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Archive for July 23rd, 2014

Kenmore 158: NPN Transistor vs. Rectified 120 VAC

Eks found some heavy-duty ET227 NPN transistors in his heap and put them on the basement steps for me … months ago, because he knew I’d be needing them.

Mounting an ET227 on a massive CPU heatsink with thermal compound and wiring it in place of the failed MOSFET produces this lashup:

Kenmore 158 - ET227 FW drive

Kenmore 158 – ET227 FW drive

The base drive comes directly from a bench supply and the collector sees full-wave rectified 120 VAC from the isolated Variac. The maximum base current rating of 40 A at DC suggests it’ll be difficult to screw this one up. The rectifier bridge doesn’t dissipate enough power to warm up, even without a heatsink.

The SOA plot from the ET227 datasheet has the expected 1 kV and 100 A limits that you can’t actually reach under most conditions:

ET227 - Safe Operating Area

ET227 – Safe Operating Area

The Kenmore 158 motor has a DC resistance of about 50 Ω, so the locked-rotor current won’t be more than about 3 A. The motor current runs around 700 mA with a voltage drop across the transistor ranging from 20 V to 50 V at normal operating conditions, so it’s just barely within the DC SOA. So far, my efforts to kill it by stalling the motor have been unavailing; I have four spares and Eks has at least five more in his heap.

The ET227 has a 960 W (!) maximum dissipation on an ideal heatsink, so the piddly 35 W it might see here doesn’t amount to much. The heatsink should have a quiet demand-driven fan.

The operating current is offscale low along the left edge of the DC Current Gain plot, which suggests a DC gain under 10:

ET227 - DC Current Gain

ET227 – DC Current Gain

As it turned out, the gain was around 7, with 100 mA base drive producing 700 mA of collector current at VBE = 0.9 V, although that comes from the bench supply’s low-res meters. There being an exponential relation between the bench supply’s voltage output and the transistor’s base current, along with the motor’s square-law positive feedback, speed control was mmmm touchy.

So the challenge will be stuffing 100 mA into a 1 V base voltage, with much better resolution and much less ripple than the usual Arduino PWM output, from an isolated supply. Given the amount of power I’m willing to burn in the ET227, a few more watts of base drive won’t make a bit of difference.

Perhaps the best way to handle all the nonlinearities in the current control path will be an isolated current feedback monitor. Hello, Hall effect sensors … [sigh]

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