# wheatstone bridge: dc null indicator

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Hi

Regards
PG

#### ericgibbs

##### Well-Known Member
hi PG

The resistor Rx is the resistor being measured and the variable resistor is used to bring the bridge into balance.

The imbalance in the bridge due to Rx can be negative or positive, a Null meter centre balance meter would indicate the polarity of the imbalance signal.

E

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#### MrAl

##### Well-Known Member
Hi,

Obtaining a null reading is a way of getting extremely accurate results. That's actually part of how the op amp works.

Once the reading is null, that means the resistances are certain ratios to each other and so if three of them are known the fourth can be calculated, but without the null this doesnt hold.

But that's not the only reason for the null. The fact is, that a 'null' in and of itself is a very accurate mechanism. That's because when there is no current flow (meter out of circuit) the meter reads EXACTLY the same as when there is zero current flow (a null). Thus all we have to do is get the meter to read the same as when it is not being used at all. This means no calibration required for the meter. If it doesnt read 0.000000 when it's out of the circuit but reads 0.000001, then we dont use 0.000000 as the null we use 0.000001.
So measuring zero voltage or current is actually quite accurate. When we try to read 1.000000 volts for example we need a calibrated meter but to measure 0.000000 volts we just have to know where the 'needle' was before we started, then get it to return by nulling.
This mechanism has profound applications in science and engineering.

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#### unclejed613

##### Well-Known Member
the calibrated resistance dial in practice is often several decade banks of resistors with switches and one continuously variable resistor with a calibrated dial. the unknown resistance is connected to the test terminals and the decade switches adjusted to get close to a null, and then the continuously variable one adjusted for a complete null. then the resistance reading is taken from the switch and dial settings. one purpose of having a center null on the meter is that you can tell from which way the meter is deflected whether the present setting is higher or lower than the resistor being measured. the tolerance of the resistances in the bridge box are usually something like 0.01% or better. such a device is used to calibrate a secondary transfer standard in a calibration lab.

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#### PG1995

##### Active Member
Thank you, Eric, MrAl, UncleJed.

Please have a look on the updated copy of the attachment from my previous post. Thanks.

A clear copy of the meter shown in FIGURE 4-3 in the attachment can be found here.

Regards
PG

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#### JimB

##### Super Moderator
Why is E0 always +ve or -ve ??
A conventional moving coil meter has the zero at the left hand end of the scale.
So if a +ve voltage moves the needle upscale, a -ve voltage will move the meter downscale.

So if E0 goes from +ve to -ve as we adjust the arms of the bridge to get a balance and hence zreo volts out, it can be a bit difficult to see what is going off when the meter is pinned at the downscale endstop.

So in an ideal academic world we need a centre zero meter so we can see the +ve and -ve voltage swings as we balance the bridge.

However, in the real world where sometimes you just have to go with what you have got, it is possible with care and practice to use a meter with a left hand zero.
Most moving coil meters have the mechanical stop set such that the pointer can move below zero for a few percent before hitting the end stop.

JimB

#### steveB

##### Well-Known Member
PG,

I agree with the above. Basically, I think that the section of text you are talking about is just saying that you will null out the meter in either case (bidirectional, or unidirectional). It's just that some applications might only be concerned with unidirectional changes. For example, if you have a weight scale made with a strain gauge, you would first zero out the meter, and then place the object to be weighed on the scale. Since you are never going to place an object of negative mass on the scale, you don't have to worry about reading large negative excursions.

However, as pointed out by JimB above, you will still need to pick up small negative excursions in order to zero out the meter.

Applications that do not require a nulling at some point in the operation are probably not well suited for a bridge circuit. A simple voltage divider or a voltage reference might work better in such cases. Of course, every rule has an exception. But, I can't think of one right now.

#### PG1995

##### Active Member
Thank you, Jim, Steve.

Could you please help me with these queries? I hope I'm not frustrating you guys too much! Because now I myself seem to be thinking that I'm trying to look into the text more than is really needed. Thank you.

Regards
PG

#### steveB

##### Well-Known Member
Q1 does not seem to be a question, but merely a critique of the text. Perhaps the text could be worded better, or perhaps we are all missing a subtlety that the author intended. I think you have probably captured the important aspects of this section.

Q2 ...

Basically, a lower range for the meter means you can pick up small amounts of imbalance on the bridge. The only thing to watch out for is that you don't make the meter range so small that you can't get a zero because of the step size in resistance changes in the bridge. In other words, if the meter range is too small, the nulling operation will be too sensitive. The meter will ping from negative full scale to positive full scale, with no ability to read zero. If this happens, it's better to have a larger range on the meter so that you can see when you are nulling out the bridge.

Q3 ...

I would say yes to that. That meter seems to be a volt meter (although it may have current measurement capability too).

Note that the difference between a voltage meter and a current meter is the value of the resistor R5. If R5 is very large, then the I5 branch current is small, and you are measuring voltage. If R5 is small, you are shorting out the terminals and measuring current. The nice thing about the null point is that when the meter is null , there is no current flow, so the distinction between voltage or current measurement is no longer important.

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#### JimB

##### Super Moderator
DC Null Indicator - a voltage or current meter which does not have to be very accurate, its only function is to tell when there is NO VOLTAGE (or current).

Resolution/Sensitivity
A 100μA meter is more sensitive that a 1mA meter is more sensitive than a 10mA meter, etc.

Consider a meter scaled 0 to 100mA, there are 100 scale divisions on the meter, that meter has a resolution of 1mA.
A silmilar meter scaled 0 to 100mA has 20 scale divisions. That meter has a resolution of 5mA.

Also be aware that resolution and accuracy are not the same thing.
Say the meter above with the 100 divisions had the scale drawn by a guy who was drunk at the time
when the pointer is on the 50 mark there is actually 52mA flowing therough the meter.
Yet the second meter with only 20 divisions, with the pointer on the 50 mark has 50.3mA flowing through it.
The second meter with the lower resolution is more accurate.

Where is the HP meter used?
Wherever it is useful for the job in hand!
That meter could be used as a null detector on the wheatstone bridge.
If it is on the bench and ready to use, you could use it to test the batteries in your radio/ipod/paddy thing/must have toy of the day!

JimB

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