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sensing different voltages isolated from each other

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abhichaotic

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hi,
i'am working in a project in which i have to sense voltages of different devices and view these on a lcd. these voltages should be isolated from each other and the output should vary according to input. linearity should be maintained.
plz suggest ideas for the design.
 

Diver300

Well-Known Member
Most Helpful Member
If you need full isolation, you need transformer or opto isolation.

Opto isolators are very non-linear. To use them, you either need to convert to digital or you need to use a dual opto isolator. In those, there are two outputs which are as close in characteristics as possible to each other. An op amp is used to make sure that the output current is what is expected, and the other output should be the same.

If there is a limit to the range of the voltage difference between the devices, you could use a differential amplifier.
 

abhichaotic

New Member
If you need full isolation, you need transformer or opto isolation.

Opto isolators are very non-linear. To use them, you either need to convert to digital or you need to use a dual opto isolator. In those, there are two outputs which are as close in characteristics as possible to each other. An op amp is used to make sure that the output current is what is expected, and the other output should be the same.

If there is a limit to the range of the voltage difference between the devices, you could use a differential amplifier.
hi,
what i had done is that i had converted input voltage of a single device to pwm pulses and passed through an opto(mct2e) after that i had averaged the signal by a filter but the problem is the output is not linear, also my input is variable from 12v to 40v. i want as the voltage changes the output should also vary and the measurement should be precise.
 
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ericgibbs

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hi,
what i had done is that i had converted input voltage of a single device to pwm pulses and passed through an opto(mct2e) after that i had averaged the signal by a filter but the problem is the output is not linear, also my input is variable from 12v to 40v. i want as the voltage changes the output should also vary and the measurement should be precise.
hi,
There is this linear opto. HCNR200

I have used reed switches in scanners, simple to to drive and suitable for analog or digital signals.

EDIT:
This is circuit drawn for an OP, it worked fine.
 

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Reloadron

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OK I see this as you are sensing voltages from a variety of devices or sensors that don't and can't share a common ground or common bus. You need signal conditioning with isolated analog inputs as well as being differential (rather than single ended) inputs.

Devices like this are frequently used for signal conditioning and isolation. You don't mention what you are doing with the final data? If this was part of a data acquisition system there are other PC based options.

How many channels and budget determine where you go with this project.

Ron
 

abhichaotic

New Member
hi ron,
yes , i am making a data acquisition system and i have to sense different devices voltages. each device gives output dc and there output is to be measured and each device should be isolated from each other. the output is viewed in a software and we have more than 50 devices . we can use microcontrollers.
plz, suggest any method
 

abhichaotic

New Member
hi,
There is this linear opto. HCNR200

I have used reed switches in scanners, simple to to drive and suitable for analog or digital signals.

EDIT:
This is circuit drawn for an OP, it worked fine.
hi ericgibbs,
actually i need a cheaper solution than this, it will be a costly enterprise for me
 
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ericgibbs

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ericgibbs

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hi ericgibbs,
will you plz tell me how to sense voltage using a reed switch and also keeping isolation in mind
hi,
Look at the Farnell reed relay link I posted.
The principle is that you use a 'scanning' method.
Say you have 8 reed relays and you want to measure 8 different voltage sources.
You common the normal open contact of all 8 reeds, this is the line thats connected to say a PIC's ADC input.
The free ends of the 8 reed contacts go to the voltage sources you wish to measure.
The PIC drives the reed relay coils sequentially as each reed is closed the PIC reads the voltage on the common input pin.

Is this what you are asking.?
 

abhichaotic

New Member
hi,
actually i want to measure the voltages and they all should be isolated from each other, also there are 50+ devices so i think think this will not work
 

ericgibbs

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hi,
actually i want to measure the voltages and they all should be isolated from each other, also there are 50+ devices so i think think this will not work
In the way I described, they are isolated or do you mean measure all 50 at the same instant.??
 
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abhichaotic

New Member
actually these devices are dc-dc converters of various ranges, and i am designing a transducer circuit to measure there voltages for a data acquisition system. in that case plz give any fruit ful suggestion
 

Reloadron

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actually these devices are dc-dc converters of various ranges, and i am designing a transducer circuit to measure there voltages for a data acquisition system. in that case plz give any fruit ful suggestion
Doing this I don't know of an inexpensive way to go about it beyond some of Eric's suggestions. I generally would run with several A to D modules in a 16 bit range (unless you want less accuracy) from a company like NI (National Instruments). Cards or devices like these. The rest depends on what you are writing for software and how it communicates with the choice of data acquisition. When you start looking at 32 channels of A to D with isolated and differential channel inputs and you want 16 bit A/D it can get pricey.

I would suggest if time allows to start with an inexpensive A/D starter kit and communicate with it (many use a simple USB interface). Something even 10 or 12 bit A/D would work and formulate a plan from there. Something inexpensive with maybe 4 channels to play around with. As I said, aside from building it yourself with Eric's ideas I don't know of an inexpensive way to go about it.

Ron
 

ericgibbs

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actually these devices are dc-dc converters of various ranges, and i am designing a transducer circuit to measure there voltages for a data acquisition system. in that case plz give any fruit ful suggestion
hi,
Would a simple drawing help you understand the solution I am suggesting.??
I am assuming 0V will be common to all the dc/dc converters under test.?, if not it will mean switching the 0v lines, which can be done but its more expensive
 

ericgibbs

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surely sir it will do. just a rough schematic of ur idea will b enough. thanks 4 taking interest in my question
OK,
I wont do all 50 channels;), say 4.

I will assume that the 0V on the remote units under test is common through all the units also that you will provide the reed relay coil drive.
 

ericgibbs

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hi,
Look at this block diagram, if there is a problem, edit the drawing and repost.
 

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crutschow

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Do you need isolated grounds for your measurements? Are the commons of the various voltages possibly at different voltages?
 
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