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Measuring small inductances
Hi all,
I've got quite a huge stash of inductors in my parts bin. The colour codes don't always seem to relate to the values I've been able to measure, with my multi-function DVM, however, and I can't accurately measure any inductor about 10uH. Is there a circuit anywhere that would enable me to get a reasonably accurate idea of the values I've got down to say 100nH or thereabouts? Thanks, p. -- The BBC: Licensed at public expense to spread lies. |
"Paul Burridge" wrote in message
... Hi all, I've got quite a huge stash of inductors in my parts bin. The colour codes don't always seem to relate to the values I've been able to measure, with my multi-function DVM, however, and I can't accurately measure any inductor about 10uH. Is there a circuit anywhere that would enable me to get a reasonably accurate idea of the values I've got down to say 100nH or thereabouts? www.google.com |
"Paul Burridge" wrote in message
... Hi all, I've got quite a huge stash of inductors in my parts bin. The colour codes don't always seem to relate to the values I've been able to measure, with my multi-function DVM, however, and I can't accurately measure any inductor about 10uH. Is there a circuit anywhere that would enable me to get a reasonably accurate idea of the values I've got down to say 100nH or thereabouts? www.google.com |
"Paul Burridge" wrote in message ... Hi all, I've got quite a huge stash of inductors in my parts bin. The colour codes don't always seem to relate to the values I've been able to measure, with my multi-function DVM, however, and I can't accurately measure any inductor about 10uH. Is there a circuit anywhere that would enable me to get a reasonably accurate idea of the values I've got down to say 100nH or thereabouts? If you have a signal generator you can connect them to a suitable capacitor and measure the resonant frequency. Leon |
"Paul Burridge" wrote in message ... Hi all, I've got quite a huge stash of inductors in my parts bin. The colour codes don't always seem to relate to the values I've been able to measure, with my multi-function DVM, however, and I can't accurately measure any inductor about 10uH. Is there a circuit anywhere that would enable me to get a reasonably accurate idea of the values I've got down to say 100nH or thereabouts? If you have a signal generator you can connect them to a suitable capacitor and measure the resonant frequency. Leon |
On Mon, 15 Mar 2004 12:48:43 -0000, Leon Heller wrote:
"Paul Burridge" wrote in message ... Hi all, I've got quite a huge stash of inductors in my parts bin. The colour codes don't always seem to relate to the values I've been able to measure, with my multi-function DVM, however, and I can't accurately measure any inductor about 10uH. Is there a circuit anywhere that would enable me to get a reasonably accurate idea of the values I've got down to say 100nH or thereabouts? If you have a signal generator you can connect them to a suitable capacitor and measure the resonant frequency. Leon Nyuk, nyuk, nyuk. I bet if he had a signal generator, he wouldn't be looking for inductors to shotgun that multiplier :) In fact, he wouln't even be building a multiplier. -- Best Regards, Mike |
On Mon, 15 Mar 2004 12:48:43 -0000, Leon Heller wrote:
"Paul Burridge" wrote in message ... Hi all, I've got quite a huge stash of inductors in my parts bin. The colour codes don't always seem to relate to the values I've been able to measure, with my multi-function DVM, however, and I can't accurately measure any inductor about 10uH. Is there a circuit anywhere that would enable me to get a reasonably accurate idea of the values I've got down to say 100nH or thereabouts? If you have a signal generator you can connect them to a suitable capacitor and measure the resonant frequency. Leon Nyuk, nyuk, nyuk. I bet if he had a signal generator, he wouldn't be looking for inductors to shotgun that multiplier :) In fact, he wouln't even be building a multiplier. -- Best Regards, Mike |
On Mon, 15 Mar 2004 12:48:43 -0000, "Leon Heller"
wrote: "Paul Burridge" wrote in message .. . Hi all, I've got quite a huge stash of inductors in my parts bin. The colour codes don't always seem to relate to the values I've been able to measure, with my multi-function DVM, however, and I can't accurately measure any inductor about 10uH. Is there a circuit anywhere that would enable me to get a reasonably accurate idea of the values I've got down to say 100nH or thereabouts? If you have a signal generator you can connect them to a suitable capacitor and measure the resonant frequency. Hi Leon, That's what I have been doing, in fact. It's just I'd prefer to have a more convenient, portable method to get instant read-outs of coil values... -- The BBC: Licensed at public expense to spread lies. |
On Mon, 15 Mar 2004 12:48:43 -0000, "Leon Heller"
wrote: "Paul Burridge" wrote in message .. . Hi all, I've got quite a huge stash of inductors in my parts bin. The colour codes don't always seem to relate to the values I've been able to measure, with my multi-function DVM, however, and I can't accurately measure any inductor about 10uH. Is there a circuit anywhere that would enable me to get a reasonably accurate idea of the values I've got down to say 100nH or thereabouts? If you have a signal generator you can connect them to a suitable capacitor and measure the resonant frequency. Hi Leon, That's what I have been doing, in fact. It's just I'd prefer to have a more convenient, portable method to get instant read-outs of coil values... -- The BBC: Licensed at public expense to spread lies. |
"Bill Turner" wrote in message ... On Mon, 15 Mar 2004 10:46:53 +0000, Paul Burridge wrote: I've got quite a huge stash of inductors in my parts bin. The colour codes don't always seem to relate to the values I've been able to measure, with my multi-function DVM, however, and I can't accurately measure any inductor about 10uH. Is there a circuit anywhere that would enable me to get a reasonably accurate idea of the values I've got down to say 100nH or thereabouts? __________________________________________________ _______ I'd recommend a grid dip meter and a known capacitor, but be sure to make the test at a frequency well below the self resonant frequency of the inductor alone. Another thing to be wary of: Some inductors are wound on a resistor to reduce the Q for a specific purpose. Be suspicious of any which have an unusually broad dip. -- Bill, W6WRT QSLs via LoTW This is what I do; it works well even on my questionable dip meter. Google for "LCR" meter; see what you come up with (mostly high $ instruments, probably). |
"Bill Turner" wrote in message ... On Mon, 15 Mar 2004 10:46:53 +0000, Paul Burridge wrote: I've got quite a huge stash of inductors in my parts bin. The colour codes don't always seem to relate to the values I've been able to measure, with my multi-function DVM, however, and I can't accurately measure any inductor about 10uH. Is there a circuit anywhere that would enable me to get a reasonably accurate idea of the values I've got down to say 100nH or thereabouts? __________________________________________________ _______ I'd recommend a grid dip meter and a known capacitor, but be sure to make the test at a frequency well below the self resonant frequency of the inductor alone. Another thing to be wary of: Some inductors are wound on a resistor to reduce the Q for a specific purpose. Be suspicious of any which have an unusually broad dip. -- Bill, W6WRT QSLs via LoTW This is what I do; it works well even on my questionable dip meter. Google for "LCR" meter; see what you come up with (mostly high $ instruments, probably). |
On Mon, 15 Mar 2004 07:40:36 -0800, "Tim Wescott"
wrote: Google for "LCR" meter; see what you come up with (mostly high $ instruments, probably). This one is great: http://www.aade.com/lcmeter.htm John |
On Mon, 15 Mar 2004 07:40:36 -0800, "Tim Wescott"
wrote: Google for "LCR" meter; see what you come up with (mostly high $ instruments, probably). This one is great: http://www.aade.com/lcmeter.htm John |
Hi,
http://www.aade.com/lcmeter.htm Or, as you have a counter, you could save yourself the money by copying his oscillator and use a calculator to find the value of L or C from the change in frequency. Cheers - Joe |
Hi,
http://www.aade.com/lcmeter.htm Or, as you have a counter, you could save yourself the money by copying his oscillator and use a calculator to find the value of L or C from the change in frequency. Cheers - Joe |
This has been answered fairly often here before. You might try a
search. The AADE L/C Meter II does quite an adequate job down into the nanohenry region, very quickly, for excitation at a few hundred kHz, and for relatively little cost. For better info at specific operating frequencies, try a vector network analyzer such as the N2PK one I believe you already know about. Paul Burridge wrote in message . .. Hi all, I've got quite a huge stash of inductors in my parts bin. The colour codes don't always seem to relate to the values I've been able to measure, with my multi-function DVM, however, and I can't accurately measure any inductor about 10uH. Is there a circuit anywhere that would enable me to get a reasonably accurate idea of the values I've got down to say 100nH or thereabouts? Thanks, p. |
This has been answered fairly often here before. You might try a
search. The AADE L/C Meter II does quite an adequate job down into the nanohenry region, very quickly, for excitation at a few hundred kHz, and for relatively little cost. For better info at specific operating frequencies, try a vector network analyzer such as the N2PK one I believe you already know about. Paul Burridge wrote in message . .. Hi all, I've got quite a huge stash of inductors in my parts bin. The colour codes don't always seem to relate to the values I've been able to measure, with my multi-function DVM, however, and I can't accurately measure any inductor about 10uH. Is there a circuit anywhere that would enable me to get a reasonably accurate idea of the values I've got down to say 100nH or thereabouts? Thanks, p. |
On Mon, 15 Mar 2004 07:12:42 -0800, Bill Turner
wrote: I'd recommend a grid dip meter and a known capacitor, but be sure to make the test at a frequency well below the self resonant frequency of the inductor alone. I'm afraid my experience of using GDMs is *very* unfavourable. They were fine in the days of big old HF kit, but nowadays with PCB/surface-mounted stuff, they're *ucking useless. Another thing to be wary of: Some inductors are wound on a resistor to reduce the Q for a specific purpose. Be suspicious of any which have an unusually broad dip. ISTR some valve anode chokes used this arrangement in HF rigs a couple of decades ago... -- The BBC: Licensed at public expense to spread lies. |
On Mon, 15 Mar 2004 07:12:42 -0800, Bill Turner
wrote: I'd recommend a grid dip meter and a known capacitor, but be sure to make the test at a frequency well below the self resonant frequency of the inductor alone. I'm afraid my experience of using GDMs is *very* unfavourable. They were fine in the days of big old HF kit, but nowadays with PCB/surface-mounted stuff, they're *ucking useless. Another thing to be wary of: Some inductors are wound on a resistor to reduce the Q for a specific purpose. Be suspicious of any which have an unusually broad dip. ISTR some valve anode chokes used this arrangement in HF rigs a couple of decades ago... -- The BBC: Licensed at public expense to spread lies. |
On Mon, 15 Mar 2004 08:10:02 -0800, John Larkin
wrote: On Mon, 15 Mar 2004 07:40:36 -0800, "Tim Wescott" wrote: Google for "LCR" meter; see what you come up with (mostly high $ instruments, probably). This one is great: http://www.aade.com/lcmeter.htm Crumbs! Thanks, John! -- The BBC: Licensed at public expense to spread lies. |
On Mon, 15 Mar 2004 08:10:02 -0800, John Larkin
wrote: On Mon, 15 Mar 2004 07:40:36 -0800, "Tim Wescott" wrote: Google for "LCR" meter; see what you come up with (mostly high $ instruments, probably). This one is great: http://www.aade.com/lcmeter.htm Crumbs! Thanks, John! -- The BBC: Licensed at public expense to spread lies. |
On Mon, 15 Mar 2004 15:12:24 +0000, Paul Burridge wrote:
On Mon, 15 Mar 2004 12:48:43 -0000, "Leon Heller" wrote: "Paul Burridge" wrote in message . .. Hi all, I've got quite a huge stash of inductors in my parts bin. The colour codes don't always seem to relate to the values I've been able to measure, with my multi-function DVM, however, and I can't accurately measure any inductor about 10uH. Is there a circuit anywhere that would enable me to get a reasonably accurate idea of the values I've got down to say 100nH or thereabouts? If you have a signal generator you can connect them to a suitable capacitor and measure the resonant frequency. Hi Leon, That's what I have been doing, in fact. It's just I'd prefer to have a more convenient, portable method to get instant read-outs of coil values... So why beat yourself to death building a sweep gen to test filters when you got that there sig gen? -- Best Regards, Mike |
On Mon, 15 Mar 2004 15:12:24 +0000, Paul Burridge wrote:
On Mon, 15 Mar 2004 12:48:43 -0000, "Leon Heller" wrote: "Paul Burridge" wrote in message . .. Hi all, I've got quite a huge stash of inductors in my parts bin. The colour codes don't always seem to relate to the values I've been able to measure, with my multi-function DVM, however, and I can't accurately measure any inductor about 10uH. Is there a circuit anywhere that would enable me to get a reasonably accurate idea of the values I've got down to say 100nH or thereabouts? If you have a signal generator you can connect them to a suitable capacitor and measure the resonant frequency. Hi Leon, That's what I have been doing, in fact. It's just I'd prefer to have a more convenient, portable method to get instant read-outs of coil values... So why beat yourself to death building a sweep gen to test filters when you got that there sig gen? -- Best Regards, Mike |
On Mon, 15 Mar 2004 18:40:07 +0000 (UTC), Joe McElvenney
wrote: Hi, http://www.aade.com/lcmeter.htm Or, as you have a counter, you could save yourself the money by copying his oscillator and use a calculator to find the value of L or C from the change in frequency. Cheers - Joe To save $100, I'm not going to copy *anybody's* circuits. John |
On Mon, 15 Mar 2004 18:40:07 +0000 (UTC), Joe McElvenney
wrote: Hi, http://www.aade.com/lcmeter.htm Or, as you have a counter, you could save yourself the money by copying his oscillator and use a calculator to find the value of L or C from the change in frequency. Cheers - Joe To save $100, I'm not going to copy *anybody's* circuits. John |
Hi,
To save $100, I'm not going to copy *anybody's* circuits. An unfortunate choice of words on my part perhaps; possibly 'copy his technique' would have been better. A technique, I should add, that has been in the public domain for a very long time. The basic idea being - 1. Take an oscillator tank circuit with fixed C1 or L1. 2. Measure frequency F1. 3. Connect unknown Cx across C1 or Lx in series with L1. 4. Measure new frequency F2. 5. Calculate Cx (Lx) using a scientific calculator, remembering that F2/F1 = sqrt(C1 + Cx) or sqrt(L1 + Lx) then we have, Cx = C1([F1/F2]^2 - 1) or Lx = L1([F2/F1]^2 - 1). For best accuracy you would need to know the value of the strays which may easily be worked out using the same technique in reverse, or swamped, as the fancy takes you. On the subject of copying, I would guess that 95% of homebrew projects rely on pre-published information; there must have been hundreds of 'chinese copies' of commercial rigs produced by hams over the years. And it goes without saying that homebrewing not buying is what we are supposed to be talking about in this group. Cheers - Joe |
Hi,
To save $100, I'm not going to copy *anybody's* circuits. An unfortunate choice of words on my part perhaps; possibly 'copy his technique' would have been better. A technique, I should add, that has been in the public domain for a very long time. The basic idea being - 1. Take an oscillator tank circuit with fixed C1 or L1. 2. Measure frequency F1. 3. Connect unknown Cx across C1 or Lx in series with L1. 4. Measure new frequency F2. 5. Calculate Cx (Lx) using a scientific calculator, remembering that F2/F1 = sqrt(C1 + Cx) or sqrt(L1 + Lx) then we have, Cx = C1([F1/F2]^2 - 1) or Lx = L1([F2/F1]^2 - 1). For best accuracy you would need to know the value of the strays which may easily be worked out using the same technique in reverse, or swamped, as the fancy takes you. On the subject of copying, I would guess that 95% of homebrew projects rely on pre-published information; there must have been hundreds of 'chinese copies' of commercial rigs produced by hams over the years. And it goes without saying that homebrewing not buying is what we are supposed to be talking about in this group. Cheers - Joe |
OOPS!!
5. should have read - Calculate Cx (Lx) using a scientific calculator, remembering that F2/F1 = sqrt([C1 + Cx]/C1) or sqrt([L1 + Lx]/L1) thus we have, Cx = C1([F1/F2]^2 - 1) or Lx = L1([F2/F1]^2 - 1). Cheers - Joe |
OOPS!!
5. should have read - Calculate Cx (Lx) using a scientific calculator, remembering that F2/F1 = sqrt([C1 + Cx]/C1) or sqrt([L1 + Lx]/L1) thus we have, Cx = C1([F1/F2]^2 - 1) or Lx = L1([F2/F1]^2 - 1). Cheers - Joe |
On Mon, 15 Mar 2004 19:01:01 -0500, Active8
wrote: So why beat yourself to death building a sweep gen to test filters when you got that there sig gen? Because it has to be small and light enough to be mounted on our robot for field checks. Try doing that with a 120lb Marconi. :-) -- The BBC: Licensed at public expense to spread lies. |
On Mon, 15 Mar 2004 19:01:01 -0500, Active8
wrote: So why beat yourself to death building a sweep gen to test filters when you got that there sig gen? Because it has to be small and light enough to be mounted on our robot for field checks. Try doing that with a 120lb Marconi. :-) -- The BBC: Licensed at public expense to spread lies. |
On Tue, 16 Mar 2004 13:02:44 +0000, Paul Burridge wrote:
On Mon, 15 Mar 2004 19:01:01 -0500, Active8 wrote: So why beat yourself to death building a sweep gen to test filters when you got that there sig gen? Because it has to be small and light enough to be mounted on our robot for field checks. Try doing that with a 120lb Marconi. :-) That's more macaroni than even *this* eyetalian can eat. You're confusing me again. If you have a filter on a bot in the field, it's the same filter that was on the bot before you got there and should do the same thing. If you change a filter in the field, you'd want to sweep it before it's installed in der bot. -- Best Regards, Mike |
On Tue, 16 Mar 2004 13:02:44 +0000, Paul Burridge wrote:
On Mon, 15 Mar 2004 19:01:01 -0500, Active8 wrote: So why beat yourself to death building a sweep gen to test filters when you got that there sig gen? Because it has to be small and light enough to be mounted on our robot for field checks. Try doing that with a 120lb Marconi. :-) That's more macaroni than even *this* eyetalian can eat. You're confusing me again. If you have a filter on a bot in the field, it's the same filter that was on the bot before you got there and should do the same thing. If you change a filter in the field, you'd want to sweep it before it's installed in der bot. -- Best Regards, Mike |
On Tue, 16 Mar 2004 10:40:27 -0500, Active8
wrote: If you change a filter in the field, you'd want to sweep it before it's installed in der bot. Sigh.. Yes, I know this has been raised (and answered at considerable length before). Raised so often, in fact, that I wrote a full explanation and stuck it on our website. Sadly, that's not up and running at the moment so I can't refer you to it. Take it from me, however, that there *is* method in my madness. :-) -- The BBC: Licensed at public expense to spread lies. |
On Tue, 16 Mar 2004 10:40:27 -0500, Active8
wrote: If you change a filter in the field, you'd want to sweep it before it's installed in der bot. Sigh.. Yes, I know this has been raised (and answered at considerable length before). Raised so often, in fact, that I wrote a full explanation and stuck it on our website. Sadly, that's not up and running at the moment so I can't refer you to it. Take it from me, however, that there *is* method in my madness. :-) -- The BBC: Licensed at public expense to spread lies. |
On Tue, 16 Mar 2004 20:26:17 +0000, Paul Burridge wrote:
On Tue, 16 Mar 2004 10:40:27 -0500, Active8 wrote: If you change a filter in the field, you'd want to sweep it before it's installed in der bot. Sigh.. Yes, I know this has been raised (and answered at considerable length before). Raised so often, in fact, that I wrote a full explanation and stuck it on our website. Sadly, that's not up and running at the moment so I can't refer you to it. Take it from me, however, that there *is* method in my madness. :-) I'll make a mental note to visit yer site once in a while. same domain as the gifs you've been posting? -- Best Regards, Mike |
On Tue, 16 Mar 2004 20:26:17 +0000, Paul Burridge wrote:
On Tue, 16 Mar 2004 10:40:27 -0500, Active8 wrote: If you change a filter in the field, you'd want to sweep it before it's installed in der bot. Sigh.. Yes, I know this has been raised (and answered at considerable length before). Raised so often, in fact, that I wrote a full explanation and stuck it on our website. Sadly, that's not up and running at the moment so I can't refer you to it. Take it from me, however, that there *is* method in my madness. :-) I'll make a mental note to visit yer site once in a while. same domain as the gifs you've been posting? -- Best Regards, Mike |
On Tue, 16 Mar 2004 16:18:30 -0500, Active8
wrote: I'll make a mental note to visit yer site once in a while. same domain as the gifs you've been posting? (Answered by e-mail) -- The BBC: Licensed at public expense to spread lies. |
On Tue, 16 Mar 2004 16:18:30 -0500, Active8
wrote: I'll make a mental note to visit yer site once in a while. same domain as the gifs you've been posting? (Answered by e-mail) -- The BBC: Licensed at public expense to spread lies. |
"Paul Burridge" wrote in message
... Hi all, I've got quite a huge stash of inductors in my parts bin. The colour codes don't always seem to relate to the values I've been able to measure, with my multi-function DVM, however, and I can't accurately measure any inductor about 10uH. Is there a circuit anywhere that would enable me to get a reasonably accurate idea of the values I've got down to say 100nH or thereabouts? "splat" test V=L*dI/dt charge up a large cap to some voltage V short the cap thru the inductor, and measure the current with a scope (digital is great, as long as the single-shot sample rate is high enough - I did this once with an HP 56000 series scope, and got 1 data point - very easy to fit a line to, very hard to fit the right line to!) knowing dI/dt and V, calculate L then, you can see saturation from where dI/dt increases rapidly (invaluable for power electronics) also, ensure that 0.5CV^2 0.5LI^2 at the current of interest - this ensures that V is approximately constant I have a couple of 33mF 35V caps in parallel, with 10 x 1R MRS25 resistors paralleled as a current sensor. It takes about 45s to set up for a measurement. I often use the shaft of a screwdriver to make the "splat" - a nice hard material is good. I have used this technique successfully to measure inductors ranging from the tiny ( 500nH) to huge (3mH 2,000A chokes). Actually, big chokes tend not to read true on LCR meters, as the magnetic material permeability is often much higher at very low currents - especially true of iron powder. if you dont have a digital scope, make a tiny, LF oscillator with a 555. Drive a grunty FET, with a small (I use 0.1R - 10R) source resistor. Then, you'll get a repetitive waveform, which works nicely on any analogue scope. If Rsense is nice and low, use a 50R BNC cable to connect to scope, with a 50R terminator at the scope - lovely clean waveforms. I built a little tester like this to test air gaps in planar cores for a 55W smps that lives inside an LED video screen (actually, thousands of them live inside). works a treat, and cost $5. One of my techs once built a splat tester for big chokes - he had several 1200V 600A IGBT's in parallel running as a series pass regulator, with about 100mF of capacitance (charged to 700Vdc thru several lightbulbs) to control V, even at currents on the order of 5,000A. A honking great LEM DCCT measured the current, and the "switch" was purely mechanical - he used the pole faces from an HV contactor, as the material is astonishingly arc resistant, so no pitting/welding |
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