Friday, September 16, 2011

A very noisy table lamp

Using the ATX switching power supply I was testing a circuit consisting of a PIC12F1822 and SN75176 transceiver, plus a couple of LEDs. The circuit has 10uF filter caps and the MCU has a 0.1uF bypass cap. Much to my chagrin the fluorescent lamp on my bench was affecting the circuit--the MCU would sometimes do a power-on reset when I turned the lamp on or off. The desk lamp has an electronic ballast/starting circuit and has an 18-watt circular fluorescent tube. Adding more caps--100pF, 1nF, 10nF, 100uF--were for naught.

To find out how what the heck was happening I hooked up the oscilloscope to the power rails. I was horrified by what I saw.

The transient would sometimes reach 20V peak to peak! Fortunately it would last for less than 20us. To determine the frequency, I turned on the FFT. As you can see below the range is between 20 to 100Mhz.

I wondered whether a linear power supply would be immune to the transients. I hastily cobbled up one using a 220VAC to 12VAC transformer, W01M bridge rectifier, 470uF filter cap, 78L05 5VDC voltage regulator and a 200-ohm resistor as load. Here are scope readings:

Most of the noise is in the range <40Mhz. The amplitude of the transients seem less than for the ATX supply. However, this supply wasn't connected to the MCU circuit. That may have had an effect.

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