# Building a solenoid need advice

• rjs123
In summary: He is using a disposable camera's flash capacitor to power the solenoid. He has a question for the class...what is the best "setup" for the solenoid to increase the magnetic force on the bb without increasing its current? Mike has found that the force on the BB is very small by comparison to a plunger. The reason being that the BB is making no significant impact on the length of the air gap. To improve on this, he starts with the existing solenoid concept, with steel wrapping around the outside. Then, he drills out a second hole opposite the one used by the

#### rjs123

I'm intending on creating a solenoid that will project a bb for a class project. I'm using a flash capacitor from a disposable camera as my current source. Here is my question...what is the best "setup" for the solenoid to increase the magnetic force on the bb without increasing its current?

I'm not a physics major...so correct me if I'm wrong...but will putting iron washers on the ends of the wrapping concentrate the force more?

like this...

[PLAIN]http://img193.imageshack.us/img193/1615/washerl.jpg [Broken] [/PLAIN]

Also will putting a layer of thin iron sheet metal inside the plastic tube aid in the force (see image below)?

[PLAIN]http://img269.imageshack.us/img269/4861/solenoid.jpg [Broken]

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My suggestion.
Make the bore of the solenoid just big enough for your BB.

The Magnetic field of Solenoid B = uNI.
So, you can Increase I and Decrease N to have the same effect.
Use, Thick wires to reduce resistance and hence increase I. You may have to trade this by reducing no of Turns N.
But that has its benifit that the Inductance will reduce and your capacitor can discharge quickly.

I don't think its wise to use the iron sheet/tube.
EXPERIMENT.

What makes a solenoid actuator pull in the plunger, is that the system is attempting to reach it's lowest energy state. Closing the flux path through the air represents more energy than is required to close the flux through mild steel. This is because the steel has a greater permiability than air. Hence the field flows more freely through the steel.

If you inspect a solenoid actuator, you'll find that the only steel inside the solenoid is the plunger, and a steel bracket is wrapped around the solenoid, leaving a hole for the plunger to move into the center. The force on the plunger increases as it eliminates the air gap within. Hence the plunger pulls weakly at first, but becomes much stronger as the last of the air is displaced.

In the case BB's, and other small parts, the pull is typically very small by comparison to a plunger. The reason being that the BB is making no significant impact on the length of the air gap.

To improve on this, start with the existing solenoid concept, with steel wrapping around the outside. Then, drill out a second hole opposite the one used by the plunger. It doesn't need to be as large as the plunger's hole, because we're, of course, working with the BB.

Now, to improve the force on the BB, place a steel tube inside the solenoid such that it contacts the steel of the housing on one end, and almost reaches the other end. Now, the gap remaining at one end should be the only substantial air gap in the path. The BB should be very strongly attracted to fill this gap.

The next trick is timing. You need the current through the coil to dissipate before the BB attempts to exit the gap, or the BB will lose momentum. The length of the gap and initial placement of the BB will aid with timing, as will the capacity / voltage of the cap and number of windings on the coil.

The volume you have for windings should be filled. However, you can change the gauge of wire to a large extent. If you are planing to use a high voltage, low value capacitor, more windings are beneficial. If you are using low voltage, high value capacitors, a heavier gauge wire is beneficial. To start, I would simply purchas an existing solenoid on the web. Look for actuator solenoids, and don't bother with ebay, because the prices are too high. 3-5\$ is about right.

Best Luck,

Mike

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## 1. How do I determine the number of turns for my solenoid?

The number of turns for a solenoid is determined by the desired magnetic field strength and the current flowing through the coil. A higher number of turns will result in a stronger magnetic field, but also requires a higher current. It is important to consider the power supply and the material of the wire when deciding on the number of turns.

## 2. What type of wire should I use for my solenoid?

The type of wire used for a solenoid should have a high electrical conductivity and low resistance to minimize heat dissipation and maximize the strength of the magnetic field. Copper wire is commonly used due to its high conductivity and availability in different gauges. However, other factors such as cost and operating temperature may also influence the choice of wire.

## 3. How do I calculate the inductance of a solenoid?

The inductance of a solenoid can be calculated using the formula L = (u * n^2 * A)/l, where u is the permeability of the core material, n is the number of turns, A is the cross-sectional area of the coil, and l is the length of the coil. However, for more accurate results, it is recommended to use an inductance meter or simulation software.

## 4. Can I use a ferromagnetic core for my solenoid?

Yes, a ferromagnetic core can be used to increase the strength of the magnetic field in a solenoid. However, the core material must have a high saturation point to avoid magnetic saturation, which can decrease the inductance and affect the performance of the solenoid.

## 5. How do I protect my solenoid from thermal overload?

To protect a solenoid from thermal overload, it is important to properly size the wire and power supply to avoid excessive heat dissipation. Additionally, using a thermal fuse or heat sink can help to dissipate any excess heat. Regular maintenance and monitoring of the solenoid's temperature can also prevent thermal overload.