how do u test an inductor?

[Louie]

can someone tell me how to test/measure an inductor on a power supply
circuit board. can it be done in circuit with a VOM?

TIA
louie

 

[Stepan Novotill]

It should measure pretty close to zero ohms.
That won't tell you if it has shorted turns though.
Stepan

On Thu, 14 Aug 2003 04:46:28 GMT, Louie <beavisnbutthead@softhome.net>
wrote:

can someone tell me how to test/measure an inductor on a power supply
circuit board. can it be done in circuit with a VOM?

TIA
louie

 

[Louie]

well i do have an analogue scope and i'd rather not fork out for an ESR
mater. i'll try what you describe. it should be plenty accurate for my
purposes.

thanks Terry, that's what i needed.
Louie

Terry Given wrote:

The best way to test an inductor is the so-called "splat" test.
Its based on the differential equation for an inductor: V = L x dI/dt

If you stick a constant voltage, say V across an inductor of inductance L,
then the rate-of-change of current dI/dt = V/L amps-per-second. If L is in
uH then dI/dt is in Amps-per-microsecond

How to do it:
you need to take the inductor out of the circuit.
you need a current-limited power supply and a scope.

1) Stick a nice low resistor in series with with the inductor - how low
depends on the current you will end up measuring. 1Ohm is a good start, BUT
make it out of 10 x 10 Ohm 1/4W resistors in parallel (theres a good
reason...)

2) connect the resistor to the power supply -ve terminal - 0V.

3) put the scope probe (use a x1 probe if you can, you'll get a cleaner
low-voltage signal), across the resistor - obviously the scope "ground" lead
goes the the -ve supply side of the resistor. BE CAREFUL - clip the probe
directly across the resistor, lest you screw up the measurement.

4) set the power supply to +10V, and limit the current to about 1-10mA. If
you dont have a current-limited supply, use a 12V battery with a 10k series
resistor in the +ve lead.

5) attach a decent size cap - say > 2200uF, > 16V - across the current
limited supply. You can use a second scope channel to measure the voltage on
the cap if you like.

6) connect a short piece of wire to the end of the inductor-under-test (the
end thats not connected to the resistor). Strip, twist & tin about 3cm of
the end of the lead.

7) set the scope to rising edge triggered, NORMAL mode (it really has to be
a digital scope, sorry)

8) when the cap is fully charged, whack the tinned wire across the cap's +ve
lead. You'll soon see why its called a splat test.

9) if you do it right, you just connected a inductor across a "constant
voltage source" ie the cap. the current will LINEARLY ramp up from zero, a
nice straight line, until the inductor saturates (ie the inductor can handle
no more current), at which time the current measurement (the R's converted
the current into a voltage which we see on the scope) slope will become very
steep.

From this measurement we can:
a) calculate the inductance: find a nice straight piece of the waveform with
the lowest slope. for some time interval dt, measure the change in resistor
voltage dV. Calculate dI = dV/Rsense (1 Ohm in my example). Then calculate
dI/dt. As V/L = dI/dt and we know dI/dt and V, calculate L = V/(dI/dt). If
dI in A, dt in s, V in volts then L is Henries. Usually use volts, amps &
microseconds, so L is in microHenries.

b) measure the saturation current - the current at which the magnetic
conductor (ie the inductor core) becomes saturated, or full. This is simply
where the measure I-vs-t curve slope becomes very steep.

To give you an idea:
say L = 125uH, Isat = 2A (I use thousands of this part) but it runs at 1A
If V = 10V and dI = 1A then dt = L*di/V = 125uH*1A/10V = 12.5us

so I'd set the scope to 2us per division. Rsense = 1Ohm so Vsense = 1Ohm*1A
= 1V, I would use 200mV/div. I'd set the trigger level to about 500mV,
trigger position in the center of the scope screen.

often you have to "splat" it a couple of times to get a nice clean waveform,
due to contact bounce - tinned wire & your finger make a crappy switch.

You can also do this with an analogu scope, BUT its harder. I actually built
a core gap tester that works this way:

Use a 555 timer to build a free-running oscillator. you'll want to twiddle
the frequency a bit but around 100Hz is FINE. Its really handy if you set up
a pulse-width twiddly knob too - you want a pulse width longer than the time
it takes (calculated above) for the current to ramp up to where you want it.

Use the output to drive a FET gate, thru a 100 Ohm resistor. Put the 1Ohms
sense resistor in series with the FET Source, then to 0V. as above, measure
the voltage across the sense resistor with a scope.

Stick the inductor between the FET Drain and the 2200uF cap, with about a 10
Ohm resistor from the cap to the power supply (the 555 circuit can run from
the same supply). just make sure that 5*Rcap*C < period of oscillator. If it
was 2kHz, then need a 100us time constant.....

then you'll get a lovely repetitive waveform that a humble old analogue
scope will happily trigger on. Also you can use a 2nd channel to trigger on
the 555 output, which is even better...

I have successfully tested inductors ranging from 100nH to 500mH, and up to
5,000A (no, not a typo) using this setup. Of course you need to pay
attention to stray inductance for very low values, and how big the cap is,
and current sensing etc.

brief guideline: energy in L = 0.5*L*I^2 at current I.

energy in cap = 0.5*C*V^2 at voltage V

make sure energy in cap > 20*energy in inductor - this ensures the cap
voltage does not change too much during the test, so "V" remains constant in
our formulae.

why use a cap & a current limiter? so you dont blow the shit out of
everything, ESPECIALLY when using high voltages to test monster chokes.

This is one of the few ways to test really huge inductors, and to test for
saturation - even the whizz-bang HP $20,000 inductance meters only do about
20A.......







"Louie" <beavisnbutthead@softhome.net> wrote in message
news:EuE_a.96093$cF.29956@rwcrnsc53...
can someone tell me how to test/measure an inductor on a power supply
circuit board. can it be done in circuit with a VOM?

TIA
louie

 

[tim kettring]

"Terry Given" <the_domes@xtra.co.nz> wrote in message news:<Vx__a.116924$JA5.2641015@news.xtra.co.nz>...

The best way to test an inductor is the so-called "splat" test.
Its based on the differential equation for an inductor: V = L x dI/dt

If you stick a constant voltage, say V across an inductor of inductance L,
then the rate-of-change of current dI/dt = V/L amps-per-second. If L is in
uH then dI/dt is in Amps-per-microsecond

How to do it:
you need to take the inductor out of the circuit.
you need a current-limited power supply and a scope.

1) Stick a nice low resistor in series with with the inductor - how low
depends on the current you will end up measuring. 1Ohm is a good start, BUT
make it out of 10 x 10 Ohm 1/4W resistors in parallel (theres a good
reason...)

Whats the good reason , closing tolerance by a factor of 10 ? In
effect a precision 1 ohm resistor ?

tim

2) connect the resistor to the power supply -ve terminal - 0V.

3) put the scope probe (use a x1 probe if you can, you'll get a cleaner
low-voltage signal), across the resistor - obviously the scope "ground" lead
goes the the -ve supply side of the resistor. BE CAREFUL - clip the probe
directly across the resistor, lest you screw up the measurement.

4) set the power supply to +10V, and limit the current to about 1-10mA. If
you dont have a current-limited supply, use a 12V battery with a 10k series
resistor in the +ve lead.

5) attach a decent size cap - say > 2200uF, > 16V - across the current
limited supply. You can use a second scope channel to measure the voltage on
the cap if you like.

6) connect a short piece of wire to the end of the inductor-under-test (the
end thats not connected to the resistor). Strip, twist & tin about 3cm of
the end of the lead.

7) set the scope to rising edge triggered, NORMAL mode (it really has to be
a digital scope, sorry)

8) when the cap is fully charged, whack the tinned wire across the cap's +ve
lead. You'll soon see why its called a splat test.

9) if you do it right, you just connected a inductor across a "constant
voltage source" ie the cap. the current will LINEARLY ramp up from zero, a
nice straight line, until the inductor saturates (ie the inductor can handle
no more current), at which time the current measurement (the R's converted
the current into a voltage which we see on the scope) slope will become very
steep.

From this measurement we can:
a) calculate the inductance: find a nice straight piece of the waveform with
the lowest slope. for some time interval dt, measure the change in resistor
voltage dV. Calculate dI = dV/Rsense (1 Ohm in my example). Then calculate
dI/dt. As V/L = dI/dt and we know dI/dt and V, calculate L = V/(dI/dt). If
dI in A, dt in s, V in volts then L is Henries. Usually use volts, amps &
microseconds, so L is in microHenries.

b) measure the saturation current - the current at which the magnetic
conductor (ie the inductor core) becomes saturated, or full. This is simply
where the measure I-vs-t curve slope becomes very steep.

To give you an idea:
say L = 125uH, Isat = 2A (I use thousands of this part) but it runs at 1A
If V = 10V and dI = 1A then dt = L*di/V = 125uH*1A/10V = 12.5us

so I'd set the scope to 2us per division. Rsense = 1Ohm so Vsense = 1Ohm*1A
= 1V, I would use 200mV/div. I'd set the trigger level to about 500mV,
trigger position in the center of the scope screen.

often you have to "splat" it a couple of times to get a nice clean waveform,
due to contact bounce - tinned wire & your finger make a crappy switch.

You can also do this with an analogu scope, BUT its harder. I actually built
a core gap tester that works this way:

Use a 555 timer to build a free-running oscillator. you'll want to twiddle
the frequency a bit but around 100Hz is FINE. Its really handy if you set up
a pulse-width twiddly knob too - you want a pulse width longer than the time
it takes (calculated above) for the current to ramp up to where you want it.

Use the output to drive a FET gate, thru a 100 Ohm resistor. Put the 1Ohms
sense resistor in series with the FET Source, then to 0V. as above, measure
the voltage across the sense resistor with a scope.

Stick the inductor between the FET Drain and the 2200uF cap, with about a 10
Ohm resistor from the cap to the power supply (the 555 circuit can run from
the same supply). just make sure that 5*Rcap*C < period of oscillator. If it
was 2kHz, then need a 100us time constant.....

then you'll get a lovely repetitive waveform that a humble old analogue
scope will happily trigger on. Also you can use a 2nd channel to trigger on
the 555 output, which is even better...

I have successfully tested inductors ranging from 100nH to 500mH, and up to
5,000A (no, not a typo) using this setup. Of course you need to pay
attention to stray inductance for very low values, and how big the cap is,
and current sensing etc.

brief guideline: energy in L = 0.5*L*I^2 at current I.

energy in cap = 0.5*C*V^2 at voltage V

make sure energy in cap > 20*energy in inductor - this ensures the cap
voltage does not change too much during the test, so "V" remains constant in
our formulae.

why use a cap & a current limiter? so you dont blow the shit out of
everything, ESPECIALLY when using high voltages to test monster chokes.

This is one of the few ways to test really huge inductors, and to test for
saturation - even the whizz-bang HP $20,000 inductance meters only do about
20A.......







"Louie" <beavisnbutthead@softhome.net> wrote in message
news:EuE_a.96093$cF.29956@rwcrnsc53...
can someone tell me how to test/measure an inductor on a power supply
circuit board. can it be done in circuit with a VOM?

TIA
louie

 

[Terry Given]

Good question: see below...

The best way to test an inductor is the so-called "splat" test.
Its based on the differential equation for an inductor: V = L x dI/dt

If you stick a constant voltage, say V across an inductor of inductance
L,
then the rate-of-change of current dI/dt = V/L amps-per-second. If L is
in
uH then dI/dt is in Amps-per-microsecond

How to do it:
you need to take the inductor out of the circuit.
you need a current-limited power supply and a scope.

1) Stick a nice low resistor in series with with the inductor - how low
depends on the current you will end up measuring. 1Ohm is a good start,
BUT
make it out of 10 x 10 Ohm 1/4W resistors in parallel (theres a good
reason...)


Whats the good reason , closing tolerance by a factor of 10 ? In
effect a precision 1 ohm resistor ?

tim

No. A "grunty" 1Ohm resistor, with very low inductance is the reason.
Lots of people forget to calculate the Peak Pulse Power seen by a resistor,
and this can be crucial. Consider for example a 5k6 10W wirewound resistor
connected directly across 230Vac 50Hz mains: We all know that the power
dissipation = (230V)^2/5600 Ohms = 9.45W. But if we tried it, using this
part, it wouldnt live very long. The peak pulse power is of course
Vpeak^2/R, and given that Vpeak = sqrt(2)*Vrms in this case the resistor
power dissipation is actually an 18.9W peak sine wave, at 100Hz.

A classic (and fatal) examples of this mistake are FET gate drive
resistors - 10R, 12V gate waveform, the peak pulse power = 14.4W. I have
seen plenty of people using 0805 or 1206 smt resistors for this job, and
they work fine, for a while....

the other good one crops up in EMI filters - people often stick small
resistors in series with the filter caps, to damp out resonances, and
effectively convert HF energy into heat. BUT they forget that the resistor
will see Vpeak^2/R peak power. I did this myself in an AC motor controller,
using 2W Philips PR02 1R resistors, running from 400V three phase supply.
During development/testing we noticed a bright flash, and upon close
inspection discovered the resistors were totally snotted. A little thought
showed why: sqrt(2)*400Vac = 566V peak, so the poor old resistors saw a
worst-case 320kW peak pulse power. The datasheet (philips are good for this)
has a pulse overload curve, and the part could handle about 100W for a short
while (100us), so 320kW rooted it completely. The solution there was to use
Carbon Composition resistors, with an appropriately wicked P.P.Power rating.

Ultimately it all boils down to thermal mass. Most resistors are a
non-conducting drum, with a thin coating of resistive material, often cut
into a spiral. For a very short length of time, the resistive material will
absorb energy "adiabatically" i.e. no heat will flow into the surroundings.
If you know the energy of the pulse, and the mass m & specific heat capacity
of the material c (Joules/(kg*Kelvin)), it is easy to calculate change in
temperature dT = E*m/c. This is true for ANY thermal problem. With a FET, if
dT + Tambient > 200 degrees C (473 Kelvin) then the silicon goes "intrinsic"
i.e. turns into a conductor, and the FET self-destructs.

A carbon composition resistor is made from a solid lump of suitably
resistive material, so there is LOTS of mass compared to a normal resistor,
so it can absorb a bigger belt. Companies like IRC and Philips make
resistors designed specifically for this job, eg IRC's CHP series, Philips
MMA/MMB series etc.

Be careful with totally enclosed wire-wound resistors. Good brands like
Vitrohm have the wound element totally enclosed in thermally conductive
ceramic goop, so there is no air and hotspots dont form - you can beat the
hell out of this type of resistor, and it will cope. Cheap, crappy resistors
usually have a groove cut in the ceramic body, into which the resistive
element is dropped. They then pour ceramic goo over the top, usually
resulting in a lot of air around the "back" of the element. These will often
emit a nice orange flash when you thump them with a big pulse - thats the
wire heating up red hot, as air is a lousy conductor of heat. These will
crap out, sooner or later.


so back to the original question:
If Vsupply = 12V and R=1 Ohm, then Ppeak = 144W, which will fry an ordinary
resistor. I fwe make it from 10 * 10R in parallel, then each one only sees
14.4W peak, so will probably not die. And dont forget, when the Inductor
under test saturates (it will unless its air cored) the inductance will
plummet to the air-cored value, so the current will rise very rapidly,
ultimately limited by the current sense resistor.

I actually did this a few years back, trying to measure an inductor in a
physics lab. I got odd results, which got weirder every time i re-tried the
experiment. eventually i realised what was going on, and used a lot of 22R
resistors to do the job......only to discover that the HP56000 series scope
i was using had way to slow a single-shot-sample rate, and all i got was 3
dots for my whole waveform.....*sigh*.......out with the 30 year old
analogue scope....

2) connect the resistor to the power supply -ve terminal - 0V.

3) put the scope probe (use a x1 probe if you can, you'll get a cleaner
low-voltage signal), across the resistor - obviously the scope "ground"
lead
goes the the -ve supply side of the resistor. BE CAREFUL - clip the
probe
directly across the resistor, lest you screw up the measurement.

4) set the power supply to +10V, and limit the current to about 1-10mA.
If
you dont have a current-limited supply, use a 12V battery with a 10k
series
resistor in the +ve lead.

5) attach a decent size cap - say > 2200uF, > 16V - across the current
limited supply. You can use a second scope channel to measure the
voltage on
the cap if you like.

6) connect a short piece of wire to the end of the inductor-under-test
(the
end thats not connected to the resistor). Strip, twist & tin about 3cm
of
the end of the lead.

7) set the scope to rising edge triggered, NORMAL mode (it really has to
be
a digital scope, sorry)

8) when the cap is fully charged, whack the tinned wire across the cap's
+ve
lead. You'll soon see why its called a splat test.

9) if you do it right, you just connected a inductor across a "constant
voltage source" ie the cap. the current will LINEARLY ramp up from zero,
a
nice straight line, until the inductor saturates (ie the inductor can
handle
no more current), at which time the current measurement (the R's
converted
the current into a voltage which we see on the scope) slope will become
very
steep.

From this measurement we can:
a) calculate the inductance: find a nice straight piece of the waveform
with
the lowest slope. for some time interval dt, measure the change in
resistor
voltage dV. Calculate dI = dV/Rsense (1 Ohm in my example). Then
calculate
dI/dt. As V/L = dI/dt and we know dI/dt and V, calculate L = V/(dI/dt).
If
dI in A, dt in s, V in volts then L is Henries. Usually use volts, amps
&
microseconds, so L is in microHenries.

b) measure the saturation current - the current at which the magnetic
conductor (ie the inductor core) becomes saturated, or full. This is
simply
where the measure I-vs-t curve slope becomes very steep.

To give you an idea:
say L = 125uH, Isat = 2A (I use thousands of this part) but it runs at
1A
If V = 10V and dI = 1A then dt = L*di/V = 125uH*1A/10V = 12.5us

so I'd set the scope to 2us per division. Rsense = 1Ohm so Vsense =
1Ohm*1A
= 1V, I would use 200mV/div. I'd set the trigger level to about 500mV,
trigger position in the center of the scope screen.

often you have to "splat" it a couple of times to get a nice clean
waveform,
due to contact bounce - tinned wire & your finger make a crappy switch.

You can also do this with an analogu scope, BUT its harder. I actually
built
a core gap tester that works this way:

Use a 555 timer to build a free-running oscillator. you'll want to
twiddle
the frequency a bit but around 100Hz is FINE. Its really handy if you
set up
a pulse-width twiddly knob too - you want a pulse width longer than the
time
it takes (calculated above) for the current to ramp up to where you want
it.

Use the output to drive a FET gate, thru a 100 Ohm resistor. Put the
1Ohms
sense resistor in series with the FET Source, then to 0V. as above,
measure
the voltage across the sense resistor with a scope.

Stick the inductor between the FET Drain and the 2200uF cap, with about
a 10
Ohm resistor from the cap to the power supply (the 555 circuit can run
from
the same supply). just make sure that 5*Rcap*C < period of oscillator.
If it
was 2kHz, then need a 100us time constant.....

then you'll get a lovely repetitive waveform that a humble old analogue
scope will happily trigger on. Also you can use a 2nd channel to trigger
on
the 555 output, which is even better...

I have successfully tested inductors ranging from 100nH to 500mH, and up
to
5,000A (no, not a typo) using this setup. Of course you need to pay
attention to stray inductance for very low values, and how big the cap
is,
and current sensing etc.

brief guideline: energy in L = 0.5*L*I^2 at current I.

energy in cap = 0.5*C*V^2 at voltage V

make sure energy in cap > 20*energy in inductor - this ensures the cap
voltage does not change too much during the test, so "V" remains
constant in
our formulae.

why use a cap & a current limiter? so you dont blow the shit out of
everything, ESPECIALLY when using high voltages to test monster chokes.

This is one of the few ways to test really huge inductors, and to test
for
saturation - even the whizz-bang HP $20,000 inductance meters only do
about
20A.......







"Louie" <beavisnbutthead@softhome.net> wrote in message
news:EuE_a.96093$cF.29956@rwcrnsc53...
can someone tell me how to test/measure an inductor on a power supply
circuit board. can it be done in circuit with a VOM?

TIA
louie

 

[Terry Given]

Some very good points. The unusual devices require unusual measuring
techniques. Have you ever figured out a way to measure "loss" in an
inductor?

yep.

Two basic techniques: The splat test in reverse - apply a current source
through the inductor in parallel with a cap, then disconnect it - use a good
cap, and the damping is due entirely to the inductor losses. You really need
to use a FET to disconnect the current source, to get a nice waveform. Very
hard to do at 1,000A. Then fit an exponential decay envelope to the ringing
waveform and voila. Horribly hard to get better than about 20% accurate
measurements.

The best technique is of course calorimetry. Dump the inductor into a
container of suitable liquid, of known specific heat capacity c, mass m and
temperature. Make sure the container is in an isothermal environment (i.e. a
chilly bin made of polystyrene). Run it for a suitably long time - 4-5
thermal time constants are required; this can be hours for a decent sized
choke. Then stir the liquid and measure its temperature. from dT, m & c,
know energy E, and if time t is also recorded, can calculate average power
loss. this can be done quite accurately, but in real terms 10% is good and
5% is outstanding for thermal measurements. Careful you dont set the
container on fire though - estimate the loss and thermal time constant first
(measuring the therm. tc is easy) then use that to figure out how much
liquid is required. De-ionised water is good, but then so is cooking oil.
(note: dont use a flammable liquid. You can easily calibrate your setup -
bung a power resistor in there, and stick a known amount of power thru it
for a known period of time, measure the temp rise, and voila. Shit, if you
want, just keep fucking with the power supplied to the resistor until the 2
temperature rises are the same......)

Its not too difficult to get a very good idea of what the losses are going
to be in an inductor - just make sure you take skin & proximity effect into
account for the copper loss, and core temperature for core loss. But what
you get is not always what you asked for - we had a problem with some 150uH
1,500A chokes for a 10kHz UPS that had astonishingly high core loss -
between 3 and 20x what we calculated. It took us a while to figure out why -
and it was the core manufacturers fault. The core was a tape-wound "C"
core - an oval, but with 2 cuts = 4 x 1 inch air gaps, to minimise fringing
flux. The material was basically metglass, 0.001" thick. When Honeywell cut
the cores, in a fit of inspired stupidity they neglected to lap & etch the
cut faces - result was metal smearing across the faces, therefore effective
tape thickness ranged from 0.001" to 1" hence eddy current losses were all
over the show. We fixed it with hydroflouric acid, and they were VERY
apologetic when we pointed out what they had done.

I also once re-designed a transformer for a 1500W ups PFC front end - boost
converter + dc-dc converter down to 24Vdc (battery level). The idiot who
designed it (ME, MIT - ha!) used 12 layers of 0.6mm Cu foil at 100kHz. skin
depth at 100kHz is 66mm/sqrt(1e5) = 0.208mm, so the foil was about 3 skin
depths thick. At 12 layers, split-wound (1/2P, S, 1/2P) that is 6 effective
layers, the ac-dc resistance ratio Fr = 150. So his AC resistance was about
150 times the DC resistance, which kind of explained all the fires (and i do
mean fires). I reduced the copper thickness to 0.1mm, so the dc resistance
went up by a factor of 6, BUT Fr dropped from 150 to 1.2, so the total loss
went down by a factor of 150/(6*1.2) = 21. And no-one at worked believed me
when i said that would solve the problem, but it did.

see "soft ferrites" by e.c. snelling OR switchmode power supply handbook, K.
Billings, OR the philips app notes by Jongsma for the relevant
nomographs/formulae. I once messed around with calculating it analytically,
but its easier to read it from a table......I'm lazy.

 

[JeffM]

apply a current source through the inductor in parallel with a cap,
then disconnect it - use a good cap,
and the damping is due entirely to the inductor losses.

I was waiting for someone to say that.
What no one has yet said is that a common failure mode for an inductor
is a shorted turn (think "step-down transformer, then think "autotransformer".
When this happens, the Q (quality factor) goes to hell and the thing won't ring.

...use Carbon Composition resistors

A carbon composition resistor is made from a solid lump of suitably
resistive material, so there is LOTS of mass compared to a normal resistor,
so it can absorb a bigger belt.

My Russian physicist boss calls these "bulk resistors"
and while they can absorb a lot of abuse, they're getting harder to find.
I did find a distributor who bought up a bunch of Allen-Bradley parts
after they quit making them, but the guy makes me pay dearly for them.