Cambridge Technology Model 6220H Instruction Manual
8
3.2 Mounting Scheme
Special attention should be given to the mechanical integration of the scanner into the optical
system. The customer must provide an adequate path for conducting away heat generated by the
scanner body. The maximum temperature that the scanner body should be allowed to attain is
5 °C. This is below the temperature at which a person feels pain; thus the scanner should never
get too hot to touch! The XY mount should ideally have very low thermal resistance to the
ambient temperature. The heatsink must dissipate the full heat generation of both scanners in an
XY application while only allowing the scanners to rise to that 5 °C maximum case temperature.
An example of a mount that has adequate heatsinking is shown in Section 5.1. The exact
amount of heatsinking required directly depends on the scanner, the customer’s load, and the
customer’s application.
To calculate the necessary heatsinking for the 622 H, there are two approaches.
1. Worst case analysis: This assumes that there are two scanners bolted to a common heatsink
and both are dissipating their full power. At 6 watts each, that is 12 watts. We must also
assume that the ambient temperature is below 5 °C, the maximum case temperature the
scanner should ever be allowed to attain. Let’s assume the ambient temperature is 4 °C.
Then the heatsink must have a thermal resistance from the scanner body-to-ambient equal to
RTH = (5 – 4 )°C/ 12 watts = . 83°C/watt
Another representation is the thermal conductivity of the heatsink instead of its thermal
resistance. This is just the reciprocal of the thermal resistance or
GTH = 1/RTH = 1/( . 83°C/watt) = 12 watts/°C
2. If it is known that the scanners will not be run at their maximum power, then the actual
dissipation can be used. This will result in a smaller heatsink. The same rules apply, i.e. the
scanner body cannot go higher than 5 °C, but since they aren’t dissipating the full rms
power, the heatsink can be smaller.
To use this method, the maximum rms power must be known. The simplest way to do this is
to measure the maximum rms current and then calculate the power.
a. Run the XY system using the application’s command waveform (using an adequate
heatsink. If necessary, use method 1. above).
b. Measure the maximum rms current at the “Current Monitor” using a “true rms” voltmeter.
c. Square this rms current, multiply this by the coil resistance (2.4ohms), then finally
multiply this by 1.4. (Note: The 1.4 is a multiplier to account for the coil’s increase in
resistance with temperature. It has nothing to do with the relationship between rms and
peak voltages.) Thus,
