PSU issue 2 Builder`s Guide
Contents
1. R9 R8 R12 12K R6 2W metal film or wirewound 5 or better Yamaha PA 20 IR R2 R3 Yamaha PA 30 OR82 R2 R3 Capacitors 22pF low K ceramic 2 5mm 330p low K ceramic 2 5mm 100nF axial ML ceramic 10uF 35V electrolytic 22uF 35V electrolytic 3300uF 35V electrolytic C4 C5 C1 C2 C7 C8 C3 C6 C11 C12 C9 C10 C9 and C10 are 105 degree Celsius radial types and have standard wire ended leads Lead spacing is 7 5mm I recommend Panasonic type EEUFC1V332 but any decent 105 degree part can be used that will fit on the board will do Integrated Circuits LM723CN 100mA voltage regulator OP177GPZ single precision op amp U1 U2 Good quality DIL sockets are recommended You need one 14 pin and one 8 pin Discrete Semiconductors 10V 500mW zener diode 1N4004 rectifier diode 1N5401 rectifier diode TIP35C NPN transistor TIP145 or TIP147 PNP Darlington transistor BC560 PNP transistor D3 D1 D2 D4 D5 D10 D6 D7 D7 D8 Ql Q2 Q3 Do not solder the two power transistors until the board is ready to be mounted on its panel There are three LEDs that can be fitted to the board to indicate power status However these can be optionally fitted to any front panel you are using In this case I would recommend that you fit 2 way 0 1 KK or MTA headers to the board in place of the LEDs 5mm red LED 5mm green LED 5mm orange LED Trimmer 10K multiturn trimmer AC VE VE ADJ Miscellaneous2. 56R Thirdly the printed circuit board s copper tracks are not thick enough to pass huge amounts of current Fourthly you need to consider how you are going to get rid of the extra heat the power devices will develop The amount of heat given out by those devices is not only dependant on the current taken by the output load but also the supply voltage The latter being dependant on the transformer being used and your local mains voltage The 3U wide 5U high panel design has only been tested to work with a maximum of 0 52A per rail when driven from a PA 20 or PA 30 The 4U 19 panel is capable of considerably more it has been tested up to 0 94A with 25V across the smoothing capacitors However I 10 would advise you to do your own experiments carefully otherwise you run the serious risk of overheating Finally I think modular systems work better with a number of smaller power supplies rather than one big one This is because as power is distributed around a large modular it gets corrupted by the connected modules and the cabling The cables have resistance and as current flows through the cables the voltage at the end of the cable gets reduced The more current that flows and the longer the cable the worse the problem Having multiple power supplies does create the not insignificant question of what to do with the multiple grounds you will now have in your system However I would like to deal with this in the User Manual which
3. covers the application of the power supply module Now let us look in more detail at the negative part of the power supply The negative supply works in a similar way to the positive regulator but uses an op amp U2 and discrete components The reference for the negative rails comes not from a precision reference but from the output of the 15V regulator This means that the negative output voltage will track the positive one but not vice versa I should add The op amp is wired as a simple inverting amplifier although its difficult to see this at first glance The input is via R9 which is connected to the 15V output The feedback is provided via R8 which is connected to the negative supply output The output of the op amp drives the base of the pass transistor Q2 via R11 which in turn controls the level of the negative output Q2 if using the recommended TIP145 is a PNP Darlington transistor This is actually two transistors in one enclosure but it can treated as one device with a larger than normal current gain hfe and twice than normal base emitter voltage of 1 2V Having a high gain requires only a small amount of drive current from the op amp via R11 even when the load on the power supply is relatively high U2 will act so that its output will force all the current flowing through R9 to be passed onto R8 This is one of the op amp golden rules that is no current shall flow into the input pins so long as feedback is maintained To do
4. this PSU could possibly be supplying nearly 1A the capacitors will need to have twice that to be on the safe side A cheaper and less rugged capacitor will overheat and won t last as long C7 and C8 are there to reduce any high frequency noise on the line The bigger smoothing capacitors tend to stop behaving like capacitors at high frequencies and the little fellas can take over at that point The voltage across the smoothing capacitors is fairly constant but its not stable enough to drive a modular synth Modular synths need to have a regulated supply so that they stay in tune and behave in a predictable fashion whatever patches are selected The regulators smooth out the voltage across the big capacitors even more and will produce a constant low ripple 15V and 15V output Both of the regulators work in a similar way You can think of them like a person controlling the speed of a water wheel If the wheel runs too fast the water flowing over the wheel is cut back by closing the sluice gate If it runs too slowly then the water flow is increased by opening the gate The person uses his own eyes to monitor the speed of the wheel and will make adjustments to the sluice accordingly Now the wheel speed will depend on two things primarily firstly the water rate but also the load on the wheel itself If the wheel needs to do more work it will slow down unless the water rate is increased The regulated power supply must also control its output
5. this the op amp must force the pass transistor to pass enough current though it to establish exactly 15V at the output Q3 and R3 form part of the current limit circuit If the voltage across R3 exceeds 640mV then Q3 will turn on dragging current away from Q2 s base and thus lowering the output voltage of the negative rail accordingly D1 D2 D4 and DS protect the 15V and 15V output rails from a variety of naughties On power down any excess current left in the modular will be shunted to the smoothing caps and not damage the now un powered power supply components Also they also prevent the negative rail from going positive should the negative rail die for some reason And vice versa too 11 Panel Assembly and Attaching the Power Devices The powered devices shown here are two MOSPEC devices The whole assembly has been removed from the heatsink panel to show the insulated mounting holes of the TO 3P devices Note that both the big transistors have been soldered from the top side of the board with the leads coming up from the bottom This section assumes you have purchased a 3U Schaeffer or Frontpanel Express panel or similar For the four PCB mounting holes insert a 20mm countersunk screw through each of them Fit a star washer over each of the exposed screws Now fit a hex spacer over each of the screws and tighten firmly Check that the PCB fits over all four screws without too much of a fight but don t fit the PCB permanentl
6. with changes of load ie the number of synth modules in your system and the input voltage ie fluctuations in the line lump s output Before we talk about the circuit in more detail I ought to say why I haven t used the more common and I might add cheaper 7815 and 7915 regulators The main reason is down to controllable current limiting The 78XX and 79XX series do not feature this In fact if you were to short out a 7815 to ground it would take around 1 7A from the supply and would heat up until the inbuilt thermal limiter kicked in 1 7A may well be high enough to damage any connected transformer or line lump to say nothing of the rectifiers and smoothing capacitors that would have to supply this current By using the LM723 and a few external components we put the control back in the hands of the builders of the circuit Let us look at the positive regulator first since this actually also controls the negative rail too The circuit is built around the venerable LM723 chip It contains pretty much everything you need to make a small regulator including precision voltage reference feedback control systems and a current limiter However it doesn t have a very big output current capacity so to use it for anything greater than 100mA one needs to use an external pass transistor Q1 The pass transistor is analogous to our sluice gate The resistance of the pass transistor is effectively changed by the voltage at its base pin 1 Decrease t
7. 2A antisurge 20mm fuse 20mm fuseholder PC mount 4 way screw terminal 5mm F1 F2 F1 F2 POWER SWITCH OP1 OP2 Heatsink compound if you are using the TIP devices with a clear mica insulating pad TO 3P mounting kits 2 off for mounting the TIP power devices Note if you have bought MOSPEC power devices the mounting hole is already insulated from the substrate of the transistor This means you do not need to purchase a top hat bush all you need is a TO 3P insulating pad Connecting wire of your choice I use 22AWG 0 5 sq mm appliance wire for all power connections M3 20mm Countersunk screws M3 10mm Countersunk screws M3 star washers M3 washers M3 hex threaded 10mm spacers M3 hex nuts M3 6mm pan head screws M3 20mm pan head screws M3 hex threaded 10mm spacers M3 star washers M3 washers M3 hex nuts Mounting hardware for Master Panel 4 off 2 off 10 off 2 off 4 off 6 off Mounting hardware for 19 rack panel 4 off 2 off 4 off 10 off 2 off For power devices 2 off For power devices And any mounting hardware for the Dizzy boards if needed Optional Extras Power switch DPST 3 way connector LED clip 1 off Power on 1 off Connector to the line lump 1 off Master Panel Jack Multiple CV gate and Attenuator Option Switchcraft 1 4 sockets 112A 12 off 47K 5 or better 0 25W resistors 1 off 4K7 5 or better 0 25W resistors 1 off 3 way Molex MTA housing 1 off 20AWG tinne
8. Oakley Sound Systems Power Supply Unit PSU PCB Issue 2 Project Builder s Guide V2 3 1 Tony Allgood B Eng PGCE Oakley Sound Systems CARLISLE United Kingdom Introduction This is the Project Builder s Guide for issue 2 of the PSU circuit board from Oakley Sound This document contains the circuit diagram of the completed board a full parts list for the components needed to populate the board and some basic testing methods You will also need the issue 2 PSU User Manual which gives details on how to use the module as well as example wiring diagrams This can be found on the Oakley PSU webpage For general information regarding where to get parts and suggested part numbers please see our useful Parts Guide at http www oakleysound com parts pdf For general information on how to build our modules including circuit board population mounting front panel components and making up board interconnects please see our Construction Guide at http www oakleysound com construct pdf Jana acef s eA ee t cier D c3q p i Je Allla R4 Qt RS RS Sat Cat Rit eA s Ravers c2 as R1 gje n aye 2 a coos e ewoo a ah 5 5 Se S Zo Neretai 15 0Y 0Y 15 HSV ev 15 IF ROO The issue 2 Oakley PSU board awaiting fitting to the front panel Which Power Devices The two off board transistors are awesomely chunky devices in a TO 3P package The devices I recommend are TIP35C for the NPN and TIP145
9. Rlimit imit is the current limit value in amperes and the R imit is the value in ohms of the resistor However there are several things to be taken into consideration should you decide to increase the current limit above the values I have chosen Firstly you need to ensure your transformer will be able to handle the additional current you are asking from it The simple rule of thumb is lac 1 8 x Idc Where lac is the steady state AC current in the transformer s secondary coil and Idc is the current taken by the load connected to the 15V and 15V rails For example The Yamaha PA 20 can supply 0 94A of alternating current into a simple resistative load from each of its two outputs But by the time the output of the PSU has been rectified and smoothed the DC current taken should not exceed 0 52A on a continuous basis Secondly the amount of current needed to drive the pass transistors Q1 and Q2 is directly related to the amount of current draw on the supply lines Thus to ask for more from the supply would be to ask more from the 723 on the positive rail and the op amp on the negative rail The actual relationship between load current and drive current is related to hfe or the gain of the pass transistor Choosing a pass transistor with too little gain will mean that you cannot obtain your required current The recommended TIP35C and TIP145 devices have been tested to work with a supply that can supply up to 1 1A ie R2 and R3 are both 0
10. V terminals This should read 15 0V or very close to it Check that the voltage across 15V and OV is 15 0V It is advisable that the VE and VE LEDs are fitted for this adjustment or that at least one Oakley or MOTM module is connected up to the power supply board This is to ensure that there is some current running through the series pass devices Q1 amp Q2 which allows them to work properly If all is well then it is time to check the current limit Put your multimeter into current mode Use the 2A or 10A setting Now put your probes across the 15V and OV terminals The meter should read 630mA for current limit resistors of 1R or 780mA for current limit resistors of OR82 The red LED should go out only when the probes are in place There is a problem if the LED does not come on again when the probes are removed Now do the same for the 15V and OV terminals This time we should get a slightly higher reading of around 720mA for 1R resistors or 870mA for OR82 resistors This time the green LED should go out While doing the current limit check you may notice the power devices getting warm This is perfectly normal If all is well everything is probably working and you can now connect your new supply up to your modules The output voltages will vary a little with load That is it will change marginally depending on how many modules you connect up to the power supply board Feel free to re adjust the trimmer when you add more mod
11. d copper wire Approx Im This shows the two fuse holders fitted to the prototype board R13 is not fitted on this particular build but it would normally be a wire link Circuit Description The design is based around some very traditional methods so there are no claims for originality with this board The line lump outputs a two phase AC signal each one with a peak of nearly 25V with respect to its centre tap output The difference between the two phases is that one is completely out of phase with the other It s a bit like an audio balanced output in that respect Both phases should be present for the power supply to work correctly Each phase is separately fused Note that the fuses should be anti surge or slo blo The standard convention is for a T in front of the value where T is for time lag For example T2A would mean a two ampere anti surge fuse Anti surge fuses have a higher thermal mass than fast blow fuses to prevent them blowing when they see a short burst of current The inrush current when the supply is first switched on is completely normal and we don t want our fuses blowing at that point D10 C12 and R12 provide a low current DC feed to a standby AC in LED This lights up whenever the line lump is connected and powered up It is wired in before the modular s power switch to remind you that the power on switch is actually a standby switch and does not switch the line lump off It is possible to run the Oakl
12. ey PSU module on a single phase This means that the unit could be connected to a single output AC adapter ie one with just two leads to pins 1 and 2 However the regulators and the AC adapter will have to work very hard in this application It is therefore not recommended to run the PSU in single phase unless the output current is going to be less than 250mA or so The raw AC is fed to a bridge rectifier based around D6 D7 D8 and D9 This is the classic bridge rectifier circuit Although it is drawn somewhat differently in the schematic than the usual bridge style If you think about a diode as passing current through it just one way you should be able to work out why the voltage across C9 ends up as only positive and across C10 as only negative I have used 1N5401 diodes in this place These are 3A devices and are plenty large enough to cope with any abuse the power supply is given The outputs of the rectifiers supply current to the two smoothing capacitors These act as reservoirs of charge when the AC voltage dips below its peak output You can think about the rectifiers as merely topping up the reservoirs 100 times a second 120 in North America whilst the capacitors actually provide the power to keep the modular powered The smoothing capacitors are generously rated both in terms of voltage and capacitance It is essential that you use good quality components here and that they have sufficiently high ripple current rating Since
13. he voltage at its base with respect to its emitter pin 3 and the transistor will increase its effective resistance and the output voltage of the PSU will drop The base voltage is controlled by the internal electronics of the LM723 but this in turn responds to the feedback from the output of the power supply The feedback path is analogous to the sluice gate man s visual record of the water wheel s motion In this case a fraction of the output signal is fed back into the LM723 at pin 4 The electronics will determine whether to make the base drive for Q1 higher or lower depending on what it sees The ADJ trimmer adjusts the fraction of the output voltage pin 4 will see and thus controls the overall output voltage R2 forms an important part of the current limiting circuit When current travels through this resistor a voltage develops across it When the voltage approaches 630mV ie 630mA through the resistor the internal electronics in the LM723 starts to pull down the base voltage on Q1 This effectively lowers the output voltage to make sure the current doesn t climb much above that value The value of R2 and also R3 will determine the actual value of the current limit For example a 1R resistor will set the current limit to be around 630mA and a 0 82R resistor will make it around 770mA The resistor value is chosen so that it will develop 630mV across it when your chosen current limit would be going through it Limit 0 63 V
14. ly and the current that is needed to drive the transistor I would recommend devices with hfe of 80 or more Using a Darlington device for the PNP transistor will ensure that you have enough gain although too much gain could lead to instability Remember also that older TO 3 packages will need special mounting kits with bushes solder tags and insulating pads Parts List For general information regarding where to get parts and suggested part numbers please see our useful Parts Guide at the project webpage or http www oakleysound com parts pdf The components are grouped into values the order of the component names is of no particular consequence A quick note on European part descriptions For resistors R is shorthand for ohm K is shorthand for kilo ohm M is shorthand for mega ohm For capacitors 1uF 1 000nF 1 000 000pF Sometimes the F is not included on the circuit diagram to indicate a capacitor s value ie 100n 100nF To prevent loss of the small as the decimal point a convention of inserting the unit in its place is used eg 4R7 is a 4 7 ohm 4K7 is a 4700 ohm resistor 4n7 is a 4 7 nF capacitor Resistors R13 is not a resistor but a wire link You can use solid core copper wire or a discarded resistor lead clipping Simply solder the wire so that it connects both pads of R13 together 5 1 4W carbon or better 1 4W 1 metal film is recommended 1K R1 Ril 4K7 R4 R5 R10 1 1 4W metal film 10K R7
15. or TIP147 for the Darlington PNP I buy Multicomp s MOSPEC ones available from Farnell If you are using the standard TIP devices you may also need two TO 3P mounting kits This comprises usually of an insulating pad or plate and an insulating bush which looks like a little top hat The kit may also have a sachet of some heatsink compound if the insulation material is made from mica Newer kits contain a pad made from a fibre glass cloth which is usually grey but I have seen white and pale green ones too This pad has two functions Firstly it insulates the metal tab of the housing which is internally connected to the collector of the transistor from the panel or heatsink Secondly it improves thermal transfer because it squashes itself into the two surfaces so it doesn t normally need any heatsink paste The insulating bush is like a little top hat made from a very brittle plastic This will sit in the hole of the device and insulate the mounting screw from the metal tab The MOSPEC TIP35 and TIP145 devices need no insulating bush only the insulating pad since the mounting hole is already insulated as part of the device Alternatively you can use any large power NPN and PNP device you fancy but just make sure you have wired the correct pins to the right pads on the PCB Remember for the PNP device you need to ensure that the minimum hfe is sufficiently high This is because there is a direct relationship between the current draw on the power supp
16. t four of the right hand column of sockets Then do the same on the left hand row You should now have four vertical wires two running down the top four sockets and two running down over six Your multiples are now complete The KEY CV and GATE sockets now need to be connected to the Oakley Dizzy buss Use a three way 0 1 Molex or MTA100 as described in the Dizzy User Guide to connect the signal lugs on these two sockets to the CV and Gate buss The 10 1 attenuator is simply made Connect the signal lug on the right hand socket IN to the signal lug of the left hand socket OUT with a 47K resistor Then solder a 4K7 resistor from the signal lug of the left hand socket to the earth lug of the same socket And that s it 14 Testing and Calibration Note all testing must be done with the heatsink or panel attached to the power devices After wiring the unit according to the instructions given in the Users Manual you should apply power to the unit Check that no device is running hot Any sign of smoke or strange smells turn off the power immediately and recheck the all the external wiring first and then the components on the board Both the VE and VE LEDs should be lit and neither should be too bright or too dim With a multimeter check the DC voltage across the 15V and 15V output terminals You should get around 30V or so Adjust the trimmer so that you get exactly 30 00V Now measure the voltage across the 15V and one of the O
17. ules to your system Tony Allgood at Oakley Sound Cumbria UK October 2010 updated February 2012 15
18. ut do not tighten fully Do the same for the TIP145 in the left hand hole Now if you have done all this correctly you should find that the when the power supply PCB is fitted back onto the four screws you can coax the power devices legs through the respective pads on the board With both the board and transistors secured to the panel with their mounting hardware you can solder the transistor leads from the top side of the board Note if you have bought the recommended Multicomp s MOSPEC TIP devices you do not need the insulating bush as the case itself has insulation around the mounting hole This makes mounting considerably easier You still have to use an insulating pad on each device though since the metal underside of the MOSPEC device should never come into electrical contact with the panel Cae Seite gt PEN The MOSPEC devices mounted onto their panel in this case it s the 19 4U high rack panel You can see the grey insulating sheets between the power devices and the panel 13 Master Module Jack Multiple and Attenuator Option Fit twelve Switchcraft sockets into the spaces on the Master Panel Align them so that the bevel faces to the top right as you look at the back of the unit With a single piece of 20SWG tinned solid core wire connect all the earth lugs together on the right hand column Now do the same on the left hand column Now connect all the signal lugs marked as T on the body of the socket on the firs
19. y in just yet Now you need to prepare the leads on the two power transistors The three legs need to be bent upwards so that the PCB can be fitted over them Note that the top surface of the device is marked with the name of the component and it is the flat side on the bottom of the device that will be in contact with the panel You should be able to see that the leads have a thicker section close to the body of the device Make a 90 degree bend upwards at a point 1mm away from this thicker section Do this for all three legs of the device If you are using TIP device with a metal mounting plate check that the mounting bush top hat fits through the two holes in the panel that the power devices will be attached to If it doesn t fit don t force it in but widen the hole slightly with a reamer or suitable twist drill Make sure all the swarf is cleaned away Now place the bush into the hole of the transistor 12 with the flange of the bush lying on the top side of the device Take one of the insulating pads and place it against the rear of the TIP35C Match up the hole in the pad with the bush that is sticking out from the underside of the tab Now place the power device bush and pad flat against the rear of the panel so that the bush fits into the right hand side hole in panel Make sure the pad does not slip out of place when you do this Insert a 10mm countersunk M3 screw into the hole from the front and fit a washer and nut onto the screw b
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