power supply04

40A PSU

■Cmsgb nceio

Fig 4: Rear panel detalls.

PCB and it is advisable to liberally apply solder ovcr these regions. Finally. notc that che PSU must not be started under load. If you try to start it under load Ulen you are likely to bum out Rl.

TEST & CALIBRATION

CHECK AND RECHHCK your w i ring; if you*vc madę a mistake it might tum out to bc morę frightening (han you would e\pect.

Set R11 (currciu limit) fully eloekwise and RI4 (voltage) fully countcr clockwisc. Apply the mains - there should not be any voltage at the output icrminals. Press and hołd momen-tary action START swiich S2 - after about five scconds the relay should puli in and LED DS2 should lighl up. The output voltagC will be below 13.2V - soadjust RI4 to obtain 1.1.2V output.

You need to adjust Rl 1 sothat thePSUjust gocsintocurrcnt limit modę whcn40A isbeing drawn. For this you require a 0.33Ł2 variable resistor that can handle 550W. This might seem to be a problem but in reality il’s fairly easy to make a variable trans i stance using

Fig 5: Heat sink covor dotails. Dotted linos are 90" lolds.

some2N3055 transistors mountedon a heatsink as shown in Fig 6.

You will also need a 40A Ammetcr or a smal ter Ammeter and shunt. Set the variable transistancc to maximum resistancc and eon nęci it to the PSIJ output. Adjust the variable transistancc until cxactly 40A is flowing and then adjust R11 to the point where the PSUjust about goesintocurrent limit modę, (ie, the PSU output voltagejust begins to fali). Remove the variable transistance.

The short circuit test involves connecting a substantial connector in linę with your 40A Ammeter across the PSI.' outputs. The PSU output eurrent should drop to just about zero and DSI (FAULT) I.F.D shouid light up. Re-movc the connector and check that DS1 goes out and the I3.2V output returns.

CONCLUSIONS

THE MOST EXPENSIVE part of this project is the transformer; the cheapcst place to obtain one is from a rally. The transformer can be madę up from Iwo transformers w ith sccondar-ies of I5V at 25A. Make surę they are con-nected in the correct phase. The secondary vo!tage must not be morę than ISV RMS off load and not less that 15V under fuli load.

Capacitor CI can bc madę up by connecting Iwo 47.000pF capacitors in parallcl or one lOO.OOOpF. Diodę Dl can bc madę from four 50A diodes arranged in a bridge if a 50A bridge is not available as a complete device. Likely sources for relay Rl.l are commercial vehicle or truck dealership Parts Departments or commercial vehicle repair works.

MI. in Fig I. is a fan motor and not a meter as shown. Additional Information can bc ob-tained from the aulhor provided an SAE is enclosed.    ♦

Fig 6: Variable transistance.

COMPONENTS LIST

Resistors

All 0.25W 5% unless otherwbe noted

I Rl	10R 5W
2 RUS.R20	1 OR
4 R2, R5. R7, RS	Ik
I R3	2R7 3W
I R4	Seetest
I R6	220R
3 R9.RIO.R19	470R
I RU	IGOR
3 RI2.RI6.RI7	I20R
I R13	5K6
I RI4	Ik
1 R15 5 R21.R22, R23,	6kS
R24.R25	0RII7W
1 RTI	4k7 Ri-adthcnnistor
Capacitors 1 Cl 7C2.C3.C4.C6.	94000uF 35V Blectrolytic
C7. CIO, CII	lOnF 50V Ceramic
1 C5	luF 35V Blectrolytic
2C8.C9	IOGuE35V Blectrolytic
1 Cl 2	M)0nE50V (bramie
Semiionductois 1 Dl	50A 50V 1*1 V Bridge rectifier
1 1)2	4V7400niW /ener diodę
1 D3	IN4I48 Signal diodę
1 IM	9V1 4(Khn\V Ze ner diodę
1 D5	15V 4(K)m\V /ener diodę
1 DSI	Red 5mm LF.D
1 1)82	Green 5mm LED
1 TRI 6TR2, TR3.TR4.	BC1S4 NPN
TR5, 1 Rf>, TR7	2N3055
1 SCRI	13A Thyristor
1 Ul	723 LM723 Volt-
	age Regulator
Misceliimeous

I 50A binding post, hlack, -OUT I 50A binding post, red +OIIT I 5A binding post. red (Rento teSense positivcj I 5A binding post, blackl Kemolc Sense negalhe)

I IIIuw    SA

I I useholder. panel niount I Fusehulder. rubber boot I IEC mains turo Socket witli integral EMC liltcr błock

I IEC Socket rubbet IkkiI I Ml 24 VDC0.2A mui.mum IX' Arial fan I PCB

I Project ca«

I KI I 24VIX 4(1 A IruckrcIaHor 12V40Acar relay & R4)

I SI ON/OFFSPST switch 240V    6A

I S2SMAKI Momentary action switch 3A

1    Tl Transformer 240/ 16V (at 5t)A 5011/ single windingprimary, single w indingsecondary speci-lied at fuli load

2    Heatsink, 1.2in C/W or bet ter beat siała capable of holding .1T03 deeices each

I Home-brew heatsink cover

6 I wt for project case

6 T03 style tbcrmal mounting kils

16 PCPins

llook up wirc

Heatshnnk sleeving

Heavy duły wire

Metal gauze and filier for fan entry hole VI3 Solder tags VI3 spacers

64

RADIO COMMUNICATION May 1996