5-V ovens (some assembly required)—part 1

The ovens in this two-part Design Idea (DI) can’t even warm that leftover half-slice of pizza, let alone cook dinner, but they can keep critical components at a constant temperature. In the first part, we’ll look at a purely analog approach, saving something PWM-based for the second.

Perhaps you want to build a really wide-range LF oscillator with a logarithmic sweep, using no more than a resistor, an op-amp, and a diode for the log element. That diode needs to be held at a constant temperature for accuracy and stability: it needs ovening (if there is such a verb).

Wow the engineering world with your unique design: Design Ideas Submission Guide

I made such a device some years ago, and was reminded of it when spotting how a bead thermistor fitted rather nicely into the hole in a TO-220’s tab. (Cluttered workbenches can sometimes trigger interesting cross-fertilizations.) Now, can we turn that tab into a useful temperature-stabilized hotplate, suitable for mounting heat-sensitive components on? Ground rules: aim at a rather arbitrary 50°C, make the circuitry as simple as possible, use a 5-V supply, and keep the consumption low.

This is a practical exploration of how to use a transistor, a thermistor, and as little else as possible to get the job done. It lacks the elegance and sophistication of designs that use a transistor as both a sensor and a source of heat, but it is simpler.

Figure 1 shows the schematic of a simple version needing only a 2-wire connection, along with two photos indicating its construction. It was slimmed down from a more complex but less successful initial idea, which we’ll look at later.

Figure 1 A simple oven circuit, heated by both R2 and Q2. The NTC thermistor Th1 provides feedback, the set point being determined by R1. Note how critical components are thermally tied together as they are all built onto the TO-220 package, as shown in the photos. Also note the fine lead wires to reduce heat loss once the assembly is heat-insulated.

Both R2 and Q2 can contribute to heating. On a cold start (literally) Th1’s resistance is high so that the Darlington pair Q1 and Q2 has enough base voltage to saturate it, with (most of) the rail voltage across R2. As the assembly heats up, Th1’s resistance drops, reducing the drive to Q1/2. The rail now appears across both R2 and Q2, with the latter taking over as the main, though now reduced, source of heat. This gives a degree of proportional control, reducing the drive as the set-point is approached. That base drive depends not only on the ratio of R2 to Th1 but also on Q1/2’s effective V_BE, which needs to be temperature-stabilized—as indeed it is. Consumption varies from ~90 mA when cold to ~30 mA when stable.

Setting and measuring the temperature

R1 sets the stabilization temperature, the target being 50°C. Experimentally, 12k worked best, giving a stable hotplate temperature of 49.6°C for an ambient of 19.5°C. Cooling the surroundings to -0.5°C left the hotplate at 48.8°C, so that the hotplate temperature falls by 0.04°C for each degree drop outside. Better thermal insulation would have reduced that.

The measuring probe was a 10k thermistor equipped with fine wires and stuck to the hotplate with thermal paste, the module being wrapped in ~12 mm of foam—and we’ll come back to that. Thermal paste and heat shrink could have been used for the main assembly but dabs of epoxy worked well and kept the hotplate surface flat. Metal-loaded, high-temperature epoxy conducts heat several times better than the plain-vanilla variety while still being an electrical insulator, though that may make little difference given reasonable physical contact.

Other resistors and transistors

R2 is fairly critical. A higher value than 47R heats up slower than is necessary, while a lower one does so too fast, leading to the temperature overshooting because of the limited proportional control. Experiments showed that 47R was close to optimal, with minimal overshoot and thus the fastest stabilization time. The hotplate temperature settles to within a degree in around two minutes and is almost spot-on after three minutes.

Neither Q1 nor Q2 is critical, but the E-line package of a ZTX300 (for example) fits better than a TO-92 would. But why not use an integrated Darlington like the TIP122? Alas, such devices incorporate base–emitter resistors, nominally 10k and 150R, which load Th1 unpredictably. Trying one picked at random showed that R1 needed to be ~7k8 for a set-point of 50°C.

Similarly, this also works with Q1/2 replaced by a MOSFET, with R1’s value now depending on the gate threshold; 3k9 was close for a BUK553. BJTs are far more predictable: build this as drawn, and it should be within a degree, with Q1/2’s V_BE settling at ~1.18 V; use a random MOSFET, and it could be anywhere.

Access all areas

The next variant, shown in Figure 2, is electrically similar but provides access to useful circuit nodes to help monitor its performance. It was also easier to experiment with.

Figure 2 While electrically the same as Figure 1, this brings out most circuit nodes to help with experimentation and monitoring, including the LEDs on “pin 3”.

Now we can see what we’re doing! The LEDs give a simple status indication, the green one lighting when it’s close to the set-point rather than fully stable. Figure 3 shows the effect, along with traces for Q1/2’s Vcc—allowing us to read the current in the transistors and R2—and the hotplate temperature. The latter is accurate, but the voltage and current scales are less so because they assume a precise 5-V supply and a 50-Ω load rather than the measured 4.94 V and 47Ω plus stray resistance. This module stabilized at ~50.6°C.

Figure 3  Measurements taken from Figure 2’s circuit for about three minutes after a cold start.

So much for the basic circuit. Now, it needs thermal insulation to keep the heat in, a block of foam being the obvious choice. But foams have widely differing thermal conductivities. Expanded polystyrene or polyethylene will work, but the foamed polyisocyanurate or similar used for wall insulation panels is around twice as good—and offcuts are often freely available from builders’ skips/dumpsters! Figure 4 shows the module from Figure 2 mounted on/in a block of it, with at least 10 mm of foam around any part of the circuit module.

Wikipedia has an illuminating plot of the thermal conductivities of many materials, including our foams and epoxies. The article of which it is a part has a lot of useful background, too.

Figure 4 The module from Figure 2 mounted on a block of foam. The intermediate connecting wires are meandered across its surface to minimize heat loss. Note the diode, typical of a component needing stabilization, stuck to the hotplate, ready for its new connections to be treated similarly.

The fine lead wires—0.15 mm diameter, as used with wiring pencils—are meandered over the surface to lengthen the thermal paths. Copper has a thermal conductivity some 19,000 times greater than the foam: 384 W/m·K vs ~0.02 W/m·K. In very crude terms, for a given thermal path length and temperature gradient, a single, short 0.11-mm-diameter copper wire will leak heat at about the same rate as the entire surface area of our foam block (~6000 mm²). Ironically, perfect insulation would be bad, as the innards could never cool to recover from an overshoot. This build took 620 seconds to cool by 63% of the way to ambient.

Hot stuff

Disconnecting Th1 in Figure 2’s circuit let the module heat up to the max while still allowing monitoring—or would have done, had I not chickened out when its resistance dropped to 720 Ω, for just over 100°C. (The epoxy was rated to 110°C.) That was with the full insulation; in free air, it struggled to reach 70°C—the rating for other components.

One subtle problem is the inevitable mismatch between the sensing thermistor and the target device, as analyzed in a Stephen Woodward DI, which also implies that the position of the target on the hotplate will affect its actual temperature. We’ll ignore that for the moment, because we’re more interested in constancy than precision, but will return to it in Part 2.

Finishing at the starting point

The foregoing circuits were actually simplifications of my starting point, which is shown in Figure 5. When the temperature is stable at ~50°C, point A is at half-rail. R3 is chosen so that U1’s output will turn Q1/2 on just enough to maintain that. However, while the extra gain improves the temperature regulation, it also causes some overshoot. R3 or R2 must be trimmed to set the temperature: fiddly, and not really designable. R3 was calculated at 4k12 but needed ~5k6 in reality. That’s why I gave up on this approach.

Figure 5 The original circuit that suffered from overshoot. The LEDs give a too-high/too-low temperature indication.

The long-tailed pair of Darlingtons (Q3, Q4) sense the difference between the thermistor voltage—half the rail when stable, as noted—and a half-rail reference, so that the red LED will be on when the temperature is low, the green one lighting while it’s high, with both on at the stable point. Full-red to full-green takes ~300 mV differential, or ~±3°C. This works but gives no better indication than the LEDs in Figure 2. (The low-power Darlingtons used seem to omit those extra, internal resistors. Q1/2 could now be replaced by that TIP122, as it’s driven by a low-impedance source. R4 is purely to protect against current surges.)

Figure 6 plots its performance when starting from cold, showing the overshoot and recovery. Compare this with Figure 3.

Figure 6 The start-up performance of Figure 5’s circuit.

If I were building something similar in any quantity, I wouldn’t do it like this: SMDs and a flexible circuit would be much cleaner. For example, a 2512 power resistor for R2 (or R5 in Figure 5), pressed flat, with some insulation, against the power transistor’s tab would probably be ideal.

In Part 2, we’ll see how even a simple PWM-based circuit can give better proportional control and hence generally better performance. The bad news: we may eventually abandon the TO-220 tab in favor of another way of assembling our hotplate.

Related Content

• Fixing a fundamental flaw of self-sensing transistor thermostats
• Self-heated ∆Vbe transistor thermostat needs no calibration
• Take-back-half thermostat uses ∆Vbe transistor sensor
• Dropping a PRTD into a thermistor slot—impossible?

The post 5-V ovens (some assembly required)—part 1 appeared first on EDN.