USING NITINOL WIRE
Nitinol wire is not easy to use. When it's all boiled
down, there are a number of restrictions that make it awkward to use.
The main problem is the current required to activate the product.
For the fine wire, the current cannot be higher than 30mA, for the thicker
wire, the current must be greater than 2 amps.
Achieving the exact current is not easy and the voltage you must apply depends on
1. The gauge of the wire,
2. The length of the wire.
There are two extra factors that need to be taken into account:
1. The stability of the supply voltage (the voltage of the battery may
fall as it gets older)
2. The voltage drop across the activating device (transistor, FET relay
The standard circuit for activating the wire
is shown below:
Determining the voltage for a particular length of wire or the most suitable
length of wire for a particular voltage, can be obtained from the following
cm to inches, use the conversion table below.
This calculator takes into account the MAXIMUM current that should be passed though
the wire for long life. The table works in INCHES. To convert
Conversion Table: Convert
inches to cm
or cm to
To prevent Nitinol wire being damaged by excessive current, a constant
current circuit can be used to power the wire.
This is a very clever idea as the length of the wire is not important and
you can concentrate on:
1. The amount of movement you require for the application (the length of the
2. The number of cycles of movement
The circuit below uses an LM 317 in a constant-current arrangement and the value
of resistor R is selected according to the gauge of wire used.
A 7805 can be used in a similar arrangement but the value of R must be changed
as the 7805 detects 5v between the "adj" and "out"
terminals while the LM317 detects a voltage of 1.2v between these terminals. (The
7805 has a different pinout).
for circuit above:
LM 317 is the common name for a 3-terminal
adjustable regulator. It comes in three packages:
The T0-3 package must be
isolated from earth.
1.2 to 37v
1.2 to 37v
1.2 to 37v
The LM 317T is rated at 1.5 amp and this means the output
voltage will be smooth and steady for a current up to 1.5 amp
(smooth voltage is not needed for Nitinol
You will notice from the table on Page 1,
the activation time for the wire is 1 second and the off time
is from 1sec to 13 seconds, depending on the gauge of the
wire. This gives the duty cycle a maximum of 50%, down to
less than 10%. This duty cycle also applies to the regulator
supplying the current so that any heat produced by the
regulator will have time
to dissipate into the air via the heat sink when the circuit
is not activated. In most cases the regulator will not get
very hot and not need a heatsink.
you are supplying the circuit from a DC supply, the input
voltage to the regulator needs to be
6.1v higher than the voltage required by the Nitinol wire. You
can find out the voltage needed by the wire by using the
The 6v1 additional voltage is made up of 1.4v drop across the
bridge (not needed if supplying DC to the circuit), 3.5v across
the regulator (to prevent it "dropping out") and
1.2v across R.
If you are supplying the circuit with AC, the value of AC
needs to be determined from the following:
An AC transformer is rated at an output voltage at FULL LOAD.
A 12v AC transformer, for example will produce 15v to 17v
AC on no-load and drop to 12-14v AC on full load. When
rectified, this will produce about 16v DC after the bridge. In
other words, this type of transformer will be suitable for a
length of Nitinol wire requiring up to a 12v supply.
You can use the 12v AC transformer (or a 12v DC
supply) for any length of wire requiring up to 12v.
This is where you really have to know your electrics!
You can use any length of any gauge wire that requires up to
12v and/or up to 1.5 amp. The LM 317T does all the adjusting for
Providing you fit the correct value resistor for R (according
the the current requirement of the wire you are using), you cannot
damage the Nitinol wire. You can also use almost any power
transformer from 12v AC to 18v AC. Again, the LM 317T will adjust the current to the
correct value. The only problem with using a power
transformer with a voltage rating higher than 12v AC is the
additional heat generated by the regulator chip (the LM 317T). However,
as we mentioned above, the duty cycle for Nitinol wire is very
small and very little heatsinking will be needed.
THINGS TO COME
don't want to upset the applecart but there are new
developments in the wind using plastic (ceramic) substrates
with metallised terminations to produce deflective
movement. These items are still in the experimental stage
but require very little current. The
life-span seems to be almost infinite and the
can be in the kHz and higher. All the limitations of Nitinol
will be answered!
It seems amazing but you can understand, it's only a
development away, just like the LED, super-bright LED and
the glow-panel. New technology is constantly arriving and
sweeping away the old.
For an enormous website on the development of this new
product, click HERE. Then
Polymers (EAP). It is also being turned into Piezo
motors and a surface wiper, as shown below:
Piezo motor simulation
Surface Wiper using EAP technology