What Is Wrong In Fig. 6 For Measuring The Resistance Of R1using And Ohmmeter? Make A Correct Drawing

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Resistance of wire seems to go downwards when part of a circuit?

Discussion in 'Full general Electronics Discussion' started by jwallis, Nov five, 2017.

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1. Hullo all,
   I'm a software guy who wishes he knew more nigh hardware and have played with circuits some but am very oftentimes stumped or find myself saying "I don't know how this is possible, but this is indeed happening" which tin become pretty frustrating.

   So here's my latest matter:
   I take a circuit in my 33 year old car that is getting very hot and I'm agape will start a fire. It'due south the connection to the 450W radiator fan. At 12V, it seems like it should be carrying 37.5A, but the correct fuse, per the manual and anybody online, is 16A, which itself doesn't make whatever sense (comments welcome).

   My bodily question however was I'k thinking the age of the wires + the crusty and slightly burned GBC fuse holder are causing increased resistance. I was thinking this would cause higher current just as I write this I realize in theory the college resistance should lower the current. Regardless, it's getting hot, then I wanted to mensurate the resistance of the components in the circuit. I measured parts of the circuit and and so had (what I idea was) a bright idea to just measure the resistance from the battery to the fan, expecting it to be high. I thought I would then make clean parts and supervene upon old connectors, re-measure and see a lower resistance. I needed a length of wire then I measured its resistance first. Below is what I saw and why I am dislocated.

   

2. Hi, welcome to the forum! It appears that your fan is cartoon also much current; how practise you know the fan motor is rated at 450W? A 12V motor needs to exist rated at less than 192W to go on 16A fuses from blowing (actually lxxx% of that or about 154W for reliability). Does the fan plough freely & also are the bearings loose and so the armature can drag confronting the field poles? These conditions would cause overcurrent as would arcing brushes & shorted armature sections.

   Signs of rut stress on wiring & the fuse holder as well bespeak probable overheating. As for the discrepancy between resistance readings, did you lot make both resistance checks on the same resistance scale, preferably R Ten 1 or lower or with an autoranginging meter?

   Some other thought, is the engine hot when you tested the wiring resistance? If not, I'd look the relay contacts to be open between the fuse & the connector so the meter should read infinite or if the fan motor's in the circuit during the exam & the relay contacts are open, current would flow from the chassis common, through the fan to the connector through the ohmmeter to the positive battery terminal & bypass the open relay contacts & fuse. In this instance the meter may be pegging the left end pivot or otherwise giving an erroneous reading. Try disconnecting 1 of the battery terminals. Promise this helps.

3. When measuring such low resistances yous have to exist aware of the effects of poor contact resistance - even pressing the test meter probes 'harder' can make a difference to your readings.

   At loftier currents ALL connections need to be very make clean and very secure. The fuse holder itself is often an area where contact resistance is neglected and trouble can exist found. Muddied relay contacts, the in-line connector etc, ALL must be scrupulously make clean.

   If you lot have an accurate and reliable test meter you lot should examination the fan current consumption using the shortest leads to a bombardment supply equally y'all tin get if only to eliminate the fan motor itself as beingness a trouble.

   Are you positive the fan is rated at 450W?? Seems an awful lot!

4. For ten anxiety of wire to have a resistance of 2 Ohms it would have to be 33 AWG.

   Bob

5. Literally "hair thin" in other words. And if you did manage to find some 33 AWG wire, it would probable be "magnet" wire, insulated and difficult to strip the insulation from.

6. Thank you all so much for your replies and warm welcome, this seems like a groovy community!I believe information technology'south 450W per various part suppliers (here and here) and the part number still legible on the fan.
   Information technology turns pretty well - without wobble, noise or any reason to recall the bearings are going out or that things are dragging, although it is at to the lowest degree twenty years former.

   I take a digital autoranging meter.

   The engine is not hot. I am jumping the temperature sensor to manually power/close the relay. It reads infinite, then when I close it, goes to 0 Ohms. The fan motor is disconnected for the test (step three).

   Tell me nigh it, this is why I was hoping the culprit was the burned fuse holder to fuse connexion. When I measure resistance from one side of the fuse holder to the other side with the fuse in, it jumps effectually some. I'one thousand going to get a new fuse and clean the fuse holder very well in hopes this fixes it... I was performing this test as an experiment/exam of my knowledge in hopes of seeing actual results: resistance measures high, clean the parts, resistance measures lower.

   I am besides trying (so far without luck) to pull the relay and check it for heat/corrosion damage.

   I tried to mensurate the current to the bombardment assuming that it took < 10A because yeah, 450W is a lot...
   Blew the fuse in my 10A meter. Hopefully didn't injure whatsoever internals at the aforementioned time. That's a great tip though if I can find a meter than tin handle college currents.

   The wire is one-time speaker wire, probably 18ga. I stripped insulation off the ends and they were a little dark-green so I spread the strands and used a blade to scrape in i management, flip over do the other side, then spread the strands laterally and did the same affair. Crap, I'1000 wrong though, in that location was more on the spool and so it was greater than x'.

   That beingness said, when I just connect the two probe leads to each other it reads 1.5 Omhs.

7. Ok, I but realized that my van has some customizations, and is using a subsequently model fan than typical for the year. The documentation I could find for the after model years says to employ a xxx A fuse, which is certainly a lot closer than 16A. Nonetheless doesn't quite work out with a 450W fan, and I plan to inquire the vw community (thesamba.com). Thank you for making me think harder on that one...

   I have been using this 16A fuse as information technology'due south how it came from the previous owner v years ago. Now this is a dumb question, simply maybe the fact that the fuse is too minor is causing it to get hot? I'd really promise it'd just blow! That fuse holder has always shown some signs of heat damage simply I never gave much attention until I smelled it the other 24-hour interval. I'thousand wary of putting in a college-rated fuse!

8. Fuses protect the CABLING, not the equipment, and then for a fan requiring 450W (37.5A) you would need 8awg for the primary feed to/from the relay/fan.

   The relay Command wiring tin can, of course, be much smaller.

   a 40A fuse would be appropriate for 8awg.

9. Interesting, I had never thought of it that way merely it makes sense. I was but told at that place should exist a separate 50A strip fuse adjacent to the relay, which is backside the dash and very hard to become to. Going to check them out now...

10. On measuring low-valued resistances with the ohmmeter function on your digital multi-meter: don't even think about doing this! When resistance values begin to approach one ohm or less, the resistance you are trying to measure becomes less (often much less) than the total resistance in series with the measurement examination leads. This "external" resistance includes the resistance of the examination leads, resistance of the connections to the multimeter, and resistance of the connections between the test leads and the unknown resistance. This "external" resistance, which has aught to do with the modest resistance you lot are trying to mensurate, becomes the major contributor to what is essentially an unreliable and usually erroneous resistance measurement. You have nigh NO command over the "external" resistance, nor whatsoever cognition of what value it adds to your resistance measurement.

    In that location are several ways to attempt to avoid this situation, which includes shorting the examination leads together to record the "external resistance" prior to making a resistance measurement, and using the "relative" button on the multimeter to "zero" the meter with the test leads shorted together only prior to making the resistance measurement. Nonetheless, as the resistance you are trying to measure becomes smaller and smaller, all the uncomplicated techniques ultimately neglect. The only process that produces guaranteed results for any value of resistance yous cull to measure is the Kelvin 4-terminal resistance measurement. See figure below.

    

    To make a resistance measurement of Rsubject, y'all must identify an ammeter in series with the resistance you want to measure, apply a depression voltage to that series connection, and record both the current read on the ammeter and the voltage developed beyond the resistance you are trying to measure. That measured voltage divided by the measured current equals the unknown resistance you are trying to measure. With a sensitive voltmeter and a stable current source, very low resistances can exist accurately measured, despite values of Rwire that are several times larger than Rsubject. Annotation the Rwire values do non take to be equal, and generally never are.

    The bombardment can exist your machine battery (engine off, and so battery is not charging), and the Rwire resistance can be real resistances that serve to limit the current through Rsubject. You can use just i real resistor in serial with the battery to represent both Rwire resistances. Value isn't disquisitional, but information technology should be low enough to provide sufficient current through Rsubject to permit the voltage developed beyond Rsubject to exist accurately measured. I would offset with most 120 ohms (providing 100 mA into a shorted Rsubject) with a 12 V car battery. Then the voltmeter will measure Rsubject with a sensitivity of 100 milli-volts per ohm. Make sure you use some other contained multimeter to mensurate the electric current through Rsubject.

    Attempt the in a higher place method on your fuse holder block with fuse inserted. From what i accept read in previous posts, the fuse holder should probably be replaced. Try measuring random lengths of wire of various diameters to get a "feel" for how very low resistances represent to various wire diameters and wire materials. If you can find some (a discarded old toaster perhaps?) measure the resistance of various lengths and sizes of nichrome heating elements.

    Good luck making a smooth transition from software weenie to hardware weenie. A hotdog that can be both is a rare treasure to work with.

    Well-nigh all digital multimeters employ a synthesized constant-electric current source to utilize a constant current to the exam leads during the ohmmeter function. The multimeter then measures the voltage this abiding electric current source produces in the unknown resistance connected to the test leads. Every bit mentioned earlier, notwithstanding, this unknown resistance also includes "external resistance" from test lead wires and test atomic number 82 connections over which you have no control and fifty-fifty less cognition nearly. The simply reliable measurement of unknown resistance must use the Kelvin four-terminal method. Non even a Wheatstone Bridge is better.

11. Thank you, I wonder if yous were a professional person teacher. Most people who have advanced subject area knowledge don't have patience for beginners, and so I am very appreciative : ).

    I am glad to hear you lot say such things nigh trying to measure out small resistances with an amateur-grade meter. It has been so challenging learning, subsequently the fact, the many gotchas like floating values on high impedance digital inputs, or that voltage dividers only working for low electric current applications. Software seems more deterministic, whereas with hardware, if you have a resistor, it might be "shut enough" to what you demand to exercise the task... and I never before appreciated the luxury of beingness able to completely pause something and take it *not* go upwardly in smoke.

    It took me reading this near 6 times, but I think I understand now. I only have i meter, but perchance I could measure the current, and so measure the voltage drop across Rsubject.

12. A decent clamp ammeter (DC) is essential for currents over 10 or 20A. Whilst these types of devices aren't as commonly used every bit ordinary multimeters they are now quite a lot more affordable than they were a few years ago.

    Yous tin can likewise buy (or build) a milli-ohm meter designed, every bit the name suggests, for depression ohms measurement.

    http://www.scullcom.uk/category/projects/milliohm-meter/

    But all this is speculative as most average users of tests equipment wouldn't or couldn't justify the expense for such rare/occasional usage.

    As yous propose, measuring the volt drop beyond the load is perhaps easier method but accuracy may be an issue for generic test meters.

    Knowing and measuring the 'problem' is probably less useful than but fixing it - and knowing all the areas where problems ascend and how to deal with them is more useful too.

    As the OP already seems to sympathize, the crucial aspects of joint cleanliness and security makes all the difference and whatever problems can often exist cured without knowing the actual 'reason' backside the heat effect - although well-nigh members on forums such as this are more than curious for the technical reasons than the actual solution!

14. No, I was offered a "job" teaching basic electronics at Lowry AFB, Denver, CO afterwards graduating from the full electronics form at that place in 1964. Turning that offer down was maybe the stupidest decision I had made upward to that point in time, but I was anxious (at age nineteen) to "go into the field" and really work on the weapons organisation I was trained to maintain and service. Something nigh 20mm Gatling guns with radar controls for aiming and tracking excited my interest.Software is e'er deterministic, but so is hardware... sometimes in more subtle means. I was attracted to software running on embedded processors (back then, minicomputers) in the tardily 1960s as a replacement for discrete hard-wired logic. It is very hard to make changes in hard-wired logic, but I thought that software could permit a solution to that problem by presenting a lot of "what if" programmable alternatives. As you note, when software breaks at that place is unremarkably no release of smoke and you just re-write the code and endeavor once more. This statement can and often does fail with embedded systems. Software errors can cause an embedded controller to go berserk and create monumental damage to the peripherals and the equipment the peripherals operate.That will work. Since nothing much is changing during either of the two measurements, so you tin can utilize your one and only multimeter to measure the voltage drop across the series current-limiting resistor, whose value is presumably well-known, and summate the electric current in Rsubject. So you move i of the probes and conform the range (or let the autoranging function practice information technology) to measure out the (much smaller) voltage drib across Rsubject. Summate its resistance from the 2 measurements. Some gotchas: any subtract in battery voltage while making the voltage measurements will lead to errors. Whatever increase in value of the serial current-limiting resistor, because of self-heating, while making voltage measurements volition atomic number 82 to errors. Be quick making the two voltage measurements, and disconnect the battery power betwixt measurements. Yous should easily be able to determine very low resistance values to an accurateness of meliorate than ±5%, which is all you lot demand for troubleshooting analysis.I agree they are more affordable, now that Hall-Issue current sensors are so ubiquitous. I disagree that they are essential. Convenient, certainly. Essential, no. Current shunts are readily available with millivolt per ampere "burdens" or you tin make your own by paralleling wire-wound precision power resistors. Of form there is the extra piece of work of opening the circuit to insert the current shunt, and then a clamp-on DC ammeter has the advantage for that. Be careful near "zeroing" them. The ones I have used tend to migrate and are sensitive to orientation in the World's magnetic field. Even so, given my druthers, I would rather use an accurate clamp-on DC ammeter than spiral around trying to open up a high-current circuit to wire in a shunt.Agreed. I have never felt the "urge" to construct a milli-ohm meter, considering it is so easy to cobble upward a test rig on the spot with sufficient accuracy for what I demand.The essence of problem solving is first knowing what problem you take. Sure, cleaning concluding connections is appropriate, but knowing that a connection is faulty is fifty-fifty better.Agreed. Sometimes having total understanding of the technical reasons for a mistake isn't fifty-fifty practical, especially if that cognition contributes nothing to a solution. For instance, knowing that thermal fuses are subject to poor internal connections... for whatever reason... makes it prudent to either (1) replace the fuse with a known good fuse, or (2) measure out the internal resistance of the fuse and its holder and replace it if likewise large a value is measured. If I had a box of spare fuses, I know which choice I would make first. OTOH, if the fuses cost upwards of fifty bux each (as some high-speed fuses used to protect semiconductors exercise), maybe some careful resistance measurements are in order.

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What Is Wrong In Fig. 6 For Measuring The Resistance Of R1using And Ohmmeter? Make A Correct Drawing,

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