🚀 go-pugleaf

3T39 wrote:
> 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: rubbishrat@hotmail.com 

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