power supply03

40A Power Supply Unit

The concluding part by Ben Spencer*

W iWw Kuią Si, flttth. •tvi.il RAI 2Ul.

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Fig 3: (a) Component overlay and (b) PCB tracking [Enclosed].

THE PCB HAS ONE corner truncatcd at 45°, to aid component location & Identification. Refcr to the component location diagram sbown in Fig 3a with tracking shown in Fig 3b.

Usc wirc that is KAWG, (IOSWG, O.I28in OD, 3mm OD) (brali hcavy duły conncctions. Doubling up 2.5mnr doinestic ring main black & red will do. R4, shown in Fig 1 (part I) not used. Fit thick wirc link on P('B board. Rcsis-tor R2 and capacitor C2 are soldcred directly across the soldcr tags for CI with C1 bolted to the chassis with a cl ip. Rclay RL1 isa40A 24V truck relay or a 40A 12V car rclay could be used and R42 included.

The two heat sinks (with transistors TR3 to TR7 & R2I to R25) arc fitted to the insidc of the rear pand, sce Fig 4, along with the DC fan and mains 11X2 pług. Transistors TR3 to TR7 musi be fitted with T03 thermal washers and spigots. Usc hcatsink pastę to givc good thermal contact. Check that the collectors, emitters and bases of TR3 to TR7 arc a vcry high resistancc to ground (ie, not short circuited).

The cooling fan nceds to be sufficicntly largo to cool the heat sinks under worst case conditions, that is when drawing 40A and dropping 350W across the power transistors. The easiest choice here is to Fit the largest you can. Transistor TRI can sink up aboul 200mA.

The PSU fan forccs air into the PSU. Aftcr circulating around the insidc of the case the air is foreed out of the rear panel through the arcas marked CUT OUT (sec I ig4) at the base of the rear panel beneath the heat sinks. To duet the air over the PSU heat sinks two homc madę heat sink covers are re<|uired. These can be manufacturcd by folding either plastic or aluminium shect at 90° as shown in Fig 5.

The cover is slid ovcr the heat sink with the lower fold of the covcr aligning with the base of the PSU case. The two outer folds align with and arc affixed to the outer heat sink fins. The cover thus forms a vertical chimney prevent-ing the aireseaping sideways or downwards.

II using metal for the covers ensure that it docsnT short out the power transistors.

Uead thermistor RTI has its leads cut to about 0.25in (6mni) and two flying leads sol dered. The soldcred joints are then covered with hcatshrink $ieeving and the head is mounted on top of TR3 (or TR4 orTR5etc). A smali piece of expandcd polystyrene is then placed over the bcad to prevent draughts from the fan affecting it and a smali home-brew metal clip is used to hołd it in place.

The PC pins on the PCB connect as follows:

1)    A to DSI anodę (light duty).

2)    B to DSI cathode (light duty).

3)    C to TR3 emitter/R21 nodc (light duty).

4)    Dto+OUT positivc terminal (light duty).

5)    K to DS2 anodę (light duty).

6)    F to TR2 base (light duty).

7)    Gto+Sremotescnscterminal(lighłduty). S) 11 to DC fan positive (light duty).

9)    .1 to DC fan negativc (light duty).

10)    K to DS2 cathode (light duty).

11)    U to CI positivc terminal (heavy duty).

12)    M to RUI coil positive (light duty).

13)    N to OUT rcmote sense terminal (light duty).

14)    P to RUI coil negatise.

15)    TR to CI negatiw terminal (heavy duty).

16)    RtoCI negativc terminal (heavy duty). Therc arc a fcw large copper arcas on the

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RADIO COMMUNICATION May 1996