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Tip for measuring heater coils or motor windings


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6 replies to this topic

#1 iceman

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Posted 15 April 2007 - 03:11 AM

When you need to accurately measure low resistances using a digital multimeter such as a motor winding, a temperature sensor, a thermal fuse, or a heating element, there are two things you can do to improve the accuracy of the measurement:

 

1) Put the instrument on the manual range setting and select the range with the most number of significant digits.

2) First short the meter probes, note the lead resistance and subtract that from your final measurement by one of:

  i) Use the meter's RELative reading function (some FLUKE Instruments have this feature)

 ii) Use the meter's ZERO reading function (as described in the application note below)

 iii) Make a mental note of the lead resistance and subtract it from your actual resistance reading using your actual brain.

There is an application note from FLUKE on how to use your meter to do this here:

http://assets.fluke....meters/7003.pdf

 

 

 


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#2 Samurai Appliance Repair Man

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Posted 28 April 2007 - 07:47 AM

Great tip, Iceman!  Always good to read your enlightening pearls of appliantological wisdom.  :dude:

Any tips for measuring the forward and reverse bias of high voltage rectumflyers (you Canadians might refer to them as "rectifiers")?  Using normal resistance settings on my Fluke doesn't do it, *I think* because the P-N junction requires more voltage than a typical ohm meter puts out.  I usually end up either using my megger or setting my Fluke 87 on conductance and measuring siemens.  Do you know of a better technique?

#3 iceman

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Posted 25 May 2007 - 08:16 PM

Greetings Honorable Samurai,

I received your question while consuming a Grasshopper Beer, carefully brewed by the Big Rock Brewery here in the heart of the Rockies.  To be honest, not the first one of the evening.

Accordingly, I was loath to leave the central command chair, and I sought to answer your excellent question wirelessly, by use of the miracle of the Internet.

http://us.fluke.com/...A(FlukeProducts)

(User Manual Link.)

This was not to be. 

I drained the remainder of the beverage, and descended into the lab, whereupon I energized the Fluke 87V, a recent gift from my youngest male offspring, (and now my good friend).  Using a second DMM, I measured the open circuit voltage on the Fluke 87V positive test probe referenced to the black or negative or ground lead.  (Diode scale selected)

I am astonished to report to you that even though the user manual states the meter is on the 3V range in this mode, in fact the reading was 7.31V!  I then decided to measure an LED, which would be expected to have a junction voltage outside the typical range of 0.5 to 0.8V of your average rectumfire.

Upon applying the positive lead of the Fluke 87V to the anode of the LED and the negative lead to the cathode, I was able to read 1.811V and see the LED illuminate.

So, in the future, should you require to measure the junction of a high Vfwd rectumfire, I recommend the trusty Fluke 87V.  Of course the megger is always an option, but would it not cause damage?

Incidentally, I don't normally drink Grasshopper, preferring the Traditional Ale, but again, my son was kind enough to lend them to me, and I did not want to offend him by refusing his generous and libatious offer.  In fact I had several, just to be sure.


Might I inquire as to the part number and junction voltage of the rectumfire in question?

BR


(hic)  iceman


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#4 Keinokuorma

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Posted 28 May 2007 - 12:06 AM

Diode measure scale will try to look for a voltage at which the current is around 50 microamperes. On a standard diode, that will be in the ballpark of 0.5 to 0.8V.  Unloaded voltage on the meter leads may be close to the battery voltage. Probably your other meter draws 50uA at that voltage. Also the meter itself will be set for voltages up to 3V, but the unloaded voltage is simply out of that range.

Normally it is not necessary to megger diodes that aren't going to work in the kilovolt range. You don't need one to measure forward, and it can actually destroy a diode that is designed to hold, for example, 200V backward.

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#5 Titania

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Posted 13 June 2007 - 07:29 PM

[user=1]Samurai Appliance Repair Man[/user] wrote:

Great tip, Iceman!  Always good to read your enlightening pearls of appliantological wisdom.  :dude:

Any tips for measuring the forward and reverse bias of high voltage rectumflyers (you Canadians might refer to them as "rectifiers")?  Using normal resistance settings on my Fluke doesn't do it, *I think* because the P-N junction requires more voltage than a typical ohm meter puts out.  I usually end up either using my megger or setting my Fluke 87 on conductance and measuring siemens.  Do you know of a better technique?

When you say HV, what sort of range are you talking about? If a hv diode is failing it's reverse rating under load it usually doesn't last long!
For measuring anything with a forward drop of more than a couple of volts, you could try making a simple test rig with two 9 volt batteries and a 1K resistor in series with the diode under test and measure the forward drop across the diode. A box with the batteries and resistor inside and a couple of banana posts for the diode could work. That way, you could test the doide in circuit with a pair of meter leads and out of circuit by putting it across the posts.
Just an idea. :)

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#6 Samurai Appliance Repair Man

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Posted 15 June 2007 - 01:17 AM

These HV rectumflyers in microwaves work with the HV capacitor to build up a 2,000 volt charge in the cap. 


#7 Titania

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Posted 15 June 2007 - 01:45 AM

[user=1]Samurai Appliance Repair Man[/user] wrote:

These HV rectumflyers in microwaves work with the HV capacitor to build up a 2,000 volt charge in the cap. 

Ah...dem ones!!!
I've never really found a reliable method of measuring those (unless they are shorted which is pretty obvious!!). If I suspect one, I just replace it and see if that fixes things.

The best part of working on MWO's is discharging the HV cap with a screwdriver beforehand :D

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