Maplin auto electronics projects P7

Maplin auto electronics projects

Figure 7.3 PCB legend and track

î

Figure 7.4 Fitting unit into box

Take note — Take note — Take note — Take note When carrying out any form of electrical work on

a vehicle always disconnect the battery and

never work inside the engine compartment with

the engine running!

• door s w i t c h e s are fitted to the 0 V side of the

cour-t e s y lighcour-t, for inscour-tallacour-tion follow Figures 7.5(a) or 7 5 ( b ) ,

• door s w i t c h e s are fitted breaking the +12 V supply

to the c o u r t e s y light, for installation follow Figures 7.5(c)

or 7.5(d)

In its simplest configuration, the unit c o n n e c t s directly

a c r o s s a door switch; PI c o n n e c t i n g to the more tive side of the switch and P3 to the more negative

posi-If ignition override is required then P2 must be c o n n e c t e d

to a s o u r c e that has power while the ignition is on (for example, + SW terminal of the ignition c o i l ) If no e a s y

c o n n e c t i o n can be made to the ignition circuit then P2

can be c o n n e c t e d into the accessory circuit

As the c o m p l e t e unit is small and unobtrusive it can ily be mounted inside a door-post, behind an existing door switch T h e b o x can be held in place using a self-adhesive pad ( s u c h as HB22Y) or bolted down using the two mounting h o l e s provided in t h e b a s e of t h e b o x Check behind panels before drilling any holes and en-sure that no wiring h a r n e s s or o t h e r c o m p o n e n t s are located behind panels that would otherwise be damaged

eas-Min Typ Max Units

Table 7.1 Specification of prototype

Courtesy +12V ü 3 ht

Fueebox ® I I I I ι lp1

y S S J Extender

Courtesy

+ 1 2 v LÎQht To ignition switch From QO * j · » 1 P1 ( o r other source of Fueebox r-—if- 1 —ι +12 V when engine

Courtesy light parts list

Resistors — All 0.6 W 1% metal film

R l , 3 4k7 2 (M4K7) R2 5 6 0 k 1 (M560K)

c o u r t e s y light leaflet 1 (XK96E)

c o n s t r u c t o r s ' guide 1 (XH79L) All of the a b o v e are available as a kit

c o u r t e s y light kit 1 (LP66W)

Optional (not in kit)

16/0.2 black hook up wire 1 (FA26D) 16/0.2 red hook up wire 1 (FA33L)

mode psu

Take note — Take note — Take note — Take note

This project requires a fair degree of expertise and tience to build Please read through the article thoroughly before undertaking construction The power supply design described in the following article is intended to power two Maplin 50 W bipolar amplifiers and a stereo pre-amplifier

pa-It is possible for the power supply to be used to supply other amplifiers, however, it is outside the scope of this article to detail the necessary modifications The supply

is capable of delivering instantaneous power levels much higher than the continuous rating, which is ideal for au- dio applications where the peak current requirement, due

to transients, is much higher than the average current quirement Higher levels of power may be drawn as long as the average power is maintained at 120 W The figure of 120 W

re-is based on maintaining a heatsink temperature at less than 65°C

For many years the motorist has not been able to efit from hi-fi quality sound while travelling in the car For the long-distance traveller, b u s i n e s s e x e c u t i v e or hi-

ben-fi buff o n - t h e - m o v e , t h e c a r is a far from i d e a l

e n v i r o n m e n t for l i s t e n i n g to m u s i c ; t h i s is due to a number of r e a s o n s First, the c a r ' s interior is designed for conveying p a s s e n g e r s and not for ideal location of

c o n v e n t i o n a l box design l o u d s p e a k e r s S e c o n d , t h e

sound replay/receiving equipment has to be

miniatur-i s e d and c a p a b l e of o p e r a t miniatur-i o n miniatur-in a v e r y h a r s h environment Dashboard temperatures often e x c e e d 60°C

in hot weather (yes, even in the English climate!) and fall to several degrees below zero in cold weather Vi-bration and humidity also add to the s t r e s s e s that the equipment must endure Third, the low, noisy and some-what v a r i a b l e s u p p l y v o l t a g e m a k e s life e v e n m o r e difficult for the e l e c t r o n i c circuitry

The environmental and size p r o b l e m s of the c a r ronment have largely been solved by cleverly designed equipment Car loudspeakers are optimised for opera-tion in rear parcel s h e l v e s and door panels instead of

envi-c o n v e n t i o n a l s e a l e d or p o r t e d e n envi-c l o s u r e s Car radio,

c a s s e t t e , CD ( c o m p a c t d i s c ) and DAT (digital audio t a p e ) equipment is very c o m p a c t Such equipment is designed for either mounting in the d a s h b o a r d / c e n t r e c o n s o l e or

r e m o t e mounting in the b o o t or under a seat, with just the c o n t r o l s l o c a t e d within the driver's e a s y r e a c h

It is however, the third point that is the main reason for this project, the v e h i c l e e l e c t r i c a l supply T h e 12 V elec-trical system is far from ideal when it c o m e s to powering audio amplifiers T h e e l e c t r i c a l s y s t e m itself, although generally referred to as being 12 V, usually o p e r a t e s at

around 1 3 - 1 4 V when the engine is running By tion, the voltage when the engine is running is a s s u m e d

around 22 W r.m.s into a 4 Ω load Most high power

ra-d i o / c a s s e t t e players have an output power of arounra-d

22 W r.m.s., regardless of how many watts the ing b r o c h u r e s boast!

advertisFor hifi quality sound reproduction in a c a r it is n e c e s sary to have the capability of higher power levels This

-not being required for blowing out the windows (although

often used as such by drivers of ageing Ford Cortinas with pink fluffy dice hanging from the interior rear-view mirror), but simply b e c a u s e a high power amplifier op-erating at m o d e s t power levels will introduce far less distortion and handle t r a n s i e n t s far b e t t e r than a me-dium power amplifier running almost flat out This is especially true if the sound s o u r c e is CD, where the dy-namic range of the recording is often very wide

T h e r e are two ways in which the output power can b e

i n c r e a s e d , by either d e c r e a s i n g the loudspeaker

imped-a n c e o r i n c r e imped-a s i n g t h e s u p p l y v o l t imped-a g e T h e m imped-a i n disadvantage of the former m e t h o d is that c a r s p e a k e r s are not c o m m o n l y produced with i m p e d a n c e s below 4 Ω and that power l o s s e s in c a b l e s are i n c r e a s e d T h e lat-ter method of increasing the supply voltage is c o m m o n l y

used in high power c a r boosters and in hi-fi c a r audio

amplifiers — this is the method that is d e s c r i b e d here

Circuit description

Figure 8.1 shows a b l o c k diagram r e p r e s e n t a t i o n of the power supply and Figure 8.2 shows the full circuit dia-gram

T h e supply input to the power supply is via P I (+V) and P2 (0 V ) T h e power supply is c o n n e c t e d directly to the vehicle b a t t e r y via high current c a b l e s , therefore the off-

b o a r d supply fuse FS1 is e s s e n t i a l in c a s e of a fault causing a short circuit directly a c r o s s the battery Re-

m o t e p o w e r s w i t c h i n g is a c h i e v e d by T R I , RL1 and

a s s o c i a t e d c o m p o n e n t s T h e c o n t r o l input P3, when

t a k e n to +V, b i a s e s T R I on and o p e r a t e s R L 1 , t h u s powering-up the rest of the supply LD1 s e r v e s to indi-

c a t e power on T h e c o n t r o l signal is provided by the electric aerial output found on most radio-cassette units

Diode Dl clamps the voltage spike produced by RL1 when

it de-energises Diode D2 provides polarity p r o t e c t i o n

by blowing fuse FS2 and preventing the r e m o t e power switch from operating

Capacitors C I , C2, C3, C4, C5 and toroid LI form the put ji-filter, the output of which supplies the push-pull output stage T h e power MOSFETs are arranged in two pairs (TR2 and T R 3 ) and ( T R 4 and T R 5 ) , e a c h driving one half of the transformer primary R e s i s t o r R8 and ca-

in-p a c i t o r C6 form a s n u b b e r n e t w o r k to i n c r e a s e t h e rise-time of switching spikes Components ZD1, D3 and TR2 and ZD2, D4 and TR5 form an active spike clamp, employed to p r o t e c t the MOSFETs' d r a i n / s o u r c e junc-tions from high voltage switching spikes This o p e r a t e s

by feeding the spike back into the gate of the relevant

Figure 8.2 Circuit diagram of the switching PSU

— ^ P a g e l ö e

13 I ! ι J 16

3 für

n Ί is ' ! ! "

Figure 8.2 Continued

MOSFET thus turning it on and clamping t h e spike Gate

r e s i s t o r s R4 to R7 help to b a l a n c e current flow through

e a c h MOSFET pair and a l s o help to r e d u c e switching noise

T l is a s t e p up transformer comprising six windings, two

c o n n e c t e d to form a c e n t r e tapped primary winding and four are c o n n e c t e d in two pairs to form two c e n t r e tapped

s e c o n d a r y windings

C o m p o n e n t s R36, C27 and C28 form a simple R-C filter for ICI which a t t e n u a t e s supply b o r n e noise C29 and R20 set t h e soft-start time period for ICI At switch on, C29 is discharged and I C l ' s outputs are inhibited As C29

c h a r g e s via R20, t h e pulse width of the PWM drive nals are allowed to i n c r e a s e from zero D14 prevents I C l ' s soft-start input from being pulled negative at switch-off and also s e r v e s to d i s c h a r g e C29 more quickly T R 6 dis-

sig-c h a r g e s C29 and inhibits I C l ' s outputs in r e s p o n s e to a thermal shutdown condition or a s t a n d b y input (low) from P19 D18 and D13 form a d i s c r e t e AND circuit When the shutdown condition and standby inputs are removed, TR6 allows C29 to c h a r g e again and t h e power supply

r e s t a r t s

R e s i s t o r R21 and c a p a c i t o r C31 s e t t h e o s c i l l a t o r

fre-q u e n c y , P 2 4 m a y b e u s e d t o m o n i t o r t h e o s c i l l a t o r waveform Care should b e e x e r c i s e d to e n s u r e that this pin is not s u b j e c t to undue c a p a c i t i v e loading, o t h e r w i s e

t h e o s c i l l a t o r frequency will shift

could break into oscillation R e s i s t o r s R15 and R16 form

a potential divider which is used to apply over voltage feedback to ICI, with the values as shown, the maximum output voltage is ±30 V

T r a n s i s t o r s TR7 to TRIO and a s s o c i a t e d c o m p o n e n t s form two high speed driver c i r c u i t s which are able to

c h a r g e and d i s c h a r g e the gate c a p a c i t a n c e of e a c h of the MOSFETs very quickly Circuit operation for one of the (two i d e n t i c a l ) drivers is as follows: R23 is t h e pull-up resistor for the open c o l l e c t o r output of ICI (pin 8 ) When pin 8 goes low (output o n ) TR7 is b i a s e d on by R25 (C34

s e r v e s to i n c r e a s e switching s p e e d ) , D15 c o n d u c t s and TR2, T R 3 turn on quickly At this time T R 8 is switched off When ICI pin 8 goes high (off) TR7 s w i t c h e s off and TR8 b a s e is pulled low; as t h e gates of TR2 and TR3 are charged to a positive potential, D15 is r e v e r s e biased and TR8 c o n d u c t s This action rapidly s w i t c h e s off TR2 and TR3

Integrated circuit IC2 is a c o m p a r a t o r with its inputs

c o n n e c t e d to two potential dividers R e s i s t o r s R31 and R32 form a r e f e r e n c e potential divider and t h e r m i s t o r TH1 and R30 form a t e m p e r a t u r e sensing network R33 and D17 provide a large degree of h y s t e r e s i s when t h e output c h a n g e s s t a t e Normally the output from IC2 (pin 7) is high and the voltage on pin 2 is around l

i n c r e a s e When the voltage on pin 3 e x c e e d s the voltage

on pin 2, the output of IC2 goes low LD2 illuminates dicating thermal shutdown and the power supply shuts

in-down At this point D17 c o n d u c t s , this adds R33 to the lower half of the r e f e r e n c e divider reducing the refer-

e n c e potential on pin 2 to around V3 supply (ignoring D17 voltage drop and saturated output voltage of IC2)

T h e voltage on pin 3 will now have to fall below V3 ply before the circuit will r e s e t and the supply allowed

sup-to restart Correspondingly the r e s i s t a n c e of TH1 will have to rise and its t e m p e r a t u r e fall before supply op-eration is resumed With the circuit values as shown, the trip t e m p e r a t u r e is 80°C and the r e s e t t e m p e r a t u r e is 60°C

Diodes D5 to D8 form a bridge rectifier (main output), the devices used are high s p e e d types, essential for use

in switch mode applications Capacitors C7 to CIO help

to reduce transients and switching noise Components

C i l , C12, L2, L3, C13, C14, C15 and C16 form jt-filter works for the main outputs R e s i s t o r s R9 and RIO s e r v e

net-to provide a minimum load lor the power supply and also

discharge the filter c a p a c i t o r s quickly after switch-off Fuses FS3 to FS6 provide p r o t e c t i o n against s h o r t cir-cuits and overloads Positive 30 V outputs are available from P4, 5, 6 and 7 Negative 30 V outputs are available from P12, 13, 14 and 15 Pins 8, 9, 10 and 11 provide a zero volt return

Diodes D9 to D12 form a s e c o n d bridge rectifier iary output), again high speed types are used Capacitors C17 to C20 help to reduce transients and switching noise Capacitors C21 and C22 are the r e s e r v o i r c a p a c i t o r s for the auxiliary output R e s i s t o r s R l l and R12 s e r v e the same purpose as R9 and RIO in the main output circuitry Voltage regulators RG1 and RG2 regulate the supply rails and attenuate switching noise on the auxiliary output

(auxilCapacitors C23, C24, C25 and C26 are decoupling c a p a c i

-t o r s and e n s u r e supply s-tabili-ty Posi-tive and nega-tive

12 V auxiliary outputs are available on P16 and P18 spectively P I 7 provides a 0 V return

re-Construction

T h e PCB is of the single-sided glass fibre type, with a printed legend to a s s i s t insertion of t h e c o m p o n e n t s T o

i n c r e a s e the current rating of s o m e of the t r a c k s it is

n e c e s s a r y to tin the e x p o s e d a r e a s of t r a c k on the derside of the PCB T h e s e t r a c k s will b e c l e a r l y s e e n as they are not c o v e r e d by the s o l d e r r e s i s t layer Tinning

un-of t h e t r a c k s should actually be the final a s s e m b l y task Removal of misplaced c o m p o n e n t s can be very difficult,

e s p e c i a l l y on a densely populated board s u c h as this, s o

p l e a s e double c h e c k c o m p o n e n t type, value and tion (where a p p r o p r i a t e ) before inserting and soldering the c o m p o n e n t

orienta-Referring to the following c o n s t r u c t i o n a l n o t e s , the parts list and Figure 8.3, begin construction It is r e c o m m e n d e d

t h a t t h e following c o n s t r u c t i o n o r d e r is a d h e r e d t o closely, o t h e r w i s e it will b e found e x t r e m e l y difficult, to fit s o m e of the c o m p o n e n t s

Start by inserting t h e t h r e e 22 SWG wire links, t h e s e are indicated on t h e PCB by a single straight line and an

adjacent LK mark

Next insert the 1N4148 signal diodes, ensuring c o r r e c t orientation