Volume control for Plastic Power amplifier
I have brought a kit from Jaycar Electronics for the Plastic
Power amplifier which featured in the April 1996 issue of SILICON CHIP. It is rated at 175W into
4Ω loads. I want
to put a volume control on this unit. I have been told to remove the
resistor from the board and replace it with a 1kΩ logarithmic pot. Can you let me know
where the pot is to be installed? (G. T., via email).
should be left on the PC board and you should use a 50kΩ pot (not 1kW). Connect it the same way as we have in
the Ultra-LD amplifier in the May 2000 issue.
Erratic readings on PIC tachometer
I’ve built the tachometer project in the April 2000 issue. It
works fine when connected to my power supply and sinewave generator but when
connected to my car’s ignition coil, the reading was not accurate and the least
significant digit (100s) is always jumping a few digits.
For example, the reading might be 1200 RPM but it will jump to
1700, then 1300 then 1200 RPM; not stable at all. The reading is also higher
than the actual RPM. The length of the wires connected to the tachometer has
been reduced to only 15cm long. Can you advise? (T. V., via
could try increasing the value of the .056μF capacitor following the
coil input resistor. This will damp down any oscillations from the ignition coil
which may be causing the high count on the display. Try a 0.1μF capacitor as a first
attempt. Reducing the value of the 47kΩ resistor between pins 3 of IC2a and pin
6 of IC1 to 22kΩ
may also help.
Engine immobiliser flattens the battery
I bought the Engine Immobiliser kit as published in the
December 1998 issue of SILICON CHIP and connected it to my 1997 Barina Swing. All voltages/currents were as
per the article and the operation was OK after wiring. However, the car battery
went flat overnight.
I put in a new battery and that was discharged too. Otherwise
the engine immobiliser seems to work OK. On the workbench, it draws only about
The car wiring was done by a qualified electrician (cost me
$200 for the $30 kit!) and is very thoroughly done. Are there any errors in the
kit? (G. K., Wollstonecraft, NSW).
Almost certainly, your Engine Immobiliser is incorrectly wired to your
vehicle. This is because the circuit should not draw current from the battery
until the ignition is switched on. Your current measurements indicate that it is
Check that the supply to the hidden switch, S1, is from the fused side of the
ignition supply and not from the battery side. In this way, when you set the
ignition immobiliser, current will only be drawn from the battery when the
ignition is switched on.
IR remote control with 30 keys
I was reading about the remote controller for trains in the
April 1992 issue (I know it’s ancient) and I would love to use the chips you
used for infrared transmission/reception. These are the MV500, MV601 and the
SL486 but I can’t find them anywhere. I am guessing they have been outdated, so
could you please tell me the chips you would use for this same type of
application today. I need to have about 30 keys on the keypad of the
transmitter. (P. K., via email).
you seen our more recent design from the October, November & December 1999
issues? This uses a cheap remote from Oatley Electronics and has eight buttons
in all. We don’t know of any chipset which will allow you to use 30 keys.
Temperature control for vacuum forming
I want some information on your 240V 5A motor controller kit
published in September & November 1992 and also on your heater controller
kit published in the July 1998 issue. I want to control the temperature on the
heating elements in my home-made vacuum forming machine. Or do you have
something more suitable? (S. A., via email).
We’re not sure how you wish to control the heating elements and whether
they run from 240VAC or not? If they do, the heater controller described in July
1998, a zero-voltage switching design, is the right choice. If you are using a
vacuum cleaner motor to produce the vacuum, the 5A motor controller is suitable;
you can use it for any 240VAC brush-type motor.
Load requirements for the Ultra-LD amplifier
In your write-up on the ultra low distortion amp project in the
March & May 2000 issues you recommend not using low impedance speaker loads
because of increased distortion caused by current sharing imbalances in the
One fact you can be sure of in audio amplifier design is that
the output load will be highly reactive over the audio frequency range. Even
good speaker designs like the Vaf DC-7 provide a widely varying load, ranging
with a nominal impedance of 6Ω. Have a look at
How can we expect your amplifier design to behave when
connected to such loads? Will distortion be the only issue or will there be
other problems as well? (M. D., ANU, via email).
Ultra-LD will handle nominal 6Ω and 8Ω loads OK but we don’t recommend 4Ω loads because as you point out,
loudspeaker loads are highly reactive and they can dip to very low values. We
wouldn’t recommend driving electrostatics either.
Avoid stupidity with 240V speed controller
I seek your advice concerning the 10A 240VAC speed controller
described in the November 1997 issue of SILICON CHIP. I have blown several BUP213s
(usually owing to my own stupidity) and have noticed the aforementioned
semiconductor usually fails before or at the same time as the 10A fuse
Consequently, I have wondered whether an IGBT upgrade might be
the best option (as my stupidity is probably a permanent, but fortunately
intermittent, factor!). Since the price of higher rated IGBTs appears to rise
exponentially with their current ratings, would it not be feasible to connect
two BUP213s in parallel? Of course, some physical modifications to the layout
would be required, but nothing too challenging. Would any other component
modification be required? (J. B., Burragate NSW).
are intrigued as to what particular form of stupidity causes the BUP213 IGBT to
blow. If you are intermittently stalling the power tool in question, we would
expect the over-current limiting to take care of the situation. Perhaps you
should check the circuit at low voltage, as described in the article.
On the other hand, if you are making some sort of connection to
the circuit while it is powered up, there is always the risk of blowing the
BUP213 and other parts.
Having said that, the BUP213 can be upgraded to a BUP314 which
is a larger TO-218 package and has a current rating of 52A rather than 32A for
the BUP213. These are available from Farnell Components; phone 1300 361 005. We
don’t recommend parallel connection of IGBTs since they will not share the
current equally. In an overload situation this might mean that you blow one and
then the other, within milliseconds of each other.
Insufficient signal from reluctor
I have one problem with the Multi-Spark CDI system described in
the September 1997 issue. I connected it up to a reluctor from a Chrysler and
there is no output from the coil. Then I shorted out the wires for the pickup
and got a spark from the coil. I then tried it on a Falcon XF with reluctor and
it worked perfectly. The resistance on the Chrysler pickup is about
460Ω and on the
XF it is about 1.2kΩ.
I looked at the trigger input for the ignition system featured
in the June 1998 issue and noticed there was a 2.2kΩ resistor in it but the equivalent
resistor in the September 1997 circuit was 10kΩ. I made the change anyway but still no
go. There doesn’t seem to be enough pulse from the Chrysler pickup. Can I modify
the circuit so that it will work properly? (M. K., via email).
reluctor signal is usually quite substantial at around 30V peak-to-peak and so
the circuit should operate. The actual resistance of the reluctor coil does not
indicate much since the output is dependent upon the strength of the magnet
associated with the reluctor, the number of turns on the coil and the reluctor
gap, as well as the rotational speed.
The sensitivity of the reluctor trigger circuit can be
increased by reducing the value of the 47kΩ resistor at the cathode of ZD5. This
will reduce the level of current holding Q8 on when there is no signal from the
reluctor. You could try using a 220kΩ trimpot and adjusting it until the
circuit works or try various values from 68kΩ to 220kΩ.
The 2.2kΩ resistor you refer to is used in the collector of the
transistor and will not affect the sensitivity of the reluctor circuit. Either
value could be used here.
By the way, Chryslers are pretty old now; it is possible that the reluctor is
faulty or you are using reversed connections to the circuit. Try swapping the
Coolant alarm query
I have built the Coolant Level Alarm from the June 1994 issue
CHIP and I feel
that there is a problem with the connection of the "hot" side of the indicator
lamp. Fig.1 has the connection between the 33Ω resistor and the anode of diode D2 but
Fig.2 has the 12V and the lamp commoned. Which one is right? (D. H., via
Fig.2 is right; Fig.1 is wrong. This was noted in the Errata we published
in the February 1995 issue.
Multipurpose fast battery charger
I’ve built the Multi-Purpose Battery Charger described in the
February and March 1998 issues and it is not quite up to spec.
I’ve tried it on a couple of 2.3A.h 12V SLA batteries and the
charger only stays on FAST for about three seconds before switching to 100% even
though the battery voltage does not correspond to the "full" voltage.
At a battery terminal starting voltage of 11.3V, the voltage
across TP CELL and TP GND is 0.97V/cell but ramps up quickly during the initial
3-second period to about 1.8V, before stepping back to a steady 1.65V during the
100% stage. The charge terminates some time later but I’m guessing this issue
relates to time-out rather than the batteries reaching full charge. Admittedly,
they are older batteries so this might well be the problem.
Do you know why the TP CELL voltage for a 12V SLA battery is
not equal to the terminal voltage divided by six cells? I read something about
scaling it back to a NiCd equivalent. I’ve downloaded a copy of the application
notes from the Philips semiconductor site and this reading has prompted a couple
(1) Given a 27kΩ Rref resistor on pin 20 and an 820pF capacitor on OSC/pin 14, I read from the
chart that the oscillator frequency will be about 50kHz. This is consistent with
your notes. However, reading across the chart, I get time-out periods of
45/90/180 minutes or three times the values that you arrived at. When I ran the
SLA battery described above I had the switch on 30-minute time-out but am sure
that it was reasonably longer than this. Am I misinterpreting the chart?
(2) Given Rb of 3.3kΩ, Rsense of 0.05Ω and Rref of 27kΩ, I calculate an Ifast of 3A rather than the 6A
quoted in the article. Am I overlooking something here? (P. J., via
Voltage regulation at Vbat (pin 19) is 1.63V for an SLA battery. Since
the end-point for a 12V SLA battery is 14.6V, the 0.111 division by the
resistors for the 12V position on S5b will give 1.6V for the cut-off
The fact that the voltage on your SLA batteries reaches 1.8V
before dropping to 1.65V suggests that either the divider resistors for the 12V
position on S5b are incorrect or the batteries are high impedance and so
The timeout period is also set by the POD voltage at pin 6.
This is actually tied low via a 33kΩ resistor (see overlay diagram) for a divide by 1 selection
and so the 15-minute timeout is set to about 22 minutes. The timeout is
tosc x POD x
PTD. PTD is set to divide by 1, 2 or 4 by the selection at pin 7 via switch
The charging current does not follow the formula detailed in
the data sheets because we are using an unfiltered DC voltage to charge the
600W DC-DC converter questions
A few years back, I bought the 600W DC-DC converter kit from
Jaycar in Perth and I didn’t have the time to start on it until recently. Before
I start, I have some questions: (1) Do I need to upgrade the car’s alternator to
a higher capacity? My present car is a 1600cc Toyota Corolla CSI. I think the
output of the alternator is about 70A. (2) Will there be a surge that will
destroy any of the car’s electronics if I wired the converter to come on as the
ignition is switched on? (3) Is there any requirement for a slow start circuit?
(4) Do I need to increase the capacity of my present battery? (A. W., via
will not need to upgrade your alternator unless you are expecting to drive your
amplifiers at full power when all the headlights and accessories are on. You can
switch the inverter on via the ignition and this was shown on the
The car battery should not need changing since the circuit is intended to
operate only when the engine is running, so as to keep the battery topped
Problem with TENs unit
I recently built one of your TENS units (SILICON CHIP, August 1997) and found
it terrific for pain relief for an inoperable spinal injury I suffered about 10
years ago. The problem is it was working fine one day then the next day it just
would not work.
I am in my first year of a diploma in electronics course in
which I have passed my hand soldering with a 97% pass mark. I have checked all
resistors, capacitors and diodes and they are fine, so I was just wondering if
there have been any problems with the units after they had gone into production?
Alternatively, could you advise me on where to look for any possible faults? (J.
C., via email).
TENS Unit does not appear to have any particular problems. The problem with your
particular unit could be with the transformer (T1) or any other component such
as IC1 or IC2, or the Mosfets Q1 and Q2. Or simply, the batteries may be
We suggest you check the operation of each part of the circuit
to isolate the problem. First, check the voltage at the drain of Q1 – you should
get a reading of 80V. If this is not present, check the supply to IC1 at pin 6
(6V). If this is OK, the windings in T1 may be shorted or open circuit.
If you do measure 80V, check the supply to IC2 which should be
around 15V. You should also check the output voltage at the electrodes – this
should give readings up to 40V (as measured with a DC-reading multimeter),
depending on the pulse width setting. Lack of output voltage may mean that IC2
is not operating or that Q1 or Q2 are short circuited.
Further checking can be made with an oscilloscope. You should
obtain similar displays to those published.
Notes & Errata
RoomGuard Intruder Alarm, April 2000: the 100kΩ resistor at the junction of
D1, D2, D4 and pin 9 of IC1 on the circuit on page 31 should be 10kΩ. The wiring diagram on page
32 is correct.
LED Dice, May 2000: LED6 is shown back to front on the PC
board component overlay on page 60. Its cathode (the flat side) should go
towards the top of the page. The circuit diagram on page 58 is
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