siliconchip.com.au Australia’s electronics magazine June 2019 107
Tools (siliconchip.com.au/Article/601)
will provide soft starting in a situation
where the mains supply is permanent-
ly connected to a device, and it switch-
es on and off by itself. That design also
has two thermistors in series so it may
be more effective than the April 2012
Soft Starter.
But given the fact that your pump
would rarely be running from the in-
verter, you would be better off simply
purchasing several NTC thermistors
similar to what was used in the Soft
Starter and see how many you need
to connect in series before the pump
will start reliably on inverter power.
Assuming that this works, you could
then mount them in a generously sized
box (ideally a vented metal enclosure)
with a mains plug and socket at each
end. It would waste a bit of power as
the thermistors would run hot while
the pump is operating, but as long
as they have enough air space, they
should be OK.
The thermistors could be connected
between the inverter and UPS switch-
ing relays, so that they are out of circuit
when running from mains, as would be
the case most of the time. They would
only come into play on the rare occa-
sion that the pump was running and
mains power was absent.
Alternatively, it would be possible
to modify the UPS design to leave the
inverter off and switch it on when
mains power had failed, and it detect-
ed that the pump wanted to start (via
load sensing circuitry).
But that would add quite a bit of
extra complexity; it would probably
require a secondary 12V battery to
run the electronics until the inverter
started (and maybe a separate charger).
So we think you should try the ther-
mistor approach first, as it’s much sim-
pler and may well do the job.Building UPS using 12V
battery and inverter
I want to build your UPS de-
sign from the May-July 2018 issue
(siliconchip.com.au/Series/323) but
using a 12V battery bank rather than
the 24V bank that you used.
I assume that since the Arduino
board and relays in your design run off
a 12V supply, I could simply change
the charger, inverter and batteries to
12V and then run these other compo-
nents straight off the battery.
Obviously, the DC battery cabling
would need to be upgraded to handle
double the current. The reason I want
to do this is that I already own a 12V,
20A charger, a 12V 32Ah battery and
a 12V Giandel inverter.
My other question is: how well will
the UPS kit handle a nearby lightning
strike or another type of pulse on the
mains? The Arduino could be the weak
link here. (N. M., Yass River, NSW)- The software should work with-
out changes if you use a 12V battery
instead of 24V, as the voltage thresh-
olds can be set during the setup phase.
However, the battery thresholds will
need to be changed to suit.
The relays and Arduino board in our
design are fed from a separate 240V-
12V power supply, which is powered
from the inverter output.
This is not just because we used
a 24V battery bank; it was designed
this way so that the load is totally re-
moved from the batteries if the unit
shuts down.
Running the Arduino and relays di-
rectly from the battery will change its
behaviour; in particular, it will not be
able to shut itself down entirely, as it
will be powered even after command-
ing the PSU to turn off.
How the unit responds to irregu-
larities on the mains will depend on
the robustness of the individual com-
ponents.
The sensing transformer and voltage
divider resistors isolate the Arduino,
and a brief surge should not cause any
problems. It’s designed to be able to
handle higher voltages than it would
usually be exposed to, anyway.
So we wouldn’t expect the Arduino
to be damaged except by a particularly
bad spike or surge. You could add ex-
tra surge protection components (eg,
a mains filter) if you are concerned
or in a particularly lightning-strike
prone area.DCC Programmer not
working with decoders
I have built the Arduino-based
DCC Programmer (October 2018;
siliconchip.com.au/Article/11261),
and it works fine as a programmer.
But when I tried to run it as a base sta-
tion, it works with NCE decoders but
will not work with Digitrax decoders.
This is a problem as I have a mixture
of these decoders.
Could this be a timing issue related
to the 116μs pulse width? If so, is there
any way this could be corrected? (G.
P., Stafford Heights, Qld)- There’s no particular reason why the
DCC Programmer should work with
NCE decoders but not Digitrax. That
it’s working fine as a programmer sug-
I want to build your new Remote
Controller Preamp with Tone Con-
trols (March-April 2019; siliconchip.
com.au/Series/333) using the rec-
ommended Universal Power Sup-
ply board. You haven’t given a part
number for transformer T1 in the
parts list, and it is not shown on
the circuit board. I guess it must be
mounted off-board? Do you have a
part number for it? (L. E. B., Beer-
burrum, Qld)- You are right that the transform-
er does not mount on the Universal
Regulator board. You will need to
use a chassis-mounting transformer.
We didn’t give a specific part
number because there are many dif-
ferent transformers which could be
used. For example, you could use a
toroidal transformer, EI-core trans-
former or even an AC plugpack.
We recommend that you use a
15V or 15-0-15V (30V centre-tapped)
transformer rated for at least 15VA to
power the preamp via the Universal
Regulator. Jaycar Cat MT2086 or Al-
tronics Cat M4915B would be ideal.Which transformer to use with Universal Power Supply
If you want to save a bit of money,
Jaycar Cat MM2008 and Altronics
Cat M6672L are a bit cheaper, but
will have more flux leakage than a
toroidal type.
Jaycar Cat MM2002 and Altronics
Cat M2155L are cheaper again, but
you would only get half-wave rec-
tification. That’s probably good, but
this will result in more ripple at the
regulator inputs.
If you want to use a plugpack in-
stead, try Jaycar Cat MP3021 or Al-
tronics Cat M9325A.