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 Post subject: How to measure a coil uh or henerys
PostPosted: Apr Sun 18, 2010 12:30 am 
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Location: Richmond Virginia U.S.A. 23224
Hi guys, what would be used to measure a coils uh?
Thank you
Gary


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 Post subject:
PostPosted: Apr Sun 18, 2010 12:34 am 
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Location: Albuquerque, NM 87123
There are modern inductance meters that are fairly cheap, or you can use an older inductance bridge.


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 Post subject:
PostPosted: Apr Sun 18, 2010 1:25 am 
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I just bought one on Epay for $20.00 - shipped from China. I figured it would either be useful or junk. I was surprised. It seems to work quite well. Time will tell. I also measures capacitance and when I tested it on some of my capacitors, it appeared to be giving good readings. I first changed out the cheeeeeep battery and now I have to wait and see how long it lasts.

Pat


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 Post subject:
PostPosted: Apr Sun 18, 2010 1:59 am 
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Location: Raleigh NC USA
My Wavetek DM27XT measures inductance up to 20 henries.

Larry

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 Post subject:
PostPosted: Apr Sun 18, 2010 2:22 am 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
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Location: Sandpoint, IDAHO 83864
Back when I was in college, we had an L/C bridge. I can't recall who made it, but it was first class stuff. Either Tektronics or maybe Hewlett Packard. But it would to to something like 20 or maybe 30 Henries for inductance.

You could also apply a DC bias across the inductor being measured and see how it affects the inductance. Needless to say, this hams junkbox chokes were all labeled with their values and current that did not result in complete saturation of their cores.

I found it very interesting how a filter choke's inductance dropped off as the current passing thru it increased and the difference between a smoothing choke and a swinging choke.

Heck, I bet the majority of ARF members don't have a clue to what I am talking about! Just showing my age, guys!
Curt

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 Post subject:
PostPosted: Apr Sun 18, 2010 2:36 am 
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Location: Westland, MI
I just use a Sencore LC53 meter. It measures from 1uH to 10H.

In all honesty though, I've never really had a need to know how many Henry a transformer was. Most of the time the schematics always showed a resistance reading and that was usually it.

I suppose if I were more into designing electronic circuits, I would be interested to know.

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 Post subject:
PostPosted: Apr Sun 18, 2010 3:08 am 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
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Location: Sandpoint, IDAHO 83864
I was mostly interested in what was the values of the chokes and such that I had in my junque box for many years. Parts that were robbed out of old TV sets and surplus equipment and such.
Curt

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 Post subject:
PostPosted: Apr Sun 18, 2010 4:25 am 
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Location: Elmira, NY
For measuring inductance of filtering chokes such as in power supplies, or speaker crossovers I've used this procedure:

I connect a resistor decade box in series with the unknown inductor, and put the series combination across a small 24v AC secondary of a powered transformer (any AC voltage will work, what ever you feel comfortable with for safety).

Start with a high resistance and work down. (This way you don't draw too much AC current and burn up your resistance.)

For example: ( Start with 5k ohms for a 8 Henery choke, higher resistance if your unsure)

I then measure the ac voltage across the resistance of the decade box,( a variable resistor will work as well) and then the ac voltage across the inductor. (Its easiest if you have two identical volt meters and measure the two values simultaneously. Then adjust the decade resistance until the two voltages are equal.

When the two measured AC voltages are equal the decade box resistance is equal to the reactance of the inductor.

Disconnect the decade box and measure the resistance with your ohm meter.

Use the formula for Inductive reactance :

XL= 2 (3.14)(f)(L)

Reactance = (6.28) (frequency in cycles) (Inductance in Henries)

or:

Heneries = Measured Resistance devided by (6.28) x (60 cycles)

For low (audio frequencies) I use 60 cycle line AC,

for higher frequencies I use an audio signal generator, for rf I use a rf generator etc. The only limitation is the volt meter frequency response.

This procedure assumes the actual dc resistance of the inductor is neglegible

I have wound many speaker crossover inductors this way years ago. I would wind a coil around a Iron shaft and connect to this test circuit, then add or subtract turns until I got the indutance
I wanted.

Eddie


Last edited by Y2KEDDIE on Jul Wed 25, 2012 11:46 pm, edited 1 time in total.

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 Post subject:
PostPosted: Apr Sun 18, 2010 4:37 am 
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Joined: Aug Tue 04, 2009 2:15 pm
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Location: Elmira, NY
The formula for inductive reactance in ohms: XL= (6.28) (f) (L)

6.28 is the constant: two pie

f= frequency in cycles

L = inductance in Heneries


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 Post subject:
PostPosted: Apr Sun 18, 2010 5:23 am 
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Location: Richmond Virginia U.S.A. 23224
WOW thank you guys!
i have a few rf coils in the junk box to learn from ,
thanks again Gary


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 Post subject:
PostPosted: Apr Sun 18, 2010 12:51 pm 
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Location: Advance, NC USA
Curt Reed wrote:
Back when I was in college, we had an L/C bridge. I can't recall who made it, but it was first class stuff. Either Tektronics or maybe Hewlett Packard. But it would to to something like 20 or maybe 30 Henries for inductance.

You could also apply a DC bias across the inductor being measured and see how it affects the inductance. Needless to say, this hams junkbox chokes were all labeled with their values and current that did not result in complete saturation of their cores.

I found it very interesting how a filter choke's inductance dropped off as the current passing thru it increased and the difference between a smoothing choke and a swinging choke.

Heck, I bet the majority of ARF members don't have a clue to what I am talking about! Just showing my age, guys!
Curt



You talking about a General Radio Type 650-A Impedance Bridge???

I purchased one about 25 years ago but have never used it.

Carl


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 Post subject:
PostPosted: Apr Sun 18, 2010 2:05 pm 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
Posts: 34328
Location: Sandpoint, IDAHO 83864
Carl- that was more than 35 years ago, as I graduated from college 35 years ago next month. It was an older piece of equipment then.

I have also used the method Eddie advocates, which is fine for power supply chokes and such. But for audio frequency transformers and the like, I used an audio sine wave signal generator and a scope. The generator was placed somewhere in the audio range, usually 400 to 1000 cps and the measurements made at that frequency.

If you have a good calibrated dual trace scope, you can view the amplitudes across each component at the same time. I also enjoyed watching the phase shift as the amount of R was varied. This eventually led me to study phase shift audio oscillators, which I built many of. Mainly for code practice oscillators and for monitoring my sent CW signals when gated on and off by another tube that picked up stray RF and rectified it to do the gating job.
Curt

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(Connoisseur of the cold 807) CW forever!


Last edited by Curt Reed on Apr Sun 18, 2010 2:14 pm, edited 1 time in total.

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 Post subject:
PostPosted: Apr Sun 18, 2010 2:13 pm 
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Curt Reed wrote:
Carl- that was more than 35 years ago, as I graduated from college 35 years ago next month. It was an older piece of equipment then.
Curt


Curt, hey I can empathize with you -- I graduated from college in 1971 :-)

Those old Impedance Bridges were impressive pieces of equipment.

Carl


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 Post subject:
PostPosted: Apr Sun 18, 2010 2:39 pm 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
Posts: 34328
Location: Sandpoint, IDAHO 83864
Yes, our college had a fine two year Industrial Electronics program back then, and was rated very high in the recommendations of two year colleges in the western United States. We had very good equipment to work with. Of course there were the Heathkit and Paco VTVM's and R/C substitution boxes and the like, but when it came to the heavy duty stuff, we had Tektronix 535 and 545 scopes, Hewlett-Packard signal generators that were the size of the Tektronix scopes and a lot of Fluke stuff.

I was very fortunate that I got to use that old equipment and to enjoy its capabilities. About a month before graduation, we got word that the school had received some sort of government grant to update their Industrial Electronics course and the money was to pay for a lot of new test equipment.

Me and a classmate, due that we were the class leaders, were given the task of making up a dream sheet of what we thought the school would most benefit with for the new equipment. Boy, that was fun. Like taking a six year old kid to a candy store and telling him to help himself to whatever he wanted.

We had something like $65,000 to spend, and if it were not all spent, then any further grants would be reduced by that untaken amount. So we made sure we spent it all! Things like benchtop digital meters for every student made by Fluke. A flock of new Tektronix scopes, signal generators by Leader, spectrum analyser by Sinclair, and a service monitor for working on two way radio communications systems, which I think was by Singer. Our total bill came to a bit over $63,500, so we did good.

That was right in the middle of the booming CB market and rage of the mid 1970's so we thought they ought to look in that direction. The only sad part was that none of us got to use the new equipment, as we were graduating in a month and the new equipment was for the next crop of students coming thru. I sure hope they enjoyed it!

I kept in touch with my two instructors, Jack Steve and Harlan Siebert for a few years after I graduated. Both of them started the class back in 1962 when the course was started and they said I was the first ham going thru the course that they knew of. Jack Steve went on and got his novice ham license and had a Heathkit HW-8 QRP rig installed in his sailboat that he had fun with. And he regretted not getting into ham radio much earlier. By 1980, they both had passed away, but the school named the Industrial Electronics building Siebert Hall in Harlan's name. The college was North Idaho College in Coeur d' Alene, Idaho, by he way.
Curt

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(Connoisseur of the cold 807) CW forever!


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 Post subject:
PostPosted: Apr Sun 18, 2010 3:46 pm 
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Joined: Aug Tue 04, 2009 2:15 pm
Posts: 115
Location: Elmira, NY
My Gosh! I thought you all were a bunch of Old Guys![/b] Your young like me! I graduated from Williamsport Community College, PA., back in 1970. (Formerly Williamsport Tech)

I worked on the R390 and 392, and GRC106's in the Army 71-73, modern state of the art electronics![/i]


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 Post subject:
PostPosted: Apr Sun 18, 2010 4:11 pm 
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Location: Elmira, NY
Gary,

Just to clarify based on your and Curt's comments:

I was under the impression you wanted to measure Iron Core chokes. Although as mentioned you can use the method on rf coils as well.

Please be aware, the value of reatance measured depends on the frequency you use to test.

Most filter chokes are measured with 60 hz assuming your filtering AC line frequency in a power supply, most audio chokes are measured at 1000hz. As I mentioned with speaker crossovers you are shooting for the reactance at a speific crossover frequency, etc...so the value in henery's depend on the application.

Eddie


Last edited by Y2KEDDIE on Jul Wed 25, 2012 11:49 pm, edited 1 time in total.

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 Post subject:
PostPosted: Apr Sun 18, 2010 4:36 pm 
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Joined: Mar Sat 08, 2008 6:08 am
Posts: 262
Location: Alberta Canada
Measuring inductance of iron or ferrite cored devices is -as people have said- is problematic. The inductance will be dependant on any DC curent flowing.

The three voltmeter method is probably the simplest for occasional use and is of acceptable accuracy. The formulæ given in previous replies do ignore the DC resistance of the inductance. If you need to eliminate this then sum the voltages using the cosine rule and using the angle calculated derive the effective resistive (in phase component).

Or if you have some drawing instruments construct it graphically.

I check out motor start capacitors this way and derive the loss angle as a very rough check.

Mike


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 Post subject:
PostPosted: Apr Sun 18, 2010 7:43 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3454
Location: Advance, NC USA
General Radio Bridge Type 650-A:
Image

I purchased this about 25 years ago.
Its a pretty impressive piece of test gear.

The insides of this thing is the most impressive:
Image

Lined with copper. Beautiful craftsmanship.
I never had a manual for this until a couple years ago I did download it. Still have never used this instrument.


Curt, do you remember these? I don't recall them being in the lab at the Tech School I attended.

Carl


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 Post subject:
PostPosted: Apr Sun 18, 2010 7:53 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2380
Location: Seattle WA US
GR-650A manual with schematics available at Andy Moorer's site:

http://www.jamminpower.com/main/GR%20Bridge.html


--Chuck


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 Post subject:
PostPosted: Apr Sun 18, 2010 9:54 pm 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
Posts: 34328
Location: Sandpoint, IDAHO 83864
Nope, I don't recall that one. I seem to recall that the one the school had was greenish in color and it may have been H-P.
Curt

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(Connoisseur of the cold 807) CW forever!


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