A hot-surface warning light is a safety device, so it must operate reliably whenever the surface is hot.
Heat, however, complicates the light’s power source, because high-temperature power wiring is difficult to install and not really flexible. Moreover, high-temperature batter ies (hard to procure) need frequent maintenance. A better approach is a selfpowered warning device taking its power from the heated environment.
Semiconductor junctions can convert electricity to heat, and vice versa. Passing current through a thermoelectric cooler (TEC), for instance, heats one side of the junction and cools the other through a phenomenon known as the Peltier effect. Conversely, the Seebeck effect lets a thermoelectric generator (TEG) create a voltage across a junction (and an electric current if the circuit is closed) when one side is forced hot and the other remains cold.
TECs and TEGs are essentially the same. Being reversible from a thermoelectric point of view, TEGs can perform as TECs and vice-versa. Both consist of a large number of pn junctions, connected electrically in series and thermally in parallel, forming a thin semiconductor wafer bonded between thin ceramic wafers to enhance heat transfer. TEGs are made with higher-temperature materials (up to 200°C), and are optimized for electricity generation. On the other hand, the maximum temperature to which TECs can be exposed is about 100°C.
The voltage output from a TEG depends on both the temperature difference applied to its ceramic sides and on the number of internal junctions in series. This voltage is small when the temperature difference is small. Most circuits require higher operating voltages than a TEG can produce, but a step-up switching regulator with low start-up voltage can boost the TEG output to a useful level.
The accompanying circuit employs the principles mentioned above. With the TEG attached to the monitored surface, the LED flashes whenever the surface temperature exceeds a threshold: When the monitored surface is hot enough to produce a temperature differential of about 20°C, the super-bright LED begins to flash at 2 Hz and keeps flashing until the differential subsides to 10°C. There’s no external power needed because a step-up switching regulator (MAX1676) boosts the TEG output to about 5 V.
The flashing oscillator is formed by the 2N3906 transistor and a precision comparator inside the MAX1676 (accessible via LBI and LBO). The LBI/LBO terminals are connected in a positive feedback loop that creates hysteresis at the input (where the 10-μf capacitor connects to common).
Two resistors connected at the inverted-LBO output (available at the 2N3906 collector), one to common and the other to the 10°F capacitor, add a negative feedback loop with asymmetric charge/ discharge paths, as required for the 2 Hz, 50 msec-ON flashing pulses. The pnp transistor also drives 30 mA through the superbright LED. The low operating voltage (about 5 V) limits the choice of LED color to red, green, yellow, or orange. The circuit can’t drive white or blue LEDs.
You should mechanically mount the TEG to maximize the temperature difference between its surfaces: A heat sink transfers heat away from one side, keeping it cool, and the other side tracks the monitored surface to which it is thermally attached. The TEG output voltage is proportional to the temperature difference across it, so the circuit begins to flash when it sees a sufficient temperature differential at the TEG. The accompanying scope snapshot shows the 2N3906-collector waveform present while the LED is flashing.
The maximum temperature of operation, usually about 200°C, is set by the TEG’s absolute maximum ratings. If the circuit is built on top of or close to the TEG, all circuit components must have adequate ratings for operating temperature. If the circuit mounts at some distance from the TEG, the connecting wires must be rated for high temperature operation. A complete datasheet and other information on the MAX1676 converter can be found at www.maxim-ic.com.
When the monitored surface is hot, this step-up regulator (MAX1676) boosts the TEG’s low output voltage to a level suitable for flashing the LED.
When the monitored surface heats up, a heat sink minimizes temperature rise on the other side of the TEG. One side of the TEG must be in intimate thermal contact with the heat sink and the other with the hot surface. Thermal-contact compound and flat surfaces on both sides of the TEG ensure intimate contacts.
— Alredo H. Saab and Bich Pham, Maxim Integrated Products Inc.
This waveform appears at the 2N3906 collector.