Relationship between volts and a coil?

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.

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

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

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

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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

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?

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James T. White

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

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

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

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

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

That would be resist O r then ...

Terry Given wrote in news:a5CFe.3788$PL5.352331 @news.xtra.co.nz:

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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