Optimizing Solenoid Design

[Callipygous]

I am trying to fit a solenoid into a greeting card to animate parts of it. I would like it to run on the sorts of power sources that could also be squeezed into a greeting card, so coin cell batteries, or very thin flexible solar panels. To that end, I have been trying to learn about how to optimize the design of a solenoid to get the most bang for my wattage, and mostly running into more and more complications.

I am hoping someone can help me understand sort of practical rules of thumb for solenoid design. For example, in trying to keep my solenoid very small in diameter, I wondered if I could make up for it by making it longer. If I understand correctly, the equation for the maximum pull force has number of coils in the numerator and length of the solenoid in the denominator, so making a longer solenoid would be a wash.

Right now my current doesn't drop if I measure just the solar panel, vs the solar panel with my solenoid in the loop, so I wondered if maybe I should be using even thinner wire to fit more coils in. Then I found a source saying that as you decrease wire size you fit more coils in but also increase the resistance and decrease the current, so it ends up being a wash.

Pulling force is proportional to the number of coils, but also proportional the cross sectional area of the armature. So if I'm limited to 1/4" or so diameter, how do I balance armature size vs space available for more layers on the coil? Is that a wash too?

The only thing I've found that seems sorta solid is that I should wrap the whole thing in backing iron, but I cant seem to find an answer about how to size that either. Is iron foil sufficient, or do I really need more like 1/8" of iron to have an impact? Where's the point of diminishing returns?

[studiot]

9 minutes ago, Callipygous said:

I am trying to fit a solenoid into a greeting card to animate parts of it.

If you are going to put it or them into a card then you need to increase the turns not by the usual linear lineup of one alongside the other but in a flat spiral fashion.

In order to mechanically couple this to the effort use a mechanical membrane, rather than a central push rod.

[Callipygous]

6 hours ago, studiot said:

If you are going to put it or them into a card then you need to increase the turns not by the usual linear lineup of one alongside the other but in a flat spiral fashion.

In order to mechanically couple this to the effort use a mechanical membrane, rather than a central push rod.

I'm down to experiment with any such configurations.

Right now I am working with the usual linear arrangement and struggling to get enough force out of it. I can tell that its working in that it will pull a paperclip sideways if I dangle it like a pendulum, but it is not generating enough force for me to even feel it. So I think the first hurdle is going to be figuring out what my minimum power supply is going to have to be. Since I want it to be as small as possible, that means figuring out how to design the most efficient coil.

 

I was thinking last night with regard to wire size and number of coils. My current solar panel generates 3.2v and will output about 0.2mA. I think this means my coil will need about 16000ohms before my current is restricted by the coil rather than the solar panels output. Since hooking up the coil does not seem to decrease the current vs a short circuit, I think this means I can afford to switch to a smaller wire size to get more coils without losses due to resistance. Can anyone confirm if that logic checks out?

 

I am aware that this solar panel may be entirely too small to serve this purpose. I would like focus on the physics and solenoid design for now.

Edited Wednesday at 07:31 PM by Callipygous