# Bridge problem

#### diab

Joined Sep 9, 2006
1
hello
i have a problem with a bridge circuit that has in the 1st side a potentiometer and in another side a RTD detector(35kohm) and in remaining both sides are a two resistors (35 kohm)
HOW CAN I MAKE A CURRENT THAT VALUE IS (150mA)

#### pebe

Joined Oct 11, 2004
626
hello
i have a problem with a bridge circuit that has in the 1st side a potentiometer and in another side a RTD detector(35kohm) and in remaining both sides are a two resistors (35 kohm)
HOW CAN I MAKE A CURRENT THAT VALUE IS (150mA)
Your question is not clear. Why do you want to make a current of 150mA in a Wheatstone Bridge?

#### beenthere

Joined Apr 20, 2004
15,819
Hi,

I moved your post here as it appeared to be a homework problem.

You also stuck this on the end of another post, which is a rude thing to do. We refer to it as hijacking a thread. Please do not do this agan, or you will be removed from the membership list.

Without knowing the value of the pot or the applied voltage, it is impossible to arrive at an answer. What are these values, and what results have you been able to arrive at?

#### hgmjr

Joined Jan 28, 2005
9,029
hello
i have a problem with a bridge circuit that has in the 1st side a potentiometer and in another side a RTD detector(35kohm) and in remaining both sides are a two resistors (35 kohm)
HOW CAN I MAKE A CURRENT THAT VALUE IS (150mA)
I think that you are going to need to supply us with a diagram of your wheatstone bridge before we can be of any assistance.

hgmjr

#### rosey_soul

Joined Sep 10, 2006
7
salams...
i'm diab's project partner... our circuit supposed to control a specifice amount of current to flow into the bridge by the potintiometer ... WHY?
our RTD values in here will be changed accordingly to the passed gas through it... why is that ? to enter different gases and each gas needs certin value of current to provide changing in TEMP.
NOW,,, by contolling the amount of the potintiometer current we can then we can determine the flowed gas through the RTD... these current values will be chosen By the PIC microcontoller chip...
Simply: our problem how to build the cir. that can control the potintiometer electrically away of man's hand.

WHY Bridge??
we have two parallel sides of resistors each side includes the following :
1-RTD and 36Kohm resistance
2-potintiometer and also 36Kohm

this bridge supplied by 9V dc source and we'll measure the current that fits the entered gas...

with all our regards...
hear from u soon

#### pebe

Joined Oct 11, 2004
626

#### hgmjr

Joined Jan 28, 2005
9,029
I'm unable to access the attachment as well.

hgmjr

#### beenthere

Joined Apr 20, 2004
15,819
Hi,

I question your resistive values in the bridge. Most RTD's are less resistive than your 35 kohm figure - more like 35 ohms for a platinum RTD. You might find a thermistor in that range of resistance.

At any rate, a 35 k bridge can only produce .000257 volts output - hardly enough for your 150 milliamp output. Bridges are voltage devices - pushing current causes them to self heat, and alter the output in an uncontrolled manner.

This sounds a bit like a home-brew GasC. If you want to try a motor-driven pot to rebalance the bridge, look at mouser.com for the Alpha motor driven pots. They're on page 536, part numbers 310-1600f-xxx and 316-1020f-xxx.

#### rosey_soul

Joined Sep 10, 2006
7
donno why it refused to be opened!!!
thnk u for your fast reply.... i've attached the picture of the circuit for u...

well i'm computer eng. !!! BUT i'm not that well with Motors !!!
sorry our super moderators i forgot to say that RTD=35 k ohm! but what do u suggest about the two 36kohm resistances?? do u think i supposed to replace 'em!

thanks for the site i'll check it now! and kindly glade to tell u what happen with me...

best regard

#### Attachments

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#### hgmjr

Joined Jan 28, 2005
9,029
Now that we have a diagram, can you shed light on what you mean by "How can I make a 150 mA current"?

Do you have a part number and manufacturer for the RTD so that we can search for a manufacturer's data sheet.

hgmjr

#### rosey_soul

Joined Sep 10, 2006
7
i meant how to build the cir. that can give me a reading of the drown ameter as 150mA which is used for inserted He gas through the RTD after the Gas Chromotography process (which is not our job for sure )

according to the RTD at the moment i don't have it !! But i can bring it to u soon with in 20 hours max.

So what do u think now???

best raegards

#### rosey_soul

Joined Sep 10, 2006
7
well ppl i've just read the homework help forum rules Sure i should have read it before begin posting...

i'm really thankfull for all who trying to help and also those saw my question and wanted to give a hand. wish i still considering the rules....

thanks ur help again and your valued time...

best regards

#### rosey_soul

Joined Sep 10, 2006
7
ok it was a long time since the last time i heared from u
i'm afraid i didn't post my prob. correctly!!
SO, our RTD in here as u know doesn't have a fixed value,and we have apotintiometer too, i must control the potin. automatically inorder to achieve spesifice current values (150,80,230 mA) here these values of current are necessary for detecting the masss volume for the flowed gas through RTD (to have the most suitable current that gives the most correct change of voltage into the RTD)

hear from you soon
best regards

#### chuckey

Joined Jun 4, 2007
75
You have connected up your circuit incorrectly.From your + terminal, there should be two fixed reisistors in series , their junction going to the meter. The other lleg should have the low value variable and PTC in series, their junction goinng to the other side of the meter. Now the meter reads zero when the bridge is balanced. the currennt through the PTC can be varied by altering the +9V. Your circuit would have needed a voltage in excess of 35,000 X .15 ~ 5,200 V at a power of ~780W!!!!
or is 150mA wrong shouldit be 150 micrA?
Frank