Lake Shore 480, Руководство пользователя, Страница: 21 / 132

Lake Shore Model 480 Fluxmeter User’s Manual
Magnetic Measurement Overview 2-5
The Magnetizer Pulse (Continued)
The time “t
p
” to reach peak magnetic field “B
p
” is considered the rise time of the pulse. These are
two important parameters to consider when selecting or designing the sense coil for the 480.
2.1.7.2 Coil Output Voltage Limits
Because of slew rate requirements and safety considerations, the maximum voltage at the coil
output should be limited to 60 volts. The Model 480 Fluxmeter is capable of measuring the fastest
of magnetizer pulses, so long as the 60 volt limit is not exceeded. Therefore, the area turns of the
coil must be matched to the peak field and rise time of the magnetic field pulse.
The equation for calculating the coil voltage in CGS units is:
where V = volts, A = cm 2 , B = gauss, N = number of coil turns, and t = seconds.
The equation for calculating the coil voltage in SI units is:
where V = volts, A = meters
2
, B = tesla, N = number of coil turns, and t = seconds.
2.1.7.3 Calculation of Minimum Rise Time
What is the fastest pulse allowable? When the area·turns (NA) of the coil and the desired peak field
(B
p
) are known, the above equations can be used to calculate the minimum rise time.
t
p
= NAB/V (SI units)
Calculations of minimum rise times are given for two standard Lake Shore probes.
V = 60 volts and cm 2 × 10 -4 = meters 2
NA = 30 cm 2
If B
p
= 3 tesla, t
p
> 150

s
If B
p
= 5 tesla, t
p
> 250

s
If B
p
= 7 tesla, t
p
> 350

s
NA = 100 cm 2
If B
p
= 3 tesla, t
p
> 500

s
If B
p
= 5 tesla, t
p
> 833

s
If B
p
= 7 tesla, t
p
> 1200

s (1.2 ms)
2.1.7.4 Calculation Of Area·Turns
Often the user will make his own coil to be used with a specific magnetizing fixture. The maximum
area

turns (NA) needs to be calculated, to ensure the 60 volt input limit is not exceeded. The
equation below can be used.
NA

Vt
p
/B
p
meters
2
(SI units)
For example, if the rise time t
p
is 5

s and the peak field B
p
is 3 tesla, then the following is a
calculation of the maximum area

turns (NA) to ensure the coil voltage will not exceed 60 volts.
NA

(60V) (5 × 10 -6 s) / 3 T = 1 × 10 -4 meter 2 = 1 cm 2