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Special circuit for vintage radio restorers! Build this step-up voltage converter This little unit converts 9-12V DC from a mains plugpack up to a maximum of 70V DC at 40mA. It can be used with battery operated valve radios or any device that requires a supply voltage of more than 12V DC. rate intercom system, this project can also be tailored to provide the 50V required to operate them. In general, this circuit can be made to generate any DC voltage up to about 70VDC at modest output current levels. Circuit details By DARREN YATES If you're into vintage radio then you'll know that 'B' batteries are impossible to get for those battery-operated valve receivers. It would be a pity if melodic sounds were never to be heard again from these grand old sets for lack of a power source. In a recent Vintage Radio column (December 1991), John Hill suggested a 'B' battery eliminator using five 9V batteries. This provides a 45V supply but some old valve radios need a +6 7. 5V DC rail. Stacking seven or eight 9V batteries to provide around +7DV DC is not really practical and it would be quite expensive too. As a result, many old battery powered radios have been left to gather dust because the batteries to drive them are no longer available or the alternatives are just too expensive. This is where our little high voltage supply comes to the rescue. It allows you to run an old battery powered radio direct from the mains and it is quite cheap. If you have a couple of old Telecom phones that you wish to run as a sepa- The circuit for the High Voltage Converter is based on a Motorola MC34063A DC-DC converter IC (Fig.2). This was previously featured in the Portable SLA Battery Charger published in the July 1992 issue of SILICON CHIP. The MC34063A was designed to convert a DC voltage up to a level of about 40V maximum but by using a high voltage external pass transistor, the IC can be made to produce at least 7DVDC. The input voltage source can be just about anything from dry cells to a plugpack or your car battery but it 170 µH L r------------ --- 8I i I1 --+--◄ 180 I I I I I 12 Rsc 1N5819 0.22 V1n 0 - - - - - - l I 12 V 1.25 V Ref Reg 1100 jcrJ I 1soo : pF 14 I L-- --- -- ------------ -~ R2 Vout - - ---.,1t1,------------0 28 V/175 mA 47k The High Voltage Converter is ideal for generating the B+ rail for a battery powered vintage radio receiver, or for any application requiring up to 70VDC at 40mA. The output voltage can be adjusted by changing two resistor values. 56 SrucoN CHIP Fig.1: block diagram of the Motorola MC34063 DC-DC controller IC. It uses an internal oscillator to drive an RS flipflop & this in turn drives a Darlington transistor pair to switch an external inductor. S1 + 0.221l SW F1 2A o-/ Vin -+ 1801l 7 8 1 ~ IC1 MC34083A B 4.7k H~ K A BCE .0047+ 470 + ~ ~ 63VW+ ~ L1 : TWO LAYERS OF 0.63mm DIA ENCU ON NEOSID TOROIDAL CORE 17/732/22 RB 1k ~ HIGH VOLT AGE DC-DC CONVERTER Fig.2: the final circuit uses external pass transistor Qt to switch inductor Ll, so that the output voltage can be boosted to 70V. Resistors RA 1 & RA2 in the negative feedback network set the output voltage to the required value. must be able to supply around 250300mA. To understand how the circuit of Fig.2 works, it is useful to have a look at Motorola's own step-up circuit for the MC34063A, as shown in Fig, 1. Here there is no external transistor, as internal transistor Q1 takes care of all the load current. Briefly, the circuit works as follows. An input voltage of 12V is applied to pin 6 of the IC and also to the 170µH inductor L via resistor Rsc· This resistor provides current monitoring and the IC shuts down if the voltage across Rsc exceeds 0.3V. An internal oscillator, with its operating frequency set by the capacitor at pin 3, .drives Q1 and QZ and thus switches current through the 17DµH inductor. Each time transistor Q1 switches off, the collapsing magnetic field associated with the inductor will try to maintain the current through it but since Q1 is off, the only available current path is via the 1N5819 diode to the 150µF capacitor C0 • Thus, the capacitor charges to a considerably higher voltage than the input of 12V. Negative feedback around the circuit is used to set the output voltage to a predetermined value. This is set by a voltage divider network consisting of Rl and R2. This feeds the inverting input (pin 5) of an internal comparator, while a 1.25V reference feeds the non-inverting(+) input. Thus, when the voltage at pin 5 is just above 1.25V, Ql is not driven and when it is below 1.25V, Ql is driven at a high frequency. Typically, the circuit of Fig.2 can regulate the output voltage to within about ±50mV. Now take another look at our circuit of Fig.2 and note the differences between it and Fig.1 which we have just discussed. In the original circuit of Fig.1, pin 1 was connected to the junction of the inductor and the diode but this presented a problem for our application. Pin 1 is actually the collector of Ql inside the IC and it has a collectoremitter voltage (Vee) rating of 40V. This would be far exceeded if we pushed the circuit to produce the 70V or so we require. The solution is to use Ql inside the PARTS LIST 1 PC board, code 11102931, 100x 55mm 1 self-adhesive front panel label, 54 x 99 mm 1 plastic zippy case, 130 x 67 x 42mm 1 SPST miniature toggle switch 1 33mm OD toriodal core (Altronics Cat. L-5120) 1 3.5mm socket 1 2-pin DIN panel socket 1 2-pin DIN plug 3 metres of 0.63mm enamelled copper wire 2 M205 (2AG) fuse clips 1 2A M205 fuse Semiconductors 1 MC34063A DC-DC converter (IC1) 1 TIP31 C NPN transistor (01) 1 BY229-400 fast recovery diode (D1) 1 33V 1W zener diode (ZD1) Capacitors 2 470µF 63VW electrolytics 1 .0047µF 63VW MKT polyester Resistors (1 %, 0.25W) 2 27kO 1 1800 1 4.7kO 1 0.470 SW 1 1kO A small heatsink must be fiUed to power transistor Qt to aid heat dissipation. There's no need to isolate Qt 's tab from the heatsink but make sure that the latter does not touch any other components. · Miscellaneous Solder, screws, washers, nuts, hook-up wire . APRIL 1993 57 DC INPUT SC11102931 ~ _Qj Fig.4: check the PC board for defects against this full-size artwork before mounting any of the parts. Fig.3: make sure that all polarised parts are correctly oriented when installing them on the PC board & don't forget the wire link that sits directly under the back of the heatsink. IC as an emitter follower which then drives external transistor Ql (a bit confusing, that) . This external transistor has more generous voltage and current ratings. Pin 1 is now moved to the supply rail side of the inductor and so the internal transistor sees no more than about +12V on its collector. Apart from the use of an external transistor, the circuit of Fig.2 works in an almost identical fashion to the original circuit of Fig, 1. However, there are a few other differences which we will explain. Note the two 470µF capacitors connected in series across the output. We would have preferred to use just one output capacitor rated at 100V but 63V capacitors are much more read- ily available; ergo , we have used two in series. Connecting two capacitors of nominally the same capacitance and voltage rating does not mean that they will equally share the voltage; the actual voltage across each capacitor will mainly depend on their leakage resistance and we have no control over this factor. We solved that this problem by "swamping" the leakage resistances of the capacitors with parallel connected 27kQ resistors (RA 1 & RAz) and these do double duty by forming part of the voltage feedback network to pin 5. Output voltage As we said before, the output voltage can be adjusted to any value up to 70VDC. This is achieved simply by RESISTOR COLOUR CODES 0 0 0 0 0 0 58 No. Value 4-Band Code (1%) 5-Band Code 1% 2 27kQ 4.7kQ 1kQ 180Q 0.47Q 5W red violet orange brown yellow violet orange brown brown black red brown brown grey brown brown not applicable red violet black red brown yellow violet black red brown brown black black brown brown brown grey black black brown not applicable 1 SILICON CHIP Once the IC is in, install the 5W resistor, the fuse clips and the electrolytic capacitors. The HIGH VOLTAGE fast recovery diode (D1) and CONVERTER power transistor Ql can then be fitted. Note that Ql must be fitted with a small heatsink and this is best attached before the HVOUT DCIN transistor is installed. The last component to be installed is the toroidal inductor. ON This is wound in two layers using 70 turns of 0.63mm enamPOWER elled copper wire. Begin with a 3-metre length of wire and thread it half-way through the centre of the toroid. Now, using one half Fig.5: this full-size front panel artwork can be used as a drilling template for the of the wire, wind on 35 turns. on/off switch. Spray the finished label with a hard-setting clear lacquer before Make sure that you wind the attaching it to the lid of the case. turns as neatly as possible and keep the turns tight and close together. adjusting resistors RA 1 and RAz in the the power switch and the two power The other half of the wire is then used divider string. The equation to derive sockets are installed on a PC board to wind on the other 35 turns. Once the inductor has been wound, the correct values for resistors RA 1 measuring 100 x 55mm and coded strip and tin the wire ends, then sol11102931. and RAz is as follows: RA1 + RAz = (Vout/1.25 -1) x 1000 Before beginning any construction der the complete unit to the board. You can secure the inductor in a This is the total value for RA 1 and work, check the board carefully for RAz• You need to then divide this any shorts or breaks in the tracks by number of ways: by using a small value by two to get the value for RA 1 comparing it with the published pat- nylon cable tie that passes through and RAz• These resistors should be tern. Repair any defects that you do the toroid and two holes in the board; equal in value if possible, so that equal find, then start the assembly by in- by pouring a little hot wax over the toroid; or by using an adhesive like voltages are developed across the ca- stalling the resistors and ·wire links Blu-Tac®. see Fig.3. pacitors. The .0047µF capacitor on pin 3 sets Next, install the 33V zener diode, Switching on the switching frequency, while the the .0047µF MKT capacitor and the 33V zener diode (ZDl) protects the IC IC. It is usual to install ICs last but, in Check that all the wiring is correct this project, it's best to install it before before applying power to the board. from over-voltage. some of the bigger components go in. You will need a power supply capaConstruction Be sure to orient the IC exactly as ble of delivering 12V. This should be shown on the wiring diagram. connected via your multimeter which All of the components except for should be set to the 2A range. Make sure that you have the supply polarity correct before switching on. After an initial surge, the current should drop back to a few milliamps at most. If the current drain is appreciably more (with no output load connected), switch off immediately and check carefully for assembly errors on the board. If all is OK, disconnect your multimeter, select the 200V DC range, reconnect the power supply and measure the output voltage. You should get a reading of about 70VDC (RA 1 = RAz = 27kQ), depending on the tolerance of your resistors. Finally, the board can be mounted on the bottom of a plastic utility case and appropriate sockets fitted to accept the input and output connecThe PC board is secured to the bottom of the case using machine screws & nuts, tions. The on/off switch can be fitted with additional nuts used as spacers. A 3.5mm socket accepts the input from the plugpack, while the output is fed to a 2-pin DIN socket. to the lid of the case. SC =flW =Ui/Jl!J+ APRIL 1993 59