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# how to connect 4 wire RTD (PT100) in wheatStone Bridge?6

## how to connect 4 wire RTD (PT100) in wheatStone Bridge?

(OP)
i want to connect a 4 wire RTD PT100 in a wheatStone Bridge.
i tried searching on internet but only upto 3 wire configurations in bridge are available or circuits using other ADS IC's are available. but i want to do it in a Bridge Configuration. can any one suggest how i can do it?

### RE: how to connect 4 wire RTD (PT100) in wheatStone Bridge?

you don't

you treat it as a two-wire device

### RE: how to connect 4 wire RTD (PT100) in wheatStone Bridge?

(OP)
really ? have you tried anytime to connect pt400 in dat configuration?

### RE: how to connect 4 wire RTD (PT100) in wheatStone Bridge?

The 4-wire RTD is the bridge circuit. A bridge circuit's function is to accurately convert a resistance to a voltage signal. With a 4-wire RTD you send a fixed current down two wires and measure a voltage on the other two wires. This voltage is proportional to the RTD resistance and is not affected by the lead wire resistance.

### RE: how to connect 4 wire RTD (PT100) in wheatStone Bridge?

(OP)
means i dont need to connect it in one of the arm of the bridge?
on wiki one diagram they have given, have a look at it.
can you please explain me that?

(OP)

### RE: how to connect 4 wire RTD (PT100) in wheatStone Bridge?

To accomplish that diagram you would have to cut two wires at the RTD and connect them direct together to form a simple wire loop. This loop then acts as compensation for lead resistance using the external bridge circuit.

### RE: how to connect 4 wire RTD (PT100) in wheatStone Bridge?

(OP)
i dint get you , can you please draw it in paint and show me?

### RE: how to connect 4 wire RTD (PT100) in wheatStone Bridge?

It is not possible to wire a 4-wire RTD as shown in your diagram. Your diagram really shows a 2-wire RTD wired to a bridge along with two additional wires used for lead wire resistance compensation. This uses four wires, and does the same thing as a 4-wire RTD, so it may be called a 4-wire RTD installation. But the two implementations are just different.
As I said, a 4- wire RTD is its own bridge circuit, and you don't place one bridge inside another.

### RE: how to connect 4 wire RTD (PT100) in wheatStone Bridge?

(OP)
means the output of this pt100 through sense wires can be directly given to anaogue to digital converter? will it work that way?

### RE: how to connect 4 wire RTD (PT100) in wheatStone Bridge?

(OP)
i want to use 4 4-wire RTD's (Pt100) to sense say temperatures in 4 different rooms.
which ADS IC will be good for this one? which ADS ICs which is low in cost bt cant take input from these 4 RTDs ?

### RE: how to connect 4 wire RTD (PT100) in wheatStone Bridge?

Interesting. I will have to figure that one out later. I have a plane to catch.

### RE: how to connect 4 wire RTD (PT100) in wheatStone Bridge?

If you're just measuring "room" temperature you do not need 4-wire RTDs. Use junky 2 wire RTDs or better use a thermistor like everyone else does. Or better yet, get yourself a handful of LM34 or LM35 parts. They make the ENTIRE task much simpler.

LM34

Keith Cress
kcress - http://www.flaminsystems.com

### RE: how to connect 4 wire RTD (PT100) in wheatStone Bridge?

The first diagrams show a standard THREE wire RTD connection. This is the common connection used in industry. It is suitable for distances of hundreds of feet. The four wire connection may be used in laboratory environments, but not in the field.

Bill
--------------------
"Why not the best?"
Jimmy Carter

### RE: how to connect 4 wire RTD (PT100) in wheatStone Bridge?

We're talking a "room". Rooms don't warrant extra accurate sensing since they aren't even thermally homogenous. A room can easily have a couple of degrees of variation spread around it and it changes constantly with set-points and external ambients.

Keith Cress
kcress - http://www.flaminsystems.com

### RE: how to connect 4 wire RTD (PT100) in wheatStone Bridge?

(OP)
i want a way to connect a 4 wire RTD to a WheatStone Bridge.
is it possible to do that or no?

and what about the kelvin configuration show in the below image ?

i want to take input from 4 4-wire RTD's and then give it to a ADC and then to a MicroController.
but all of this i have to do using WheatStone bridge. So i want a way to do so.

### RE: how to connect 4 wire RTD (PT100) in wheatStone Bridge?

Is the obession with a Wheatstone Bridge because that is a requirement of your college homework question? I can see little practical reason for needing a 4-wire connection at all in this application, and even less to insist on using a Wheatsone Bridge to the exclusion of other accepted techniques. And cheating on your home work is against the rules of the site, plus it doesn't help your education.

### RE: how to connect 4 wire RTD (PT100) in wheatStone Bridge?

(OP)
no.. i just want to know whether i can connect a 4 wire pt100 in one of the arm of the wheatstone bridge.
if yes then how to go about it?

### RE: how to connect 4 wire RTD (PT100) in wheatStone Bridge?

(OP)
if it is really not possible to connect a 4 wire RTD in any bridge then is there any IC which can take input from upto 4 4-wire RTDs ? with lowest cost ? to which i can feed these 4 RTDs..? ADS1246/47/48 can take upto 3 4-wire RTDs ,but is der any IC which can take 4 4wire RTDs?

### RE: how to connect 4 wire RTD (PT100) in wheatStone Bridge?

Have a read of Analog Devices' application note: http://www.analog.com/static/imported-files/semina...

4-wire connection is for use with a Kelvin connection. 3-wire is used for a bridge with one varying element.

### RE: how to connect 4 wire RTD (PT100) in wheatStone Bridge?

(OP)
hey Scotty , nice link...i read on it.. but in the kelvin Section they have not given about which element is varrying and all , means in case i am connecting a 4 wire RTD in that kelvin section how the circuit will look like? can u please tell me?
the ckt is this :

(OP)

### RE: how to connect 4 wire RTD (PT100) in wheatStone Bridge?

(OP)
@skottyUk i meant where is the location of the 4 wire RTD in that circuit?

### RE: how to connect 4 wire RTD (PT100) in wheatStone Bridge?

That circuit would use a 2-wire RTD, and it would not compensate for lead resistance. There is no way to deploy a 4-wire RTD in that circuit. You could make a 2-wire RTD from a 4-wire type if you wanted to. Just be careful to cut one lead off each end, not both leads off the same end.

### RE: how to connect 4 wire RTD (PT100) in wheatStone Bridge?

Now you're asking for an IC that can accept 4 wire RTDs.. They're not cheap! I gave you a link to an IC that avoids you needing any other ICs and minimizes the code math.

What class IS this for?

Keith Cress
kcress - http://www.flaminsystems.com

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