Megger/Wheatstone Bridge to a good home?

Reply to
**THE-RFI-EMI-GUY**
Loading thread data ...

The only way of selecting an "appropriate voltage" would be by resistive dropping. This is a non-electronic device, with accurate resistors fashioned from coils of wire.

The meter movement is interesting - it has two separate coils, one to respond to voltage and the other to respond to current. there is no return spring on the needle!

Reply to
Airy R.Bean

You jest, sire!

Reply to
Airy R.Bean

Moving coil (and moving) iron meters respond to current, not voltage. The operate on the principle of magnetic fields- a which are not created by a voltage, only a current flow. (qv Ampere's Law.)

To measure a voltage, a resistor is placed in series with the meter, with its value chosen (using Ohms Law) to give the required scaling.

This is covered in the current Amateur Radio examinations, as it was in the old RAE. Until now, I'd expect it to have been covered even in Essex University engineering course.

Reply to
Brian Reay

Your deeply-seated desire to sneer makes you look such a fool. The meter in the Megger-Bridge has two coils, one in series with the resistance being tested to respond to the current in the test circuit, and one across the generator to respond to the voltage. That one has a series resistor to make it respond to voltage; a pretty standard technique that I am surprised is so new to you that you feel the need to comment upon it. But then, you have or had a licence issued under the gangrenous degeneration that is the M3/CB Fools' Licence scheme so such silliness or recently-resolved ignorance is to be expected.

So, what do you gain by your uncontrollable sneering?

You really are a complete ass. You initiate rather silly and childish sneers, and then, when you hear something in direct response to your goading that you don't like, you run away sobbing and screaming to the plod pod looking for a shoulder to cry on. What would the children for whom you are responsible make of your "telling tales to teacher"? What would your own daughters, daughters whom you have _TWICE_ introduced to us by your boasting in Bad Con, make of your behaviour in this NG when they reach the age at which you can no longer censor their Internet access? Would they find a disparity, hypocrisy even, between the standards that you impose on them as they grow up, and your rather silly and childish example portrayed with great frequency in this NG?

BTW - your claim to be using a kill-file seems to be another example of downright lying by you.

Reply to
Airy R.Bean

If it's not an electronic device, what is it? It's got resistors, coils etc. forming a circuit. What's the FSD of the meter?

Reply to
Frank Turner-Smith G3VKI

As I said a few days ago, there is *always* a logical flaw in a Bean argument. Well spotted! Standy by for bluster and spin-doctoring, with a little history-rewriting thrown in.....At least references to this latest blunder, having been so recently made, can't be referred to as 'raking over the coals'. Give it a few days.....

Reply to
Spike

You said "The meter movement is interesting - it has two separate coils, one to respond to voltage and the other to respond to current. there is no return spring on the needle!"

You need to learn to write clearly, if this is the best defence you can muster against your poor wordcrafting.

Don't be such a sore loser.

Reply to
Spike

Dear Mr Bean, I tried to email you but it didn't work. I am a beginner to electronics and have just studied the Wheatstone Bridge at school. I would like very much to see a real example. I live in Christian Malford near Chippenham, Wiltshire. Do you live anywhere nearby? Aled Jones, (no relation to the singer)

Reply to
Aled Jones

And another person completely misses the point.

The meter movement in question has *two* coils. One is connected *across* the hand-cranked generator output with a suitable resistor in series. The other coil is connected in series with the generator output and the test leads.

So one coil responds to the voltage output of the hand-cranked generator, and the other responds to the current in series with the component under test.

Since the generator output is 'hand-cranked', its exact voltage is variable. A variable voltage will cause the current in series with the component under test to vary as well. So instead of using a fixed-torsion spring, the voltage-senstive coil is used for restraining torque. Higher voltage, higher test current, but also higher restraining torque, same meter deflection.

There *are* things beyond what is found in 'Amateur Radio examinations'

daestrom

Reply to
daestrom

I think it's rather sad that such seemingly innocent offer should degenerate to such childishness

Reply to
JB

For once Airy has a point. One coil is connected across the generator output. Yes, it has a resistor in series with it. The resistor is fixed, so that coil is responding to changes in generator output *voltage*. The other coil is in series with the generator and the test leads connected to the component under test. The two coils are arranged so the torque developed by the one directly connected to the generator output moves the needle toward the infinity mark. And the coil in series with the component under test develops torque to move the needle towards the zero ohm mark.

So the restraining torque developed by the first coil is proportional to generator voltage, and the promoting torque developed by the second coil is proportional to the quotient of generator voltage and test component resistance. This has the rather nice advantage of canceling out the affects of variation in generator voltage. Reduce generator voltage by 10% and both torques developed are reduced proportionately and the needle deflects to the same position.

All iron-vane or D'Arsonval movements are magnetic in nature, that is true. But this rather unique arrangement using a second coil to develop restraining torque instead of a mechanical spring is noteworthy. With the restraining torque varying with the applied voltage from the generator, there is no need to even zero the movement before measuring. Unlike the more conventional 'ohm-meter' setup where you must short the leads and adjust the internal resistance to obtain a zero reading before each use (to compensate for variations in the internal battery's state of charge).

daestrom

Reply to
daestrom

Indeed, either way, the meter moves due to the current. In this case, the couple (the term for two forces such as you refer to) results from two magnetic fields, both due to currents. One current having passed via the the circuit under test, the other via the fixed resistor.

-- Brian Reay

formatting link
formatting link
FP#898

Reply to
Brian Reay

There is no NTL cable service in that area.

Reply to
Airy R.Bean

: > The meter movement is interesting - it has two separate coils, : > one to respond to voltage and the other to respond to current. : > there is no return spring on the needle! : >

: : Moving coil (and moving) iron meters respond to current, not voltage. The : operate on the principle of magnetic fields- a which are not created by a : voltage, only a current flow. (qv Ampere's Law.)

brian, once again you show lack of practical knowlege.

the meter movement Airy describes is of the type used for grounding-tests on the BAC1-11 which as we all know was built in GREAT BRITAIN.

: : To measure a voltage, a resistor is placed in series with the meter, with : its value chosen (using Ohms Law) to give the required scaling. : : This is covered in the current Amateur Radio examinations, as it was in the : old RAE. Until now, I'd expect it to have been covered even in Essex : University engineering course. typical reply from an rsgb lead instructor who avoided a morse test for twent years but could sit a morse assessment and have his picture taken with his BIG YELLOW PIECE OF PAPER.

Reply to
ZZZPK

I am afraid that the childishness is representative of the decline in the standards of Ham Radio in Britland since the gangrenous degeneration that is the M3/CB Fools' Licence scheme was introduced.

You can get one of those licences within two days from a starting point of knowing nothing about Ham Radio or about electronics.

Regrettably, setting the hurdle so low has resulted in a torrent of ne'er-do-wells becoming licensed and polluting the Ham Bands.

What was Ham Radio has now become multi-band CB.

Nobody who holds, or who has ever held, an M3/CB Fools' Licence will ever make it into the ranks of Radio hams with their gentlemanly traditions.

Sic transit gloria Mundi.

Now, if nobody is interested in the Bridge Megger, to give it its correct title (after checking in "Telephony" by Herbert & Proctor), regrettably is will go into the bin (and no doubt be discovered in an archaeological dig on the site of the municipal rubbish tip by a 25th Century Tony Robinson ("Baldrick" to aficionados of Blackadder))

Reply to
Airy R.Bean

Thanks for your comprehensive reply.

The real problem is that Bean writes in a sloppy, ill-considered fashion, and his comment "The meter movement is interesting - it has two separate coils, one to respond to voltage and the other to respond to current. " leaves out the rather crucial fact that something else is necessary to make the device work - the resistor, as you pointed out - and it all to be suitably connected up.

He hasn't yet learned to weasel-word his way round statements like this, and if he won't learn, then he can't be helped.....

A mere six additional words would have rendered any follow-up (apart from " I'll have it.....send it here " irrelevant.

I hate to rake over near-cold ashes, but in a previous life Bean produced such howlers as 'heat bands', 'mm waves are due to nuclear emissions', 'gears transmit movement not power', 'dehumidifiers absorb heat' - and only a few days ago ' a spring and a shock-absorber are the mechanical equivalents of a coil and capacitor hence that is why both have a resonant frequency'. All claptrap but liable to lead the unwary astray. Hence the need for corrective action.

Reply to
Spike

Subject: Re: Megger/Wheatstone Bridge to a good home? Newsgroup: alt.engineering.electrical => Jock. >>Perhaps you could get the Yanks, who are responsible for most of

Yes you do. Iraq is not in close proximity to Haiti.

Reply to
G. Morgan

Dear Mr Bean, I am not writing this from home, at weekends I stay with my father in Farnham. He has an NTL cable service and has set up my laptop to work on either his service or a dial up from my mother's house in Christian Malford. Aled Jones, (no relation to the singer)

Reply to
Aled Jones

I wonder why, if you are in Farnham, that tracert shows you to be in the Cambridge/Aldeburgh sector?

Reply to
Airy R.Bean

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.