with 12V, but since the device shuts down automatically anyway, then no worries, right?Quote from: nickm on February 22, 2014, 06:51:52 pmYes, the zero current output voltage of the AC712 is Vcc X 0.5I considered using an opamp to buffer that end. It would be more elegant, but it added more components and complexity, and 50mA seemed okay to me so I decided bugger it. The resistors and volt. reg. remain in spec. I can still raise the value of those resistors a bit without sacrificing too much accuracy. (I only wanted 5%) Thoughts?I cannot tie this meter to ground. It would only read -1 to +1V. I would have to get a 0-4.0V meter, which I may do in the future. On second thought... galvanometers aren't fast enough to show most AC signals anyway, and I can just flip the input to measure a negative DC. For AC, it's more useful to show RMS values anyway, which gets way beyond the scope of this project. Yeah, I'll probably stick to DC which was my original motivation anyway ... measuring auto circuits.I really appreciate the feedback so far.