Lake Shore 370 User Manual
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Lake Shore Model 370 AC Resistance Bridge User’s Manual3.7.2 Heater Output ConnectionBNC receptacle provides connections for the temperature control Heater Output. Signal is carried on the centerconductor and isolated heater output measurement common is on the outer conductor. The heater output has a maximumvoltage of ±10 V and can source 100 mA. Mating connector not included.3.7.3 Heater Output CablingCoaxial cable is recommended for the heater output, outside of the test Dewar. The center conductor should be largeenough to carry the maximum 100 mA current to prevent accidental burn out of the conductor. The outer conductor,heater output common, should be terminated at the bulk head connection or other conductive point on the Dewar toprovide the single connection between isolated heater output circuitry and the Earth grounded Dewar.3.7.4 Calculating Control Heater PowerThe control heater output current source on the Model 370 has eight ranges scaled in approximate one-third steps. Eachrange will source one-tenth the power of the next largest range with the highest range being 1 W. The current sourcevoltage compliance is 10 V and limits the heater resistance to 100 Ω when operating at full power. The restriction onheater resistance is not as severe on lower power ranges and higher heater resistance is often preferred. Heater output canbe displayed either as percent of full scale current for a range or as power in watts. Power calculation requires anaccurate heater resistance be entered into the instrument.Some examples of heater resistance and output range are given in Table 3-1. Maximum power for a range is the lower ofthe two P = I2R and P = V 2/R where P is heater power in watts, I is the maximum current for the range and V is thecompliance voltage of 10 V. Heater output resolution is lost any time the compliance voltage limits power.If a permanently installed heater has a very high resistance value, a resistor may have to be put in parallel with it on theoutside of the Dewar to increase the number of useful output ranges and maintain good control resolution.Table 3-1. Heater ResistanceHeater Resistance10 Ω 100 Ω 1000 Ω 5000 ΩOutputRange100 mA31.6 mA10 mA3.16 mA1 mA316 μA100 μA31.6 μA100 mW10 mW1 mW100 μW10 μW1 μW100 nW10 nW1 W*100 mW10 mW1 mW100 μW10μW1 μW100 nW100 mW †100 mW †100 mW10 mW1 mW100 μW10 μW1 μW20 mW †20 mW †20 mW †20 mW †5 mW500 μW50 μW5 μW* Indicates maximum power range and resistance† Indicates power limited by compliance voltage3.7.5 Heater Output ProtectionThe heater output hardware is internally protected against open circuit and short circuit conditions. The output shouldnever be connected to an external power source. There are additional firmware features in the Model 370 to help preventdamage to the load. The heater will automatically turn itself off if an open circuit condition is detected. The instrumentwill not allow the control setpoint to be larger than the setpoint temperature limit stored with the temperature responsecurve. The heater range can be limited so it is not accidentally set too high from the front panel.Installation 3-13 |
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