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Registered Member #3429
Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288
Hi guys, This is my first post, and I am very happy to have found this web site. I have been a hv hobbyist and experimenter for more than 40 years, and I own a small company that repairs dental X-ray equipment (up to 100KV).
I recently purchased a Chinese LCR meter on Ebay. It's a model TH2821B (if you search that model on Ebay, you will find numerous sellers that have it for sale or for auction). I purchased this particular model because of its impressive inductance range (1uH to 9,999H)! The kind of X-ray transformers that I work with have typical secondary inductances of around 2000H to 4000H, and so I thought that this meter would be an excellent addition to my work bench. The problem that I'm having is, it does not seem to be able to measure large inductors that have a high DC resistance ( typically >100K Ohms). The meter works fine for inductors with DC resistance of, say, 65K or 75K, but right around 100K it ceases to read the inductance. The meter shows "-----" in those cases. I tried to contact the manufacturer through the seller that I bought the meter from but hit a dead end. So, I was wondering if anyone else has had a similar problem measuring an inductance with a high DC resistance. Does this problem occur with a high-buck, well made LCR meter such as one made by hP or other quality brand? OR can anyone explain what might be happening? Is this "normal" behaviour for an LCR meter?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
I had this problem with a Peak Atlas LCR meter, which was unable to read the inductance of ignition coil secondaries.
To measure this high resistance inductance, I made the secondary into the lumped constant LC frequency determining element of an emitter-coupled oscillator, which I bodged together in a few minutes. I worked out the self-capacitance of the secondary by adding a capacitor in parallel, and observing the change in fo with and without the extra capacitance.
Q was low, but the oscillator did its best and fo wandered up and down between 950 and 1050 Hz like the Grand Old Duke of York.
Next time I have to do this, I plan to make one of those funky FET negative resistance oscillators
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Since it's a laminated steel core you can't measure at much higher than line frequency (50/60Hz) The 1kHz and 10 kHz ranges would be inaccurate with laminated steel cores meant for 50/60 Hz due to eddy-currents in the steel and in this case winding capacitance. Judging from the specifications, your meter should be able to measure 4000H at 100/120Hz Assuming the range is 1uH to 9999H 100/120 Hz i.e. 1uH to 9999uH is a typo. Unfortunately that meter is at it's limits (low current) where you need it. Do you need to know the inductance? You could apply ac mains to the secondary and measure whatever you need; voltage or turns ratio, impedance with open/short primary .. ) I'd use a mains filament lamp in series in case turns are shorted etc. If you really need inductance, drive the primary with a signal generator looking for resonance with say 1.8nF (high voltage) across the secondary.
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
You can also apply an AC voltage to the secondary and measure the voltage on the (unloaded) primary to figure out the turns ratio, then measure the primary inductance (with nothing connected to the secondary) with your LCR meter, and then multiply that by the square of the turns ratio. I don't know how accurate this method is, but I think it would give you a pretty good ballpark figure.
Registered Member #3429
Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288
Anders M. wrote ...
You can also apply an AC voltage to the secondary and measure the voltage on the (unloaded) primary to figure out the turns ratio, then measure the primary inductance (with nothing connected to the secondary) with your LCR meter, and then multiply that by the square of the turns ratio. I don't know how accurate this method is, but I think it would give you a pretty good ballpark figure.
Anders M.
Hi Anders,
I thought about doing that too, but I wasn't sure if the primary inductance had a linear corellation to the secondary inductance. I'll give it a try and see how it works out.
FYI - I do not need to accurately measure the secondary inductance of these X-ray transformers because I only need to know if there are any shorted turns. Just one shorted turn on the secondary will dramatically lower its inductance and its Q factor.
Also, I was thinking more about how LCR meters work, and the problem I'm experiencing might be due to the unit's drive current or voltage. If a particular inductor has a very high DC resistance (due to very fine wire) then the LCR meter may not be providing enough "drive" to get a reading (just guessing here). But in my case, since I'm only concerned about shorted turns rather than the actual inductance, then measuring the turns ratio and calculationg the secondary inductance based on what I read on the primary should work for me.
Thanks to everyone who posted a reply! You were all very helpful.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Welcome to 4hv. If your real name is Mike then I'm probably an ebay customer of yours.
Have some very specific ideas about practical, resonant measurement of secondary windings to detect shorted turns, but they must wait till after an XRT measurement report that brings me here today.
Real quick and dirty idea: Put a little neon glow lamp across the secondary. Connect, then disconnect a 1.5-volt battery to the primary. I did this to establish winding polarities, and was surprised to see how long the lamp glowed. I bet a shorted turn would greatly reduce or eliminate the glow time.
Let me know if you want a NE2 by first class mail. Or get one from a lighted rocker switch or a disposable flash camera.
Registered Member #3429
Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288
Klugesmith wrote ...
Welcome to 4hv. If your real name is Mike then I'm probably an ebay customer of yours.
Have some very specific ideas about practical, resonant measurement of secondary windings to detect shorted turns, but they must wait till after an XRT measurement report that brings me here today.
Real quick and dirty idea: Put a little neon glow lamp across the secondary. Connect, then disconnect a 1.5-volt battery to the primary. I did this to establish winding polarities, and was surprised to see how long the lamp glowed. I bet a shorted turn would greatly reduce or eliminate the glow time.
Let me know if you want a NE2 by first class mail. Or get one from a lighted rocker switch or a disposable flash camera.
-Rich
Yup, my real handle is Mike, and my Ebay name is xray-repair. I used to sell used X-ray tubes and hv transformers on Ebay, but the market has been saturated lately, and so I don't bother doing that any longer.
I like your idea of placing an NE2 across the secondary and shock-exciting the primary. I don't know if that would catch a single shorted turn or not, but it's worth a try! Just a single shorted turn in an X-ray transformer can make the difference of the machine working within specs or not. I'm old enough to have a pretty complete electronics junk box which includes various neon bulbs. Thanks for the offer to send me one anyways!
Just for everyone's information..... I repair specifically dental X-ray tube heads. Some are 60Hz ac type (i.e., large iron core transformer) and some are DC type (i.e., small high frequency ferrite core transformer). They operate in the range of 60KV to 100KV, but the most common is 70KV. If anyone has questions about X-ray tubes, transformers, and associated equipment, I may be able to answer them. My experience is with "stationary anode" tubes (as oposed to rotating anode, wich are the more high powered medical X-rays).
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
The absolute DC resistance of the secondary at a given core temperature can be measured, although it takes a bit of time.
With that the LR time constant can be determined at various DC excitation voltages. Graph the settling time of the current. Thats how some LCR meters measure.
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Problem measuring very large inductors.
