Relationship between volts and a coil?

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Just a quick question.
Please I need clarification on this:
Would a voltage of say 12volts running through a coil of 10 uh be lower
after the coil?.
Does a coil infact drop the voltage?
Thanks
Al


Re: Relationship between volts and a coil?


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Every coil has a resistance, which is what drops the voltage.
You can measure that with a DC meter and work out the drop.

Apart from that, the output voltage will change whenever the
current changes, so that for example, when you first connect
the coil to a resistive load, the current is zero and the
voltage on the resister is zero - all the 12V is across the
coil. The current (and hence voltage on the resister) climbs
as the magnetic field increases. The bigger the coil, or the
smaller the resister, the slower the increase (and the slower
the switch-on). So we talk about the "L/R" time constant.

When you remove the resister, the voltage might well jump to
hundreds or thousands of volts, whatever it takes just so the
current doesn't have to change instantaneously. The energy in
this pulse comes from the stored energy in the magnetic field,
and is what destroys badly-designed switching circuits. It's
also what drives your car's spark plugs.

Clifford Heath.

Re: Relationship between volts and a coil?


Well,
I am getting 12 volts DC before and the same after the coil; No voltage
change;
Does this mean something is wrong?
I am testing as many point around the circuits on a TV TEAK M687 and I
suspect this coil to be faulty, however it does have a resistance of
1.1 ohms
Al


Re: Relationship between volts and a coil?



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Al,

An inductor (coil) is almost pure inductance (hence the name) & very little
resistance, thus on dc it presents a low resistance (as you've indicated).
Thus if there is minimal load there will be minimal voltage drop across it
(what's the dc current?).  What is the application the inductor is being
used for?

Also, are you sure its 10uH and not 10 mH.

Regards



Kevin




Re: Relationship between volts and a coil?


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I doubt the coil is your problem unless it looks burned.

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Re: Relationship between volts and a coil?


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Assuming the coil is powered by DC, then whatever is downstream of the coil is
not passing any (or very little) current.  What is in the circuit after the
coil?

--
James T. White



Re: Relationship between volts and a coil?



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That's normal, the coil is fine. It's probably just there to filter out high
frequency noise.



Re: Relationship between volts and a coil?




James Sweet wrote:
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Many thanks for all the replies; troubleshooting for coils will be much
easier now.
Al


Re: Relationship between volts and a coil?


That would be resist O r then ...


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Re: Relationship between volts and a coil?


Hello, alitonto!
You wrote  on 24 Jul 2005 20:48:48 -0700:

 a> Just a quick question.
 a> Please I need clarification on this:
 a> Would a voltage of say 12volts running through a coil of 10 uh be lower
 a> after the coil?.
 a> Does a coil infact drop the voltage?
 a> Thanks
 a> Al


You should really study the difference in behaviour between DC and AC to
properly understand the effect you are trying to measure. As previous
posters have pointed out the DC resistance of such a coil is so low that a
simple multimeter is unlikely to provide a sensible reading as it is using
DC. The resistance you have "measured" is likely to mostly be the contact
resistance of the probes. To answer your question though, the DC volts
before and after the coil will be pretty well the same. The coil only
inteferes with the voltage for a very brief time, as the magnetic field
builds up around the coil. An inductive component like this, is an AC
animal.

With best regards, 3T39.  E-mail: snipped-for-privacy@hotmail.com



Re: Relationship between volts and a coil?


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Its also worth mentioning that a relay coil does the exact opposite - DC
resistance is so high (many, many turns of very fine wire) it limits the
coil current to, say, 30mA or so (depends of course on the relay). kOhms
are common with relays.

If the resistance of a coil (air core, so cant saturate) were zero, then
applying a DC voltage across it would cause the current to ramp up from
zero to infinity, slope dI/dt = Vdc/Lcoil. It would also take an
infinitely long time to reach infinite amps.

In practise there is always some R, limiting the current, although
superconducting coils do exist and are used for energy storage - bung
many, many amps into a superconducting coil, then short the 2 ends
together and the current flows in a circle without decaying - google
SMES (Superconducting Magnetic Energy Storage).

A fun thing to do is get a voltage-mode smps and heat the core up to the
curie point, whence the core effectively disappears, inductance
skyrockets and *bang* the smps self-destructs.

Cheers
Terry




Re: Relationship between volts and a coil?


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Visions of a huge SMPS, Buster holding a flame thrower and a smirking
Adam behind the blast screen ....

Re: Relationship between volts and a coil?


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Hows this then:

A company I used to work for got a UL rep to come out from USA to
perform tests on a range of large motor controllers (giant smps). One
test involves packing the product with cotton wool, shorting one of the
2 series DC bus caps (actually 18 2200uF caps in parallel), and turning
it on - no flames can be emitted (hence the cotton wool).

We argued this was not a good idea at 400kW, 2m from a 2MVA supply
transformer, but the UL guy insisted. Best not to think about the
250,000A fault current :)

When it came time to perform the test, he found himself about 10m closer
to the drive than everyone else, and hastily backed away. Perhaps
un-surprisingly, the result was a colossal explosion, flames shooting
out several feet (caused by plasma/molten metal setting the cotton wool
on fire), and the total destruction of the drive.

He then spoke to his manager, and UL agreed it was not a particularly
meaningful test at high power.

Cheers
Terry

Re: Relationship between volts and a coil?


@news.xtra.co.nz:

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When they were 'charging' the new 700 MHz NMR superconducting magnet
[slowly running the current up], the magnet 'quenched'. Boiled off 2000 Ltr
of liquid He in just a few seconds. The fog set off the fire alarm and
cleared the building.

The magnet wasn't damaged.



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bz        73 de N5BZ k

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