power supply01

40 A Power Supply Unit

The first of a two part article by Ben Spencer, G4YNM¹

Tl IIS PROJECT dcscribcs a Protcctcd 13.2Y40A Power Supply Unii which. providcd ihc heal sinkingand cooling fan arc suitublc, iscapableofrunning at fuli power al I (K)% duły tycie. H features soft starting,ovcrvoltage, current limiting,short Circuit protection and automatic fan control. It is a substantial project that is not for the novioe eon struć tor.

CIRCUIT DESCRIPTION

THE HEAVY CURRENT carrying conncc-tions arc shown in bold in Fig 1. The mains input passes through an EMC filier, protection fu.se FI and ON/OFF switch S1 to transformer Tl. The sccondary output of Tl is rcctified by DI but cannot pass through open relay contacts RLla. To start the PSU switch. S2 is operated allowing limited current to pass via R1 to slowly charge the reservoir capacitor Cl. As Cl charges, the RLla pullsinclosing the relay contacts and shorting out RI and S2 placing the PSU in the ON State.

Voltage regulator 1CI is the popular 723 . A 7.2 V reference on IC1:6 is fed to non-invcrting IC1:5. Tliis is coinpared with a sample of the PSU output voltage via R13, R14 and RI5 to inverting input at IC1:4. The 723 can source !50mA at ICI: 10, so transistor TR2 aets as a Darlington drivcr for the pass transistors TR3 to TR7 inclusive. Resistors R2I to R25 are current sharing resistors and the voltage across them is proportional to the current drawn by the PSU load.

When the current through R21 reaches 8A (ic. a total PSU current output of 40A) the voliage across Rl I and R12 is 0.88V and is tapped from Rl I to biasatransistorinside ICI which robs IC 1: 10 of some current. forcing the PSU into a current iimiting modę.

If the PSU load is such that the current limited output voltage falls to about 6V (ie, a short Circuit) diodę DS1 bccomcs forward

lr* if 2CV relay used

g PKIB kio;

Fig 1: The Circuit diagram of the complete 40A Power Supply Unit.

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RADIO COMMUNICATION April 1996

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i) New King Sl, Bath. Avon HA I 2BL