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By NICHOLAS VINEN 100W Hybrid Switchmode/ Linear Bench Supply, Pt.3 In this third and final instalment on our 40V/5A DC input bench supply, we take the completed PCB and fit it into the case, along with the chassis-mounting hardware and wiring. We also answer some reader questions about the supply. O VER THE LAST two months, we have described the operation of our new bench power supply and given the construction details for the PCB. This supply is somewhat unusual in that it runs off a 12-24V input such as a 12V battery or old PC or laptop power supply. It also combines a switchmode buck/boost circuit with a linear regulator to give a wide output voltage range, low noise and fast-acting current limiting. It’s built into a case from Altronics which will be supplied with two pre-fitted LED panel meters for dual metering, ie, simultaneous voltage and current read-out. The voltage and 84  Silicon Chip current are adjustable in 0-40V and 0-5A ranges using multi-turn pots for accuracy. There is also a pushbutton to view the current limit setting to make it easier to adjust. Since the current limiting is linear in nature, the supply can be used as a voltage or current source. Now let’s go over the final steps to complete and test the power supply. Preparing the panel meters In addition to trimming the leads and fitting plugs to suit the connectors on the PCB, we need to tweak the two LED panel meters slightly. This is best carried out by first removing them from the front of the case, which is done by squeezing the clip on one side and then pushing that side forward until it pops out the front. You can then squeeze in the clip on the other side and remove the unit. The actual panel meter is inside a plastic housing with a rear plate that’s held on by four more clips, two on either side. Gently push these in with the tip of a flat-blade screwdriver; you don’t want to snap the plastic. Once you’ve popped one side up, the rear panel should then come off easily and you can pull the PCB assembly out. The first thing to do is remove the short circuit between pins 2 & 3 of the siliconchip.com.au header connector. This can be done by simply running a hot iron between them a couple of times, taking care not to damage any of the surrounding components. Do this for both panel meters. We also need to change the position of the decimal point on one panel meter. By default they read up to 199.9 which suits us for voltage but for current, we need it to read up to 19.99, ie, with the decimal point between the second and third digits rather than third and fourth. This modification is done by clearing a solder shorting ‘link’ on the board and making another one. These solder ‘link’ positions are between an exposed track and three small rectangular pads near R4 at lower-right, next to the MKT capacitor. Left-to-right they are labelled S, B and Q (see the instruction sheet supplied with the meter). You will need to clear the short from track L to pad S and instead short track L to pad B. That’s just for the ammeter; leave the other meter (for voltage) with L and S connected. If you’ve installed trimpots VR7 & VR8 on the PCB you can put the meters back into their plastic housings and snap the backs on. Otherwise, leave the backs off as you’ll need access to the meter trimpots later. Connecting cables The next step is to fit polarised header plugs to the bare ends of the supplied hook-up wires. Trim them all to the same length of around 100mm, then strip the ends and crimp them into the pins which are supplied with the polarised header plugs. This is Running The Supply From A Higher Voltage We’ve already had enquiries as to whether it’s possible to run this unit from a higher voltage DC supply and the answer is yes, with a few small modifications. As stated in the previous articles, old laptop and PC power supplies are quite suitable and will typically supply 12-17V, while a typical 6-cell lead-acid battery is also suitable, giving a supply of 13-14.5V while being charged and 12-13V the rest of the time. However if you have a 24V (12-cell) lead-acid battery or battery bank, as used in many trucks, boats, caravans and off-grid power systems, it’s not a good idea to connect the bench supply as originally designed. That’s because the battery will approach 30V during charge, well above the recommended maximum supply of 24V. There are a few simple changes which will allow operation up to 40V, although we recommend keeping the supply below 30V to avoid excessive dissipation in REG1 due to the relatively high current drawn by the LED panel meters. These are as follows: (1) The nine 10µF 25V SMD input bypass capacitors for the switchmode section should be replaced with nine 4.7µF 50V capacitors (ie, identical to those used in the output filter bank). You could use 10µF 50V capacitors instead, to maintain the same capacitance but we don’t think this is necessary. (2) The 100µF 25V input bypass capacitor for REG1 should be replaced with a 47µF 50V/63V electrolytic capacitor. (3) Zener diode ZD2 should be changed to a higher voltage type. The recommended value to use is 39V however with the above example (ie, running from a 24V leadacid battery), 33V would also be an acceptable choice. While REG1 will run hotter with a higher input voltage, under load the switchmode section will likely run somewhat cooler (due to the lower input current) and it may be able to supply a little more current at higher output voltages than would be available with a regulated 24V DC input. done by folding the two small metal leaves over the exposed portion of the wire and the larger ones over the insulated section and then squeezing them down hard with needle-nose pliers to hold the wires in place. Note though that unless you have a specialised crimping tool for this kind of pin, this will be insufficient to retain the wire so you will then also need to solder the exposed copper in place. Use only a small amount of solder and don’t get any on the outside of the pin or it won’t go into the plastic block. Once all four wires have pins attached, slide them into the slots in the header block and push them in until they click into place. The wires must be ordered as shown in Fig.7 last month (see photo below). If you get them wrong, you will have to use a small tool to push gently on Below: the view inside one of the panel meters. For both meters, you need to remove the short circuit between pins 2 & 3 of the header connector at left. You also need to move the position of the decimal point on one meter (used to indicate current) by clearing the short between track L and pad S at bottom right and instead shorting track L to pad B (see text). Above: the panel meter with the cover back in position. Both meters are connected to the main PCB via a 4-way cable fitted with polarised header plugs at each end. siliconchip.com.au June 2014  85 − 0-40V VR1 and VR2 need to be connected 86  Silicon Chip + On SILICON CHIP Off 12-24V DC . 0-5A Limit View Connecting VR1 & VR2 Fig.8: these full-size front and rear panel artworks can be copied or downloaded in PDF format from the SILICON CHIP website and used as drilling templates. Another set can then be laminated and attached to the case. Output + Load on/off Set Voltage Voltage Set Current Current SILICON CHIP the metal flange which retains each pin in the block so that you can slide them out. to the board in order to test it. You can temporarily fit two 10kΩ 9mm linear potentiometers if you have these on hand; there are pads to do so and this is quite convenient but expensive if you have to purchase them. The alternative is to wire up the chassis-mount pots you intend to use with the unit and use them off-board. The 10-turn types generally have three solder lugs arranged front-toback, with the two on the pot body being the ends of the track and the one at the rear the wiper. However, this isn’t necessarily a standard so you really do need to measure the resistance between the terminals to determine which is which. Basically, with the pot fully anti-clockwise, there should be minimum resistance between the left-most and centre pins on CON5 & CON6. The most convenient way to wire the pots up is to get cables with 3-way female headers on the end, chop them in half and solder the bare ends to the pot, however this does mean that the plugs can go into CON5 and CON6 either way around so it would be easy to accidentally reverse the action of one or both pots. A better but more laborious approach is to make up cables using ribbon cable or light-duty hookup wire with a polarised plug on the end, as described above for the panel meters but with three wires this time. Initial checks Having wired up VR1 and VR2, turn them both full anti-clockwise. Fit LK2 but leave the shorting block off LK1 entirely. With S1 off (up), connect a 12-24V power supply to CON1 and measure the current drain. You can do this by leaving F1 out and connecting a DMM in amps mode across the two fuse clips. There may be a small pulse of current when power is first applied but this should quickly drop to just a few microamps after a second or so; ie, the DMM should read zero unless set on a low current range. Assuming that’s OK, switch on S1 and check the new current reading. It should be just under 100mA. If it’s over 200mA or unstable, switch off and check for faults (eg, incorrectly orientated parts, bad solder joints, etc). If the current reading is acceptable, you can then check some voltages. The mounting screws of Q1, REG1 and REG2 make convenient ground points (ie, for the black probe). These voltages should be as follows: bottom-most pin of REG1 = 11.6V to 12.4V (nominal 12V); top-most pin of REG2 = 4.8V to 5.2V (nominal 5V); either end of the 10Ω resistor above siliconchip.com.au The PCB fits neatly inside the instrument case and is secured using self-tapping screws into integral mounting posts. Be sure to modify the supplied panel meters as described in the text. D5 = approximately -10V; bottom-most pin of REG3 = -4.8V to -5.2V; left-most lead of the 470Ω resistor below VR4 = -2.5V. Once you have finished these checks, switch off S1 and disconnect the supply. If any of the voltages were wrong, check the circuitry around the regulators and IC2. Note that with the power switch on and LK1 out, the output of the switchmode regulator section will be pulled negative by the boost supply charge pump but it should be clamped by D16 to a safe level of no lower than -0.3V, to protect IC1. Assuming all is OK so far, with the power off, fit LK1 in the “TEST” position, then switch it back on. Check the supply current; it should now be stable at around 150mA. Turn VR2 clockwise, perhaps 10% of the way through its rotation, then adjust VR1 and monitor the output voltage (ie, between the -OUT and +OUT terminals). The output should change as VR1 is rotated and be fairly stable up to the input supply voltage, at which point rotating VR1 further clockwise will have little effect. Note that the supply current will drop somewhat when the output is ‘pegged’. If VR1 doesn’t seem to do anything, try turning VR2 clockwise a bit, as the current limit siliconchip.com.au has not been trimmed yet. You can now plug in the panel meters and check that they operate correctly. Start with the voltmeter and check that its reading can be adjusted with VR1; note that it won’t be accurate though, we have yet to trim it. You may notice REG1 and REG2 getting warm with the panel meter connected as it draws a fair bit of current (around 130mA). You can also now connect the ammeter and check that you don’t have the meters mixed up, ie, it should have two decimal places rather than one. But note that it will only read zero because (a) there is no load and (b) S2 is not connected yet. If you really want to check it out, you can short pins 1 & 3 of the header for S2 and then check that you can adjust it through a range of (roughly) 0-5A with VR2. Final tests Now to finally check that it’s all working properly. First, switch off and remove power, then switch LK2 over to the “RUN” position. Adjust VR1 to minimum and VR2 a little above minimum. If possible, connect a pair of DMMs or a scope to monitor the voltage across D16 as well as the voltage at the output. You may want to insert the 10A Part List Errata In the parts list last month, we specified 8 x BC547 transistors and 12 x BC557 transistors. While these would seem to have a sufficient voltage rating (45V for a 40V supply), due to the boosted voltage rails, some transistors may be damaged during operation at high output voltages. As such, we suggest all constructors substitute BC546/BC556 transistors respectively for maximum reliability. Also, we omitted a 200mm length of 10mm diameter heatshrink tubing. fuse now, if you haven’t already. If you have a third DMM to measure the amps, connect it across the fuse clips but make sure it’s in amps mode (not milliamps). Re-apply power with S1 off and then switch on. If possible, check the current drain. Without the panel meters connected it should settle at around 120mA but with the meters connected it will be closer to 400mA. There should be around 1.2V across D16 (the minimum output of the switchmode regulator) and close to 0V at the output. Now turn VR1 clockwise slowly. June 2014  87 Can The Supply Be Used As A Battery Charger? In short, yes, this supply can be used for charging batteries which use a constant current/constant voltage charge cycle. This includes Lithium Ion (Li-Ion), Lithium Polymer (Li-Po), Lithium Iron Phosphate (LeFePO4) and (with some manual input) lead-acid batteries including sealed/gel cells (SLA) and absorbed glass mat (AGM). Essentially, all you need to do is set the supply’s output voltage to the charge termination voltage for your battery pack, set the current limit as high as you can within the capability of the battery itself, connect the supply’s output to the battery terminals and turn the load switch on. The supply will then attempt to pull the battery’s terminal voltage up to the set voltage. If it can’t, it will deliver the amount of current you have requested until the voltage rises to the set point, then it will keep it there indefinitely. Caution should be used with lead-acid batteries since generally the maximum voltage that can be applied permanently is around 13.8V (slightly higher for SLA). Higher voltages Higher voltages can be used with lead-acid batteries for more rapid charging; up to about 14.4V for wet cell and 15V for SLA. But the supply can’t be left on permanently; the cells will begin to gas once they reach this voltage and the battery will be damaged if this continues for a long time. Typically, you would switch the supply off once the charge current has dropped to about 10% of the set level, or 30-60 minutes after the maximum voltage has been reached. While no damage should occur if the supply’s input power is interrupted (or switched off) with the battery connected and the load switch on, the supply will draw some current from the battery. Therefore, once the battery has finished charging, turn the load switch off before shutting down the supply entirely. This current is approximately 8-16mA, depending on battery voltage. This flows from the battery, through Q23’s body diode and into the output capacitor bank of the switchmode supply. The linear regulator automatically shuts down when the -5V rail is not present so relatively little current will flow in this condition. However, it may eventually flatten a battery left connected. As before, the output voltage should increase but the reading across D16 should also increase at the same time, remaining about 0.7V above the output. You should also now find that you are able to turn the output voltage up above the input supply voltage. But do not turn it up much past 40V; we haven’t set the maximum voltage yet and this may be possible. Of course, in theory, the circuitry should limit the output to a safe level but it’s best not to test your luck. If you’ve gotten this far, chances are everything is working properly but before putting it in the case, it’s probably a good idea to do a load test and check that the current limiting operates correctly. For this, you will need to solder a length of tinned copper wire into the “-OUT” terminal (you can re-use this wire later to connect it to the binding post). Having done that, use clip leads to connect a 5W resistor of say 10-100Ω between -OUT and +OUT (the easiest way to connect to +OUT at the moment 88  Silicon Chip is to clip on to the cathode of D13). Next, turn VR1 and VR2 fully anticlockwise and switch the power back on, then advance VR1 clockwise – the current meter should still read (near) zero. You can then rotate VR2 and check that the current flow increases linearly. Check that the unit is able to supply at least a couple of amps but note that the resistor may get quite hot as you turn the voltage and current up. When you’re satisfied it’s working properly, switch the power off. Calibration The next step is to adjust the trimpots. This includes VR3-VR6 on the main board and either VR7/VR8 (if fitted) or the calibration pots on the panel meters. First, set the output voltage range. Turn VR1 fully anti-clockwise and VR2 to about halfway. Measure the voltage across the outputs with a DMM and adjust VR4 for 0V. Now turn VR3 anticlockwise, then rotate VR1 fully clockwise and adjust VR3 for 40V. These controls should not interact but you can re-check the zero voltage setting if desired. Now adjust VR1 for a non-zero output voltage (5V say), VR2 fully anti-clockwise and wind VR6 all the way anti-clockwise, then slowly advance VR6 until the output voltage returns to the set voltage. That done, connect a DMM set to read amps across the output. The current flow should be low (a few milliamps). Turn VR5 fully anti-clockwise and then advance VR2 fully clockwise. Adjust VR5 to get a reading of 5A, then disconnect the multimeter (don’t take too long on this step). To calibrate the voltmeter, set the supply for a 40V output and adjust VR7 or its onboard pot until that is what it reads. For the ammeter, connect a DMM in amps mode across the outputs as before, dial in a couple of amps and then adjust VR8 or the ammeter pot until the readings match. Case preparation The case for this project is a 1U half-rack plastic case; Altronics part number H4996. However, Altronics have produced a special variation of this case, which has two rectangular cut-outs on the front panel for a pair of their 3.5-digit Q0588 LED Digital Voltmeters, which are supplied with it. They also supply and install an SPST rocker switch. The catalog number for this halfrack case with the two panel meters and the mains switch is K3205. It’s available at the special price of $59.50. Since the case will be supplied with these parts already installed, all you have to do on the front panel is drill the extra holes for the two pots and current limit view pushbutton switch. There are four holes required on the rear panel, for the DC input socket, power switch and output binding posts. Front & rear panel artwork is provided in Fig.8 and these labels can be attached to the front and back of the case to aid in operation. These diagrams can also be used as a guide for drilling the front panel holes. The front panel hole locations aren’t especially critical but for the sake of neatness, it’s best to position them where shown. The rear panel hole locations do need to be accurate however, as the DC input and switch holes must line up with the components mounted on the PCB. The binding posts holes can be siliconchip.com.au moved if required but be careful that the internal portion of the posts won’t interfere with Q23’s heatsink fins. We haven’t placed them the usual 19mm apart for this reason but depending on how far your binding posts project into the case, you may be able to move them closer together. Drill each hole with a small pilot drill then enlarge to them size using either a series of larger drills or a taper­ ed reamer. Remove any swarf using a deburring tool or oversize drill bit. If you want to attach labels to the front and rear panels, do so now, after cutting out the matching holes. Putting it together Before proceeding, disassemble the case so that you have four separate pieces – front, back, top and bottom. Don’t lose the screws. Having already soldered leads to the pots, you can now mount them on the front panel and attach the knobs. It’s a good idea to terminate the wires with polarised headers so that they can’t be plugged in the wrong way around. The wiring diagram (Fig.7) in Pt.2 last month showed how our unit was wired but your pots may have different connections so check these first. Similarly, solder wires terminated in a 3-pin female header plug to the pushbutton before fitting it to the front panel and pushing the cap on. That done, having prepared the panel meters earlier, pop them back into their plastic housings and clip them into the front panel. Remember that they are configured differently; the meter with track L shorted to pad B (ie, the one you changed) is the ammeter and this goes between VR2 and S2. There is one more thing to do before putting the board in the case and that is to make up a cable to connect the output to the load switch. Cut two lengths of extra-heavy-duty hookup wire, 240mm and 260mm long. Strip about 6mm of insulation from each end of both wires and crimp a 6.4mm female spade connector onto one end of each wire. Now place them side-by-side in a 200mm length of 10mm diameter heatshrink tubing so that there is about 10mm between the base of each spade connector and the end of the tube, then shrink it down. Solder the free end of the shorter wire to the +OUT terminal on the PCB (near Q23). The other, longer wire can then pass through the siliconchip.com.au The rear panel carries the power switch (S1), a hole to access the DC socket and the two output terminals. adjacent hole and should stick out the top of the board by about 30mm. Strip this end back a bit further, leaving around 15mm of bare copper strands. Now secure the PCB to the bottom of the case using four No.4 x 6mm self-tapping screws; don’t use longer screws or they could damage the case. While doing this, you will need to make sure that the heavy-duty wire runs diagonally under the board to emerge near the opposite corner and that the wires sit side-by-side and avoid any posts or protrusions, otherwise it will be difficult to screw the board down. Push the crimp connectors onto S1’s terminals (either way around), then fit the front panel to the bottom of the case using the self-tapping black screws removed earlier. With that in place you can plug in the two panel meters, the two pots and S2. Pay careful attention to the orientation of any connectors that aren’t keyed, especially that for S2. This requires you to determine the pushbutton switch’s common, normally open and normally closed terminals. That’s done by setting a DMM on continuity mode and finding the two terminals which are shorted when it is not pressed (COM & NC). You then press the button and the two that are shorted must be COM & NO. You can then plug its connector into the header with the COM, NO & NC connections as shown on the PCB overlay diagram. Rear panel connections Now fit the binding posts to the rear panel, making sure their wire entry holes are aligned vertically and that their nuts are done up tight. That done, slip the rear panel over S1 (enlarge the hole if it doesn’t fit) and secure it to the base but don’t use the supplied screws; use two black M3 x 5mm machine screws instead. The supplied screws are too long and would interfere with projections from the bottom of the PCB. Check that a standard DC connector will pass through the remaining hole and mate with the socket on the board; if not, remove the panel and enlarge that hole. You can then wrap the bare ends of the hookup wire attached earlier around the red (+) output binding post and solder it in place. For the negative output terminal, loop a short section of tinned copper wire around it, solder it in place, then pass this down through the -OUT pad and solder it there. If you need to remove the PCB from the case in future (eg, to troubleshoot it) then you will need to desolder the binding post connections. Finally, check that the fuse is in place, You can then fire the supply up for a final operational check. It’s a good idea to wind the voltage and current knobs down to minimum before powering up and to monitor the input current initially. However, assuming all the earlier tests were OK, as long as the chassis wiring is correct, it should operate correctly. Check that it works by varying the output voltage and current and perhaps connecting a testing load. It’s then just a matter of fitting the lid using the screws you kept from earlier and the SC supply is complete. June 2014  89