Design Idea (DI) contributors have recently explored various possibilities for ON/OFF power control using just a momentary contact “shiny modern push-button,” many of which build off of Nick Cornford’s “To press on or hold off? This does both.”

These ideas are interesting, and they’ve suggested a different notion. Figure 1 takes the one-button power control concept a bit further. It uses its button to provide six bits of resolution to a kilowatt of variable AC power, addressing adjustable applications like heating blankets, lamp dimming, motor speed, etc. I like it because, well, shouldn’t there be a bit (or even six) more to life than just ON/OFF?

Figure 1 Variable AC power control with a simple pushbutton. When S1 is pushed, counter U1 ramps through the 64 DAC codes in a 2¹⁰ / 120Hz = 8.5-second cycle and stops on any selected power setting when S1 is released.

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Power control method

The power control method employed in Figure 1 is variable AC phase angle conduction via thyristor Q3. It’s wired in the traditional way except that the 6-bit DAC driven by CMOS counter U1 fills in for the usual phase adjustment pot. Because, unlike Q3, the DAC circuitry isn’t bidirectional, the D1-4 rectifier is needed to feed it DC and keep it working and counting through 60-Hz alternations.

Full power Q3 efficiency is around 99%, but its maximum junction temperature rating is only 110 °C. Adequate heatsinking of Q2 will therefore be necessary if output loads greater than 200 W are expected.

Adjusting U1 to the desired power setting is accomplished by pushing and holding switch S1. This connects the 120-Hz full-wave rectifier signal from the D1-D4 bridge to the Schmitt trigger formed by R2, R3, and U1’s internal non-inverting q0 input buffer.

The subsequent division of the 120 Hz signal by U1’s ripple divider chain makes flip-flop q5 toggle at 120/2⁵ = 3.75 Hz, q6 at 120/2⁶ = 1.875 Hz, and so forth down to q10 at 120/2¹⁰ = 0.117 Hz. This gives a ramp time of 8.5 seconds for the full 0 (= full OFF) to 63 (= full ON) code cycle. Meanwhile, digital integration of the raw signal from switch S1 by U1’s counters suppresses switch contact bounce.

When the desired power setting (lamp brightness, motor speed, etc.) is reached, release the button, i.e., just let go! However, due to the fairly rapid toggle rate of the lower counter stages, a bit of practice may be required to accurately hit a target setting on the first try.

DAC topology

The DAC topology is straightforward. Just six (R4 through R9) binary-weighted resistors make up a summing network that produces a 0-V to 15-V input to the Q1 Q2 complementary current-mode output buffer.

Q1 provides nominal compensation for Q2’s Vbe offset and tempco, as well as sufficient current gain to allow use of multi-megohm resistances in the summation network. This is important because operating power for the DAC is basically stolen from Q3’s phase control signal.

This (as you probably noticed), nicely avoids the need for a separate power supply, but it provides only microamps of current for U1 and friends. So, a power-thrifty topology was definitely needed.

DAC reference Z1 is remarkably content with its meager share of this starvation diet. It maintains a usefully constant regulation despite only a single-digit microamp bias, which is impressive for an 11-cent (in singles) part. Meanwhile, U1 daintily sips only tens of nanoamps.

R11 and C3 provide an initial reset to OFF when power is first applied.

At this point, you might reasonably ask: Is this scheme any better than a simple pot with a twistable knob? Well, don’t forget the “shiny modern push-button” factor.

Stephen Woodward’s relationship with EDN’s DI column goes back quite a long way. Over 100 submissions have been accepted since his first contribution back in 1974.

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