X10 TW523/PSC05 Power Supply Modifications
Jeff
I have seen several reports of problems with the power supply in the X10
TW523/PSC05 interface. Since I've designed capacitor-coupled supplies
in the past, I decided to take a closer look at this one to see what can
be done to improve its performance.
I built a simulation in CircuitMaker using the standard values with a
pulsed load lasting the duration of one X10 transmission. It became
evident the suggested capacitor modification from 100uF to 470uF on the
-15V supply doesn't help significantly because of the duration of a
standard X10 transmission.
The circuit seems to be starved for current because of the size of the
AC dropping capacitor. This allows only 1 watt of energy to be
delivered to the circuitry. It appears the first step in improving the
power supply is to increase the value of this capacitor to 3.3uF at
250V. This increases the available energy to 1.5 watts. Several
suitable capacitors are available from Digikey. The most critical
factors are voltage (250V) and thickness (10 mm is a tight squeeze).
The second modification does two things. It dissipates the extra
half-watt when the interface is not transmitting, and provides a
secondary current path to power the -15V circuitry when the transmitter
is pulling down the -30V supply. This modification adds a resistor in
parallel with 15V zener diode Z5. The optimum value for this resistor
appears to be in the 330 to 470 ohm area, but is dependent on the
average -15V current load. This resistor should be 1 watt to safely
handle the dissipation.
A third modification that helps handle the transmitter load is to
increase the 470uF -30V filter capacitor C13 to 1000uF. There is no
need to increase the value of the 100uF capacitor C12.
Rather than clip the LED lead as some have suggested, the LED function
can be retained, and the current drain can be reduced substantially by
increasing the value of the 1K dropping resistor R6 from 1K to 3.3K.
Obviously, the LED will be much dimmer, but the operation can still be
observed if necessary.
These changes should solve problems caused by the marginal TW523/PSC05
power supply.
The component numbers refer to the excellent schematic found at:
http://www.geocities.com/ido_bartana/220tw523.gif
I hope this helps solve some of your problems.