TECHNICAL TERMS
Duty Cycle

Duty cycle is the term used to deﬁne the heating
factors of a solenoid to prevent thermal damage. Dutycycle is normally expressed as a percentage and isof the “on time” plus the “oﬀ time” :ON Time
Duty Cycle(%)=
ON Time+OFF Time100

Duty cycle is important because a by-product of applying electrical power to the coil is heat. This excess heatcan thermally damage the coil. Thus the term duty cycle
is used to assure that only the safe maximum amount ofelectrical energy is applied to the solenoid.
As an example, if a solenoid is rated at 7 watts continuous
duty, this means that the solenoid coil can operatecontinuously (100% on time) with an input power levelof 7 watts with no thermal damage. This continuous
duty rating is arrived at by lab testing. However, ifincreased energy output is required, then this can beaccomplished by increasing the input power. However,this leads to increased temperature rise which can leadto permanent thermal damage in the coil. To prevent
this, the solenoid must be operated in an “on” and“oﬀ” mode. Therefore if the input power level isincreased to 14 watts (double the continuous duty rating)the solenoidʼs duty cycle must be dropped to 50% toprevent thermal damage. Additionally, the maximum“on” time needs to be considered.
Maximum “On” Time

When a solenoid is actuated, current is applied to thecoil. A by-product of this is temperature rise in the coil.
When a solenoid is energized continuously, heating of
the coil increases until a saturation level is reachedwhich is equal to the ambient heat radiation. When asolenoid is operated in an intermittent (on-oﬀ) manner,
the “on” time becomes critical if higher voltages (thuscurrents) are applied to the coil. These higher currents
cause higher coil temperatures which can exceed thecooling capacity of the solenoid through ambient
radiation. This heat rise can thermally destroy thesolenoid's coil. To avoid such a catastrophic failure,
there is a maximum “on” time, which is the longest
time the solenoid can be energized without thermaldamage. The maximum “on” time has been determinedfor each duty cycle, and is shown in the coil data.

2Ampere TurnAmpere turn describes the electro-motive force thatcauses a solenoid to actuate. Ampere turn is the product of the applied current to the coil, multiplied by thenumber of turns of magnet wire, as shown in the formula
below:
Ampere Turns= Voltage
Coil Resistance

Coil Number Turns

Energy output of a solenoid is proportional to the number of ampere turns. The larger the number of ampere
turns, the larger the mechanical energy output. However,
the ampere turns should not exceed 10% duty cycle, asabove this level may cause coil thermal damage. Also,
since all solenoids reach magnetic saturation at 10%
duty cycle, increasing ampere turns will not increaseenergy output.
Heat Sink

In order to reduce coil heating and to enable a larger
number of ampere turns to be employed, a heat sink is
attached to a solenoid. Coil data of rotary solenoids,
push-pull solenoids, and tubular solenoids are measured and tested with a standard heat sink attached.
the coil data charts, or a lower duty cycle, must be used
to avoid thermal damage to the coil. The coil data ofsmall push-pull solenoids, self-holding solenoids, open
frame solenoids and proportional/on-oﬀ solenoids, isshown without use of a heat sink.
Safety Factor

To reduce potential problems due to energy less throughsafety factor of 1.5 is recommended (required output1.5). When using open frame solenoids and self-holding
solenoids, the safety factor should be 1.3. It is possible
to use a smaller safety factor, but precise application
testing should be performed under all possible conditions.
Temperature Eﬀects on Performance

Coil and performance data shown in this catalog aremeasured at an ambient temperature of 20˚C. When avoltage is applied to a coil, the coil temperature rises,

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TECHNICAL TERMS
This temperature increase causes an increase in the coilresistance. As a result, the energy output will decrease.
The relationship between the coil temperature and thecoil resistance are shown in the following table.
Coil temp.
(˚C)-40-2002040
60
80
100120Resistancecoeﬃcient
0.7640.843
0.921
11.079
1.157
1.236
1.314
1.393

Ampere-Turnratio
1.309
1.186
1.086
10.927
0.864
0.809
0.761
0.718

To select the appropriate solenoid, it is necessary toapplication. To determine the value of force or torqueafter the coil temperature increase, the ampere turns
must be calculated from ambient temperature based on
the following formula :t= R −R (234.5+t )+t −t
Rt : temperature rise (deg.)
R : initial coil resistance (ohms)
R : ﬁnal coil resistance (ohms)
t : initial ambient temperature (˚C)
t : ﬁnal ambient temperature (˚C)

This circuit is used when a large energy output fromthe solenoid is required, but the space is limited. The
circuit applies an, extremely high voltage to a coil during
energizing (pick) and then applies a voltage to the coilto reduce potential coil heating problems (hold). The
most frequently used method is to use a simple timing
circuit that applies a high voltage signal for approximately
50 msec. and then drops oﬀ to a holding voltage. There
are also dual coil designs whereby one coil is used for“pick” and the other for the “hold”.
Power Supply

All of our standard solenoids are DC type. If the power
rectiﬁed type. Consult the factory for recommendations.
Relation Between Duty Cycle and Voltage

A relationship exists between duty cycle and voltage.
In the table below, the relationship between ampere
turn, power (watts), and voltage is shown. For example,
the voltage at 25% duty, cycle is twice the voltage at
100% duty cycle.
Duty cycle

100%

50%

25%

10%

Ampere-Turn122

10Voltage12210Watts

12410

How to Calculate Non-standard Duty Cycle andVoltage

Response TimeThe response time for solenoids is the time from whena voltage is applied to the coil until a given stroke has
been achieved. The data in this catalog uses a coil temperature of 20˚C as a reference point, without any load.
Response time will become longer when a load isapplied to the armature.
Arc Suppression

Pick and Hold Circuit

The solenoid coil is an inductive load which has a highinductance. As such, caution should be taken in theform of arc suppression when energizing and de-energizing the coil.

The voltage of a solenoid at a duty cycle not shown inthe coil data, is determined by the following formula :
E =E

100

=E

1100
100
E : Voltage at 100% duty cycle (VDC)
E : Voltage at (VDC): 100% duty cycle: duty cycle to be used

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