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Subject: Re: Simple adjustable 0 - 400V regulator, with flaw
NNTP-Posting-Date: Mon, 20 Jan 2003 01:42:43 GMT
Organization: AT&T Broadband
Date: Mon, 20 Jan 2003 01:42:43 GMT
Kevin McMurtrie wrote:
> Time to read the specs before making everything high impeadance.
> The minimum load is 0.755 to 0.770 mA, depending on temperature and
> input voltage. Ratings say to expect 1.00mA minimum load.
> The ADJ current is 0.053 to 0.075mA, depending on temperature. Ratings
> say to expect 0.080mA maximum ADJ current.
> In the first diagram, the output voltage will vary by up to 5.5 volts
> with temperature when the pot is centered. Drift could be annoying even
> if it's below the accuracy of mechanical adjustment.
> In the second diagram, the output will vary by as much as 40 volts when
> in the "fixed" mode. Might as well get rid of the fixed mode and paint
> a dot outside the adjustment knob where it's 300V.
Yup. Excellent points.
It seems to me that there are at least three approaches to dealing with
the pot kill problem in this application.
The cheapest would be to add a series diode between the regulator output
and the 10uF capacitor. Note that this also would require a bleeder load
resistor (say, 180k 1W) across the 10uF capacitor to pull down the output
voltage when switching in the zeroed out pot in the presence of no
external load. Even so, it could take a while (5 to 10 seconds) for the
output fully discharge. Also, regulation would be degraded slightly by
variations in the diode's on voltage with temperature and current, but
this would be a few tenths of volt in the face of hundreds of volts.
Not quite as cheap ($.30 in quantity), but arguably the most elegant would
be the 1k ohm, 1.5ma Supertex mosfet that Winfield Hill likes so much.
However, this approach suffers the risks associated with using a sole
sourced part. The circuit also requires about two seconds to discharge
the output when the pot is switched in while zeroed out.
Another approach would be to add an active discharge helper in parallel
with the pot to shut away the bulk of the 200ma peak current that it
would otherwise have to carry. Imagine, for example, a 330 ohm current
sense resistor inserted between the low end of the pot and circuit ground
Now further imagine an NPN transistor hooked in parallel to this circuit
with its collector connected to the high side of the pot, its base to the
low side and its emitter to ground. Now the maximum discharge current
through the pot will be reduced by the beta of the transistor plus two
miliamps. Note that, unlike with the above two circuits, discharge will
appear instantaneous to the user.
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