For the heating and cooling elements common in industrial systems, resistance is not a fixed quantity. These elements include such devices as positive-temperature-coefficient heaters and thermoelectric coolers. Their resistance can change more than 100% during operation, and the result is a change in power dissipation for elements receiving drive from a fixed voltage or current source. Worse, excessive power can damage the heating or the cooling element. Driving the element with a fixed and regulated power driver overcomes these problems (Figure 1). The circuit is analogous to a voltage or a current source but delivers fixed power levels that are independent of the load resistance. A feedback loop senses load power and automatically adjusts the output voltage to maintain the desired power level. The circuit measures the output current with a current sensor, IC2 and then determines the output power by multiplying the current by the voltage with the use of a four-quadrant analog-voltage multiplier, IC1 and IC3.
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| Figure 1. | This regulated power source delivers fixed power to a varying load. |
Because the multiplier's output is inverted, you add a unity-gain-inverting stage, IC4, to reinvert the output-power signal. Op amp IC5 then compares output power to the reference power, VPC input, and integrates any difference between them. The integrator provides an automatic power adjustment by increasing or reducing the output voltage until output power equals reference power. IC6 and Q1 form a voltage follower that drives the load. The following formula sets output power:
VPC = 10 × P × RSENSE,
where P is the desired output power in watts, RSENSE is the sensing resistor in ohms, and VPC is the reference-power input in volts. If, for example, the desired load power is 1 W, and RSENSE = 0.1 Ω, then set VPC to 1 V.
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| Figure 2. | Power delivery to the load is nearly independent of load resistance. |
Curves for load power versus load resistance for 0.5 and 1 W loads show that power delivered to the load changes less than ±7% for a change of 10,000% (two decades) in load resistance (Figure 2). If you define load regulation as the change in output power divided by the output power, then for a load change of 6 to 40 Ω at 1 W, the load regulation is ±2%. For the circuit to work properly, you must calibrate the analog multiplier as Motorola's MC1495 data sheet delineates. You repeat that calibration procedure below for convenience. Remove jumpers J1 and J2 for the calibration.
- X-input offset adjustment: Connect a 1-kHz, 5 V p-p sine wave to the Y input. Connect the X input to ground. Using an oscilloscope to monitor test point T1, adjust RX for an ac null (zero amplitude) in the sine wave.
- Y-input offset adjustment: Connect a 1-kHz, 5 V p-p sine wave to the X input. Connect the Y input to ground. Using an oscilloscope to monitor test point T1, adjust RY for an ac null (zero amplitude) in the sine wave.
- Output-offset adjustment: Connect the X and Y inputs to ground. Adjust ROUT until the dc voltage at T1 is 0 V dc.
- Scale-factor (gain) adjustment: Connect the X and Y inputs to 10 V dc. Adjust RSCALE until the dc voltage at T1 is 10 V dc. Repeat steps 1 through 4 as necessary.
