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How to run 12V CB radios from 24V 24V to 12V converter for trucks This 24V to 12V converter can deliver up to 5 amps with very little power loss. It is intended for powering CB radios and radio/cassette players in trucks which normally hove 24V battery supplies. Design by JOHN CLARKE Most larger trucks use a 24V supply for their electrical system and commonly employ two 12V or four 6V batteries connected in series to provide the voltage. This presents a problem when CB radios and radiocassette players are installed. A common method for supplying the required 12V is to tap it off from the centre point of the two (or four) batteries. This certainly works but it does have a serious long-term drawback. To describe how this occurs, let us consider the most common case where two 12V batteries are used. When a heavy current device such as a CB radio is connected, it takes all its energy requirements from the lower 12V battery in the series string. This means that the total drain on the lower battery is higher than for the upper unit. But when the batteries are being recharged this fact will not be taken into account. The two 12V batteries will still be recharged to a nominal 28.8V cutout (ie, twice the normal 14.4V setting in a 12V system) but as time goes on, the lower battery will always be undercharged while the top unit is over-charged, as the electrical system attempts to make up the required total voltage of 24V. The result will be premature failure and necessary replacement of both 12V batteries. That is a very expensive way of running 12V gear. The same problem applies if four 6V batteries are employed. The only satisfactory way to prolong battery life is to derive the required 12V from the whole 24V supply. The simplest way of doing that is to use a series regulator which can be set to deliver around 13.6 volts which is a good voltage for running 12V equipment. There is little wrong with this approach except for one problem excessive power dissipation in the regulator. Consider what happens if the 24V truck supply is running at around 28 volts (which is normal), the regulator is set to deliver 13.6V and the CB radio is drawing 5 amps when transmitting. This means that the regulator will have to dissipate over 70 watts. In the hot cabin of a truck this could be a serious problem, requiring a large and bulky heatsink. So scrub that idea, it isn't practical. Our 24-12V converter solves the dissipation problem by using a switchmode regulator. It uses a power transistor which is switched on and off at a rapid rate to provide 13.6V. -- S1 + 1ZV 20kHz OVJ1_Jl_ D1 Fig. l ':' Fig.1: this circuit cannot be used because Ql 's load is shortcircuited when the radio is connected. 28 I LOAD +0-0 D1 SILICON CHIP Fig. 2 .,.. h~ I ... i.,.. Fig.2: the circuit is based on the source-follower configuration. Vi '---4---,-+ D1 Fig. 3 Fig.3: basic operation of a switchmode supply. S1 is the transistor and turns off and on at a rapid rate. .. PARTS LIST 1 PCB code, SC111-1287, 100 x 55mm 1 folded aluminium case, 100 x 58 x 45mm 1 panel mount fuse holder 1 in-line fuse holder 2 3AG 5A fuse 2 1 0mm grommets 3 solder lugs 4 plastic PC s tandoffs 4m 1 mm enamelled copper wire 180mm 1 mm tinned copper wire 2 T0-220 mica washers and insulating bushes 1 Neosid iron cored toroid 17-146-10 Semiconductors View inside the prototype. All the parts, including the toroid coil, are mounted on a small printed circuit board to give a compact assembly. Because the power transistor is being switched on and off, it is very efficient and wastes little power. It dissipates only about 5W when delivering 5 amps. The converter has good regulation. The input voltage can range between 18V and 30V for an output voltage change of only 0.7V. The output is well filtered too. Switching ripple is 30mV peak-to-peak at the full load current of 5A, falling to 10mV p-p with no load. A small metal case houses the converter circuitry. This reduces the level of electromagnetic radiation emanating from the circuitry as well as providing a heatsink for the main switching transistor. Circuit details At first sight the circuitry looks fairly complicated but the operating principle is relatively simple. It is best understood by first referring to Fig.1. This shows a MOS (metal-oxide silicon) power transistor Ql with a square wave drive signal fed to its gate. This can be made to work perfectly well to provide a 12V output. The only drawback is that the output has the positive line at + 24V (nominal) and the negative side at + 12V with respect to the chassis of the vehicle. The problem with this is that virtually all 12V automotive gear such as CB radios and radio/cassette players need their cases earthed to the vehicle chassis. If this was done with the circuit of Fig.1, Ql 's load would be short-circuited and it would be burnt out. So back to the drawing board. Fig.2 shows another arrangement for the switching supply with the 12V output referenced to ground (vehicle chassis). It uses Ql as a source follower with the drain connected to the 24V supply. The problem with this circuit is that the gate must be driven at least 12V above the source to ensure that Ql switches on fully . In practice, this means that the gate has to be driven 12V above the 24V rail. (Life isn't simple, is it?) To solve this problem we need a drive circuit which will swing the gate between + 12V and + 36V. We can do this but it needs its own supply circuit delivering more than 36V. We solved that problem with a voltage doubler circuit. Fig.3 shows the basic operation of the switchmode supply. Switch Sl is the transistor which turns on and off at a rapid rate. When S1 is closed, current passes through the inductor to charge capacitor Cl. When Sl opens, the inductor current is diverted through the flywheel diode Dl to charge Cl and so prevent a large back-EMF being 1 1 2 1 5 1 1 1 TL07 4 quad op amp IC BUZ71 SIPMOS trans istor BC54 7 NPN transistors BY229-400 or MUR1 550 fast recovery diode 1 N4148 , 1 N9 14 s mall s ignal diodes 39V 400mW or 1 W zener diode 30V 1 W zener diode 13V 400mW or 1 W zener diode Capacitors 1 1 OOOµF 63VW PC electrolytic 2 2200µF 16VW PC electrolytic 1 1 OOµF 35VW PC electrolytic 1 2.2µF 50VW PC electrolytic 2 0.4 7 µF metallised polyeste r 1 0.01 µF metallised polyester 1 0 .0022µF metallised polyester 1 0.001 µF metallised polyester 1 4 70pF ceramic Resistors (0.25W, 5 %) 1 X 1 OOkO, 7 x 47k0, 3 x 10k0, 1 X 4 .7k0, 2 X 2.2k0 , 1 X 3900, 1 X 2700, 1 0.5W X 1000 , 1 X 470 Miscellaneous Hookup wire , solder, screws, nuts etc . developed across the switch. The complete circuit diagram (Fig.4) brings it all together. It includes a voltage doubler, a gate driver, a switchmode oscillator to provide the square wave for the DECEMBER1987 29 470 l'"'""-..-----,----.....-----.--....--wv,,-+VOR FROM ACCESSORY IGNITION SWITCH 47k SA LINE FUSE . - - - - - - - - - u ~ - - , - + 2 4 V FROM BATTERY 1000 + 0.47+ JSVW+ 47k 47k 47k 470pF-l'. 40kHz OSCILLATOR SA -.....-------0--.0---0+ VOLTAGE DOUBLER OUTPUT 13.6V, SA i- 10k 47k 2.2k B ELJc VIEWED FROM BELOW ~R GDS D5 1N4148 K A I. SHORT IF 04 VOLTAGE HIGH 20kHz SWITCH MODE OSCILLATOR 24V-12V CONVERTER · L1 : 64T, 1mm ENAMELLED COPPER WIRE ON A NEOSIO 17 -146-10 4.7k 111-1287 Fig.4: the circuit includes a voltage doubler (ICla, D1, D2, Cl and C2), a gate driver (Q2), a switchmode oscillator (IClb) and a voltage comparator (IClc) for output voltage regulation. gate driver and a voltage comparator for output voltage regulation. It uses one quad op amp integrated circuit (ie, four operational amplifiers in the one IC package), two NPN transistors, and one field effect power transistor. The voltage doubler comprises op amp ICla, diodes Dl and D2, Cl, C2 and associated components. ICla is connected as a Schmitt trigger oscillator. This works as follows: Initially, pin 6 of ICla is low and pin 7, the output, is high. This causes the 470pF capacitor at the pin 6 input to charge via the 47k0 feedback resistor. This continues until the voltage reaches the positive threshold of the Schmitt trigger, at which point the output goes low. The capacitor then discharges via the 4 7k0 resistor until the voltage reaches the negative threshold of the non-inverting input when the output goes high again. Thus ICla is an oscillator operating at about 40kHz. The square wave output is fed to a diode pump circuit consisting of Cl, D1, D2 and C2. Initially, when 30 SILICON CHIP the output of ICla is low, capacitor Cl is charged via Dl to 24V. C2 is also charged to 24V via Dl and D2. When the output of ICla goes high to about 24V, the positive side of Cl (ie, the junction of Dl and D2) is jacked up to 48V and so Cl 's charge is transferred to C2. When ICla again goes low, Cl is again charged via Dl and the cycle starts again. The voltage developed across C2 is limited to 39 volts by zener diode D3. This is fed to the gate driver stage Ql and Q2. ICl b drives the base of QZ via a 2.2kQ resistor. When the output of IClb is low, QZ is off and Ql is switched on by virtue of the resistor between its base and collector. Ql applies about 39V to the gate of Q3 and turns it on. When the output of ICl b goes high, QZ is switched on which turns off Ql and also Q3, the main switching transistor. As Q3 turns off, the inductor L1 tends to maintain its current flow and pulls the source negative. However DB clamps the source at about minus 0.7V. D6 is included to speed up the turn-off of Q3 , by ac- Close-up view of FET Q3 (left) and diode D8. See Fig.6 for mounting details. tively pulling the gate down towards OV. D7 is included to prevent the gate-source capacitance from being charged to a large negative value and thereby indirectly improves the turn-on time of Q3. Inductor Ll , a 680µ,H toroidal choke, and two 2200µ,F capacitors connected in parallel filter the square wave output of Q3 to produce smooth DC. A small load resistor of 4. 7k0 is there to discharge the capacitors if no load is connected at the time power is turned off. The 0.47 µF capacitor improves the filtering at high frequencies. A 5-amp fuse protects the output against overloads and short circuits, while a 5A in-line fuse provides protection in the case of a circuit fault. I I \ \ Voltage regulation ICl b is another Schmitt trigger oscillator (similar to ICla) with a pulse output at 20kHz. Its trigger level is modulated by the voltage comparator IClc to give voltage regulation. IClc does this by comparing the averaged output voltage from L1 with a 13.6V reference at its non-inverting input, pin 13. If the output voltage at 11 is lower than the reference voltage, the output of IClc goes low and pulls down the voltage at pin 10 of IClb. Thus the duty cycle of IClb changes so that its positive pulses are shorter. This means that Q2 is turned on for shorter periods of time and this increases the averaged output from Q3 and 11. Conversely, if the output voltage is higher than the reference voltage, the output of IClc raises the voltage at pin 10 of IClb. This makes the positive pulses from ICl b longer, turns on Q2 for longer periods of time, and thus decreases the averaged output voltage from Q3 and 11. The 0.0OlµF capacitor between pins 12 and 13 of IClc provides filtering for the error voltage (ie, the difference between the converter output and the 13.6V reference voltage). The capacitor also enables the converter to start reliably when power is first applied. ICl b oscillates at about 20kHz. This is high enough to prevent the switching of Q3 from becoming audible but not so high that switching losses become excessive. The 24V supply to the drain of Q3 is decoupled with lO00µF and 0.47µF capacitors. The supply to the remainder of the circuit is decoupled with a 470 resistor and lO0µF capacitor and protected against voltage spikes by a 30V zener diode. Note that the 24V supply is permanently connected to Q3 while the OUTPUT CONNECTED TO EARTH LU ON OUTSIDE OF CASE 5A FUSE FROM BATTE IN-LINE F FROM ACCESSORY IGNITION SWITCH 09 ! Fig.5: parts placement and wiring diagram for the converter. Use mica washers and insulating bushes to isolate Q3 and DB from the chassis. rest of the circuitry is connected to the ignition or accessory switch. This avoids the necessity for a heavy duty on/off switch. One point should be made before we complete the circuit description and that is to tell you what happened to the fourth op amp. After all, ICl is a four op amp package. The fourth op amp, associated with pins 1, 2 and 3 of the T1074, is not used and is therefore not shown on the circuit. Pins 2 and 3, the op amp inputs, are connected to the 0V line and so the op amp latches up (ie, its output goes high, to almost the + 24V supply). Construction Our prototype 24V to 12V converter was built into a folded aluminium case measuring 100 x 58 INSULATING BUSH ~ t \ ~ MICA WASHER iI SCREW HEATSINK (REAR OF CASE) NUT 10220 DEVICE Fig.6: mounting details for transistor Q3 and diode D8. x 45mm. The circuit components are mounted on a printed circuit board coded SCl 11-1287 and measuring 100 x 55mm. Start construction by installing all the low profile components on the PCB as shown in Fig.5. These include the IC, resistors and diodes (but not DB). The capacitors and transistors can then be installed. Q3 and DB should be installed with their leads about 10mm long so that they can be later bolted to the side of the metal case. The inductor, 11, is wound on a Neosid powdered iron core toroid, type 17-146-10. This requires 64 turns of 1mm enamelled copper wire, evenly wound around the toroid. Strip the enamel off the two ends of the winding before mounting the toroid on the PCB. The toroid is secured using three Ushaped tinned copper wire links as shown on the wiring diagram (Fig.5). The PCB is supported in the case using plastic standoffs. You will have to mark out and drill the necessary holes for these, along with holes for the cable entry, fuse holder, earth lug mounting screw, and the mounting screws for Q3 and DB. Deburr all holes using an oversize drill bit. Take extra care to ensure that the mounting surfaces for Q3 and DB are smooth and free DECEMBER1987 31 The circuit is housed in a compact folded aluminium case. Install grommets at external wiring points. 0 ...... 00 N """ I ........""" (.) en Oo Fig. 7: here is the full-size artwork for the PC pattern. of metal swarf. Q3 and D8 are bolted to the side of the case using TO-220 mounting kits - ie, mica washers, insulating bushes and screws and nuts. Fig.6 shows the details. Smear the mating surfaces on the devices and the case with heatsink compound prior to installation. Finally, use your multimeter to check that the metal tab of each device is indeed electrically isolated from chassis. If you do find a short, it should be corrected immediately. Once the unit has been completely assembled, it is ready for testing. Connect a 24V supply and check that the output is at about 13.6V. If the voltage is higher than this check 32 SILICON CHIP the voltage between ground and the anode of D4. This voltage should be the same as the output voltage. If necessary, the output voltage can be reduced by 0.6V by shorting out D5. Troubleshooting Double, double, toil and trouble; fire burn and cauldron bubble. Perish the thought but it is possible that your converter may not work when you turn it on. Don't panic though, it is fairly easy to get it going if it should malfunction. The hardest part about troubleshooting is poking the prods from your multimeter or oscilloscope into the case and onto the components, since it is so tightly packed. To make it easier in this respect, you may wish to remove the fuseholder temporarily, to give better access to the case. Wrap some insulation tape around the fuseholder terminals though, to prevent the possibility of shorts. The first step in troubleshooting is to check that the supply voltage is being fed to the circuit. With 24V applied to the two input cables, check that this voltage appears across D9, the 30V zener, the drain of Q3 and pin 4 of the IC. If the 47Q resistor feeding D9 cooks as soon as you connect the supply it is likely that you have connected the supply leads the wrong way around or D9 is reverse-connected into circuit. Now check that ICla is functioning. The easiest way to do this is to measure the voltage across zener diode D3. This should be close to 39V. If this voltage is not present, check the orientation of diodes Dl, D2 and D3. If they're OK, check that ICla is oscillating. This is easy to do if you have an oscilloscope. If ICla is oscillating, a 40kHz square wave with an amplitude of 24 volts peak will be present at pin 7. If you don't have an oscilloscope, you can check the DC voltage present at pin 7. It should be close to half the supply voltage . Similarly, the voltage at pins 8, 9, 10, 12, 13 and 14 should also be at close to half the supply voltage, if the other two op amps are functioning properly. If after all those checks the unit is still not delivering correct output, it is possible that Ql, Q2, D6 or D7 is at fault. Try shorting the base and emitter of Q2. This will turn off Q2 which will let Ql turn on continuously. This should apply about 37V to the gate of Q3, allowing it turn on completely and feed the full 24V to the output. If that does not happen, it is possible that Q2 is shorted. Note: disconnect the two 2200µF 16VW capacitors for this test. On the other hand, if the output of Q3 is continuously high, it ·is possible that D6 is open circuit or round the wrong way, or Q2 is open circuit. It is highly unlikely that Q3, the most rugged semiconductor in the circuit, is damaged.