# Driving multiple coils

#### Wisehorn

Joined Feb 8, 2014
2
Hi all, I'm trying to play around with coils and I could use a bit of help

Does duty cycling DC electromagnets save on power like an LED, or do their inductive properties make it more efficient to supply a constant voltage?

On a similar note, if I wanted to fire off some coils sequentially using something like a demultiplexer with a BJT or MOSFET, would I have to have a separate transistor for every coil or is there a way of using one between them?

The coils will be quite small as what they are moving will be light, they will probably be a a few cm in diameter, 30-40 turns of thin wire, 0.5A at 12v. Does that sound like a sensible set of parameters? Could I simply buy a demux that will handle that?

Essentially I want to be able to create a simple animation using a matrix of electromagnets pulling things around, and the way I need to do it means its not quite right to use just one coil mounted on some mechanical fixture for example.
Thanks!

#### arouse1973

Joined Feb 11, 2014
4
When you first switch power to a coil the current is very small because the coil sort of pushes back on the current trying to go through it. After a time period the current starts to rise until a steady state occurs. At this point the current will be the applied voltage divided by the coils resistance. So running a coil from d.c will give you the largest magnetic field but the most current, they go hand in hand. Keeping the same voltage and coil and using pulses will reduce the overall current but also reduce the magnetic field.
Thirty to forty turn of what diameter? This makes a difference.

#### crutschow

Joined Mar 14, 2008
26,008
You are moving "light" with a small magnetic coil?

#### Wisehorn

Joined Feb 8, 2014
2
I was thinking of using around 0.5mm diameter which means I'll need to use a max of 0.5 amps by the looks of the charts.

Light as in not heavy
I want to build something similar to this you see:

I was thinking since I posted that I might use permanent rod magnets as the core, because I want to be able to hold the ferrofluid in place vertically as much as possible, and then modulate certain areas with a higher magnetic field using a microcontroller. Obviously they are using a different method in the video...