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Vintage Battery Radio Li-ion Power Supply by Ken Kranz and Nicholas Vinen Vintage Radio enthusiasts know that ‘A’ and ‘B’ batteries have been effectively unobtainable for some time. So what to do? Try this compact and easy-to-build module. Using Li-ion or LiPo cells, it can generate both the A and B supplies for most battery valve sets, and it suits sets with a wide range of HT voltages. It generates virtually no EMI (which could interfere with radio reception). It also incorporates battery over-discharge protection and reversed battery/cell protection. I wanted a power supply to run a typical battery-powered vintage radio from a set of 18650 (or similar) Li-Ion rechargeable cells. I considered developing a switchmode design for decent efficiency, but RFI from switchmode supplies can interfere with radio reception. So I designed this ‘low-tech’ supply using a low-cost PCB-mounting transformer and a few transistors and passive devices instead. This circuit was designed to power my Aristocrat Tecnico 859 from four 18650 Li-ion cells, but it would suit a great many valve sets. It is often tuned to 3WV in Victoria’s Wimmera from my home in Adelaide, South Australia, with no power supply noise evident. The set requires 90V for the ‘B’ supply and also 1.4V for the ‘A’ supply. 16 One of these was also fitted to a 1937 Velco radio that I restored, although I left off all the low-voltage cutout components as the set now runs from a 6V DC plugpack. The filament supply is 2V <at> 700mA, so I attached the low tension (LT) regulator to the diecast aluminium enclosure the PCB is fitted into. The B+ for this radio is 135V. Most battery-powered radios need a B+ supply of 90-135V DC at up to about 12mA (<1.7W). So I chose a 5W PCB-mounting mains transformer which I used ‘backwards’, with the 230V primary used as the secondary output winding. I determined that I would need a transformer with either 6V+6V or 9V+9V secondaries (acting as the primary here, configured as a single centre-tapped 12V or 18V winding). Note that you can’t just determine the transformer turns ratio by dividing the secondary voltage into the primary voltage. Consider the 230V to 6V + 6V transformer; the 6V AC output voltages are determined for a resistive load at full power. With 230V AC on the primary, the 6V windings’ open-circuit voltages measure 8.4V AC each. So the actual turns ratio is 230V ÷ 8.4V = 27.5. Therefore, 8.4V AC is the nominal input voltage when the low-voltage winding is used as the input; but if only 90V output is required, it can be somewhat lower. The small transformer’s primary winding DC resistance is around 800Ω, and the low-voltage secondaries measure around 2.5Ω each (or 5.6Ω each for the 9+9V version). This also needs to be considered, as does the high leakage inductance. Practical Electronics | January | 2022 Scope1: scope grab of the oscillator operation, with VR1 and VR2 at their midpoints. The yellow and green traces are the waveforms at the collectors of Q3 and Q4, while the blue and mauve traces are the gate voltages of MOSFETs Q1 and Q2. The duty cycle is not quite 50%, hence the need for adjustability. (Note: the MOSFET switch-on is gradual but switch-off is fast.) The relatively high secondary winding resistance (which is the primary in this application) means that the driving MOSFETs don’t need any current limiting at switch-on. The peak current is limited by the transformer itself. Respecting the current ratings for the various windings, an output of up to about 2W is possible; more than enough for this application. The best operating frequency is often above the 50/60Hz recommended for the transformers. The final design provides some frequency adjustment, to let you set the optimal operating point. The B+ supply normally needs to be galvanically isolated from the filament supply as back-bias is often employed. Because of this, and the fact that the B+ current remains constant, a simple series resistor and zener diode is used to regulate the B+ output. Remember that the 800Ω transformer winding can be used to dissipate some energy. Caution Depending on how you configure it, this supply could generate voltages above 60V DC, which is considered the limit of safe ‘extra-low voltage’ operation. While 100V or so may not seem very high compared to mains voltages, it’s certainly high enough to give you a serious shock should you come in contact with the high-tension (HT) side of the circuit. Scope2: here the blue and mauve traces are still the gate voltages of Q1 and Q2 while the yellow and green traces are their drain voltages (ie, the push-pull drive to transformer T1). The MOSFETs operate as inverters with significant inductive spikes at switch off; high enough to cause ZD5 and ZD6 to conduct So you must work in such a way that you can’t come in contact with the Supply or the HT circuitry it is powering while power is applied. When probing or adjusting the Supply, always use tools with sufficiently high voltage ratings. Once it has been set up, it must be housed in such a way that users can’t come in contact with any of the HT circuitry, and all wiring should be properly insulated. If you are already working on valve sets, chances are you will already understand the danger and have safe practices. If you are a novice, seek assistance from a more experienced technician before building or working on this Supply. Circuit description While the circuit and board are designed to operate from one or two batteries, it’s far better to have two batteries: a lower-voltage battery for the A supply and a higher-voltage battery for the B-supply. This improves efficiency and reduces heat dissipation. My recommendation is that the higher-voltage battery should consist of two Li-ion, LiPo or LiFePO4 cells in series, giving a nominal voltage of around 7.4V (or 6.6V for LiFePO4). You could use two sets of parallel cells if you wanted to – ie, a 2S2P configuration, although that isn’t really necessary. Features and specifications • Runs from two or four Li-ion, LiPo or LiFePO4 batteries (typically two series cells for the HT generator and two parallel cells for LT) • HT output (B): 24-135V DC at up to 2W • LT output (A): 1.2-2.5V at up to 3A (with a heatsink) • Low-battery cut-out voltages: 0-10V (B), 0-4.5V (A) • Quiescent current when off: around 10µA (B) and 2µA (A) • HT operating current (B): around 300mA <at> 6.2V for 135V HT • LT operating current (A): 5-10mA plus what the radio draws • Other features: low EMI, indicator LED, provision for low-current SPST on/off switch, adjustable transformer drive frequency and duty cycle. Practical Electronics | January | 2022 The lower-voltage battery can be one of the same cells, or better, two in parallel. This will then have a nominal voltage of 3.7V or 3.3V, depending on the chemistry. Assuming that two batteries are used, the higher-voltage battery is connected to CON1 and the lower-voltage battery to CON2 (see Fig.1 above). Both connections have reverse-polarity protection in the form of series 1A PTC thermistors and reverse-connected 3A diodes, D1 and D2. Should either battery be connected with the wrong polarity, the associated diode will conduct and cause the PTC to go high-resistance. The radio would then not work, and presumably, this would lead you to discover and correct the problem before the battery discharged. Those PTCs also provide a measure of over-current protection, should something go wrong on the power supply board or in the radio. The B IN + supply from CON1 then goes through P-channel MOSFET switch Q5 and runs the high-voltage B-supply generator, while the A IN + supply from CON2 goes through a similar MOSFET switch (Q6) and onto the A-supply generator. These MOSFETs provide the low-battery cut-out protection, which will be described later. If a single battery is used, CON2, PTC2, D2, D4 and Q6 are left off the board, and a wire link is soldered across LK1. The B IN + supply then goes through switch Q5 and onto both the A-supply and B-supply generators. More on this possibility later. High-voltage generator As well as the description below, the operation of this part of the circuit is depicted in oscilloscope grabs Scope1Scope5 overleaf. With MOSFET Q5 on, current flows through the 220Ω resistor to charge the 10µF bypass capacitor for 17 q q l SC BATTERY Battery Vintage RadioRADIO Power POWER Supply SUPPLY Ó VINTAGE the oscillator. The voltage across that capacitor, and thus the oscillator supply, is clamped to around 5.6V by zener diode ZD1 for consistent operation. NPN transistors Q3 and Q4 form a basic oscillator, with trimpots VR1 and VR2 providing a small amount of both duty cycle and frequency adjustment. This allows you to tune the oscillator to get a 50% duty cycle for the most efficient driving of transformer T1, and to adjust the frequency to tweak the power delivery to suit your radio. The oscillation frequency is determined by the time constant of the resistances and capacitors connected to the bases of transistors Q3 and Q4. With VR1 and VR2 centred, R = 100kΩ and C = 100nF, so the approximate frequency is 1/(1.38RC) = 72.5Hz. Setting VR1 and VR2 at the extremes, it can be varied from about 54Hz up to 96.5Hz. The duty cycle is adjusted by varying the resistance of one trimpot slightly compared to the other. Drive for the gates of MOSFETs Q1 and Q2 comes from the collectors of Q3 and Q4 via 5.6kΩ current-limiting resistors. These form RC low-pass filters 18 with the MOSFET gate capacitances, and their values may be increased if switching noise is a problem. The 18V zeners protect the MOSFETs from an excessive gate-source voltage which might be caused by back-EMF from the transformer coupling through the MOSFET parasitic capacitances. In practice, they rarely conduct. Q1 and Q2 drive the ‘primary’ of transformer T1 in push-pull fashion. The 9+9V windings are intended to be the transformer’s secondaries when it is operated from the mains, but here we are using it in the opposite manner. T1’s centre tap connects to the battery supply before the 220Ω resistor, so that the transformer has a low source impedance. It draws around 0.5A when delivering more than 2W at 100V. Note that Q1 and Q2 must be logiclevel MOSFETs as they will typically receive a maximum gate-source voltage of around 5V. The output of T1 is rectified by BR1 to charge the first 100µF capacitor. The second 100µF capacitor forms a lowpass filter with the 100Ω resistor to reduce ripple, while zener diode ZD2 limits the voltage applied to the radio Reproduced by arrangement with SILICON CHIP magazine 2021. www.siliconchip.com.au until its HT current draw comes up to normal. After that, it’s limited by the transformer and 100Ω series resistor. Note that high-voltage zener diodes have quite high zener impedances, so for example, if ZD2 is a 75V diode, the B+ OUT voltage could easily exceed 85V at light loads. This is unlikely to damage any radio, and it will drop to a more normal level as the radio draws more current. ZD2 is just there to prevent wildly high HT voltages from being applied. Filament supply The filament supply is based around adjustable linear regulator REG1. This is similar to the LM317 but can deliver more current; over 3A, rather than 1.5A. The LD1085 also has a lower dropout voltage than the LM317 at similar currents, although that isn’t important here. The A OUT voltage is adjusted using trimpot VR5, which forms a divider with the 110Ω resistor between the OUT and ADJ terminals. As there is a fixed voltage between OUT and ADJ, and a fixed resistance, that means that the current through VR5 is essentially constant. So Practical Electronics | January | 2022 Fig.1: the left-hand section of the Battery Vintage Radio Power Supply circuit provides input protection and the low-battery cut-out function, while the middle section is the HT drive oscillator and LT regulator. The oscillator drives the step-up section at upper right, with T1 providing high voltage AC that’s rectified by BR1 and filtered by two electrolytic capacitors and a resistor to give relatively smooth HT DC. by varying its resistance, we vary the voltage between ADJ and GND, and thus set a fixed output voltage. A typical 5-valve portable radio (eg, the Aristocrat Tecnico 859) with a valve compliment of 1T4, 1R5, 1T4, 1S5 and 3S4 will require 300mA at 1.4V for the filaments. So with a 3.7V battery, the regulator will dissipate (3.7 – 1.4) × 0.3 = 0.69W (690mW) so little heatsinking is required for REG1. The heatsinking of REG1 can be adjusted as required; some heatsinking was required for my Velco radio. You could use a flag heatsink, as we did on our prototype, or bolt the regulator to a piece of metal, such as the chassis. Note than some battery radios had the filaments connected in series. Those radios will need a filament supply of something like 7.5V <at> 50mA. This circuit would be suitable for such radios with a few tweaks. For example, the 110Ω resistor would have to increase to say 620Ω to give REG1 sufficient adjustment range, and the B+ battery would probably need to be three Li-ion, LiPo or LiFePO4 cells in series to give REG1 a Practical Electronics | January | 2022 sufficiently high input for regulation, even when the battery is almost flat. The resistors connected to pin 5 of IC1b would also need to change from 1MΩ/2.2MΩ to something like 3.3MΩ/1MΩ so that the low-battery cut-out adjustment range would suit that battery. Low-battery protection MOSFET switches Q5 (and Q6, if fitted) are used to provide low-battery protection. If either battery’s voltage drops below a critical level, Q5 and Q6 switch off, so the power supply and radio shut down. In this state, the circuit only draws about 10µA from the B-battery and about 2µA from the A-battery. Presumably, you would notice the radio has switched off and either recharge them or swap them for fresh cells. But if for some reason you forget and leave the radio switched on, it would be several months before this minimal current drain could damage the cells. That’s why this circuit was designed with a low quiescent current in mind. When power switch S1 is closed, current can flow from whichever battery has a higher voltage, through small signal diodes D3 and D4 and then switch S1, into the input of REG2. This is an ultra-low-quiescent-current, lowdropout 3.3V linear regulator. It powers micropower dual comparator IC1 and also serves as a voltage reference. A fraction of this 3.3V reference is fed to the two inverting inputs of the comparators, at pin 2 and 6 of IC1. The fraction that is applied to those pins depends on the rotation of trimpots VR3 and VR4. These set the low-battery cut-out voltages, and they can vary the voltage at those inputs over the full range of 0-3.3V. The actual battery voltages are applied to the non-inverting inputs, pins 3 and 5, after passing through fixed resistive dividers. While these two dividers use the same resistor values, they are in different orders. So around 1/3 of the B-battery voltage is applied to pin 3 of IC1a, while about 2/3 of the A-battery voltage is applied to pin 5 of IC1b. In combination with the nominally 3.3V reference and trimpots VR3 and VR4, that means that you can set the switch-on voltage thresholds to anywhere from 0-10V for the B-battery, and 0-4.5V for the A-battery. Those ranges are slightly wider than necessary, to allow for variations in the exact regulator output voltage between samples. Hysteresis is provided by 10MΩ feedback resistors between the comparator outputs and non-inverting inputs. This has been arranged so that the hysteresis is a fixed percentage of the voltage. The source impedance for the noninverting inputs is 687.5kΩ in both cases (1MΩ||2.2MΩ), and this forms a divider with the 10MΩ feedback resistor. It gives a hysteresis percentage of 687.5kΩ ÷ 10MΩ = 6.875% So for low-battery cut-out voltages of 3.3V and 6.6V for the A and B batteries, that would give you switch-on voltages 6.875% higher, or 3.525V and 7.05V respectively. The resulting hysteresis voltages are around 0.23V for the Abattery and 0.45V for the B-battery. When both batteries are above their switch-on voltages, output pins 1 and 7 of IC1 are high, at 3.3V. Therefore, the base-emitter junctions of NPN transistors Q7 and Q8 are forward-biased and so both conduct, pulling the gates of MOSFETs Q5 and/or Q6 low and lighting LED1. If either battery falls below its switch-off voltage, the corresponding transistor switches off and thus Q5 and Q6 switch off. High base resistors for Q7 and Q8 (2.2MΩ) are used because if one battery voltage is low but the other is high, current will still flow from the corresponding comparator output and this will increase the current drawn from the higher voltage battery (usually the B-battery). 19 Scope3: the yellow trace is the drive voltage across T1’s primary (ignoring the centre tap), while the green trace is the voltage across the secondary. Note the different scales: 20V/div for the primary and 50V/div for the secondary. The secondary shows little overshoot and no ringing. Scope4: a close-up of the edge of the waveform in Scope3. Here you can clearly see the primary overshoot is limited to around 60V by ZD5 and ZD6, which each conduct for around 3-5µs per cycle, protecting Q1 and Q2 from excessive drain voltages (although they are avalanche rated, so would likely survive). Note the 100µs delay between the leading edges of the primary and secondary waveforms. The 2.2MΩ base resistors are the highest practical values to minimise this, and determine the minimum value for an LED’s current-limiting resistor as 12kΩ. That means that LED1 has to be a high-brightness type. into a 5kΩ load (20mA), or around 100V into a 10kΩ load (10mA) at 7.5V DC. It’s only if you need more current than this, especially at the upper end of the voltage range (approaching 135V) that you might need to substitute the 6V+6V transformer, which will give you a bit more HT power. As the battery discharged, I did find that the HT dropped a bit with my test sets during use with the 9V+9V transformer. However, I didn’t notice any variation in performance as a result of this. On/off switch If you don’t need a power switch on the supply, you can simply place a shorting block on CON3. We have provided CON3 as a convenient way to switch power on and off, and you only need an SPST switch that hardly has to handle any current. But with S1 off, there will still be a small quiescent current drawn from the two batteries due to the resistive dividers which remain connected. This is around 1.5µA from the Abattery and 3µA from the B-battery. That should mean the batteries last for around a year with the radio switched off. If you need to reduce the battery drain when off, you will instead need to use a DPST or DPDT switch to cut the battery connections to CON1 and CON2. That switch will need to handle the full load current of at least 1A for each battery. Note that the batteries may still suffer from a small amount of self-discharge, so it’s still a good idea to check and charge them every six months or so. Selecting ZD2 Four 5W zener diode options are given in the parts list, to suit different radio requirements. Common radio B-battery voltages are 22.5V, 45V, 67.5V and 90V. Choose the diode type with a voltage rating just slightly higher than your B-battery voltage. Our suggestions are 24V, 47V, 68V and 91V respectively. For a 135V HT, you can use a 130V or 150V zener. Once the radio has warmed up, you can adjust the transformer drive frequency to get a voltage close to the rated B-battery voltage. The 5W zener diode (ZD2) is mainly included to limit the supply voltage before the valve filaments reach full emission. Note that it isn’t uncommon for the voltage to still rise by 5-10V or more above nominal initially, due to the relatively high zener impedance of these parts (it’s higher for higher voltage zeners). This usually should not cause any problems for most radios, given that it should still be within about 10-15% of the nominal voltage and won’t usually happen continuously unless there is a radio fault. Choosing a transformer The 9V+9V version (Myrra 44236) should suit most constructors. With a 9V DC input, it will deliver around 100V 20 PCB design All of the HT tracks and components on the PCB have been spaced apart by 2.54mm, which is enough gap to suit mains voltages (350V+ DC peak). This isn’t strictly necessary, but it was possible without increasing the board size, so I did it. There is one component (ZD2) that carries HT that’s quite close to one edge of the board, so avoid putting that edge right up against anything conductive. You could add some neutral-cure silicone sealant around its leads and the solder joints on the underside if you wanted extra insulation. But note that this part can get quite hot at times. For that reason, we’ve also increased the amount of copper on the PCB connecting to its leads on both sides; this helps to draw some extra heat away (although its 5W rating is already pretty generous). Construction The Battery Vintage Radio Power Supply, available from the PE PCB Service, is built on a double-sided PCB coded 11111201 and measures 125 × 54.5mm. It has been made as compact as possible, within reason, so you can fit it and the Li-ion cells in the space that would have been occupied by the original batteries. Refer now to Fig.2, the PCB overlay diagram, which shows where all the parts go. My original design used mostly SMD components, with many of them mounted under transformer T1, and therefore managed to be a bit more compact than this one. But I think that a lot of Vintage enthusiasts would find it fiddly to build, hence this all-through-hole version, which still manages to be fairly compact. Start by fitting all the small resistors – leave the 1W resistor off, for now. While you can determine the value of a resistor by reading its colour bands, it’s best to use a DMM set to measure ohms to verify this, as some colours can look like other colours under certain types of light. Practical Electronics | January | 2022 Parts list – Battery Vintage Radio Power Supply Scope5: the yellow trace is again the transformer primary waveform while the green trace is the voltage across the first 100µF capacitor, and the blue waveform is the voltage across the HT output, with a 20mA load (94.5V into 4.7kΩ or 1.9W). You can see that the ripple before the RC filter is very modest at 92mV RMS, and it’s even less after; just 16.7mV RMS. If you are using a single battery to power the Supply, bend one of the resistor lead off-cuts to form link LK1 and solder it to the board in place of the header shown in Fig.2; otherwise, leave LK1 off. As you read the following instructions, keep in mind that you will not be fitting the middle terminal block (CON2), PTC2, diodes D2 or D4, or MOSFET Q6. Mount the smallest diodes, D3 and D4, then all the 1W zener diodes, ZD1 and ZD3-ZD6. All of these must be oriented with their cathode stripes facing as shown in Fig.2. At this point, it’s a good idea to fit comparator IC1. Make sure its pin 1 notch and dot go towards the top of the board, as shown in Fig.2. I don’t recommend using a socket for reliability reasons, although you could if you wanted to. With that in place, mount the larger diodes D1 and D2, again watching the cathode stripe orientation. Then fit bridge rectifier BR1, ensuring that its longer positive lead goes to the pad marked ‘+’ (its other leads should also have their functions printed on the top of the package). Push it all the way down before soldering and trimming its leads. Now fit switch header CON3. You can use a regular or polarised header, or just solder a couple of wires to the PCB. If you want the Supply to always be on, you can either place a shorting block on CON3 or solder a small wire link in its place. The next step is to fit small signal diodes Q3, Q4, Q7 and Q8. They are all the same type; ensure their flat faces face as shown in the overlay diagram, and bend their leads out gently to fit the pad patterns. Follow with small regulator REG2, which is in a similar package to those transistors, then install the four ceramic and MKT capacitors where shown. Now mount the five trimpots, making sure that you don’t get the three different types mixed up. VR1 and VR2 are 50kΩ (and may be marked 503), VR3 and VR4 are 1MΩ (may be marked 105) while VR5 is 100Ω (may be marked 100). The next step is to fit the smaller electrolytic capacitors, ensuring that their longer leads go to the pads marked with a ‘+’ symbol on the PCB (the striped side of each can indicates the negative lead). Leave the high-voltage capacitors for later. Follow with the two PTCs, which are not polarised, and then the four terminal blocks. Make sure their wire entry holes face towards the outside of the module, and note that the three side-by-side blocks are spaced apart and so should not be dovetailed; mount them individually. Now fit the five TO-220 devices, which all mount vertically. Make sure you don’t get them mixed up (see Fig.2 and Fig.1 or the parts list), and also ensure that their metal tabs are oriented as shown. You may wish to bend the leads of Practical Electronics | January | 2022 1 double-sided PCB coded 11111201, 125 x 54.5mm, available from the PE PCB Service 1 Myrra 44236 9+9V PCB-mount transformer (T1) [element14 1214600, RS 173-9939] or 1 Myrra 44235 6+6V PCB-mount transformer (T1) [element14 1214599, RS 173-9923] (see text) 2 RHEF100 or RHEF100-2 1A PTC/polyswitches (PTC1 and 2) [element14 3296327, RS 657-1772] 4 2-way terminal blocks, 5.08mm pitch (CON1,2,4,5) 1 2-pin header or polarised header with jumper shunt (CON3) 1 SPST panel-mount switch (S1; optional) 4 tapped spacers (for mounting the PCB) to match screws below 8 M3 x 6mm panhead machine screws (for mounting the PCB) 1 flag heatsink with TO-220 insulating washer and bush (for REG1; optional) 1 M3 x 10mm panhead machine screw, nut and two washers (for mounting the flag heatsink) Semiconductors 1 MCP6542-E/P dual micropower comparator, DIP-8 (IC1) [element14, RS, Digi-Key, Mouser] 1 LD1085V 3A adjustable regulator, TO-220 (REG1) [element14, RS, Digi-Key, Mouser] 1 S-812C33AY-B2-U micropower low-dropout regulator, TO-92 (REG2) [Digi-Key, Mouser] 2 CSD18534KCS N-channel logic-level MOSFETs, TO-220 (Q1,Q2) [element14, Digi-Key, Mouser] 4 BC547 100mA NPN transistors, TO-92 (Q3,Q4,Q7,Q8) 2 IPP80P03P4L04 P-channel logic-level MOSFETs, TO-220 (Q5,Q6) [element14, RS, Digi-Key, Mouser] 1 high-brightness LED (LED1) 1 5.6V 1W zener diode (ZD1) 1 24V 5W zener diode (1N5359B) (ZD2) [^ ] or [^ element14, Digi-Key, 1 47V 5W zener diode (1N5368B) (ZD2) [^ ] or Mouser] 1 68V 5W zener diode (1N5373B) (ZD2) [^ ] or 1 91V 5W zener diode (1N5377B) (ZD2) [^ ] or 1 130V 5W zener diode (1N5381B) (ZD2) [^ ] (see text) 2 18V 1W zener diodes (ZD3,ZD4) 2 56V 1W zener diodes (1N4758) (ZD5,ZD6) [^ ] 1 W04M 1.5A bridge rectifier (BR1) 2 1N5404 400V 3A diodes (D1,D2) 2 1N4148 small signal diodes (D3,D4) Capacitors 2 220µF 16V low-ESR electrolytic 2 100µF 250V/400V electrolytic [eg, Panasonic EEUED2E101S] 2 10µF 50V electrolytic 2 1µF 50V multi-layer ceramic 2 100nF 63V MKT Resistors (all 1% metal film except where indicated) 2 10MΩ 4 2.2MΩ 2 1MΩ 1 100kΩ 2 75kΩ 1 12kΩ 2 5.6kΩ 2 1kΩ 1 220Ω 1 110Ω 1 100Ω 1W 5% 2 50kΩ mini horizontal trimpots (VR1,VR2) 2 1MΩ mini horizontal trimpots (VR3,VR4) [eg, element14 108244] 1 100Ω mini horizontal trimpots (VR5) [eg, element14 2859725] REG1 slightly before fitting it so that its tab is flush with or beyond the edge of the PCB, to make it easier to mount a heatsink later. Follow by mounting the two remaining capacitors. There are pads for a 7.5mm-pitch standard radial capacitor or a 21 This photo of the assembled PCB is, like the diagram below, at 1:1 scale. Not shown here are the battery holders because these will depend on the batteries you use and the way they are set up. You need 7.4V on CON1 – easily achieved with a dual ‘18650’ battery holder (two cells in series). For the 3.7V required for CON1, we used a pair of single battery holders connected in parallel. Fig.2: use this PCB overlay and wiring diagram as a guide to building the Supply and wire it up to the radio and batteries. Construction is straighforward; simply fit the components as shown here, starting with the lowest profile types and working your way up to the highest profile. Make sure that polarised components like the IC, diodes, MOSFETs, regulators and electrolytic capacitors go in the right way around. 10mm-pitch snap-in capacitor. The former suits the 250V Panasonic capacitor mentioned in the parts list, while the 10mm pads should suit the Altronics Cat R5368 100µF 400V capacitor. You might also be able to get Jaycar Cat RE6156 (100µF 400V) to fit with a bit of lead bending. Fitting the transformer The transformer has four leads on one side and two on the other, and these should be an exact fit for the PCB pads. I had to do quite a bit of ‘massaging’ of the leads to get them to go in as they are such a precise fit. I found that tweezers are a good tool for this, as you can slip them in under the transformer and gently bend and coerce the leads until they all pop into their respective holes. Make sure the transformer is pushed down all the way onto the board before soldering and trimming its leads. The transformer will now make a nice steady base as you mount the 1W resistor and 5W zener diode (ZD2). While you could push these all the way down onto the board, it will aid in convective cooling if you space them off the board 22 at least a few millimetres. I raised them by about 8mm above the top of the PCB on my prototype. Remember to choose ZD2 based on your radio’s HT voltage. You can now install LED1. How you do this depends on what your plans are with it. If you don’t need an external power-on LED indicator, you can simply push it right down (with its longer lead on the side marked ‘A’, opposite the flat on the lens) and solder it in place. Or you could bend it over at right-angles, facing away from the module. If you want it to be externally visible, it would be best to chassis-mount it using a bezel. You could then either solder flying leads from its leads to the PCB pads, or solder a 2-pin header (regular or polarised) onto the PCB and then solder leads to the LED with a plug or plugs at the other end. If your radio will be drawing more than about 500mA from CON5, especially if there is a big difference between its LT voltage and the battery supply, fit a flag heatsink to REG1. I used an insulating washer and insulation bush mainly to ensure good contact between the heatsink and regulator, but it’s also a good idea in case the heatsink could short to a metal case, the chassis or anything else. You will definitely need to insulate the tab from the case or chassis (using an insulating washer and bush) if you are mounting it directly to the case/ chassis for cooling. That just leaves the four tapped spacers, which you can attach to the provided holes on the board, for mounting the module to your radio case (or wherever you plan to use it). Testing and adjustment It’s best to test and adjust the Supply using a variable DC bench supply; ideally one with current limiting. You’ll also need a DMM at the ready, set to a high volts range. The board can generate some hazardous voltages, so make sure that it is in a location where it can’t short against anything and where you can probe it without any risk of coming in contact with the board. Start by centring trimpots VR1 and VR2, setting VR3 and VR4 at maximum and VR5 to its minimum. If you’ve built the two-battery version, bridge Practical Electronics | January | 2022 the positive inputs together (you don’t need to bridge the negative terminals as they are connected on the PCB). Set your bench supply to around 4V and the current limit to a low value, then switch it off and wire up either input (CON1 or CON2) to the supply. Switch the supply on and watch LED1. It should not light yet, and the current drawn from the supply should be low (under 1mA). If it’s significantly higher than that, you could have a board fault, so switch off and check for short circuits and incorrectly located or oriented components. If all is well, increase the current limit to around 1A and wind the voltage up to about 8V, then rotate VR3 anti-clockwise until LED1 lights up. The circuit has now powered up, so check the voltage across the CON4 output. It should be slightly higher than the voltage rating of zener diode ZD2. You may notice ZD2 and/or the 100Ω resistor getting warm. Also check the output voltage at CON5. It should be around 1.2V. Check that you can vary it by adjusting trimpot VR5. You might as well set it to your desired voltage while you’re at it. Now rotate VR3 and VR4 fully anticlockwise, set the supply voltage to your desired A-battery (Li-ion) cut-out voltage, then rotate VR4 clockwise slowly until LED1 switches off. Then increase the supply voltage to your desired B-battery cut-out voltage; LED1 should switch back on. Rotate VR3 slowly clockwise until the unit switches off. You have now set both battery cut-out thresholds. To set the ideal operating frequency, you will need to connect your actual radio to the outputs (after powering the supply down). Power it back on and wind the supply voltage back up to your nominal battery voltage (around 7.4V for two Li-ion or LiPo cells in series). Switch the radio on and after it has warmed up, make sure it is working normally. Then adjust VR1 and VR2 in lock-step (eg, making small changes in one, then the other) while monitoring the HT voltage. Adjust until you achieve the specified voltage, or as close to it as you can get. Once you have done that, if you have a scope, you can adjust for 50% duty cycle in the transformer drive. Power down the circuit and connect the scope up to the ends of the 5.6kΩ resistors closest to MOSFETs Q1 and Q2 and connect the scope’s ground to circuit ground (eg, the anode of D1 or D2). Power it back on and adjust VR1 and VR2 by small amounts in opposite directions until you achieve pulse widths on both channels that measure the same. Minimising EMI radiation While this circuit has low EMI, component variations could mean that yours radiates enough to affect radio reception. If so, try increasing the values of the two 5.6k resistors. These slow the switch-off of MOSFETs Q1 and Q2, reducing the spikes at the transformer primaries. These should ideally be below the 56V conduction threshold of zener diodes ZD3 and ZD4. Test the supply with your radio tuned off-station. If you hear hash, try increasing the 5.6k resistors to 15-22k or possibly higher. If you have a scope, check the waveforms at the cathodes of ZD5 and ZD6 to see that the spikes have been reduced or just test it again with the radio. You may need to re-tweak the frequency/HT voltage after doing that. You should eventually arrive at settings for VR1 and VR2 that satisfy both conditions. It’s then just a matter of mounting the Battery Vintage Radio Power Supply module and its batteries to your radio case or to a piece of timber you will install in the case. Or if it won’t fit inside the radio, you could mount it in some sort of Jiffy box and wire it up to the set. If doing the latter, then you must ensure both the module and the wiring are properly insulated! ESR Electronic Components Ltd All of our stock is RoHS compliant and CE approved. Visit our well stocked shop for all of your requirements or order on-line. We can help and advise with your enquiry, from design to construction. 3D Printing • Cable • CCTV • Connectors • Components • Enclosures • Fans • Fuses • Hardware • Lamps • LED’s • Leads • Loudspeakers • Panel Meters • PCB Production • Power Supplies • Relays • Resistors • Semiconductors • Soldering Irons • Switches • Test Equipment • Transformers and so much more… JTAG Connector Plugs Directly into PCB!! No Header! No Brainer! 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