hot to the touch, even with a small heat sink attached. This was
too hot for a closed plastic shell. A melt-down was coming.
Fortunately, there is relatively new technology in the form
of switching regulators to save the day. I had on hand a few
78ST105HC modules which come in a plastic frame with
no heat sink. They provided the same regulation function
and worked great, generating little heat if any. While the
78ST105HC is hard to get, their success prompted me to find
the PSU2-5 from EzSBC.com, also available from their eBay
store, an even smaller switching regulator in the same package
outline as an LM7805. The heat problem was solved. A small
capacitor was added across the output of the regulator —
about 33uf at 25 volts. This capacitance value is not critical but
use at least a 10 volt rating.
The last component in the basic power unit is a magnetic
reed switch to turn the unit on and off. A plain single-pole
6
6. LiPo batteries.
Battery-powered models - 4
single-throw switch can also be used. LiPos are supposed to
have a limited lifetime of charge/recharge cycles, as do most if
not all rechargeable batteries. So when I’m done operating, I
switch the unit off either with a tiny magnet attached to a fig-
ure [8], a small wand, or other device.
The reed switch is a single-pole double- throw switch, whose
“normally on” side is used, such that when the switching mag-
net is taken away it assumes the “on” position. I was able to
locate these easily on eBay
A
mechanical switch will also do well, but it requires more han-
dling of the model to operate. Small reed switches like these
are easily hidden, and can be activated by a small magnet
without direct contact. This is a nice feature to build into a
model. If the model is not used for a long time (say a week),
it may be necessary to use the switch to disconnect the load,
and recharge the battery back-up.
I have been able to operate the twin motors of one diesel
(described later), for over an hour with no other power source.
Current draw was about 180 ma. For lower-power LED lighting
in cars, operation is much longer. While this will not provide
7
7. Battery manager board with built-in DC-DC 5V
converter.
MRH-Nov 2014
