Does anyone know how to make a Wheatstone bridge?!?

[shadowfax00178]

Wheatstone bridge anyone... does anyone know anything about it like how to make one, how it works, or anything? :lol:

 

[TheOne]

The bridge works on the principle of ratios. From the diagram:

When balanced RV1/R3 = R1/R2

So if you have 1 unknown value for say R3, by taking ratios (when the bridge is balanced with no current flow in the uA meter) you can determine the unknown value. RV1 will be a precision adjustable component with calibrated scale or markings according to set value. R1 and R2 will also be very accurate components.

You do get bridges to measure unknown capacitors, inductors and even unknown frequencies.

Search Google for more

 

[Nigel Goodwin]

As TheOne pointed out, it's a very simple device - but one thing he didn't make clear is that for inductors and capacitors you need an AC supply, and an AC balance indicator.

A common (very simple) method is to use a 1KHz oscillator for the supply, and a simple crystal earpiece (very high impedance) for the balance indicator - simply adjusting the bridge for no sound in the earpiece.

You can, of course, use the same AC setup to measure resistors as well.

 

[TheOne]

Nigel makes a good point. I was thinking of mentioning something about this, but it was getting late.

I have done some more diagrams now.

The first diagram shows a bridge for finding unknown inductance. When the bridge is in balance there's no AC voltage on the meter. (work better with a high impedance earphone as Nigel suggested, but my program lack a symbol for it)

The next diagram shows the bridge unbalanced.
-------------

The next set of diagrams are interesting. The problem arise that when you have two inductors with the same inductance but one has lower Q (due to extra series resistance of the inductor as indicated by the 10R resistor in the block surrounding the inductor on the right) you are unable to find a null position even though the amplitude over the inductances can be adjusted to be equal by RV3. This happens because of the phase difference that exists. By using a phasing control, we can now insert extra series resistance in one of the two legs to make the bridge null out. At the same time we can determine the Q of the unknown inductance from Q = XL/(set value of RV2)

Bridge for determining unknown capacitance works in a similar way.

When the bridge is used to measure high impedances it may be neccesary to use a "Wagner ground circuit" to balance out any stray capacitances.

 

[shadowfax00178]

money promblems... sort of

the problem is... i can only spend $5 on the materials needed to make it so that is what im having trouble with. LIke, how would you get a galvanometer for less than 5 dollars?... Now, you could probably make a galvanometer but I don't know how... do you?

 

[Nigel Goodwin]

Re: money promblems... sort of

the problem is... i can only spend $5 on the materials needed to make it so that is what im having trouble with. LIke, how would you get a galvanometer for less than 5 dollars?... Now, you could probably make a galvanometer but I don't know how... do you?

Use a cheap crystal earpiece (as I suggested above), feed the bridge with an audio signal (about 1KHz or so) from a simple 555 oscillator. $5 should be plenty!.

 

[Styx]

so inside an LCR meter is there a couple of weastone bridges?

just interested, never really thought of it. I usually test inductance by pulses of volts and look at the rise in amps - not practical for low-current inductors

 

[Russlk]

Inexpensive cap meters apply a fixed current and measure the time to reach a voltage. I don't know how inductance meters work, could apply a fixed voltage and measure the time for the current to reach a known value.

 

[Nigel Goodwin]

Inexpensive cap meters apply a fixed current and measure the time to reach a voltage. I don't know how inductance meters work, could apply a fixed voltage and measure the time for the current to reach a known value.

They could measure the reactance at a certain frequency, or use the coil as part of an oscillator and measure the output frequency. Meters don't generally use a bridge, because they don't just give a reading, you have to operate them - although I suppose an automatically zeroing one could be built?.

 

[shadowfax00178]

defining the meaning of an "Electronics Newbie"

what's a crystal earpiece and a 1KHz oscillator ... and a cap meter and i don't get how that's needed and what it will do, where to put it, and all that stuff. Also, for the wheatstone bridge, don't you need like an adjustable resistor to compare it to the unknown one by a ratio thing, how would you make an adjustable resistor?

 

[Nigel Goodwin]

Re: defining the meaning of an "Electronics Newbie&quot

what's a crystal earpiece and a 1KHz oscillator ... and a cap meter and i don't get how that's needed and what it will do, where to put it, and all that stuff. Also, for the wheatstone bridge, don't you need like an adjustable resistor to compare it to the unknown one by a ratio thing, how would you make an adjustable resistor?

If you don't even know the names of basic items, it's not looking very hopeful for building anything!.

You don't make a variable resistor, you buy one - they are called potentiometers - commonly used as volume and tone controls, but you require a linear one for a bridge.

What are you actually wanting to do?.

 

[shadowfax00178]

basically..

im trying to build a wheatstone bridge that can accurately measure resistances in the range of 1 ohm to 100 ohms... i have to test the reliability by measuring the resistances of a known resistor... the overall cost of all materials must be less than $5 and i have about a month to finish it...

 

[Nigel Goodwin]

Re: basically..

im trying to build a wheatstone bridge that can accurately measure resistances in the range of 1 ohm to 100 ohms... i have to test the reliability by measuring the resistances of a known resistor... the overall cost of all materials must be less than $5 and i have about a month to finish it...

Presumably this is a school assignment then?.

Are you allowed to use a multimeter for the null indicator, without it counting towards the cost?. In which case all you need is the multimeter, a battery, a linear potentiometer, and a reference resistor.

 

[fly101]

There is something I don't really understand and am curious about.
https://en.wikipedia.org/wiki/Wheatstone_bridge
On this webpage, there is a meter in the middle of the Wheatstone Bridge and I got confused by it because I couldn't understand what kind of meter it was. :?

 

[shadowfax00178]

well, since it's not gonna be part of the wheatstone bridge, im assuming that it wouldn't count towards the price unless ur thinking of attaching it to the bridge, in that case, it would count... the tools that u use wouldn't count but if something is part of the bridge, it would count towards the price

 

[Phasor]

There is something I don't really understand and am curious about.
https://en.wikipedia.org/wiki/Wheatstone_bridge
On this webpage, there is a meter in the middle of the Wheatstone Bridge and I got confused by it because I couldn't understand what kind of meter it was. :?

The meter is a galvanometer - a type of sensitive ammeter.

When the meter reads zero, the bridge is balanced.

 

[shadowfax00178]

galvanometer

how would u make a galvanometer...? to add it to the wheatstone bridge