What constitutes a dry cell? A carbon-zinc cell,
alkaline “Bunny” battery, lithium watch cell, among others.
Anything without a liquid or gel electrolyte. While most
dry cells are primary — that is, they can’t be recharged —
the ability to recharge doesn’t draw the line between
wet and dry because today’s new crop of dry-cell Li-Ion
batteries are rechargeable. Dry cell leakage? A topic for
I just received my copy of the April ‘06 issue and
saw your suggestion of connecting a supercap across
the five-volt supply lines on the USB as a solution
for providing energy storage for a hard drive. Although
this energy storage circuit may work well, the USB
specification sets a stringent limit on transient current
draw. An uncharged 1F cap would look like a short
circuit to your hub, and the PC would most likely shut
the port down as soon as it’s plugged in — as it will do
on my computer.
I have developed several USB peripherals, and this
issue has come up before. To prevent port shutdown, I
developed the circuit shown in Figure 7. Q1 controls the
charging current for C1 and, thus, its rate of charge. Q2 —
in combination with D3, R1, and R2 — forms an error
amplifier. The voltage drop across D3 nulls the base-emitter voltage drop on Q2 (about
0.6V). This circuit forms a closed loop
so that the voltage drop across R5
will be approximately equal to the
voltage set by R4 while the capacitor
is charging. The higher the voltage
setting on R4, the higher the current,
and the faster the rate of charge.
When the capacitor is charged, it
will cease drawing any current. Q2
will turn off, R1 will pull the gate of
Q1 to the supply rail, which will turn
it fully on.
— Stephen Santarelli
The old proverb says “Seek, and ye
shall find,” and that’s certainly true
of free software. The Internet is
chock full of freebies: free software
for e-commerce, auctions, business
management, and more.
Want to become a website?
If so, then this is for you.
August 2006 29