Measuring a 0-1V 5A analog input

10 Apr 2012

ndeye aida lette wrote:

Hi everybody I have to create a circuit capable of measuring 0-5 ampereA AC currents and convert these to a (RMS?) linear 0-3.3V DC voltage with a mbed..I don't how to make the link between the mbed and the circuit. I will be very happy to get some answers thank to you.

You could go for an ACS712. It's produced in different versions(different max currents) and Sparkfun sells one with a PCB on it.

Lerche

10 Apr 2012

okeyy..I never work with the acs712..i willmake some research..so with this component I have to built the circuit that will enable me to measure the AC current?

19 Jun 2012
03 Mar 2015

Hello - I realize this posting is few years old, but I am still going to try my luck with a response.

I am trying to integrate these CTs on the mbed platform: http://www.onsetcomp.com/files/manual_pdfs/6225-F-MAN-CTV_web.pdf

There are multiple amps variants for this CT, but the output signal for all of them is 0-2.5 VDC.

I aim to integrate multiple of these CTs (varying amp ratings) with my mbed MCU, using this Multiplexer: http://www.vishay.com/docs/49949/pt0176_49949.pdf

Since there is an in-built ADC on this mux IC, I may just use that, instead of using the ADC on the microcontroller.

Can anyone please comment on the feasibility of this solution, and use of analog multiplexers on the mbed platform? If anyone can direct to some similar multiplexing applications that have been tested with mbed, that would also be appreciated.

Thanks in advance.

05 Mar 2015

We built this board around the mbed platform

http://flexscada.com/products/flexs-q3/

It has an 8 channel 24 bit simultaneous sampling bipolar adc capable of sensing both positive and negative voltage with jumper configurable modes for:

AC Current Transformers 4-20MA Current Loop Sensors 0-5V Sensors 0-10V Sensors +/- 60V AC/DC Voltage

It also has a dedicated one wire bridge for DS18S20 temp sensors and 8 solid state relay outputs

http://flexscada.com/wp-content/uploads/2015/03/ct_open_energy_monitor_sensor_ethernet_internet.png

Were in the middle of building a multiplexer addon board and I'll post the project on here when its finished if it would be of any help

-deleted-
13 Apr 2015
02 Sep 2015

Guys,

The BBC wants to put together suitable projects for year 7 ( 11-12 yr olds ) They won't have heard of Hall Probes until much later, let alone be able to argue that those things are non-invasive and don't heat up. So much of this thread has been off-putting and contrary to the educational goal.

Back to the original posting of "how to measure up to 5 Amps" with the BBC microbit. Avoid AC, as that is year 8 +. Pick a task which should be understandable to a year 7; for example to measure the DC current through a car front sidelight or indicator bulb when connected to a 12V car battery or school DC power supply. The apparatus must be safe to use with all 12V bulbs up to 120W headlamp size. Please can a science teacher modify this as appropriate for their year 7 class.

Choose a sense resistance which won't get hot, as that would complicate things and could be unsafe. I'd use 6 to 12 feet of ordinary mains cable, 15Amp rated. This can be scrap cut off any broken household appliance. You also need two lengths of thin wire; awg28 single strand would do. A teacher must supply the reel. Strip and connect the blue and brown wires to the two terminals of the device under test (possibly by screwing onto something which the teacher has made to hold 12V lamps). A scrap automotive indicator lamp housing needs spade terminals, which can be crimped which is too fiddly, so use lengths of crocodile-clip 4mm banana-plug wire instead. Tape down the wires where there is no risk of them short-circuiting if knocked.

Cut and strip brown and blue at the other end of the scrap mains wire. Twist your first thin wire onto the blue and attach those to car battery - or the black of a school DC power supply or the black of a scrap ATX desktop computer power supply. Connect this first thin wire to GND of your microbit by croc or solder.

Twist or croc your second thin wire to the lamp end of the blue wire. Croc that to pin0 of your microbit.

Visually inspect your device under test and go through the following checklist:

0. Are all connections present and as the diagram in your science notebook ?

1. Does blue at supply end connect to microbit GND and power supply {BLACK or - or 0V} ?

2. Does blue at lamp end connect to microbit pin 0 and device-under-test - ?

3. Does brown at lamp end connect to device-under-test + ?

4. Are all cables and connectors placed where they won't get knocked, taped if necessary, and firmly connected?

[next, someone is going to add a method to get AD0 readings on a microbit at the right level for a year-7 to follow] [someone from microbit needs to write here whether it is better to have a resistance such as 1k in the path to pad 0. What is the input impedance of the microbit pad0 when used as AD0 in?]

If all checks 0 to 4 were PASS then connect brown at supply end to the + terminal of the car battery or the {RED or + or +12V} of your power supply. While the supply is ON the bulb should light.

[note for teachers on using scrap ATX supplies from desktop computers] Opening the case of any scrap old desktop, it is usually possibly to undo four screws, fiddle with the various 4-pin molex connectors and 20 or 24 pin main connector, and recover an ATX power supply which is useful for experiments at 1 to 10 Amps 12 Volts, and also 5Volts. - advantages - cuts out if overloaded or short-circuited. big and cheap. - disadvantages - fixed supply, often 11.6V with no adjustment

To switch it on, provide mains power and check that any switches are "ON".

On the 20 or 24 pin main connector, link GREEN to BLACK. A paperclip can be bent to do this.

With a multimeter, check that one of the red-black-black-yellow cables with a 4-pin molex shows +5, 0, 0, +12 Volts.

That is a PASS, after which you can switch it off, cut and strip a BLACK and YELLOW wire for 12V experiments.

[questions about ADC on the microbit]

Expecting 0.1 ohms in a six-foot cable, 5Amps gives about 0.5 Volts from microbit GND to microbit PIN0.

Is there a prescaler on the ADC which can be set to an appropriate range ?

How many bits is the AD raw number ?

What is the settling time and a sensible interval between records to sum in a measurement ?

What is the precision and accuracy of the AD?

[example answers for an arduino nano v3, these would be : in setup, prescaler should be set to 1.1V range AD is ten-bit measurements every 0.01 seconds are about right there is not a lot of point in summing more than four measurements for 12bit indication]

Hopefully the "measure up to 5A" task can be made less frightening than the posts above, which are definitely not at year-7 level. So far I've written only the hardware description to present about 0 to 0.5 Volts to the microbit AD0 proportional to 0 to 5 Amps DC. I've not seen anything yet of the spec sheet or programming requirements, so can someone else do that. It needs at least some calibration, which could be as simple as writing the code with a suitable calibration constant in it. Teachers might want to introduce some mention of integer and float number types in computers, as raw AD and calibrated Amps provide clear and understandable examples of the two types.

03 Sep 2015

Where did you get the 7 year-old, and microbit part from at this years old topic? When would a 7-year-old want to measure 5A?

03 Sep 2015

Erik,

Sorry if I was a bit off-topic. I got to this forum by searching for "BBC microbit pinouts" and "BBC microbit supply current". The BBC microbit project aims to give out a lot of nicely made units similar to the more serious boards that this forum is about, to 11-12 year olds later in the year, "to make things with" [as yet not fully specified]. "year 7" is a British designation for the year of school normally attended by 11-12 year olds. Looking for the first task which I'd want to let the kids loose on (plot Amps vs Volts for an automotive 12V bulb as a science project for familiarisation with some hardware and the programming environment), I found much above to be unsuitable for BBC microbit recipients. Nevermind. Perhaps I'll find an educators forum another day.

ZT

03 Sep 2015

ZT,

Don't get me wrong: I think it was a useful post, both in information you provided, and questions you raised. However it doesn't seem to be really related to this topic, despite that it is similar. So I think it fits nicely on these forums, but I think it would be best if you give a bit of background information first, link to this topic, and then add this post with your comments and open questions. Then in a new topic everyone can discuss just purely your post.

Erik

04 Sep 2015

ZT

I think you (or possibly google) have a slight mbed / microbit confusion going on. It doesn't help that under the hood the microbit is based on the mbed platform.

If you take a look at this thread most of it dates back to 2012, that is a few years before the microbit was first announced, this forum is generally people looking to do embedded c++ programming. While there is a wide range of skill levels it's generally all people with some basic programming experience and, in this hardware specific forum at least, also some electrical experience. It's certainly generally expected that the average reader won't be an 11 year old just starting to learn the more beginner friendly languages planned for the microbit.

Perhaps it would be a good idea to add a beginners guide to mbed/c++ on the microbit to this site or a microbit specific forum for children who want to migrate from the more hand holding programming interfaces to something more real world. After all we don't want to raise a generation of programmers who don't know what a pointer is, once that happens we'll all be stuck using c# ;-)

28 Oct 2015

It is easy the voltage drop across the resistor will be proportional to the current passing through it. According to Ohm's Law V=IR. You will need to make sure that current only passes through the resistor in 1 direction in case of DC, and tie the ground leg of the resistor to the GND pin on the mbed.

http://www.7pcbmanufacturing.com/