power supply02

40A PSU

Fig 2: Using remote sensing.

SPECIFICATION

Input uiltuge: 240 VAC Output vultage: I3.2YDC Output currcnt: 40AIX'

Omcurrcnl protcction: Cuneot limiting at 40A Short Circuit protcction: Regulator shut off.

Ołcrłoltagc protcction: Shuls ofTDC input and discharges input stage resenoir.

Over temperature protcction: l an automatically operatesat heatsink temperaturo of65°C.

Indicators: Power ON I BDand Short Circuit Protcction activc LED.

biascd and switches ICI's interna! trans i stor on. robbing ICI: 10 of all currcnt and shutting the regulator down. I'liis prevents a short Circuit across thc output wrecking TR3 to TR7. Diodę DSI lights up to wam of thc fauli. If the short Circuit is removed, the 9.1V set up by R7 and D2 rcvcrsc biases lCIs interna! transistor emitterand thc PSU output will return to I3.2V.

Tenninals +S and -S are for remotc sensing. wherethe PSU might bc iocated some di stance front the equipment tt is powering and a volt-age drop occurs along theequipment DC power leads (Fig 2). The remotc sense tcrminalsdraw very liltle eurrent and are used purely to mcas-urc thc voltagc at thc load. If the delisered voltage is helów 13.2V. thc PSI will incrcasc thc voltagc at the output terminals to make up for the drop along the equipmcnt leads so that cxactly I3.2V is ddivered to the load. If rc-mote sensing is not used then shorting links must be connected across thc PSU output terminals. Resistors R18 and R20 are included for forgetful oper a tors.

Ovcrvoltage is only likely to occur if one of TR3 to TR7 fails short Circuit. Overvoltage protcction is provided by D5. R16, R17 and SCRI.1 f thc PSU output inereases above I5V (at the load) then D5 conductsand fires thyristor SCRI. Whcn SCRI fires. it short circuits the unregulated input discharging Cl via R3 and causing relay RLI to drop out. When the relay drops out it prevents capacitor Cl front heing rcchargcd on the ncxt reciified half cycle and the PSU is latched in thc OFF stale.

A PSU o! this catcgory will dcvclop a fair amount of heat in TR3 to TR7. roughly 350W at 13.2V 4QA output and this must be trans-ferred to the environment via suitable heat sinks. Previously published designs havc ci-ther had a fan that continuously runs (which after prolonged periods beconics intcnscly irritating) or one that must be switchcd on by thc operator reeptiring the operator to periodi-eally test heatsink temperaturę (scorched ftn-gers). Ot ber designs simply shut the PSU down until it bas cooled, which is pretty uscr un friendly if you happen to be working a rarc DX station. This design overcomcs both of these problems by using a simplc but effective auto-matic fan controllcr.

A 4.7V rcference is set up by R5 and D2. Bcad thermistor RTI is thertnally couplcd to a hol spot on one of the heat sinks (ie. on top of TR3). At Iow ambient temperatures the thermistor bas a high resistance and TRI is switehed off. IJnder heavy usage thc PSU heatsink warms lip and so does thermistor RTI.

As 65°C the thermistor value has dropped sufficicntly forR6todcvclop0.65Vandswitch onTR I drivingthc DCfan.blowingcoolingair ovcr the heat sinks. As the heatsink cools so does RTI and at 55X the reverse process occurs and the fan is switehed off. Switching is gradual with the result that thc PSU is not tripped out by the fan operating and thc fan runs for just long enough to cool thc PSU down.    ♦

. . . to be continued