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Handles 12V <at> 5A (or 10A with alternative inductors) by John Clarke Easy to build and store in a compact UB5 Jiffy box Effective noise and transient suppression Low standby current under 5mA Transient voltage clamping Low-pass filtering Fused supply DC Supply Filter for vehicles Many devices will run off 12V DC, so it’s pretty tempting just to plug them into a vehicle supply (via a cigarette lighter socket or similar), and away you go. But you’re likely to run into two significant problems with that: supply noise messing with the device’s performance and voltage spikes possibly frying it. This DC Supply Filter solves both those problems. P rotect your 12V equipment from voltage transients that could cause irreparable damage using this Vehicle DC Supply Filter. It connects inline with the DC supply to clamp and filter transient voltage excursions. It’s especially useful for audio gear as it reduces that horrible ignition system whine that can pass through the vehicle’s electrical system. While many 12V supplies are transient and noise-free, some are not. That’s especially true of the 12V (or 24V) supply from a vehicle with the engine running. In particular, heavy load switching such as electric radiator fans or air conditioner compressors switching on or off can produce voltage transients on top of the theoretically smooth 12V DC supply. Other noise and transient sources include the vehicle’s alternator, where alternator brushes produce electrical noise, and the ignition system with frequent pulses delivered to the coils and spark plugs. It isn’t just for vehicles, either. Mains switchmode power supplies can also have transients on their output, as well as noise. These typically have high-frequency noise due to the switching nature of the supply and Practical Electronics | November | 2023 can produce transients when the load is abruptly changed from full load to a lesser current. We have found on multiple occasions that modern switchmode plugpacks are unsuitable for powering sensitive circuits, including signal generators, instrumentation, audio preamplifiers and theremins. While some equipment powered from such sources can survive damage, others are more sensitive. The device may fail quickly due to voltage transients exceeding the internal electronics ratings, or it could fail over an extended period as sensitive electronic components accumulate damage with each voltage transient. Transients and noise can be reduced with a low-pass filter, and a transient voltage suppressor can absorb harmful spikes. The filter effectively removes high-frequency signals from the DC supply. The result is a supply with much lower noise, less high-frequency ripple and more minor voltage transients. Filtering can go a long way to protecting your valuable equipment from damage. Our DC Supply Filter (hereon, ‘Filter’) is quite compact and can be housed in a small Jiffy box. Heavy-duty screw terminals are provided for the input and output connections, plus there is an onboard fuse and a power-on indicator LED. Filter design The circuitry for the Filter is relatively straightforward, as shown in Fig.1. It uses two inductors and several capacitors. A transient voltage suppressor (TVS) is included to absorb excessive voltage spikes. The TVS specified begins to conduct at its Vbr (reverse breakdown voltage) of 14.4V and provides full voltage clamping at 23.5V, although it would have to be a mighty spike for it to allow the voltage to rise much above 16V. Note that a TVS will conduct a small leakage current at voltages below its reverse breakdown voltage. As it turns out, about half the quiescent current of this Filter is due to the TVS. But we need it to protect the downstream equipment from the worst spikes such as ‘load dumps’. If you want to use the Filter at a higher voltage, like 24V, you will need to change the TVS to one with a suitable Vbr rating, plus the two electrolytic capacitor working voltages will 25 DC Transient Filter Fig.1: the Filter has two main roles: to reduce high-frequency noise and absorb large spikes. Noise is attenuated by two cascaded LC filters (47μH/101.1μF) while the TVS between the two shunts to ground any particularly large voltage spikes that make it past the first filter stage. need to be increased (to 35V or 50V for a 24V supply). For the particular TVS we used, we measured a leakage current of about 2.6mA from 12V up to 15.6V, at which point the current increases as it begins to clamp the voltage. The leakage current through the TVS is something to consider if this is going to be the cause of battery discharge over time. An extra 2mA (approximately) is drawn by the power indicator LED. Typically, when used on a vehicle supply, the overall 4.6mA current should not discharge the battery except over a long time. Returning to the Fig.1 circuit, power is applied at CON1, and current flows through the fuse (F1) to a small bypass capacitor, then the inductor L1, rated at 47μH and 5A. Following this are three paralleled capacitors: a 100μF low-ESR electrolytic, 1μF multi-layer ceramic and 100nF MKT polyester. These bypass ripple, noise and transients to ground. The TVS is connected in parallel with these capacitors to clamp over-voltage spikes. We use a mix of capacitors to improve the filtering action over a wide range of frequencies. The non-electrolytic capacitors function better at higher frequencies, while the electrolytic capacitor provides reasonable filtering below 100kHz and better still below 10kHz. The assembled PCB for the 5A version mounts within the UB5 enclosure on the flanged lid using TO-220 insulating bushes as standoffs. Reproduced by arrangement with SILICON CHIP magazine 2023. www.siliconchip.com.au 26 A second identical LC (inductor/ capacitor) low-pass filter follows, forming a second stage to reduce noise and ripple going to the output at CON2. Note that a radio signal filter design would likely not include the capacitors across CON2 because they expect 50W source and load impedances. With our Filter, we expect the source impedance will be close to 0W, and the output impedance can be anywhere from about 1kW down to as low as 2.9W for a 5A load with a 14.4V supply. The capacitance across the output of the Filter at CON2 gives an effective frequency roll-off that is relatively independent of the external load connected to the output. In effect, the capacitors provide a low impedance down to below 10Hz. The LED indicator (LED1) is driven from the 12V supply via a 4.7kW resistor. The LED does not light if the unit is not powered or the fuse has blown. Testing the Filter We conducted a test to see how effectively the Filter Filter voltage transients using two power supplies. One supply was set to provide 14.4V DC and the other 50V DC. The 14.4V was fed to the filter input via a large inductor to isolate this from the transient voltage derived from the 50V supply. In order to create a suitable test transient, we charged up a 100nF 100V capacitor that was subsequently switched over to connect to the Filter input momentarily. The result can be seen in Scope 1. The top yellow trace shows the input voltage transient with a peak 26.4V above the steady 14.4V DC supply. At the output of the Filter, shown on the lower cyan trace, there was only a 600mV increase; that’s a reduction in transient amplitude by a factor of 44 times. Note that the oscilloscope traces were AC-coupled, so the 14.4V DC applied to the Filter is not seen on the oscilloscope traces. If we had DC Practical Electronics | November | 2023 Fig.2: the measured performance of the prototype is quite a bit better than what was predicted by simulation, with noise and ripple attenuation starting below 1kHz and already below −20dB by 2kHz. −55dB is reached just above 5kHz. coupled the traces, the sensitivity (volts per division) would have needed to be much higher to prevent the traces from going off-screen at the top. That would have made the details of the transient harder to see. We also plotted the Filter’s frequency response using the LTspice simulator and by measurement in Fig.2. The measurement was checked down to −55dB at 6kHz, and the expected roll-off above this frequency continues as an extrapolation of the measured roll-off rate. Construction The Filter is built on a double-sided, plated-through PCB, coded 08108221 , measuring 77 × 46mm and available from the PE PCB Service. The 5A version fits in a standard UB5 plastic Jiffy box. If you wish to make a 10A version, you will need a larger UB3 box. The part changes required are shown in the parts list, including the use of a 10A fuse instead of 5A. See the separate panel on winding and mounting the inductors. Refer to the overlay diagram, Fig.3, as a guide to construction. Begin by soldering the 4.7kW resistor and low-profile parts such as the TVS and capacitors. The TVS needs to be oriented correctly, with the striped PCB rather than on the underside, as the PCB needs to sit low in the Jiffy box to fit. Depending on your preference, the LED should be installed either down close to the PCB or with long enough leads to protrude through the enclosure. Scope 1: a demonstration of the effectiveness of the Filter. We purposefully created a 26V spike on top of a 12V supply, causing some ongoing oscillations at the Filter’s input. The voltage at the Filter’s output peaked at only 0.6V above the DC level. end to the top. The two electrolytic capacitors must have their striped (negative) side toward the bottom, with the longer positive leads to the pads marked with a plus sign on the PCB, towards the top. The fuse holder, CON1 and CON2 can be fitted now. Inductors L1 and L2 are installed upright, with the leads entering the smaller holes adjacent to each side of the inductors on the PCB. The cores are held using a cable tie through each core and around under the PCB via the larger holes. Ensure the cable tie joint is on the top side of the Mounting it in the enclosure If using the UB5 Jiffy box, mount the PCB to the inside of the flanged lid using countersunk head screws from the outside, with the PCB raised off the base with insulating bushes. These are the type usually used to isolate a TO-220 transistor (or similar) from its mounting screw. Cut a section of the round washer portion of the bush with side cutters to allow it to fit on the flanged lid, adjacent to the corner mount mouldings, as shown in the photo at the bottom of the next page. Holes at each end of the enclosure are required for the cable glands. The PCB has cut-outs to make room for the gland nuts, so ensure the holes are centred on the enclosure sides. The input and output wires pass through the cable glands at each end and are terminated at the screw terminals. Make sure the wiring polarity is correct, as the fuse will blow if connected incorrectly to the input. Draw the wires out through the glands as the PCB is inserted into Fig.3: the Filter assembly is straightforward; the components shown here are for the 5A version. Only the TVS, LED and electrolytic capacitors are polarised. For the 10A version, the inductors will be larger and must be mounted above the other components on longer leads. Practical Electronics | November | 2023 27 the enclosure, then tighten the gland nuts to prevent the wires from being pulled out. We have designed a panel label that can be printed and affixed to the enclosure, as shown in Fig.4. A PDF file of this label can be downloaded from the November 2023 page of the PE website at: https://bit.ly/pe-downloads Parts List – DC Transient Filter (12V, 5A version) www.poscope.com/epe - USB - Ethernet - Web server - Modbus - CNC (Mach3/4) - IO - PWM - Encoders - LCD - Analog inputs - Compact PLC 1 double-sided, plated-through PCB coded 08108221, 77 × 46mm, available from the PE PCB Service 1 panel label, 80 × 47mm (optional) 1 UB5 Jiffy box with flanged lid [Altronics HF0155, Jaycar HB6016] 2 15A 2-way PCB-mount screw terminals (CON1, CON2) [Altronics P2101] 2 47μH 5A chokes (L1, L2) [Altronics L6617, Jaycar LF1274] 1 30A blade fuse holder [Altronics S6040] 1 5A blade fuse (F1) 2 cable glands to suit 4-8mm cable diameter 4 M3 × 10mm countersunk head (CSK) screws 4 M4 hex nuts 4 TO-220 insulating bushes 2 100mm cable ties (5A version only) 1 transient voltage suppressor rated at 1500W with a Vbr of 14.4-15V (TVS1) [Jaycar ZR1170] 1 3mm LED, any colour (LED1) 2 100nF 63V MKT capacitors 2 1μF 50V multi-layer ceramic capacitors 2 100μF 25V low-ESR electrolytic capacitors 1 4.7kW 1/2W resistor 10A version changes 1 UB3 Jiffy box [Altronics HF0203, Jaycar HB6014] (instead of UB5 Jiffy box) 2 powdered iron toroidal cores (L1, L2) [Jaycar LO1244] (instead of 47μF 5A chokes) 1 2m length of 1.25mm enamelled copper wire (for winding L1 and L2) 1 10A blade fuse (F1) (instead of 5A fuse) Winding L1 and L2 for 10A use - up to 256 - up to 32 microsteps microsteps - 50 V / 6 A - 30 V / 2.5 A - USB configuration - Isolated PoScope Mega1+ PoScope Mega50 The ratings of pre-wound inductors L1 and L2 limit the standard version of the Filter to 5A. The circuit can supply up to 10A by using hand-wound inductors instead. In this case, L1 and L2 are made by winding 24 turns of 1.25mm diameter enamelled copper wire on the specified toroidal core. The ends of the wire will need to be stripped of insulation using a sharp craft knife before soldering. Keep the ends long enough so the inductors can mount raised off the PCB, as they will not fit in the space allocated for the 5A inductors. Ideally, these inductors should be secured high enough to clear the other PCB-mounted components, but low enough to allow the assembled PCB to fit inside the enclosure. A horizontal mounting will give the best clearance; the inductor leads may need extending if they aren’t left sufficiently long. You can use neutral-cure silicone sealant to secure the inductors to the PCB and adjacent components. Fig.4: you can attach this panel label to the box lid, so its contents aren’t a mystery. - up to 50MS/s - resolution up to 12bit - Lowest power consumption - Smallest and lightest - 7 in 1: Oscilloscope, FFT, X/Y, Recorder, Logic Analyzer, Protocol decoder, Signal generator 28 The insulating bushes for the PCB should be trimmed to fit the lid. This filter is suitable for use with last month’s Multi-Stage Buck/Boost Charger (October 2023) for battery charging from a vehicle power supply. Practical Electronics | November | 2023