have used TO-220 transistors in this
application without a heatsink, but a
small heatsink would be more
reliable. If you put the circuit in an
aluminum box, use the box as the
heatsink (but insulate the box). I used
a RadioShack plastic box with an
aluminum cover.
To use this circuit in a 24 volt
system, double R3, R4, R6, and R7.
Figure 4 is the parts list.
BLINKING TAIL LIGHT
series with the capacitor. Xl in this
case is calculated using the
frequency F = 1/2Pi(LC)^. 5.
I simulated the circuit and got a
peak voltage of 140V, so it appears
that there is not much energy lost in
the resistance.
In your third case, the calculated
voltage is 571 volts with C = 0.1 µF.
My simulation shows 595V but if I
put 620 ohms in series with the
capacitor, the peak voltage is 500
volts and there is no ringing. I didn’t
calculate the Q but it should be
about 1/2.
and the other
comes out to
the F terminal.
There is also
an unused R
terminal which
was to be used
with an idiot
light, but this
car has an
ammeter.
I have a
friend who
would like to
use such a
circuit to
charge a 24 volt battery, so any
modifications to make the same
circuit work in a 24 volt setting
would be welcome.
— Mike Watford
■ FIGURE 3
ALTERNATOR
REGULATOR
AI know I designed this before, but I didn’t put the schematic where I could find it so this one is new. In
Figure 3, the transistors and diodes
are rated to 100 volts in order to
survive transients of the automotive
system. D1 protects against reverse
voltage and D2 catches the
backswing of the field inductance
when Q1 turns off. I used Darlington
transistors to keep the power in the
control circuit low. IC1 is a four volt
( 4.096V nominal) shunt reference. R7
is a thermistor to temperature-compensate the charger. My battery
manual recommends 2. 5 to 3
millivolts per cell; this is intended to
do that. You will need to adjust R3 to
get 13. 8 volts at the battery. One
way to do this on the bench is to
apply 13. 8 VDC to P1 and connect a
12 volt light bulb to the field output.
Adjust R3 so the light just goes out. I
QIf you have ever been hit in the rear by a vehicle because the driver was asleep, playing with a cell
phone, or whatever, it doesn’t take
too much to make you ill at ease
with traffic closing in on you. I speak
from experience (1951, 1992, and
1997).
A low frequency flasher and
driver inserted right after the brake
light switch should get the attention
of the person closing in on your rear-end even with text messaging, cell
phones, and loud music. Do you
think their mind could process a
flashing red brake light in addition?
What ideas do you have for this?
— Bob Haskett
QI loved your circuit for an automobile generator voltage regulator from October ‘09. Do you have
something similar for an alternator?
Mine is an old General Motors
Delco-Remy alternator (1967 Pontiac)
with an external vibrating relay type
regulator. One side of the alternator
field coil is grounded to the frame
22 June 2010
ALTERNATOR REGULATOR PARTS LIST
PART DESCRIPTION PKG
D1, D2 DIODE, 100V, 5A, SCHOTTKY DO- 15
Q1 PNP DARLINGTON, 100V, 5A TO-220
Q2, Q3 NPN,DARLINGTON, 100V, 800 mA TO-226
D3 4.096V SHUNT REGULATOR TO-92
R3 1K TRIMPOT 3266W
R1 300 OHMS, 1/4W, 1%, METAL FILM AXIAL
R2 4.7K, 1/4W, 1%, METAL FILM AXIAL
R4 20K, 1/4W, 1%, METAL FILM AXIAL
R5 10K, 1/4W, 1%, METAL FILM AXIAL
R6 15K, 1/4W, 1%, METAL FILM AXIAL
R7 5K THERMISTOR RADIAL
MOUSER PART #
750-SB5100E-G
511-TIP127
512-ZTX614
595-LM4040D41ILP
652-3266W-1-102LF
271-300-RC
271-4.7K-RC
271-20K-RC
271-10K-RC
271-15K-RC
871-B57540G502F
AI have my doubts that flashing brake lights will be ffective, and you should check with local ordinances
that they are legal. In any system that
I am familiar with, voltage is applied
through the brake switch to the lights
which are grounded. To use this circuit,
you will need to cut the brake wire
and splice in the circuit and ground.
In the circuit in Figure 5, the
transistors are rated at 100 volts to
survive the automotive environment.
Some vehicles have five brake lights,
so I designed for 10 amps current. Q1
will dissipate 20 watts in the worst case
and R1 needs a heatsink for 10 watts,
so a 30 watt heatsink is required. For
a 50 deg C rise above ambient, a
heatsink rated at 1.7 deg C/W should
be used. Figure 6 is the parts list.
I think a 4 Hz blink rate would
be optimum and will use a hysteresis
oscillator. The CD4093 Schmitt-trigger
