Measuring the resistance of a hot resistor

capable

(ie.

were

It's amazing really... You take all of 4 separate posts to respond to my single post.. shows your mind isn't thinking logically. And you also seem to get angry rather quickly when your own ideas are challenged. Can't take criticism can you?

I have not disputed the DC method at all but for low resistances it has certain requirements which the OP is unlikely to have access to. I ask you, what is wrong with a suggestion of a simple single meter resistance measurement on an indirectly heated resistor anyway. There is only one error source to be considered - not 3 as in the DC method.

As to whether the 100W resistor is a slider or not... Since the OP hasn't clarified it how can you tell what it is. There is absolutely no reason why a slider type resistor can't be used without the slider being connected is there? It is still a FIXED value across the ends isn't it?

As I said, there is nothing wrong with the DC measurement/calculation method for most resistors particularly when low cureents are concerned. Once you get down to quite low resistance values around 1 -

2 ohms with this method you need to use a LOW IMPEDANCE STABLE CURRENT SOURCE. This is not too difficult where low current values are used but in the OP's case he wants to heat a 100W resistor and this means appreciable current must be passed through it. That puts an even greater importance on the stability of the PSU sourcing the current.

Any variation in current due to changes in loading (remember, this resistor is suspected of changing its value) will dramatically change the readings which are then multiplied in the calculation, therefore the current source must be inherently stable irrespective of loading. Then there is the meter resolution and tolerances in both amp and volt meters which must be considered. In total we now have 3 sources of potential inaccuracy in the calculation. By using a single good quality DMM with zero ohms cancelling means you have ONLY the meter resolution and tolerance to consider. And for all intents and purposes in this particular case, it will be at least as accurate as the DC method - and far simpler to carry out without using multiple pieces of equipment which may not be on hand or available to the OP.

On Thu, 01 Mar 2007 01:21:53 GMT, Ross Herbert Gave us:

You f****ng retard. I responded to segments of the post individually. Fuck you for even thinking that you could make a case that it is incorrect. Fuck you.

Stop stalking me.

If you have something of substance to add to the discussion do sao. Leave your petty baby bullshit assessments about me out of your horseshit, boy. You don't even deserve discourse. Fuck off!

On Thu, 01 Mar 2007 01:21:53 GMT, Ross Herbert Gave us:

As if your petty, E-1 grade, unsolicited, unprofessional, uneducated assessments mean a goddamned thing. I don't have to measure up to your retarded view of the world, you stupid f*ck!

So FUCK OFF! Then DIE!

On Thu, 01 Mar 2007 01:21:53 GMT, Ross Herbert Gave us:

You're an idiot. You DID attack it. THEN you pissed and moaned that buying a new one would be easier.

Now, you are backstepping after I told you to read the f****ng topic of the thread.

It being a low resistance value is ALL THE MORE reason that the bench setup would yield BETTER results than a f****ng ohmmeter ever could.

Get a clue.

On Thu, 01 Mar 2007 01:21:53 GMT, Ross Herbert Gave us:

And I told you, you f****ng retard, that the error in a standard ohmmeter is FAR GREATER than the error in a volt meter and a current measurement and calculation. Your method also has the unknown of the meter leads and their attachment resistances, which at that low resistance is an even greater error set.

Also, dipshit, that is only two errors to chain together... two meters. Two negligible errors. READ IT AGAIN... N E G L I G I B L E

Only a retard like you would add in an error for the calculation, because you shouldn't even be in this industry if you are so stupid you cannot keep that out of your figures.

All you had to do is take the f****ng blinders off, and read what was already written in this thread, dipshit. But NOOoooo...

You had to act even more retarded than PhilTardBoy does.

On Thu, 01 Mar 2007 01:21:53 GMT, Ross Herbert Gave us:

You are one thick twit.

You tell us, retard boy... Is it?

On Thu, 01 Mar 2007 01:21:53 GMT, Ross Herbert Gave us:

You're an idiot. THE READINGS ARE THE READINGS. If the resistance "changes" as you say, the values read on the meters will change. Take any reading at any time point and calculate, and that result IS the resistance of the load at THAT TIME POINT.

NOTHING gets "multiplied in the calculation" you total ditz!

Of course the nominal value of a slider type power resistor end to end is fixed..

Since you don't seem to have the intelligence to see it I will have to spell it out.

A slider type power resistor has a bare area of resistance wire in the middle which a metal band makes contact with. The end to end resistance is fixed at the nominal value and the metal band tightened by a screw/nut arrangement is used to tap off a potential where required. If the resistor had previously been used/abused (it was described as being old), the action of tightening the metal band may have compressed and flattened a small area of the resistance wire thus changing its x-sectional area and increasing only slightly the end to end resistance when cold. When carrying high current this flattened area of wire presents a high resistance which will have a potential drop proportional to the current flowing, which will therefore mean the end to end resistance while hot is considerably higher than it would normally be if it were not compressed. If you haven't observed this phenomenon during your career in electronics then you haven't been very observant.

change

You don't get it do you?

If Amp meter A has +/-2% error and Voltmeter B has +/-2% error and the current source has +/-2% variation due to loading, then ALL of these errors contribute to the calculation error. The overall error can be considerably greater than that of a single meter reading having a larger error tolerance than that of Meter A or B.

On Thu, 01 Mar 2007 02:57:52 GMT, Ross Herbert Gave us:

Are you really so stupid as to think I didn't know? Apparently so.

On Thu, 01 Mar 2007 03:48:00 GMT, Ross Herbert Gave us:

You're a joke.

Said meters do NOT have said error, and the current for ANY test of the resistor is that which is READ directly from the current probe of the test set up, therefore the supply matters not. When Current value A is passed by the probe, and voltage reading B is taken, calculated resistance C is Accurate.

So shut up about it. No one is relying on a stated value of some supply monitor. ALL readings are taken directly, so all figures arrived at are accurate.

why do you need a low impedance current source?

yeah, so high impedance would be better.

--

Bye.
   Jasen