check ohm meter operation

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  #1  
Old 02-27-13, 09:23 AM
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check ohm meter operation

I have a bench multimeter of which I would like to try to check the ohm meter part of it for proper operation/accuracy at the various ohms settings. Is there a common item with known resistance number(s) I might try to check, just to see if the readouts correspond as they should?
the various ohms settings are:
200
2k
20k
200k
2MK

Not too concerned about the 2MK but maybe at least some of the lower settings. thanks
 
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  #2  
Old 02-27-13, 09:31 AM
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You could try it against a resistor of known values.
 
  #3  
Old 02-27-13, 09:36 AM
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Hi sg
A "short" is the first reference, it tells you the meter can deflect full scale. As for calibration purposes I can't think of anything other than some resistors to do that. Now, all electronics circuits will have lots of those, especially older ones. My local recycling center is always loaded with electronics people are throwing away and you could have lots of fun un-soldering all kinds of components. Color codes can be confusing, but not difficult.

Bud
 
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Old 02-27-13, 09:48 AM
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Well that definitely makes sense. But believe it or not I don't have a resistor around. I could buy one, or perhaps there might be another suggestion of how I could get acquire one short of having to make a trip to the electronics store and spending much money? thanks again
 
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Old 02-27-13, 09:53 AM
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Incandescent light bulbs and toaster ovens are resistors.
 
  #6  
Old 02-27-13, 10:26 AM
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Actually I almost forgot, I have one brand new 2N3904 resistor I could try checking. It's a three-prong resistor; which prongs do I put the meter probes onto, and what resistance should I expect from this resistor?
Also, in regard to (Ray's) reply (post #5 here); dumb question I have here, what kind of resistance numbers should I expect if I were to check an incandescent light bulb or a toaster oven. Excuse my cluelessness. thanks
 
  #7  
Old 02-27-13, 11:56 AM
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Although there are resistance reading on that transistor, the actual numbers vary depending upon the voltage applied by the meter. The guy that sent you the transistors probably has a collection of resistors as well , so don't buy any. What do they do with electronic equipment being thrown away out there? An old computer, tv, radio? If you open one up, the circuit bd will have a bunch of components on it.

Bud
 
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Old 02-27-13, 11:57 AM
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If you have any old electronics you can unsolder resistors from them.
 
  #9  
Old 02-27-13, 12:09 PM
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Your 2N3904 is a TRANSistor, not a RESistor. The trouble with using any old resistor to check calibration is that the resistance is usually only +/- 20% of the color code. If the last band on the resister is silver then it is a 10% tolerance and a gold band denotes a 5% tolerance.

What you really need if you want to check calibration are 1/2% tolerance and for those you will have to order them.
 
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Old 02-27-13, 12:41 PM
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Thanks Furd for at least straightening me out that my 2N3904 is a TRANSistor, not a RESistor. Okay well I at least knew it was a something ister. Anyway what I need to do is check resistance readings from various contact points on a car ignition coil; I have the specifications. I wonder if I could get away with un-soldering an old resistor off an old piece of electronics then and check it out per color code and taking into consideration the tolerances you described.
 
  #11  
Old 02-27-13, 01:29 PM
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Sure, you can do that as a rough check on the accuracy of your meter. My own experiences over some fifty years is that ohmmeters rarely go far out of calibration but often give erroneous results when the battery goes dead. If you have a "ohms adjust" knob (usually only on an analog meter) simply short the test leads together and adjust for zero ohms resistance. If you can't achieve that then the battery needs replacing and if you still can't get it then the meter needs to be serviced or replaced.

Also remember that ALL resistance measurements are going to be approximate. If the specifications for your car state a resistance of "X" then plus or minus 10% is almost always acceptable. Often the specification will state a tolerance range in percent or even a range of numbers.
 
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Old 02-27-13, 01:44 PM
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Furd, my meter happens to be bench tester model, so plugs in, no battery. I went ahead and shorted the leads together at the various ohms settings and it the readouts are zero for each when I do that, so I think that means it's at least calibrated okay. And yes as you mentioned the resistance specifications for my ignition coil are stated in a range, probably for the tolerance factor as you also mentioned. Thanks!
 
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Old 02-27-13, 01:48 PM
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If you are just looking for a ballpark measurement, you can buy a 5% resistor box for less than $20. You can also buy individual precision resistors for less than a buck. You can find color code charts on internet.

Digital meters are significantly more accurate than their analog cousins. While you might expect a +/-3% FS accuracy, you should expect 1% full range accuracy on a basic DMM.
 
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Old 02-27-13, 02:57 PM
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thanks for that info, Wayne. My meter is digital, so should be fairly accurate reading then.
What is FS by the way?
 
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Old 02-27-13, 03:09 PM
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Full scale? ...................................
 
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Old 02-28-13, 11:24 AM
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I don't have the manual for this old Heathkit multimeter. Perhaps someone might tell me what the symbol in upper left corner of the LED readout that looks like a vertical dash (along with numbers on the readout) might mean? I get it consistently when I am trying to check ohms between between two particular terminals on my ignition coil. Specifications for those two terminals according to the manual should read out at approximately 2,090-2,310 ohms, but I get a readout of .98 (along with that symbol) when I check the contacts with range set at 2K, or 9.8 (along with that symbol) when I check the contacts with a range set at 20K. And those are the same readouts I get when I have the probes touching nothing.
 
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Old 02-28-13, 03:19 PM
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The dash is a "negative" sign. You normally wont see that on the Ohms function unless the component you are connected to is under power somehow. Or there is something wrong withcthe meter. Is the condensor still attached to your coil?
 
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Old 02-28-13, 03:53 PM
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Are you absolutely certain there is no battery in your meter? I know nothing about the Heathkit meter but I had a Knight kit (Allied Radio) vacuum tube voltmeter back in the 1960s that plugged in AND required a small battery for the ohmmeter function. Some meters had a ni-cad rechargeable battery that was constantly recharged.
 
  #19  
Old 02-28-13, 06:08 PM
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As I mentioned, the dash is vertical, not horizontal like a regular dash (or negative symbol) would be. The ignition coil I'm testing is not under power; it's removed from the vehicle, and no the condensor is not attached to the coil. I don't think (or know if) there's anything wrong with the meter, but of course that's a possibility. I could probably try to take a picture of the readout(s) as I describe, and post them here.
Furd, I am not absolutely there isn't a battery in my meter. I'll try to double-check on that.
 
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Old 02-28-13, 07:56 PM
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Here's a couple photos of the inside of my mulitimeter. I don't see anything that looks like any kind of battery in there:
http://i207.photobucket.com/albums/b...ps4ffa612f.jpg
http://i207.photobucket.com/albums/b...ps8198d4f7.jpg
But now for some reason, when I plug the meter into a receptacle, I'm getting no LED readout at all; it's just dark. I just noticed the ground prong on the plug is broken off, but I don't know if it's been that way or just happened, and whether this could be causing the problem, but I don't think so: http://i207.photobucket.com/albums/b...ps0719110f.jpg
I think something just happened to it, like the transformer went bad or something. It was working earlier today...
 

Last edited by sgull; 02-28-13 at 09:49 PM.
  #21  
Old 02-28-13, 08:50 PM
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There isn't an interlock switch that is only closed when the case is on is there?
 
  #22  
Old 02-28-13, 09:48 PM
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Nope. No such switch. I get the dark readout on the display whether the case is on or not.
 
  #23  
Old 03-01-13, 07:36 AM
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Check the fuse. On old equipment that has not been used for years, the electrolytic capacitors often go bad. Those are those aluminum cylinders on the PCB.
 
  #24  
Old 03-01-13, 08:19 AM
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Well here's a picture of the fuses. Neither looks bad on visual inspection; no apparent broken contacts within. One of them has a little resistor and spring inside and is marked 1/4A,and the other is just a straight strip of metal and is marked 3A.
http://i207.photobucket.com/albums/b...ps992007d1.jpg
At work I have this little multimeter Amazon.com: GB Electrical GDT-185A 6-Range Digital Multitester: Home Improvement I suppose I could try testing those fuses with that. And if they're good maybe try testing other stuff to see why I'm getting the dark LED readout all of a sudden.
 
  #25  
Old 03-01-13, 09:39 AM
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I'm usually opposed to unnecessarily adding to our landfills but in this case...how much time are you willing to invest in a meter than can be replaced for $20? The inexpensive small digital meters you can buy at Sears, Home Depot, etc are all way more accurate and handier than the one you're trying to resuscitate.

Now if it were a vintage meter with glowing "nixie tube" display, that might be a different story!

As I mentioned, the dash is vertical, not horizontal like a regular dash (or negative symbol) would be.
My bad. I was reading posts on my Android tablet while laying on the couch so I got vertical & horizontal mixed up
 
  #26  
Old 03-01-13, 09:52 AM
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In answer to your question guy, I know a decent and reliable and handy and modern multimeter can be had relatively inexpensively. I need to break down and buy one. Also, I might try to invest a little more time getting my old piece of junk Heathkit bench model working again. Yep you could probably say I got too much time on my hands.
 
  #27  
Old 03-01-13, 07:34 PM
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Apparently I'd need not only a decent multimeter but also an ESR meter to thorougly test the electrolytic capacitors on my Heathkit PCB. So, although I too am usually opposed to unnecessarily adding to our landfills, unfortunately it seems that's gonna be the ol' Heathkit's destiny.
 
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