This article documents an implementation of PAROT (Power Amplifier Run On Timer) using Transformerless power supply for PAROT.
This PAROT uses a 230V AC relay for 230V mains switching and includes PTT switching using an FOD852 opto coupler.
The intended application is to control power to a valve PA, providing programmable heater delay, and cool down delay of power off.
Above is the electronics built on a small piece of Veroboard. This one uses a 0.47µF cap as the power supply current requirements are a little lower than for the SSR.
Above is the copper side of the Veroboard. The layout is designed to accommodate another implementation using a small Triac to switch a 230V AC relay. The board has been given a heavy coat of acrylic PCB lacquer to improve voltage withstand.
A workbench checkout revealed the quality of cheap Chinese caps, the first one fitted was 55% of the marked value and the Parot would not run. Replacing it with a cap measuring 0.45µF solved the problem.
The PAROT was inclosed in an inexpensive Jiffy box to make it safe for an extended trial.
Measurement of the power supply on load
Above is a scope capture of the power supply Vcc rail with the power supply loaded (LED on and Triac driven), mains at 241Vrms.
Above is an expanded view of the ripple. Ripple measures 160mVpp, equivalent to 46mVrms, ripple factor is 1.3%.
A thermograph of the interior after running on full load with the cover on shows the relay coil to be the only hotspot at 30° rise. This is a low cost ($3) Chinese relay (JQX-13F with a 200-220V coil on 240V). It is a DPDT 10A AC contact set, and both contacts are wired in parallel. The temperature of the operated relay is similar with zero load current.
Brown out test
The PAROT was connected to a Variac and voltage varied from 265V down. The device worked down to 140V at which it dropped the load, but reapplied power to the load when the voltage was raised to 150V. The power supply is quite adequate to supply the current required by the electronics.
A test with a portable receiver and listening on the station receiver did not reveal any significant noise emissions from the PAROT.
Extension to other contexts
This design is for nominal 230Vrms 50Hz AC supply. The main power supply series capacitor must be designed for the voltage and frequency used.
This implementation is configured for 10s heater delay, and 900s cool down time, no other options are active.
Above is the EEPROM configuration (using Hex Editor Neo’s structure viewer).
- Duffy, O. May 2016. Transformerless power supply for PAROT.
- Duffy, O. March 2013. Power Amplifier Run On Timer (PAROT). https://owenduffy.net/module/parot/index.htm.