110 Silicon chip Australia’s electronics magazine siliconchip.com.au
put of the HV generator PCB and the
4.7kW/1W resistor connecting to the
positive test terminal. This could use
say a 470μH RF choke in series, togeth-
er with a shunt capacitor arrangement
to ground (after the RF choke).
This shunt capacitor arrangement
would be a duplicate of the existing
one at the output of the HV generator
to ground – ie, a pair of 100nF/630V
caps in series, each with a 10MW par-
allel resistor to ensure voltage sharing.
This filter would help to isolate the ca-
ble capacitance from the HV generator.
It might also be a good idea to fit a
470μF capacitor in parallel with the
existing 100nF capacitor connected
between the negative test terminal
and ground. We hope these sugges-
tions help you to achieve your goals.
Running model railway
at 24V
I am interested in building your
Model Rail Controller (April 1997;
siliconchip.com.au/Article/4890). I am
building a garden railway with one lo-
comotive which needs 24V. The 1997
design supplies 12V, and I like that it
is simple. Can this project be upgrad-
ed to 24V? If so, do you still have this
PCB available?
If this project can’t be upgraded, do
you have an alternative project? (I. S.,
Glenhaven, NSW)- The Train Controller in the April
1997 issue actually runs from a split
12V supply. One 12V supply is for the
positive (eg, forward direction) and the
other 12V supply for the negative (re-
verse) direction. It is not easily adapt-
ed for a 24V supply, and the PCB is no
longer available.
Instead, we suggest you build the
Li’l Pulser Model Train Controller Mk2
(July 2013 & January 2014; siliconchip.
com.au/Series/178). It can be powered
from 24V. If the supply could exceed
24V then you will need to upgrade the
input 2200μF supply decoupling ca-
pacitors to 35V types.
LED chaser kit wanted
for tractor
Do you have a kit for a light chaser
using LEDs that I could power from
my Ferguson TEA20 tractor? I’ve fit-
ted indicators, brake lights, headlights,
and tail lights using 12V DC powered
LEDs. (D. P., Young, NSW)- We haven’t published a light chaser
in a long time, and the PCBs and kits
for our older projects are no longer
available. However, Jaycar sell the kits
for Short Circuits projects, originally
designed by Silicon chip, which in-
cludes at least three LED chasers.
The Jaycar Cat KJ8064 10 LED light
chaser runs from 12V DC. They also
have a 10-LED Knight Rider LED scan-
ner kit, Cat KJ8236 and a 20-LED light
scanner/chaser, Cat KJ8066. You can
look up these catalog codes on their
website for more information, includ-
ing a link to the instructions for each
kit as a PDF file.Headphone amplifier
sharing power supply
I am planning a revamp to build
the Hi-Fi Stereo Headphone Amplifi-
er (September 2011; siliconchip.com.
au/Series/32) into an enclosure along
with a multi-input preamplifiers. As
the preamp runs from a ±15V supply,
can I safely run the headphone am-
plifier from the same supply? (R. K.,
Cessnock, NSW)- The headphone amplifier uses in-
ternal ±12V rails mainly for conveni-
ence, as this allows you to use a more
common 12V AC plugpack power sup-
ply. You could run it from ±15V with
little risk of overheating. The quies-
cent power would increase a bit and
so the output transistors would run a
bit hotter, but you could dial back the
quiescent current slightly to compen-
sate, if necessary.
I acquired some Silicon chip mag-
azine back issues, including the vac-
uum tube based preamplifier designs
incorporating switchmode power
supplies, published in the November
2003, February 2004 and January/
February 2016 issues (siliconchip.
com.au/Series/293 and siliconchip.
com.au/Series/295).
I can’t find any mention of the
type of 12V power source required,
eg, linear “wall wart” or switch-
mode types. The linear (transform-
er-based) types with DC outputs are
now hard to obtain.
The SMPS types are more abun-
dant. Do you know if there would be
any noise problems caused by using
a switchmode plugpack with any of
these designs?
Also, I read online that one con-
structor used the November 2003
power supply to power a stereo pair
to good effect, then he says that hebought more PCBs, which are no
longer available, only for some of the
power supplies to fail without any
reason. So I’m wondering if there
may have been an update to the de-
sign in later issues.
I have been working in electronics
design including vacuum tube audio,
and I thought that if it worked for a
year or so then the failure may have
been to assembly flaws, dry joints etc.
I have made my own PCB but
haven’t powered it up yet. I believe it
has to be loaded down with the tube
circuit. Any help would be most ap-
preciated. (J. H., UK)- In both designs you’ve men-
tioned, the mains power supply feeds
straight into the input of a linear regu-
lator which should remove any noise.
That’s why ~15V supplies were
specified, to give the headroom for
regulation down to 12V. The 2016
design was tested using a switch-
Using switchmode plugpacks with valve preamp
mode plugpack, and there was no
sign of any digital noise getting
through to the outputs.
There have not been any updates
to either of these projects (except for
the one you mentioned, in February
2004) and we are not aware of any
problems with them. If it worked
for a year and then failed, that sug-
gests a faulty component. None of
the components in these circuits are
particularly stressed.
Neither power supply design
should fail if operated without load,
as they both have feedback-based
voltage limiting. However, the Jan-
uary/February 2016 design does re-
quire the specified load to prevent
‘squegging’ which results in more
noise appearing at the output.
We sell PCBs for all the projects that
you mentioned. See siliconchip.com.
au/Shop/?article=3390 & siliconchip.
com.au/Shop/?article=9768 SC