QUESTIONS ANSWERS ■ WITH RUSSELL KINCAID &
In this column, Russ answers
questions about all aspects of
electronics, including computer
hardware, software, circuits,
electronic theory, troubleshooting,
and anything else of interest
to the hobbyist. Feel free to
participate with your questions,
comments, or suggestions. Send all
questions and comments to:
BURNED PARTS ON
WEED WHIP CHARGER
QI am not sure why the charging circuit on my weed charger fried (guessing battery went bad
as it will no longer hold a charge),
but the parts are not recognizable.
The resulting fire appears to be
localized to the transistor, and melted
adjacent parts. I would like to fix the
circuit board. My questions are:
guess for the replacement of the
TO92 transistor (or a better upgrade)
and pass diode?
The schematic is shown in Figure 1.
— Dwayne Stenlund
• What would be a reasonable
AStarting from the battery which is 13. 8 VDC at full charge and adding .5V for the diode plus another .5V
for the transistor Vbe (assuming it is
■ FIGURE 1.
an NPN), the base voltage is 14. 8
VDC. I compute the current through
the 2,700 ohm resistor to be 5. 5 mA.
If the battery is being charged at a 20
hour rate, the current is 7 Ah/20 =
350 mA. Assuming that the transistor
current gain is 100, the base current
is 3. 5 mA. The current through the
15 ohms is at least 8 mA which puts
the zener voltage at 14.98 volts; call
it 15V. The drop across 1,000 ohms
is at least eight volts, so the input
should be 15 + 8 = 23 volts. The
power dissipation is .35A*( 23-14. 3) =
3 watts — too much for a TO92.
A metal can transistor (such as a
2N2219) will have better heat
dissipation and the diode
should be a DO41 (such as a
There is no current limit in
the circuit, except the gain of
the transistor reduces at high
current. When charging a dead
battery, the current could
approach one amp.
Fortunately, the internal
resistance of the source will
reduce the input voltage — let’s
say it is 15 volts — then the
power dissipation is ( 15-11)*1
= 4 watts. That is too much for
even a heatsunk TO92, so this
is a marginal design at best.
QAfter searching the Internet, I built and tested the chaser light circuit in Figure 2.