EDN Access — 07.18.96 Temperature programmer uses digital control

-July 18, 1996

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Design Ideas:July 18, 1996

Temperature programmer uses digital control

Jordan Dimitrov,
N Poushkarov Institute of Soil Science and Agroecology, Sofia, Bulgaria

The circuit in Figure 1 is useful for precise, repetitive temperature control of small-volume objects. It offers high resolution and effective isolation between the control lines and the power mains. It generates no RFI, has adequate speed, and provides a linear relationship between the digital-input code and the average thermal power supplied by the heater. The circuit works by generating a programmable number of triggering pulses to a triac connected in series with the heater. You can set the number of pulses between 0 and 99. The circuit in Figure 1 is configured for the 220V, 50-Hz European standard, but you can modify component values to accommodate other standards.

The bridge-configured diodes, D2 through D4; the optocoupler, OC1; and the gate, G1, produce a continuous series of triggering pulses with a 10-msec period and 0.8-msec pulse width. Pulses appear when the power-mains voltage crosses the zero level. The 4518 dual-decade counter, the two 4585 comparators, and gate G2 allow only a portion of the pulses to reach the control electrode of the triac. When the number of pulses counted is lower than the number of digital inputs I0 through I7, the gate transmits the triggering pulses. It then stops the pulses until the pulse number reaches 100; then, the cycle repeats.

The heater is thus turned on for some portion of every 1-sec interval and turned off for the remainder of the interval. Because thermal processes are slow, the temperature changes smoothly, with no noticeable ripple. Transistor Q2 amplifies the current from the output transistor of the second optocoupler, OC2. Both transistors connect to a transformer-free dc power supply configured from the zener diode and associated passive components. A computer must supply control words in packed-BCD format. For example, if 50% of the nominal heating power is required, then the computer must set high levels at inputs I1, I4, and I6.

The circuit can generate an arbitrary temperature by using a suitable set of control words stored in memory or calculated by the computer. The minimum interval between two successive loadings is 1 sec .(It's advisable to synchronize the interval with the power-mains frequency.) Longer intervals yield resolution inferior to the achievable 1% of maximum heating capacity.

The author gratefully acknowledges the support of the Bulgarian Ministry of Education, Science, and Technologies for its support in developing this circuit. (DI #1892)

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