Re: Speaking of Colin Mitchell -- Circuit mistakes
- From: fredssbloggs@xxxxxxxxx
- Date: Sat, 30 May 2009 06:50:31 -0700 (PDT)
On May 29, 10:18 am, MooseFET <kensm...@xxxxxxxxx> wrote:
On May 26, 5:37 pm, "Joel Koltner" <zapwireDASHgro...@xxxxxxxxx>
wrote:
He's a very opinionated guy -- would fit in quite well here. :-)
Check out this page of his:http://www.talkingelectronics.com/projects/SpotMistakes/SpotMistakesP... ;
is the extra circuitry in the solar charger necessarily a mistake? The
saturation voltage of a transistor is perhaps 200mV, which is noticeably
better than a diode's drop of, say, 500mV, so it seems to me the extra
complexity might be more than "paid for" by the higher efficiency of the
charging.
It may also be that the diode would need a heat sink but the
transistor doesn't. The trade of mechanical for electrical complexity
can be worth it.
The transistor is sort of over current protected by not having
infinite base current. Since no values are given, we don't know if
this could matter.
He also didn't figure out how to get rid of the trim pot. Adjusting
that is likely costing more than the transistors.
Way down at the end of the page, there is a circuit that I say will
destroy its Zeners after a few times of bing plugged in.
Someone there's a column Colin wrote some years ago about how compact
fluorescent lamps would never catch on -- they just weren't good enough and
too expensive. I suspect he's claiming he was right, given how his own
Australian government is now forcing them on consumers, like it or not! :-)
---Joel- Hide quoted text -
- Show quoted text -
The essential requirement is for a current limited LDO regulating at
full charge battery voltage, with appropriate temperature coefficient
compensation, between the solar cell and the battery/load tie-in
point. A simple diode will not do this, the solar cell will attempt to
run the battery and ciruit voltage up to the highest voltage/current
supported by the irradiation. In the kit original circuit, TR3 is a
VBE multiplier, which should remain linear, and maintains the emitter
voltage of TR1 at (1+R1/R2)*VBE. This should be set close to VBATT,FC,
the full charge battery voltage. TR1 is not necessarily saturated
until the battery nears full charge, at which point TR1 base current
increases by a factor in the the range of 10-100x with a corresponding
increase of the TR3 VBE to be multiplied by 60mV-120mV, overcoming the
VEC,SAT loss of TR1. The inherent power limit of the solar cell is
relied upon for the overall circuit current limit. Although not as
precise as could be, the basic circuit shown in the kit schematic can
probably be made good enough for the intended purpose and is way more
than just an overcomplicated diode.
.
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