• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Need help with project, AC ammeter to ADC??

Status
Not open for further replies.

Neophysics

New Member
I am trying to measure the current flow in a mains wire and interfacing it with a ADC, i have a 50:5 current transformer, and have been looking at a simple ADC0804 anologue to digital converter, the problem is i dont know of a circuit that can get accurate dc input (for the ADC) from a AC 50hz current transformer, i have looked at a differential amplifier but its it fairly confusing...

Also i have to link 8 relays to the output of the ADC (8bit) ???, which cannot have a change rate of more than 2hz, and the adc sinks on the output so could you help me with a pnp/npn relay driving setup

my current solution is CT > AD637 (rms to dc converter) > LF398 (sample and hold) this apparently can get me a 2hz data output on the adc > ADC0804 8bit anologue to digital coverter, i have put lots of time into this to get this far but my limited experience cannot take me any further...:confused:

i would be very grateful for your help, i am new to this forum as i spend alot of time on the internet just trawling and gathering information, but this time i need a bit of help

many thanks
 

Neophysics

New Member
does it have a 0.7 voltage drop you get with all diodes?, i have found another non diode rectifer circuit, using a LM324... how does a rms to dc thingy work? they look like a good option as i need the rms (for a power meter)
 

ericgibbs

Well-Known Member
Most Helpful Member
does it have a 0.7 voltage drop you get with all diodes?, i have found another non diode rectifer circuit, using a LM324... how does a rms to dc thingy work? they look like a good option as i need the rms (for a power meter)
hi,
This the simplest precision rect I have seen, it works well.

Point to note it gives a precision fullwave rectified signal, so for a steady 'dc' level you require a peak detector and filter.
Unless your program is going to sample the fullwave signal [ testing for max voltage]
 

Attachments

Last edited:

Neophysics

New Member
thanks for the precision rectifier... i am trying to get from AC current to 8bit output, i am not inputing to a uC yet just relays, i have been told to put the ac coming out of the CT across a 0.1ohm resistor as a shunt and measure the ac voltage, how you any circuits for a rms to dc converter and is this even a option? thankyou for your quick replies
 

ericgibbs

Well-Known Member
Most Helpful Member
thanks for the precision rectifier... i am trying to get from AC current to 8bit output, i am not inputing to a uC yet just relays, i have been told to put the ac coming out of the CT across a 0.1ohm resistor as a shunt and measure the ac voltage, how you any circuits for a rms to dc converter and is this even a option? thankyou for your quick replies
hi,
Look at this image.
 

Attachments

crutschow

Well-Known Member
Most Helpful Member
how does a rms to dc thingy work? they look like a good option as i need the rms (for a power meter)
The "thingy" a special purpose IC that calculates the RMS value of an AC waveform (even if it's not a sinewave) and generates a DC output proportional to this RMS value. Analog Devices makes several such devices RMS to DC Converters | Other | Analog Devices.

They also make ICs specifically designed to measure AC power Energy Measurement | Analog to Digital Converters | Analog Devices.
 

MrAl

Well-Known Member
Most Helpful Member
Hi,

Since you are working with 50Hz which is fairly low and uC's ADC's are fairly
fast these days, you may even be able to program your own custom RMS
converter right in the same chip you use to read the signal.

There are a couple ways to go about this when you have to read AC.
You can read just the top half of the wave and assume it is symmetrical,
or you can go all the way and add an offset of 1/2 Vcc and that will allow
the AC signal to be read by the ADC as a totally positive sine wave with
zero at 1/2 Vcc instead of at -Vcc. You can then use subtraction in your
program to convert the signal to DC for example, and average it all within
the uC chip program.
Because you are going to have to read low level AC anyway, you are going to
need an amplifier anyway so might as well add a DC offset before it gets to
the ADC input.
Total parts would be:
1. Sense resistor (such as 0.1 ohms)
2. Op amp and associated resistors
Note there are no diodes or any rectification done outside of the uC chip.
This helps to keep parts count low.

Funny, i might actually build one of these myself.
 

ericgibbs

Well-Known Member
Most Helpful Member
i just need to adjust the gain on ericgibbs circuit... how would i do this?
What level of gain do you require.?
Making R7 [2K] a variable resistor will enable you to set the gain of the second stage.

Also what reponse time time do you require.?
 

Neophysics

New Member
i need a adjustable gain so i can change the voltage of the ac into a 0 to 5v range for inputing into my sample and hold (LF398) and acquision time doesnt matter because i need to take reading every 0.5 seconds... which i think i can get from LF 398, the gain is not decided yet as im going to experiment with different CT and load resistors, but something from 1 to 80 ish would be good

Does anyone have a good diagram for a LF398, i need to know the voltage that the sample and hold logic needs to.... sample and hold, im going to run the logic from a 555 so i can get 2 hz is this a viable option or is there a onboard clock ?

thankyou for your help
 

Neophysics

New Member
in your circuit R7 is to adjust the gain, is that in relationship with R8, so is that R7/R8+1 equalling a gain of 3...

also wot does R4 do?

thankyou
 

ericgibbs

Well-Known Member
Most Helpful Member
in your circuit R7 is to adjust the gain, is that in relationship with R8, so is that R7/R8+1 equalling a gain of 3...

also wot does R4 do?

thankyou
hi,
Look at this image.

The prec rect reduces the peak level to half Vac in.

It would help if you told me the level of the input Vac and the required 'dc' level output.

Ref the PM's I cannot post images on PM,s.?:)

EDIT: the image shows a Vac in of 1Vpk [ 2Vppk] and Vdc out of +5Vdc, smoothed.
 

Attachments

Last edited:
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top