Page 32 - 39. MRH13-05-May2013
Basic HTML Version
DCC Impulses Column - 2
(2 inputs, one + and one - output) or can be made from four
discrete diodes. Generally, I recommend the module, for small
size and ease of construction (3).
Rectifiers have two primary specifications, voltage and current.
The rated voltage is properly known as Peak Inverse (reverse)
Voltage (PIV) and is the maximum voltage that the rectifier is
designed to withstand backwards across the diode. Back to
my water analogy from my December 2012
Basic Electronics column. PIV is the maximum pressure that
the check valve will withstand when the water is trying to flow
against the closed valve.
Rectifiers are usually rated at 50 volts or higher. Since this
exceeds the maximum DCC voltage, most any will work just
fine, voltage wise.
The sum of all the current drawn through the rectifier should
be less than the rating on the rectifier. One amp is about the
minimum current rating for a rectifier and works in the vast
majority of DCC uses. In the case of this circuit, there will be
2
3: Bridge rectifier module vs. discrete diodes.
about ⅛ amp being drawn through the rectifier, so a 1-amp
unit will be fine.
There are two capacitors in this circuit.
The filter capacitor (C1) is an electrolytic (polarized) unit. Its
function is to store the pulses passed by the rectifier. It makes
DC out of the pulses of DCC that have been directed by the
bridge rectifier. 10 µF is enough to smooth out the pulses. A
25-volt rating will work with all DCC track voltages up to 20
volts. If you are in garden railroading or using track voltages
up to the 27-volt DCC maximum, use a 35-volt capacitor for
safe operation.
The other capacitor (C2) is there to remove any spikes left due
to asymmetry in the DCC signal. This should be a small mica or
ceramic capacitor. The electrolytic capacitor doesn’t filter these
small spikes, so the addition of a spike suppressor capacitor in
parallel will help you have clean DC for sensitive electronics like
the camera. A frequent value is 0.1 µF at 50 volts. This could be
anywhere from 0.01 µF to 0.5 µF, as long as the rated voltage is
above 35 volts.
Figure 5 shows the oscilloscope display reading the DCC signal
being applied to the power supply whose circuit is in figure 2.
2: Power supply schematic.
3
MRH-May 2013
