108 Silicon chip Australia’s electronics magazine siliconchip.com.au
Power Tools? (July 2012; siliconchip.
com.au/Article/601)
It’s load-sensing so is suitable for
devices which have their own pow-
er switch or where they switch on
automatically. For basic soft starting
where you’re switching on a device at
the wall, the simpler April 2012 Soft
Starter is suitable (siliconchip.com.
au/Article/705).
You might also want to look at
our March 2011 Mains Moderator
(siliconchip.com.au/Article/937), a
low-cost unit to reduce high mains
voltages for devices that draw up to
450W, although it can be built to han-
dle more power.Automotive Sensor
Modifier questions
I am thinking of building the Sili-
con chip Automotive Sensor Modifi-
er. Can you supply the PCB and if so,
does it come with the components? If
not, where do I buy those components
from? Also, can the Sensor Modifier
convert a sensor voltage from 1.2-4.5V
to 0.5-4.5V? (S. W., via email)- For this project, we sell the PCB (Cat
SC4068) and programmed PIC micro-
controller (CAT SC4069). You can get
the other parts from suppliers such as
Jaycar, Altronics etc.
If you do not have the parts list and
instructions, you will find these in
the magazine article. You can buy the
printed magazine from siliconchip.
com.au/Shop/2/4077 or access to the
empty. We suggest that you try chang-
ing the values at line 49 and line 656
and wipe the EEPROM to be sure any
remnants are gone.Battery charger inrush
current limiting
I am having troubles with my bat-
tery charger, and it occurs to me that
you could create a small project which
could be of use to me, as well as ap-
pealing to others.
I have a solar system with battery
backup and use a mains-powered bat-
tery charger as a further backup. As my
mains voltage tends to sit around 253V
AC, the charger often blows fuses or
breaks down when switched on, due
to the high surges involved. What I
need is a slow start device, which will
cope with a 10A load current, and the
surges involved.
Possibly this could be a variation
of your latest mains motor speed con-
troller, built in a small box with flying
leads and incorporating a microcon-
troller and Triac. The timing could be
controlled by a jumper, with a range
from a few seconds (large motors)
through a minute (my charger) to sev-
eral minutes (large valve amplifier). (D.
T., Yallourn, Vic)- Switch-on surges usually only last a
few mains cycles unless the device has
a big motor in it that takes time to spin
up. And even then, the worst is usual-
ly over within a second or so. Did you
consider building our Soft Starter for
plied with the opposite polarity to
how they were wired on the PCB. So, I
then cut their coil leads and used two
wires to swap them and solder them
back on the PCB.
After reapplying power, the four
relays were triggered correctly and I
could hear the (discreet) click. So the
internal flyback diode wiring was the
opposite of the specified relays. The
unit then gave accurate readings with
various test capacitors and inductors.
One thing I noticed is while I can
specify the value of CP and LP from the
terminal, I cannot enter a value for R.
No matter if I chose a unit or not, no
value will be written in the Arduino
memory. I tried changing the default
value in the code (line 656 onwards),
but it always displays 130W on the LCD
upon startup. Any idea why? Thanks
and keep up the good work! (O. A.,
Singapore)
- None of the relays that we tested for
the LC Meter had internal diodes, so
we did not suffer these problems. We
checked the data sheet for the relays
we used (TRR1A05D00) and it does
not show any diode or polarity mark-
ing on the coil.
We tested the calibration menu just
now and we were able to change the R
value by typing either “140” or “140R”
(and “S” afterwards to save). The new
value is loaded from EEPROM when
we reset the unit.
The default value for R1 is also set
on line 49 of the sketch; this sets it to a
sensible default in case the EEPROM is
I am having trouble with tran-
sistors Q1-Q4 in the DAB+/FM/
AM Radio (January-March 2019;
siliconchip.com.au/Series/330)
overheating. I replaced the transis-
tors with new ones and have thor-
oughly checked the circuit against
the schematic, and everything
checks out. I can find no short cir-
cuits. But they’re still overheating.
They get too hot to touch shortly
after switch-on, and both the -5V and
+5V rails are pulled down as a result.
With the transistors removed, the
rest of the board seems OK and the
voltage rails remain stable. Any idea
as to where to look next would be
appreciated. (D. E., Wattle Park, SA)- We had problems with these same
transistors overheating in our second
prototype and it turned out to be a
bad solder joint in one of the feed-
back resistors near op amp IC5 (from
memory, one of the 2.2kW resistors).
That was causing the op amp’s out-
put to swing wildly from rail to rail
and thus causing the transistors to
overheat. Resoldering that resistor
fixed it.
But that was just one pair of the
transistors as only one channel was
affected by this fault. It sounds like
the transistors in both channels are
overheating on your board.
Try (quickly) measuring the volt-
age across D1 & D2. You should get
a reading around 1.2V in both cas-
es. If one or both are much higher,DAB+/FM/AM Radio audio transistors overheating
they could be over-biasing the tran-
sistors.
Try removing the two 2.2kW resis-
tors at either end of D1 & D2. If those
transistors still overheat then there is
something wrong with the devices;
either they are in the wrong locations
or are the wrong types.
If that does stop the overheating,
then try fitting a higher value resistor
pair to one diode, eg, 10kW or even
22kW. Perhaps your BAV99s have
an unusually high forward voltage,
or your BC807/BC817s have unusu-
ally low Vbe.
That would result in a much high-
er quiescent current than in our pro-
totypes and could result in what you
have described happening.