Maplin auto electronics projects
Courtesy lights extender 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! 147 Auto electronics projects • door switches are fitted to the 0 V side of the cour- tesy light, for installation follow Figures 7.5(a) or 7.5(b), • door switches are fitted breaking the +12 V supply to the courtesy light, for installation follow Figures 7.5(c) or 7.5(d). In its simplest configuration, the unit connects directly across a door switch; PI connecting to the more posi- tive side of the switch and P3 to the more negative. If ignition override is required then P2 must be connected to a source that has power while the ignition is on (for example, + SW terminal of the ignition coil). If no easy connection can be made to the ignition circuit then P2 can be connected into the accessory circuit. As the complete unit is small and unobtrusive it can eas- ily be mounted inside a door-post, behind an existing door switch. The box can be held in place using a self- adhesive pad (such as HB22Y) or bolted down using the two mounting holes provided in the base of the box. Check behind panels before drilling any holes and en- sure that no wiring harness or other components are located behind panels that would otherwise be damaged. Min Typ Max Units Operating voltage 10 12 15 V Quiescent current at 12 V 3 mA Maximum switching current 5 A Table 7.1 Specification of prototype 148 Courtesy lights extender Courtesy +12V ü 3 ht Fueebox ® I I I I ι l p 1 . y S S J Extender Courtesy + 12 v LÎQht To ignition switch From QO * j · » 1 P1 (or other source of Fueebox r-—if- 1 —ι +12V when engine (b) P2 { « running). / / / / o.S°°u Extender + 12V From f f 1 1 1 P1 Fueebox ι„ ' r ' ι Courtesy Ught β »ιΓ / / / / Extender (ο ml_V JL Courtesy ® Light +12V To ignition swttch From • · 1 τ 1 P1 (or other source of Fueebox r-—if' ι +12V when engine °Ϊ5|Γ P? "running). < d) 1 111 _r JL Courtesy ® Ught Figure 7.5 Λ*Γ (a) Simple connection for vehicles with negative switched courtesy light (b) Connection for negative switched courtesy light with ignition override (c) Simple connection for vehicles with positive switched courtesy light (d) Connection for positive switched courtesy light with ignition override 149 Auto electronics projects Courtesy light parts list Resistors — All 0.6 W 1% metal film Rl,3 4k7 2 (M4K7) R2 560 k 1 (M560K) Capacitors Cl 47 μΡ 16 V minelect 1 (YY37S) Semiconductors TRI TIP122 TR2 MPSA14 Dl,2 1N4148 1 (WQ73Q) 1 (QH60Q) 2 (QL80B) Miscellaneous PI,2,3 1 mm PCB pins 1 pkt (FL24B) PC board 1 (GE81C) min box and base 1 (JX56L) courtesy light leaflet 1 (XK96E) constructors' guide 1 (XH79L) All of the above are available as a kit courtesy 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) 150 8 Car audio switched- mode psu Take note — Take note — Take note — Take note This project requires a fair degree of expertise and pa- 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. 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 re- quirement. Higher levels of power may be drawn as long as the average power is maintained at 120 W. The figure of 120 W is based on maintaining a heatsink temperature at less than 65°C. 151 Auto electronics projects For many years the motorist has not been able to ben- efit from hi-fi quality sound while travelling in the car. For the long-distance traveller, business executive or hi- fi buff on-the-move, the car is a far from ideal environment for listening to music; this is due to a number of reasons. First, the car's interior is designed for conveying passengers and not for ideal location of conventional box design loudspeakers. Second, the sound replay/receiving equipment has to be miniatur- ised and capable of operation in a very harsh environment. Dashboard temperatures often exceed 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 stresses that the equipment must endure. Third, the low, noisy and some- what variable supply voltage makes life even more difficult for the electronic circuitry. The environmental and size problems of the car envi- ronment have largely been solved by cleverly designed equipment. Car loudspeakers are optimised for opera- tion in rear parcel shelves and door panels instead of conventional sealed or ported enclosures. Car radio, cassette, CD (compact disc) and DAT (digital audio tape) equipment is very compact. Such equipment is designed for either mounting in the dashboard/centre console or remote mounting in the boot or under a seat, with just the controls located within the driver's easy reach. It is however, the third point that is the main reason for this project, the vehicle electrical supply. The 12 V elec- trical system is far from ideal when it comes to powering audio amplifiers. The electrical system itself, although generally referred to as being 12 V, usually operates at 152 Car audio switched-mode psu around 13-14 V when the engine is running. By conven- tion, the voltage when the engine is running is assumed to be 13.8 V. A singled-ended amplifier operating from a supply volt- age of this (low) level is capable of delivering around 7W r.m.s. into a 4 Ω load. If a BTL (bridge tied load) amplifier is used the power output can be increased to around 22 W r.m.s. into a 4 Ω load. Most high power ra- dio/cassette players have an output power of around 22 W r.m.s., regardless of how many watts the advertis- ing brochures boast! For hi-fi quality sound reproduction in a car it is neces- 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 because a high power amplifier op- erating at modest power levels will introduce far less distortion and handle transients far better than a me- dium power amplifier running almost flat out. This is especially true if the sound source is CD, where the dy- namic range of the recording is often very wide. There are two ways in which the output power can be increased, by either decreasing the loudspeaker imped- ance or increasing the supply voltage. The main disadvantage of the former method is that car speakers are not commonly produced with impedances below 4 Ω and that power losses in cables are increased. The lat- ter method of increasing the supply voltage is commonly used in high power car boosters and in hi-fi car audio amplifiers — this is the method that is described here. 153 Auto electronics projects Circuit description Figure 8.1 shows a block diagram representation of the power supply and Figure 8.2 shows the full circuit dia- gram. The supply input to the power supply is via PI (+V) and P2 (0 V). The power supply is connected directly to the vehicle battery via high current cables, therefore the off- board supply fuse FS1 is essential in case of a fault causing a short circuit directly across the battery. Re- mote power switching is achieved by TRI, RL1 and associated components. The control input P3, when taken to +V, biases TRI on and operates RL1, thus powering-up the rest of the supply. LD1 serves to indi- cate power on. The control 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 protection by blowing fuse FS2 and preventing the remote power switch from operating. Capacitors CI, C2, C3, C4, C5 and toroid LI form the in- put ji-filter, the output of which supplies the push-pull output stage. The power MOSFETs are arranged in two pairs (TR2 and TR3) and (TR4 and TR5), each driving one half of the transformer primary. Resistor R8 and ca- pacitor C6 form a snubber network to increase the rise-time of switching spikes. Components ZD1, D3 and TR2 and ZD2, D4 and TR5 form an active spike clamp, employed to protect the MOSFETs' drain/source junc- tions from high voltage switching spikes. This operates by feeding the spike back into the gate of the relevant 154 Car audio switched-mode psu 155 MAIN OVER VOLTAGE FEEDBACK OUTPUTS ΓΤ-77ΤΤ * . } ΓΖ3 O+V »» i \—I—o+v :„ BRIDGE ^ ' ' .11 . RECTIFIER TT—FILTER ^ | 1 q y + 13.8V Q , . . . Tr n.TFP PUSH—PULL j II [ ' ΓΖ3 Ο-V INPUT *=»p^ 0 ! J TT—FILTER OUTPUT STAGE* , || I n I Ι I Ι BRIDGE fc RESERVOIR I OSCILLATOR . «' RECTIFIER ^ CAPACITORS Έ7ν Π Π TEST 1 a L^- 1 1 1 1 -r 1 JG* LJ I LJ ? t _J FERRITE I I I j j I j TRANSFORMER , 1 . C°ONS?LΟ^ 0 % 0 Π ΡΓ ρη^ΓΓ^ 0 Η Lj r*M LU HIGH-SPEED I REGULATORS ^N^uV 01 ^ ^οΑίΓ CONTROLLER -ρ DRIVER | 1 1 1 1 1 1 ι 1 1 i AUXILLiARY I j OUTPUTS STANDBY ο ^ I 0 + 12V INPUT j nLTER —* ο — 12V THERMAL I PROTECTION Figure 8.1 Block diagram of the switching PSU Auto electronics projects Figure 8.2 Circuit diagram of the switching PSU 156 [...]... e d , t h e plus (+) sign on t h e b o d y must b e i n s e r t e d into t h e h o l e n e a r e s t that marked with a plus sign 163 Auto electronics projects Figure 8.3 164 PCB track and legend Car audio switched-mode psu Figure 8.3 Continued 165 Auto electronics projects M3 Isoshake M3 Solder Tag PCB (Ttn^ M3 Figure 8.4 /i Nut Solder M3 N u t to PCB pad ! Power input connection assembly F o r m... gain of t h e overv o l t a g e amplifier P h a s e c o m p e n s a t i o n is n e c e s s a r y t o e n s u r e good l o o p s t a b i l i t y , o t h e r w i s e t h e p o w e r s u p p l y 159 Auto electronics projects could b r e a k into o s c i l l a t i o n R e s i s t o r s R15 and R16 form a p o t e n t i a l divider which is u s e d to apply o v e r v o l t a g e f e e d b a c k to I C I , with... as R9 and RIO in t h e main output c i r c u i t r y Voltage r e g u l a t o r s RG1 and RG2 regulate t h e supply rails and a t t e n u a t e switching n o i s e on t h e auxiliary output 161 Auto electronics projects C a p a c i t o r s C23, C24, C25 and C26 a r e d e c o u p l i n g c a p a c i t o r s and e n s u r e supply s t a b i l i t y P o s i t i v e and n e g a t i v e 12 V auxiliary o... ira ' •• 1 C4 3 _ÏÎLU Figure 8.2 ISÎ • 9 II 1 44 Σ Ζ Ν16 I "> TS ,,IIf>τι R B2 U1 1 []& Β > !!^_ 3- Ρ — 1 - + - 4 H 1 I π »k U lis lu lia lia I BZ U1 1 tu 11 1 © TKs^^r II—ro— Continued 157 1 Auto electronics projects Page 157 res » I —I 5 & 0 «ι & & im**Y II • eis mm ΚΙ ι 11 Τ Τ ρί^-η ÊH—Τ 1 —Ki Π w i J L-Î^CK— - ^ " " Tt—,-5··« 1 1 ™ I w L5 +v4 l II ^ F • £ V3 I ά ι « * -τ : P4 C=3 ! ο v i I U'-w... H O L E S DRILL—OUT T O 3 5 m m Dia UNUSED EXISTING^ H O L E S DRILL-OUT TO 3 5 m m Dia Figure 8.6 Heatsink d r i l l i n g information Figure 8.7 Lead formation for BUZ11 M O S F E T s 167 Auto electronics projects Figure 8 8 Lead formation f o r B Y W 8 0 rectifiers using the M2.5 hardware as shown in Figure 8.9 and P h o t o 8.1 S o l d e r t h e l e a d o u t s of t h e t r a n s i s t o r s... components Car audio switched-mode psu Phote 8.1 Close-up of heatsink assembly Twist a n d solder connections screen PCB C P 16 Heat—shrink Thermistor Pins Figure 8.10 Thermistor connection 169 Auto electronics projects RR R R I II 1 ~|I I 1 Γ^ρ^ 1 1 L s^J -*) Main Outputs to power P P PP©™ I II 1 Ι Γ Ί P26 1 ( V ] >—»* V - > * . 149 Auto electronics projects Courtesy light parts list Resistors — All 0.6 W 1% metal film Rl,3 4k7 2 (M4K7) R2 560 k 1 (M560K) Capacitors Cl 47 μΡ 16 V minelect 1 (YY37S) Semiconductors. the engine running! 1 47 Auto electronics projects • door switches are fitted to the 0 V side of the cour- tesy light, for installation follow Figures 7. 5(a) or 7. 5(b), • door switches. with a plus sign. 163 Auto electronics projects Figure 8.3 PCB track and legend 164 Car audio switched-mode psu Figure 8.3 Continued 165 Auto electronics projects M3 Isoshake