TF3MA, Sun Sep 8 11:29:37 GMT 2002

13.8V 25Amp linear power supply

essi sa hefur veri heimstt ---- sinnum san 7. Jn 2003


Circuit details

I got the transformer from an old heater controller. Any transformer can be used, it has to provide about 19V RMS at the rated current. Extra transformer may be needed for the 30V supply for LM723 regulator. The bridge rectifier is a 35Amp type and the 40.000uF capacitor is computer grade, I used 3 12.900uF 40V surplus caps.
The LM723 regulator is used to control the 3 or 4 MOSFETs, I used MOSFETs instead of bipolar transistors as the MOSFETs can be driven directly by the LM723. The gates of the MOSFETs are protected by 15V zener diodes. 0.1 Ohm resistors are used to equalize the current between the MOSFETs and also to for current sensing. Fold-back current limiting is done by R5 and D4 which provide extra current into R14 when the output voltage is under VREF, and thereby lower the current limit set by the 0.1 Ohm resistors.
The output voltage is set by R11 and should be set to 13.8V. 3 led indicator are used, one for AC power, one for current limit indication and one for monitoring the VOUT from LM723.
The crowbar circuit is for over-voltage protection and fires at about 16 Volts. The voltage is sensed by R31 and R32 and compared to the reference voltage provided by zener diode Z5 which is 6.8 V.
The 230V MOV is used to suppress any spikes on the 230V AC line. Two fuses are used, 3.15 Amp Slow for the AC and 20 Amp for the DC.
A 10 Ohm NTC inrush current limiter should be inserted in series with the 220 V AC input line to limit the turn on surge current.
The P6K839A is a 36V zener voltage suppressor, any 1W 36V zener can be used instead, it is for protecting the LM723 which has max voltage rating of 40V.
73 de TF3MA
Schematic diagram drawn using gschem