9V Battery Current

Try connecting the three windings in series; see if you can make a meaningful comparison of the relative strengths (series VS parallel).

It places an upper limit on how much current the battery might possibly be putting into the electromagnet, or into any load. And it's interesting to me, since I now know the fault current envelope I might see if, say, I had a battery-powered gadget and a tantalum capacitor failed or an IC latched up or something.

And it was obvious that this particular measurement could not have been dangerous, even before I knew the results.

Roger the combat boots.

John

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Since it's Ampere-Turns that's doing the work and the number of turns
stays the same, if the current drops to one-tenth of what it was, so
will the strength of the field, so it's ten times less.

Um, I think it's 3.3 times less.

But the big picture is that the amount of field you can sustain longterm depends on how well you can cool the coil. So for a fixed copper cross-section and surface area, there's some maximum number of watts you can pump into the copper before you toast it, and those watts set the field strength. So series/parallel or wire size doesn't change the basic limit on field strength so long as the power supply can be adjusted.

John

Nice equations.

But it's not the current in the battery that makes magnetic field, it's the current in the windings. Series, battery current equals winding current. Parallel, current in each winding = 1/3 battery current.

Think about it this way: we have three coils. Parallel, there's 10 volts across any given coil. Series, there's 3.3 volts across each coil. So the current in any bit of wire is 3.3 times higher in the parallel case. And the total ampere-turns product makes magnetic flux.

The windings don't know that they're in series or in parallel; they just see current and make flux.

John

I think I can guess that one - 3 3.3 ohm coils in parallel makes one ohm, so the current is around 12A. In series would make about 10 ohm, with a current of around 1.2A. So the strength would be about 9 times less.

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Oops: that makes the field ratio 3:1.

John

Isn't it like this?

Rp=R/3 Rs=3R

Ip=V/(R/3)=3V/R Is=V/3R

Ip/Is=(3V/R)/(V/3R)=9

yes, or as long as you can split the coil into as many paralleled windings as you want.

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No, it's times the number of turns. In series there are 3 times as many turns, so it's like John Larkin said. The term "ampere-turns" springs to mind. :-)

And how do you get 9 when you divide 12 by 1.2?

Thanks, Rich

Do *not* guess! Your "logic" is incomplete, and thus inaccurate. I suggest you make the measurement; you will find the series configuration to be a lot stronger than your "estimate".

Your logic is incomplete and inaccurate.

AT LAST! You got it correct; ampere turns. Parallel = if each coil has 100 turns, then the parallel combination has

100 turns (looks like fatter wire, folks), and one then has 1200 AmpereTurns. Series = The turns add up (assuming the connection polarity is correct) for 300 turns, and one then has 360 AmpereTurns.

--- Your logic (or your arithmetic) is incomplete and inaccurate.

1200AT/360AT = 3.333, but:

12V 12V | | +-----------+------------+ | | | | [10T] 1R [10t] 1R [10T] 1R [10T] 1R | | | | [10T] 1R +-----------+------------+ | | [10T] 1R 0V | 0V

4pi N I Since B = K ---------, l

If we assume the same diameter and length and they're both wound over the same core, then K, 4pi, and l drop out and we're left with

B = N I

Where N is the number of turns per winding and N is the current flowing in the winding.

If we assume 1 ohm coils for convenience, then for each winding in the parallel case:

E 12V I = --- = ----- = 12 amperes R 1R

and

Bp = N I = 10 turns * 12A = 120 AT

Since there are three windings with the same diameters and lengths and they're wound over each other the fields will add, for a total of

Bp = 3 N I = 3 * 120AT = 360AT.

For the series case we'll still have the same length and diameter, but the current through each of the coils will be

E 12V I = --- = ----- = 4A R 3R Then, for each coil,

Bs = N I = 10 turns * 4A = 40AT

and for all the coils.

Bs = 3 N I = 3 * 40AT = 120 AT

So, since Bp = 360AT and Bs = 120AT, the magnetic induction for the parallel case will three (not 3.333) times greater than for the series case, just like Larkin said earlier.

-- John Fields

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Yes, you're right. Thank you.