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Tim Blythman’s Display Adaptor for the BREADBOARD PSU The Dual Channel Breadboard PSU is compact and handy for prototyping. It slots straight into a breadboard’s power rails and can run from a plugpack or USB supply. The Display Adaptor attaches to the Breadboard PSU and displays lots of handy data, such as the set and actual voltages and currents. It even has extra voltmeter and ammeter channels to help you analyse your prototype! T he Breadboard PSU is a compact unit that plugs into a breadboard, providing two voltage adjustable current-­limited supply rails. It’s a handy tool for prototyping and testing, but by itself, you won’t know what voltages you’ve set or how much current is being drawn. This add-on module solves that by providing readouts of the setpoint and actual voltage and current for each channel. Since it uses a microcontroller with many analog inputs, we have added extra voltage and current monitoring channels that give you a lot of flexibility. We’ve also included a pair of bi-­ colour LEDs to provide status indications and a piezo buzzer to sound alerts. It even calculates an estimate of the dissipation that’s occurring in the transistors in the Breadboard PSU, so you can avoid burning them out. The PSU Display Adaptor simply mounts directly above the Breadboard PSU and doesn’t take up any extra bench space. Display Adaptor When we designed the Breadboard PSU, we realised it would be pretty easy to add extra circuitry to monitor its operation. This is part of the reason for the numerous headers on the Breadboard PSU. Voltages are applied to pins on those headers that are proportional to voltages and currents in the circuit, making it easy for an Features & Specifications ∎ Uses a common 20x4 character backlit LCD ∎ Shows 11 statistics ∎ Four independent voltages and two currents displayed ∎ 100mV resolution on voltages, 10mA resolution on currents ∎ Typically 1% accurate, can be calibrated ∎ Includes indicator LEDs and over-current warning buzzer ∎ Shows dissipation estimate for PSU transistors ∎ Stacks on top of Breadboard PSU for minimal clutter 40 Silicon Chip Australia's electronics magazine add-on board to monitor the status. Fig.3 shows the circuit of the Display Adaptor. It won’t do much without the Breadboard PSU, so the components have been numbered to follow on from that circuit, except for CON5CON9, which form the inter-board connections and are effectively common to both boards. We’ll also refer to parts on the Breadboard PSU, so you might need to refer to that circuit (Fig.1 on page 32). Power for the Display Adaptor comes in via CON7, which has connections to ground, the 15V rail and the 5V rail from the PSU. It effectively combines the inputs from CON1 and CON2 on that board. The Display Adaptor only needs a 5V rail to operate, so REG2 is a 7805 linear 5V regulator accompanied by 100μF input and output capacitors. This larger TO-220 type regulator has been mainly chosen to provide the higher current needed to drive the LED display backlight. Jumper JP3 allows sourcing power from REG2 or the USB connection if preferred, but we recommend that this jumper be set to the REG position. That’s because the regulator’s output siliconchip.com.au Fig.3: this circuit interfaces with that of the Breadboard PSU (Fig.1 on page 32) via CON5-CON9. CON7 provides power to the Display Adaptor, while CON5 and CON6 supply the voltages measured by the microcontroller IC4. CON8 and CON9 feed the two extra currents that can be measured between the two PCBs. will be much more accurate and consistent than a USB supply. IC4 is a 44-pin PIC16F18877 microcontroller, chosen for its numerous input/output (I/O) pins. It’s effectively the same part used in the USB Cable Tester from November & December 2021 (siliconchip.au/Series/374), but in a compact TQFP package, which saves a lot of space. IC4 has two 5V and two ground connections, each pair bypassed by a 100nF capacitor. The in-circuit serial programming (ICSP) pins are taken to CON13 for programming and siliconchip.com.au debugging the microcontroller. If you have a pre-programmed microcontroller, CON13 does not need to be fitted. There is also a 10kW pullup resistor on IC4’s MCLR pin to prevent spurious resets. One of the great things about the PIC16F18877 is that its ports and pins are highly interchangeable. While it might look like a complicated chip with many pins, most PCB traces simply fan out in the required direction to the nearest connection point. Practically all I/O pins are internally connected to the microcontroller’s Australia's electronics magazine ADC (analog-to-digital converter) peripheral, so we can use them to read and monitor external voltages. Nine such voltages come from the Breadboard PSU through CON5 and CON6. Eight of these correspond to the actual and setpoint (target) voltages for the current and voltage of each of the two PSU channels. The remaining voltage to monitor is a divided version of the so-called 15V rail, allowing it to be measured too. This is handy to know as it is the DC supply for the PSU outputs and will dictate such things as the maximum December 2022  41 Make sure to check components for clearance with the LCD when assembling the Display Adaptor PCB. output voltage. You might find this handy to monitor if you’re running the Breadboard PSU from a battery and want to check that it’s not going flat. This reading is also used in the calculations to determine the dissipation in the Breadboard PSU’s power transistors. Using a battery is an easy way to get a floating (ie, not connected to Earth) power supply and is something that the Arduino Programmable PSU could not do without being connected to a laptop computer running on its own battery. Handy additional inputs Four more analog voltages are monitored that are derived from the four 51kW/10kW voltage dividers connected to four-way header CON11. These are the same ratios used on the Breadboard PSU, giving the same nominal 30.5V scale against a 5V reference. You can use these four independent voltage channels to check and monitor your breadboard prototype. Using the same divider ratios mean that a single (nominal) calibration factor can be used for all voltage inputs. The input impedance at these pins is much lower than a multimeter, but we think they’ll still be convenient when you need to check multiple voltages in your circuit simultaneously. CON12 is another four-way header that provides the facility to monitor two currents in your circuit. Each requires two connections as the current needs to pass in, go through the current sense resistor and back out to the circuit under test. The arrangement is the same used for monitoring the output currents of the Breadboard PSU. A voltage appears across the 100mW shunt resistor in each channel when current passes through them. That voltage is amplified by IC1 on the Breadboard PSU PCB and returns to the Display Adaptor via the third pins of CON8 & CON9, to be read by a further two ADC inputs. We can do this because IC1 is a quad-channel device and only two of its channels are used by the Breadboard PSU hardware. The voltages on the current monitor inputs must be no higher than the INA4180’s 26V limit. That seems unlikely, given that the circuit on the breadboard is presumably powered by the maximum 15V outputs of the Breadboard PSU. The 20-column, four-row alphanumeric LCD module connects to the circuit via header socket CON10. 10kW trimpot VR5 wired as a voltage divider provides a contrast control voltage into pin 3 of the LCD. 500W trimpot VR6 is wired as a variable resistor to allow the LED backlight brightness to be adjusted. This can save power by dimming the backlight when running from a battery. Six control signals go between CON10 and IC4 to control the LCD module in four-bit mode. IC4’s digital outputs drive these pins to clock data and commands into the LCD. CON10 also provides power for the LCD controller and backlight LED, the contrast voltage generated by VR5 and provides a connection to pull the RD/ WR pin low. The micro doesn’t read from the display controller, saving an I/O pin. Another four digital output pins of the micro drive bi-colour LEDs (LED1 & LED2) via 1kW dropping resistors. Each LED uses two I/O pins and, depending on which is high and which is low, either the red or green LED element (or neither) is lit. Finally, another digital output is used to drive piezo sounder SPK1. Firmware Microcontroller IC4’s main task is to read the raw analog voltages on various pins, scale them according to a calibration factor, and display them on the LCD. Screen 1 shows the resulting display. The first line shows the parameters set by the potentiometers on the Breadboard PSU, indicated by an “S”. These are the CON3 voltage (as set by VR1), CON3 current (VR3), CON4 voltage (VR2) and CON4 current (VR4) targets. As the current-limiting circuitry on the Breadboard PSU pulls down the reference voltages using Q2 and Q4, Screen 1: everything you need to know is on this screen. To fit everything in, it cycles through the incoming supply voltage and transistor dissipations in the bottomright corner, as shown in the inset. 42 Silicon Chip Australia's electronics magazine siliconchip.com.au the displayed voltage can dip slightly (up to around 0.2V) during current limiting. The value of the 100kW resistors connected to the wipers of VR1 and VR2 is a compromise between this side-effect and providing a low impedance path for the control voltage. So take care not to set these voltages while current limiting is active. Note that the “A” (for amps) at the end of the first line is implied due to the space needed for the “S” at the start. We’ve also used custom narrow characters for the units to provide visual separation. These characters use the display’s character generator RAM feature. The second line shows the corresponding measured values, marked by the leading “A” for “actual”. For the most part, the voltages should match the setpoints except when the current limit is active, in which case the current should match its setpoint. The third line shows the ‘bonus’ voltage readings from CON11, while the first two readings on the fourth line are the currents measured at CON12. The small icons that follow indicate whether audible alarms are active for the CON3 and CON4 outputs, respectively. The remaining three statistics share the last five character slots in the lower-­right corner of the screen. The dissipation in each main regulator transistor is calculated as the CON3 or CON4 output voltage subtracted from 15V rail voltage, multiplied by the appropriate current. The display cycles every two seconds between showing the 15V rail voltage (which won’t necessarily be 15V) and the two calculated dissipation figures of Q1 and Q3 on the Breadboard PSU. This is possible because the dissipation is expected to be in the range of single digit (0-9) watts, so it can be displayed very compactly. You can see this in the Screen 1 inset. If the reading is above 9W, it is clamped to 9W for simplicity. Besides driving the display, which takes up most of the microcontroller’s time, it also monitors pushbutton switches S1-S3 and lights up LED1 and LED2 depending on the prevailing conditions. The purpose of those switches and LEDs will be described later, in the section on using this unit. In brief, the siliconchip.com.au buttons allow the audible alarm for either channel to be toggled and all the values displayed to be calibrated. The LEDs indicate when either channel is in current limiting or otherwise unable to achieve the desired voltage. Construction Start by fitting out the PCB for the Display Adaptor, which measures 99 × 63mm and is coded 04112222, referring to overlay diagram Fig.4. There are five surface-mounting parts, but none are that difficult to handle. You should have flux and solder wicking braid at the very least, as the pins on IC4 are fairly close together. Flux will help the solder flow in the right places, and the braid will help remove it if it gets where it shouldn’t. We also recommend having tweezers, a fine-tipped soldering iron, good illumination and a magnifier to help you check your soldering. Start by soldering the microcontroller, IC4. Lay down some flux on the pads and align it on all four sides. The TQFP part is a bit more fiddly than, say, an SOIC part that only has pins along two sides. Roughly place it and check that the pin 1 dot matches the PCB silkscreen. Tack one pin in place and check that it is flat and that all the pins are above the correct pads. If not, apply heat to the soldered pins and gently adjust the chip’s position with tweezers until all the pins are perfectly aligned. Fig.4: the Display Adaptor is much the same size as the LCD module that sits above it. Pin headers CON5-CON9 are fitted below this PCB to connect to the Breadboard PSU. We also recommend that the ICSP header (if fitted) go underneath the PCB to give clearance for the LCD. The LEDs are installed last to align with the top of the LCD, while the trimpots and piezo should be checked for clearance below the LCD. Australia's electronics magazine December 2022  43 The Display Adaptor stacks above the Breadboard PSU to create a handy device that simply plugs into the power rails of a breadboard. It’s much more compact than a standard dual bench power supply, helps tidy unruly wiring, and you won’t have to glance away while testing your prototype. With parts like this which have closely-­spaced pins, try to keep the iron away from the top of the pins and work on where the pin touches the PCB pad. That helps to avoid solder bridges forming between the pins. With it aligned, go around and solder each pin, starting on the opposite side from the pins you initially tacked. Finish by retouching the first pin(s) if necessary. Then use solder wick to remove any bridges that have formed. Some more flux and a touch from the soldering iron can help tidy up any joints that don’t look right. Follow with the two 100nF capacitors near IC4, which are not polarised. The shunt resistors are the other surface-mounting parts; they will be much easier due to their larger size. Use a similar technique of soldering one lead, checking for alignment and then solder the other side. With all the SMDs fitted, clean off any flux residue using a flux remover or alcohol (eg, isopropyl or methylated spirits) and a lint-free cloth and/ or Nylon brush. Allow it to dry fully before proceeding. You can then fit the through-hole 44 Silicon Chip resistors. There are four different values, so check each part with a multimeter against the silkscreen printing to confirm that the correct value is placed in the correct location. Most of them have values that are powers of ten, so their markings will be similar, but they will easily be distinguished by a multimeter. The two 100μF capacitors near REG2 are polarised (the longer leads go to the pads marked +) and must be mounted on their sides to leave enough clearance for the LCD to fit above. It’s easiest to bend their leads before soldering. Check which way this will be (based on the polarity), slot them into place and confirm that the positive marking aligns with the longer lead before soldering. Although the Breadboard PSU won’t be subjected to much movement, there is no harm in securing the capacitor bodies to the PCB with a dab of neutral-­ cure silicone sealant. REG2 is fitted similarly to the transistors on the Breadboard PSU PCB. Bend the leads back 90° around 7mm from the body of the regulator, slot them into the holes in the PCB and Australia's electronics magazine then slip the heatsink underneath. Thread the machine screw through from below and loosely secure it with the washer and nut. Adjust the regulator and heatsink to be square and within the silkscreen markings, then nip up the nut, being careful not to twist the regulator. The leads can then be soldered and trimmed. Solder the three-way header for JP3 now, then fit the jumper to the REG position (across the top two pins), unless you have configured the Breadboard PSU to use USB power. Right-angle switches S1-S3 will only fit one way, with their buttons facing out from the PCB. Just check that they are lined up neatly before soldering. CON11, CON12 and CON13 (if needed) can be soldered next. We used right-angle female headers for CON11 and CON12 as these will accept jumper wires for prototyping. If you can’t get right-angle types (they will be included in our kit), you can carefully bend the pins of vertical types before soldering. We installed CON13 underneath the Display Adaptor PCB as this gave the best clearance to the adjacent spacer for connecting a programmer. Check our photos for how CON11, CON12 and CON13 look on our prototype. Final assembly Remove the screws and tapped spacers from the Breadboard PSU, then fit the tapped spacers to the LCD module, so that we can use it to align and check the next steps of the assembly. Orientate the LCD module so that the 16-way header is at upper left with the display upward. If there are text labels for the pins, these should be the right way up. This is the normal orientation of the LCD module as we describe the assembly in the following. The tapped spacers along the left (top and bottom) and top right of the LCD module should be secured with the short (5-6mm) machine screws. The spacer at lower right uses the 32-35mm machine screw as this forms the top of a stack of three spacers. Mount the trimpots similarly to the switches. They will need to be pushed down firmly against the PCB to ensure they do not foul the LCD module above. You can check this by temporarily slotting the LCD module siliconchip.com.au above, using the longer machine screw for alignment. Then fit the piezo buzzer, making sure to check the polarity markings. Some of these devices are pretty tall; check the clearance there too. If you haven’t yet fitted the 16-way header to the LCD module, do this now. You can then use it to square up the 16-way female header attached to the Display Adaptor PCB that connects to the LCD module. Solder the female header to the Display Adaptor PCB, then separate the two boards. Temporarily fit three tapped spacers above the Breadboard PSU PCB, with short screws coming up from below. This will allow you to align the headers from Display Adaptor PCB. If you haven’t fitted CON5-CON9 to the Breadboard PSU PCB, do that first. Then slot the corresponding headers into the top of them, rest the Display Adaptor PCB over them, and solder them while everything is aligned. Separate the two PCBs and remove the temporary spacers from the Breadboard PSU PCB. The final components to be soldered to the Display Adaptor PCB are the two LEDs; they are positioned to poke over the top of the LCD module’s PCB, making them just visible below the display. So we will fit them after the LCD module is fitted to the Display Adaptor PCB. The Display Adaptor PCB should have six unoccupied M3 mounting holes at this stage. The four in the corners are for the LCD above, so leave them free. Fit the other two ‘spare’ mounting holes with tapped spacers. Put a tapped spacer below the one on the left (between CON12 and CON13) and secure it with a short machine screw from above. The hole at upper right (next to S1) should be fitted with the 20-25mm machine screw and secured with a tapped spacer below. Fit the LCD module to the Display Adaptor and secure it with three short machine screws into the tapped spacers with short screws at their other ends. The bottom right corner can have another tapped spacer threaded over the 32-35mm screw that is already fitted. Orientate the LEDs so that they light up red when the left-most lead is more positive than the right. You can use a multimeter on diode test mode to check that, then solder the siliconchip.com.au LEDs so they protrude just above the LCD module. Now add the Breadboard PSU PCB to the bottom of the stack. Check for clearances and trim any leads that might foul components below. If things are still very close, you can add some insulating material between the two. Secure the Breadboard PSU PCB at its left-hand (breadboard) end by a machine screw into the underside of the tapped spacer. The last two tapped spacers cover the two exposed screw threads on the right to form the feet, similarly to the bare Breadboard PSU. This secures the other end of the PCB stack and completes the assembly. Powering it up If you wish to tread cautiously when applying power for the first time, use a current-limited PSU set to around 100mA or a 9V battery. Make sure there isn’t anything connected to CON3 or CON4. The LCD backlight should light up, but you might need to adjust the contrast trimpot VR5 to get a legible display. After that, it should look much like Screen 1, although the displayed values will probably differ. Check that the voltage at bottom right is about half a volt below the supply at CON1. With nothing connected, it should cycle between the input voltage and “0W 0W”. Pressing S1 or S2 should toggle the alert icons at lower right. If one of the LEDs is red, the piezo should sound when its alarm is unmuted. If this isn’t the case, the LEDs may be reversed. To check this, dial up the current limit to about halfway; you should get a reading of about 1.25A on the top line. Set the voltages to their minimums. This results in a state where the LEDs should definitely be green. The easiest way to force a red LED alarm state is to dial the voltage potentiometers to their maximum and the current limits to their minimum. This should also result in an audible alarm from the piezo if the alarm is unmuted. Parts List – Breadboard PSU Display Adaptor 1 double-sided PCB coded 04112222, measuring 99mm x 63mm 1 20×4 alphanumeric LCD with backlight (LCD1) 1 self-oscillating piezo transducer (SPK1) 1 10kW side-adjust trimpot (VR5) [Jaycar RT4016] 1 500W side-adjust trimpot (VR6) [Jaycar RT4008] 3 right-angle SPST tactile pushbutton (S1-S3) 2 6-way pin headers (CON5, CON6) 4 3-way pin headers (CON7-CON9, JP3) 1 jumper shunt (JP3) 1 16-way female header (CON10; for LCD1) 1 16-way header (for LCD1) 2 4-way right-angle female headers (CON11, CON12) 1 5-way right-angle pin header (CON13; optional, for ICSP) 1 small TO-220 finned flag heatsink 7 12mm-long M3 tapped spacers 1 M3 × 32-35mm panhead machine screw [Jaycar HP0418] 1 M3 × 20-25mm panhead machine screw [Jaycar HP0414] 7 M3 × 5-6mm panhead machine screws 1 M3 shakeproof washer SC6572 Kit ($50) 1 M3 hex nut Includes all the parts listed. Semiconductors 1 PIC16F18877-I/PT 8-bit microcontroller programmed with 0411222B.HEX, TQFP-44 (IC4) 1 7805 5V 1A linear regulator, TO-220 (REG2) 2 bi-colour red/green 3mm LEDs (LED1, LED2) [Jaycar ZD0248] Capacitors 2 100μF 25V radial electrolytic 2 100nF 25V M3216/1206 X5R/X7R ceramic, radial ceramic or MKT Resistors (all ¼W 1% axial except as noted) 4 51kW 5 10kW 2 1kW 1 100W 2 100mW M6432/2512 1W SMD Australia's electronics magazine December 2022  45 Finally, you can check that S3 cycles through the various calibration screens. If that’s the case, then the Display Adaptor is working as expected. If the LEDs show the wrong colour, desolder them and swap their leads. Calibration In regular use, a single screen displays all applicable information, previously shown in Screen 1. This is shown at power-up, so you can use the Display Adaptor without pressing any buttons. If the readouts you see on the Display Adaptor are off by more than 5%, we recommend checking your construction, as it should be closer than that without calibration. Start by checking all the divider resistors. The 1% tolerance components specified will be more than adequate for most purposes and within the resolution of the displayed values, so calibration is optional. Pressing S3 accesses the calibration factors for all the displayed parameters, except the transistor dissipations, which are set by their constituent voltages and currents. Each press of S3 simply cycles through each in turn until you return to Screen 1. Screen 2 shows a typical calibration page. The calibration factors are displayed in the same order as on the main screen, but the second line of text also describes the parameter. The third line shows the calculated value of that parameter using the current calibration factor, which is seen on the line below. The calibration factor is changed using S1 and S2 to adjust up and down. Thus, the simplest way to calibrate is to use a multimeter to measure the parameter (voltage or current) and then adjust the calibration factor until they agree. Because all voltages use the same 51kW/10kW divider, their default calibration factors are the same. Similarly, all currents have a different corresponding calibration factor. Use a multimeter to read the voltage or current you wish to calibrate. Note that for currents, you will need to apply some sort of load and make sure that current limiting is active to check the setpoints. Select the appropriate screen, then adjust the calibration factor up or down using S1 and S2, respectively, until the multimeter reading matches the displayed reading. Take care that you have the correct screen, as there are quite a few different parameters. After that, return to the main screen and check that the displayed values are consistent. The final calibration page (Screen Screen 2: all the main parameters shown on the main screen can be calibrated using these screens. Simply read off the actual voltage with a multimeter and use S1 and S2 to adjust the displayed voltage until it matches. Screen3: the calibration factors can be saved to non-volatile EEPROM by pressing S1 and S2 simultaneously on this page. 46 Silicon Chip Australia's electronics magazine 3) allows the calibration factors to be saved to EEPROM, meaning they will be stored permanently for future use. Simply press S1 and S2 together on this page to permanently save the data. A message will be displayed to confirm this has happened. Using it From now, the Display Adaptor simply displays the various voltages and currents set and used by the Breadboard PSU. You can mute and unmute the alarms with S1 and S2. The power display at lower right that alternates with the supply voltage will warn of conditions that might overheat the Breadboard PSU’s transistors. The display reads 0W-9W for each channel, as that’s all it can show in the available space. The design is intended to handle up to 3W continuously and up to 5W for short periods. If you see these creeping up any higher, shut down the circuit to avoid damage to the Breadboard PSU. With everything set up, you shouldn’t need to do anything with the Display Adaptor except read what it displays. On the main screen, S1 and S2 toggle the audible alarms for the CON3 and CON4 outputs, respectively. A speaker icon with an “x” indicates that the alarm is muted, which is the power-up default. Since LED1 sits above VR1 and LED2 sits above VR2, each LED corresponds to one channel of the Breadboard PSU. Usually, the green LED is lit for each channel. If IC4 detects that the actual voltage is not near the setpoint voltage, it changes the LED to red. In practice, this means that the current limiting has activated, although it can also happen if the voltage potentiometers are set above the DC input voltage. If the alarm for the corresponding channel is not muted, the piezo sounds in short chirps when the corresponding LED is red. That should get your attention without being as annoying as if it sounded constantly. While the Breadboard PSU lacks an on/off or load disconnect switch, it’s quite easy to pull out the side plugged into the breadboard, which disconnects it. It would be a good idea to do that immediately, if you notice the transistor dissipation values are unexpectedly high or something else is wrong. SC siliconchip.com.au