If the Earth is solid, then it would
be worthwhile trying a bigger mains
filter. This could be as simple as winding the mains wires through a toroidal
ferrite core.
Speaker Protector not
sensing AC
I have built the October 2011 Loudspeaker Protector module (siliconchip.
com.au/Article/1178) from an Altronics kit (Cat K5167).
I configured it for my supply rail
voltage and tested it with the prescribed method, and it seemed fine.
However, as soon as I connect the AC
sense lines from my transformer, it
refuses to work.
Connecting AC sense to the positive
rail as detailed in the instructions and
performing all other tests shows the
module to be working.
Can you enlighten me as to the likely
cause of this problem? (D. J., Mandurah, WA)
• The first thing to do is check that
you have wired up the transformer to
the AC sense terminals correctly. Typically, you would have a centre-tapped
winding with the centre tap Earthed
and the other connections going to
both your bridge rectifier AC terminals, and the terminals of CON2 on
the Speaker Protector.
Assuming the connections are correct, verify that the base voltage of Q2
is low (below 0.2V) when the AC voltage is present at the AC sense input. If
that is incorrect, then check diodes D2
and D3 and the transistor Q1 to ensure
they are orientated correctly, have
good solder joints and are not faulty.
That it worked with a DC test suggests transistor Q1 is working OK and
that at least one of diodes D2 & D3 is
functioning correctly.
It could be a problem with the resistor or capacitor values around Q1. If
the 470nF capacitor is not soldered
correctly or has the wrong value, the
circuit will work with DC voltage
applied but not AC. Similarly, if the
resistor values are wrong, the circuit
might not hold up through dips in the
AC voltage.
Flexitimer with higher
supply voltage
Many years ago, I built the PICBased Flexitimer Mk.4 (June 2008;
siliconchip.com.au/Article/1847) from
112
Silicon Chip
a Jaycar kit (Cat KC5464). Can it be
modified to run from a 24V supply?
(J. S., via email)
• You need to change the relay to a
compatible type with a 24V DC coil,
the 470μF capacitor to a 35V rated
type, and the 1kW LED current-limiting resistor to 2.2kW.
AEE ElectroneX.......................... 7
Incorrect component in
low ohms meter
Control Devices..................... OBC
Advertising Index
Altronics...............................83-86
Ampec Technologies................. 25
I have built the Low Ohms Tester by
John Clarke from the June 1996 issue
(siliconchip.com.au/Article/4987), but
I can’t get it working. The text says that
the voltage at pin 2 of IC1 should be
the same as pin 3.
Adjusting VR1, I can get 2.4V on
pin 3, so it appears REF1 is working
OK. But the voltage at pin 2 is 1.64V. I
have replaced IC1 and Q1 to no avail.
(N. L., Christchurch, NZ)
• Since you have replaced IC1 and
Q1, that seems to rule out either component being faulty (which would
explain what you are seeing).
However, if Q1 is the wrong type
or orientated incorrectly, that would
cause this sort of fault. The only other
possibilities are a lack of continuity or
incorrect value with the 2.4kW resistor, trimpot VR2 or the 200W resistor.
Try changing range switch S2 to see
if that has any effect; if it does, it is
likely one of the latter three components at fault. Note that if this part of
the circuit is operating normally, pin
6 of IC1 should be around one diode
drop (approximately 0.6V) below the
voltage at pins 2 & 3.
Dave Thompson...................... 111
Pulse generator circuit
wanted
Vintage Radio Repairs............ 111
I’m interested in building a pulse
generator. I found a pulse generator
design in Practical Electronics, February 1979. Can you suggest a circuit as
simple as that one, but up to date with
similar specifications and features? (R.
M., Melville, WA)
• We published a pulse generator
circuit in the Circuit Notebook section
of the November 1997 issue, which has
similar features to the one you refer
to (siliconchip.com.au/Article/5833).
However, there was no PCB design to
accompany that circuit.
You could also build up the Practical Electronics design as there is nothing wrong with it. All the parts used in
that circuit are still available.
SC
Australia’s electronics magazine
Digi-Key Electronics.................... 3
Emona Instruments................. IBC
Hare & Forbes............................. 9
Jaycar............................ IFC,53-60
Keith Rippon Kit Assembly...... 111
Lazer Security......................... 111
LD Electronics......................... 111
LEDsales................................. 111
Microchip Technology.................. 5
Ocean Controls......................... 11
PMD Way................................ 111
Silicon Chip Shop...............96-97
Switchmode Power Supplies....... 6
The Loudspeaker Kit.com......... 10
Tronixlabs................................ 111
Wagner Electronics................... 63
Notes & Errata
Ultra-LD Mk.4 Amplifier, July-August
2015: the circuit diagram (Fig.1)
incorrectly specifies 135mV across
the 68W emitter resistors of Q2a
& Q2b, the correct value should be
around 68mV.
The September 2021 issue is due
on sale in newsagents by Thursday,
August 26th. Expect postal delivery
of subscription copies in Australia
between August 25th and September
10th.
siliconchip.com.au
