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Old 12th September 2008, 10:06 PM   #1
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Default BUF420M Current Sink

Hello all,
I'm trying to build a very large current sink as a type of inrush limiter. My idea was to use something along the lines of the BUF420M, grounding the emitter, setting up an adjustable voltage divider for the base, and pulling a couple hundred MA of current through the collector.

The plan is to use this between the ground and the CT on my power transformer, keeping the power supply somewhat confined. Any ideas? (320v B+)

Here is the data sheet:

What kind of voltage would you all expect across this guy? Any decrease in rectified B+? Best to bypass it with a cap?
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Old 13th September 2008, 03:10 AM   #2
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Beware. All bipolar transistors suffer from a condition called "secondary breakdown". For most transistors this is the primary failure mode in linear operation. This limits the power dissipated by the device in linear mode. Most transistors have a chart showing the "SOA" (safe operating area) for several duty cycles and DC. These are not shown for this device. Only data for short pulse (.2uS). This device will probably die in your application.

I built a device similar to what you describe. I used a mosfet (no secondary breakdown) and drove it with the PWM output of a PIC chip. The pulse width is narrow at turn on gradually widening until the fet was fully turned on. A fully on fet drops about a volt in my system. You could do this with a 555 chip instead of the PIC chip.
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Old 13th September 2008, 03:16 AM   #3
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Do you happen to have a schematic for your setup? So even if I'm running this guy conservatively (200ma) and not exposing it to high voltage, it will still break down?
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