Alt-BEAM Archive
Message #02109
To: westy civic83@execulink.com
From: Steven Bolt sbolt@xs4all.nl
Date: Thu, 8 Apr 1999 10:24:59 +0200 (CEST)
Subject: [alt-beam] Re: capacitors
On Wed, 7 Apr 1999, westy wrote:
> Hi, I need some .22uf capacitors, I have a large quantity of
> ceramic disk capacitors and was wondering what number .22 is,
> eg, alot of these caps are marked 222 and 271, ect.
>
> what formula do you use to determine these numbers.
.22uF equals 220N (nanofarad), which in turn equals 220000P
(picofarad). That latter number can be written as 224, the `4'
indicating the number of zeroes.
Since many manufacturers have acquired the odd habit of using
invisibly small print and colors like light brown on yellow to make
cap values as difficult to read as possible, other manufacturers
now provide us with quite affordable multimeters which can measure
small caps. Something to keep in mind when you buy your next DMM.
> On a positive note, I got my first fled solar engine to work
> today, it uses a 0.047f memory backup capacitor from a
> camcorder, a panasonic bp-2433 solar cell, and a 2.2k resistor.
> It works ok, but I have one question, the 2.2k resistor, what is
> it's function, and what would happen if I changed it to say, a
> 4.7k resistor?
Look at the circuit. When the cap is charging, the motor can be
viewed as a short. So the base of the first (pnp) tor is pulled
high by the resistor, which keeps it switched off until the fled
triggers. At that moment the pnp tor gets some base current and
pulls the second (npn) tor into conduction. This in turn pulls the
connection between motor and resistor to almost gnd (or `-' or
whatever it says in your version of the schematic).
That not only starts the motor, but also provides the pnp transistor
with base current through the resistor. Which is necessary, because
as the motor starts, the voltage begins to drop and the fled
doesn't trigger anymore. Now as long as the pnp tor gets sufficient
base current, it will provide the npn tor with base current, so
both keep getting base current. This part of the process is called
`latching'. The latch holds until the current flowing through the
resistor is no longer sufficient, which state is reached when the
voltage drops almost to the base/emitter voltage of the transistors
(about 0.7V).
If the resistor value is larger, the latch may release sooner, or
latching may not occur to begin with. If the resistor value is
smaller, wasted and possibly even damaging excess current will flow
into the bases of both transistors. The best possible resistor
value depends on the trigger voltage, the properties of both
transistors and the fled, on the motor, and on the mechanical load.
The combination of latch and driver makes the circuit rather happy
to oscillate if things are not exactly right. In your mind's eye,
you can see the npn tor start to pull on the motor and the
resistor. Current flows, but the voltage over the motor doesn't
rise above the base/emitter voltage of the pnp tor, which means
that base current is still lost rather than gained through the
resistor. Now the stored voltage drops, the fled switches off, the
voltage rises, the fled triggers again and so on.
This unhappy state of affairs is more likely if the resistor value
is larger, but also when the npn transistor is unsuitable as a
driver, or when the motor demands excessive current to start. In
such cases the npn transistor doesn't fully switch on (saturate),
and the latch will fail.
The above is by no means a complete description of this `simple'
circuit and its pitfalls, but perhaps it begins to answer your
questions...
Best,
Steve
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# sbolt@xs4all.nl # Steven Bolt # popular science monthly KIJK #
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