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Select the features you want in this Fast Charger for NiMH batteries This is a truly versatile charger. It can charge up to 15 identical NiMH or Nicad cells. You can build it to suit any size cells or cell capacity and you can set the charge rate. It can fast charge, trickle charge and has safeguards, including temperature sensing, to prevent overcharging. S TANDARD NiMH and NiCad chargers are available just about anywhere, from hobby stores to supermarkets, the service station and even your local chemist or newsagent. However, they usually only charge two or four AA cells and at quite a slow rate – typically they will take between four and 15 hours to charge. But what if you want to charge at a much higher rate or more than four cells at a time? Or cater for C and D cells or battery packs? The only complete answer is to build the new SILICON CHIP Fast NiMH Charger. It can charge from one cell up to 15 cells simultaneously and battery packs up to 18V for both NiMH and Nicad types. Charging can be set from 34  Silicon Chip just a few milliamps up to 2.5A and it includes a reliable end-of-charge detection, with extra safeguards included to prevent over-charging. Safety is important when charging NiMH and Nicad cells and batteries because they can be destroyed, or have their life seriously shortened, if the charger is left on for too long after the battery pack has reached full charge. To see why over-charging can destroy a battery pack, have a look at Fig.1. This shows the typical voltage, temperature and internal pressure rise of a cell or battery pack with charge. by JOHN CLARKE Once charging goes past the 100% point, the temperature and internal pressures rapidly rise and the voltage initially rises and then falls. Continual overcharging will damage the cells due to the elevated temperature. This accelerates chemical reactions that contribute to the ageing process. In extreme cases during overcharging, excessive internal pressure can open their safety vents to release the pressure. These vents will re-close after the pressure is released but the cells will already have been damaged. Full charge detection Full charge can be determined in one of two ways. The conventional way has been to monitor the voltage siliconchip.com.au Main Features siliconchip.com.au Our new Fast NiMH Charger requires a small thermistor to be installed in the battery pack or cell holder, in close contact with one of the cells, so it can monitor temperature. The beauty of this system is that it will recharge any cell, regardless of whether it is flat or only partially discharged – you will not over-charge it. There is a proviso here and it applies when you charging very cold batteries – they may rapidly rise in temperature during charging. This could cause a false dT/dt end of charge condition. To circumvent this, the dT/dt measurement for end of charge detection is only enabled when the cell tem- perature is at least 25°C. Should the thermistor end-point detection fail, a timer is included that will switch off charging after a preset period. Further safeguards to protect the cells are also included. Charging will not start, or will stop, if the NTC thermistor for the cells is disconnected or if the temperature is under 0°C or over 55°C. Should the charger itself become too hot, charging will pause and the temperature is measured after two minutes to check if it has cooled sufficiently to restart charging. Select the features you want In its simplest form our new Fast NiMH Charger includes only the temperature detection feature. You 75 1.50 100 65 1.46 80 CELL VOLTAGE 1.42 60 1.38 40 35 1.34 20 25 1.30 55 45 PRESSURE across the battery pack and detect the point where the voltage begins to rapidly rise and then fall. This form of end-point detection is called dV/ dt (ie, change in voltage with respect to time). In practice, the critical end-point can be difficult to detect at low currents, particularly with NiMH (nickel metal hydride) cells. In fact, dV/dt end-point detection with NiMH cells is neither safe nor practical. The only safe way is to monitor the temperature of the cells. Very few chargers do this. This far more reliable method, especially with NiMH cells, monitors the temperature rise of one or two cells within the battery pack. During charging the cells do not heat up much because most of the incoming power is converted into useful stored energy. However, once the cells become fully charged, the charging current (and power) is converted to heat and so the cells rise quickly in temperature. Detection of this temperature change at the charging end-point is called dT/dt – change in temperature over time. The critical temperature rise is of the order of 2°C per minute. This is where normal charging should stop. Some chargers, ours included, may have a top-up charge after the endpoint to ensure full charging. After top-up, the cells can be trickle-charged to maintain full charge. In this situation, the cells are deliberately left connected to the charger, in the knowledge that they won’t be damaged but will be absolutely “full to the brim” when they are needed. TEMPERATURE (°C) These 1500, 1700 and 1800mAh ‘AA’ NiMH batteries were considered “state of the art” in our last NiMH charger (November 2002). Now 2500mAh are quite common (we’ve even seen claims of 3000+ ‘AA’). Our new charger will handle these as well as C and D cells and even battery packs. • Designed for NiMH cells but will handle Nic ads too • Charging timeout • dT/dt (temperature change) for end of charge detection • Over and under cell temperature detection • Power, charging and thermistor indication LEDs • Adjustable charging timeout limit • Adjustable dT/dt setting • Optional top-up and trickle charging • Adjustable charge current • Adjustable top-up and trickle charge curren ts • Over-temperature cut out for charger 0 0 50 100 STATE OF CHARGE (%) Fig. 1: typical charging curves for Nicad batteries (NiMH are similar). Cell temperature (green) and voltage (red) are most often used to detect the “end point” or 100% charge but in NiMH cells, the voltage is much less reliable. September 2007  35 Specifications Maximum input voltage.......................30V Maximum charge current.....................2.5A corresponding to 0-2.5V at Charge current adjustment...................From 0-2.5A, (in approximately 40mA steps) VR4 ...........................................................TP4 using , corresponding to 0-5V from Timeout adjustment.............................From 0-5 hours ...........................................................VR1 at TP1. x5 link installed (LK1) ............................................................0-25 hour with rise/minute, corresponding to dT/dt adjustment..................................From 0.5°-5° at TP2. ............................................................0.5-5V from VR2 once cells reach 25°C or more e minut dT/dt measurement..............................Once every when LK2 is installed, Top up and trickle charge.....................Top up available when LK3 is installed ............................................................Trickle enabled VR3 from 0-500mA, Trickle charge adjustment....................Adjustable using 0V to 5V at TP3 to g ondin ............................................................corresp ximately 5mA steps appro in ble ............................................................Adjusta g for 1 hour Top up charge......................................4 x trickle settin Cell over temperature cutout................55°C Cell under temperature detection.........0°C Charger over temperature cutout.........50°C can add top-up and trickle charging if you want (no extra components are required) and set all the charge parameters: full charge current, trickle charge, timeout period and dT/dt values. Full charge can be set from about 40mA up to 2.5A while trickle can be set from 10mA up to 500mA. Timeout can be set from between 0-25 hours while dT/dt can be selected from between 0.5°C rise per minute to 5°C per minute. More details concerning the adjustments are included in the setting up section of this article. Three LEDs indicate the status of the charger controller. The power LED is lit whenever power is applied to the charger (obvious!) while the Thermistor LED lights if the thermistor is disconnected or if there is an over or under-temperature detection. For over-temperature (>55°C), the Thermistor LED will flash once a second (1Hz) while for under-temperature (<0°C) the LED will flash once every two seconds (0.5Hz). Over heating of the charger itself causes the Thermistor LED to flash once every four seconds. Finally, the charging LED is continuously lit during the main charging cycle and switches off when charging is complete. If top-up and/or trickle charging are selected, the charging LED will flash at 1Hz during top-up and at 0.5Hz during trickle charge. Note that if the thermistor LED is lit or flashing, the charging LED will be off, indicating that charging has paused or stopped. A view inside our new NiMH Fast Charger. As you can see, the PC board sits in the bottom of the diecast box, as normal. But when the lid is screwed on, it becomes the base and the whole thing is turned over so the PC board is actually upside-down. 36  Silicon Chip siliconchip.com.au 7–30V DC IN D2 1N4004 POWER A K REG1 LM317T IN S1 OUT ADJ 220 F 50V 220nF 120 VR1 T/t (5V=5°C/min) TRICKLE (5V=500mA) VR2 10k +5V CELL/BATTERY TEMP 18 TH1 RB3 AN3 VR4 1 10k AN1 12 IC1 PIC16F88I/P RB4 10 AN2 RB5 11 LK2 LK3  SC 1k TH2 S K Q1 IRF540 A 0.22  5W TOPUP ENABLE D1, D2 TRICKLE ENABLE AN4 A 470 8 A OUT K K ADJ AN6 RB1 7 Vss 5 THERMISTOR LED2 IN OUT 470 LM317T LEDS A 2007 G ZD1 16V 1W 4 TIMEOUT D 1k 7 10k 13 HEATSINK TEMP IC2 6 LM358 ZD1 RB2 100nF 10 F 16V 8 LK1 TP5 3 5 1k RB6 TP4 D1 1N5822 K 8.2k 9 TP3 10k VR5 20k 47 2 VR3 CHARGE (2.5V=2.5A) AN0 A 100 F 10V 100nF Vdd TP2 9.1k CON1 14  K +5V 17 10k A POWER LED1 TP1 CON2 + TO BATTERY – 470 TPGND VR6 500 TIME OUT (5V=5h) TP +5V 10 F 16V  K A CHARGE  LED3 K D K A G D IRF540 S NIMH BATTERY CHARGER Fig.2: the circuit is based on a PIC16F88 microcontroller and apart from the components used to set and monitor the current, there’s not much more to it. Circuit details The circuit for the Fast NiMH Charger is based around a PIC16F88I/P microcontroller, IC1. Apart from the complexity of the software for IC1, there is not much else to it. Two NTC thermistors are used in the circuit. NTC stands for “negative temperature coefficient” and this means that the resistance of the thermistor is progressively reduced as the temperature rises. Thermistor TH1 monitors the cell or battery pack being charged. It is connected via a 2-way terminal block (CON1) and forms a voltage divider with 20kW trimpot VR5 across the 5V supply. VR5 is adjusted so that the voltage across the thermistor is 2.5V at 25°C. The voltage across the thermistor is monitored at the AN4 input (pin 3) of IC1 via a 47W resistor and 100nF filter capacitor. These are included to siliconchip.com.au remove radio frequency (RF) signals and noise that could be present due to the thermistor being connected remotely from the circuit. The voltage at the AN4 input is converted into a digital value and the values are compared against the over and under temperature values and for dT/dt changes. Trimpots VR1, VR2 and VR3 are used to set the timeout, dT/dt and trickle charge values. The wiper of each trimpot connects to the AN0, AN3 and AN1 inputs respectively and these inputs can receive between 0V and 5V, depending upon the setting of the trimpot. For the full charge current input at AN2, VR4 connects to the +5V supply via a 9.1kW resistor. This restricts adjustment to a nominal 2.5V maximum (for a 2.5A maximum setting). The voltage inputs are all converted to digital values within IC1 so that the settings can be processed in software. Test points TP1, TP2, TP3, TP4 & TP5 are provided for setting the trimpots when using a multimeter. There is also a TP GND terminal for the negative probe of your multimeter. The voltages measured at each test point directly relate to the setting’s value. For example, setting VR1 to give 4V at TP1 will set the timeout to 4 hours. The timeout value can be multiplied by a factor of five if jumper link LK1 is inserted. This ties pin 12 to ground. With LK1 out, pin 12 is pulled to +5V via an internal pullup resistor within IC1 and timeout is set at x1. Links LK2 and LK3 work in a similar manner. LK2 enables the top-up and LK3 enables the trickle charge modes. Outputs RB1 and RB2 of IC1 drive the Thermistor and Charge LEDs respectively via 470W resistors. September 2007  37 CHARGE LED3 LED2 100nF 8.2k D2 1k 1k ZD1 5W VR6 500 – 10 F DC IN VR2 10k VR4 10k TP2 TP4 TP5 100nF IC1 PIC16F88-I/P 120 + TO BATTERY LK2 LK3 LK1 220 F VR5 20k 100 F 10 F 470 9.1k 470 470 IC2 LM358 17090140 1k 0.22  5822 D1 Q1 (UNDER BOARD) LED1 47 TP GND TO THERMISTOR POWER THERMISTOR REG1 LM317 10k S1 TP1 TP3 VR1 10k VR3 10k TP +5V 220nF RE GRA H C H Mi N TH2 Fig.3: full-size component overlay – compare this with the photograph of our prototype at right. If the 220mF capacitor you have is higher than 14mm, it will have to be laid over to fit within the case. Constant current source Op amp IC2 and Mosfet Q1 are connected to provide a controlled current source to charge the battery (connected via CON2). Op amp IC2 compares the voltage across the 0.22W resistor (at pin 6) with the DC voltage derived from the RB3 output of IC1 (at pin 5). The output from RB3 is a 5V 500Hz pulse-width-modulated signal which is fed to a divider and filter network comprising 8.2kW and 1kW resistors and a 10mF capacitor. The filter network smooths the pulse output to give a DC voltage. It is this smoothed DC voltage which effectively sets the current level provided by Q1 to the battery. Diode D1 is included to prevent the battery from discharging via the intrinsic reverse diode inside Mosfet Q1, when the power is off. D1 is a 3A Schottky diode, specified because it has less than half the forward voltage of a normal power diode. Typically, it has about 380mV across it (at 2.5A) compared with a standard diode which has 0.84V across it at 2.5A. The lower voltage drop also means less power loss in the diode; 0.95W at 2.5A compared to 2.1W in a standard diode. Power for the circuit is taken from a DC plugpack supply via diode D2. This diode provides reverse polarity protection for the following capacitor and regulator REG1. An LM317T is used to provide a 38  Silicon Chip regulated 5V supply to IC1 and the trimpots. This was chosen in preference to a standard 5V regulator because it can be adjusted to supply a precise 5V, using trimpot VR6, to make the settings of VR1 to VR5 more accurate. Voltage requirements To fully charge a battery you will require up to 1.8V per cell from your plugpack even though the nominal terminal voltage shown on the battery pack is 1.2V per cell. Hence, to charge a 6V battery which has five cells, you will need a DC input voltage of 9V (5 x 1.8V). Similarly, an 18V battery will have 15 cells and you will need 27V (15 x 1.8V) to charge it fully. However, while the voltage requirement for charging one, two or three cells is less than 7V, in practice you need more than 7V at the input to ensure that the LM317T regulator operates correctly, ie, remains in regulation. You can operate the charger in a car, in which case the input voltage will be around 12V with the engine stopped and up to 14.4V with the engine running. With 12V in, you can charge up to six cells (ie, a 7.2V battery). With 14.4V (ie, engine running), you can charge up to eight cells (ie, a 9.6V battery). Note also that using a supply voltage that is significantly higher than required to charge the cells will cause the charger to heat up more than necessary. For example, at 2.5A and with 10V higher than the battery voltage, there is going to be 25W dissipated in the charger. The heatsink will certainly become hot and the charger will shut down when it reaches 50°C. So you may have to reduce charge current if the supply voltage is high compared to the battery voltage. Charge current Maximum charging current is limited by the mAh capacity of the cell or battery (as can be seen in the accompanying table) and the rating of the DC plugpack or power supply. So if you charge at 2.5A, the power supply or plugpack must be able to deliver this current. Note that most “transformer” type plugpacks cannot supply this amount of current while some “electronic” plugpacks (ie, those with a switchmode supply) may be able to. Construction The Versatile NiMH Charger is constructed using a 98 x 53mm PC board, coded 14109071. It is housed in a diecast box measuring 111 x 60 x 30mm. A fan heatsink (that’s fanshaped, not a heatsink with a fan!) measuring 55 x 105 x 25.5mm mounts on the case to ensure that the charger runs reasonably cool. Begin construction by checking the PC board for any defects such as shorted tracks and breaks in the copper. Check also that the hole sizes are correct. Holes for the DC socket and the 2-way screw terminals will need to be larger than the 0.9mm holes required for the other components. Also check that the corners have been shaped to clear the internal corner posts of the box and that the 6mm diameter access semicircle for Q1’s screw has been cut from the edge of the PC board. Install the resistors first. Use the resistor colour code table as a guide to each value or use a digital multimeter to check each resistor before inserting it into the PC board. Next, install the wire link, the diodes, the IC socket (for IC1) and IC2, taking care to orient each with the correct polarity. The capacitors can go in next. Note that the electrolytic types must be oriented with the polarity shown. If the 220mF 50V capacitor is higher siliconchip.com.au The complete charger, reproduced close to life size. Q1 is mounted under the PC board – you can just see its tab poking out the left side (between the terminal blocks). Inset below is the wiring of the NTC thermistor which attaches to the side of the box, monitoring temperature rise. 2-WAY HEADER PLUG HEATSHRINK SLEEVES (2) OVER WIRE CONNECTIONS OUTER HEATSHRINK SLEEVE OVER THERMISTOR, SPADE LUG & CONNECTIONS than the 14mm-high type we used, it may need to be mounted on its side (over ZD1 and D2) to allow room to fit into the box. Follow these parts with the 2-way and the 3-way headers for the jumper links, then install PC stakes for test points TP1-TP5 and for the TH2 connection. Also, install the PC stakes for S1, TP GND and TP +5V. The bases of each of the three LEDs should be 15mm above the surface of the PC board. Orient them with the anode (longer lead) toward the left of the PC board. LED1 and LED2 are the green LEDs while LED3 is a red LED. They ultimately are bent over at right angles at a point 10mm above the PC board, so that they fit through their matching holes in the side of the box. Next, solder the trimpots in place. They have different values so be sure to install the correct unit in each position. Note that the 10kW trimpots may be marked with 103, the 20kW with a 203 and the 500W with a 501 instead of the actual (Ohms) value. Regulator REG1 lies flat on the PC board with its leads bent over to insert into the appropriate holes. It is secured using an M3 screw and nut. Now install the DC socket and 2-way terminal screw connectors. At this point, apart from Q1, the PC board assembly is complete. This close-up of our prototype shows how Q1’s legs are bent up and soldered to the underside of the PC board. And here’s how the thermistor (TH2) is “heatshrinked” to a spade lug and then secured to the box side. siliconchip.com.au Mounting Q1 Q1, an IRF540 MOSFET, is not actually mounted on the PC board – it screws to the case 6mm underneath the board. As shown in the photo, its legs are bent up 90° and are soldered to the underside of the board (they just poke through the upper surface, underneath the 0.22W 5W resistor). You need to get the MOSFET into the right position so that when the completed PC board is placed in the box, a hole can be drilled through the NTC THERMISTOR (TH2) SPADE LUG case and heatsink. This is a little tricky to achieve because the centre leg, the drain, is actually shorter than its gate or source legs. Bend the drain up 90° 5mm out from the body of the MOSFET and similarly bend the source and gate legs up 90° 7mm out from the body. Now solder Q1 in position and turn the board over. The hole through Q1’s heatsink should be right in the middle of the access semicircle cut in the edge of the PC board. Boxing it Insert the PC board into the box and mark out the corner mounting holes in the base of the box and also the hole position for Q1. Drill these out to 3mm in diameter. Now place the heatsink squarely onto the base of the box and mark out the four corner mounting holes and the Q1 hole onto the back of the heatsink. Drill these out using a 3mm drill bit. The battery temperature thermistor (TH1) is mounted inside a modified battery holder so it contacts two cells. September 2007  39 the centre terminal of the switch and it doesn’t matter which terminal the other goes to – if it appears to work “upside down” (ie, off in the down position), you simply turn the switch through 180°. Assembly This view shows the mounting positions for the LEDs and switch (front) plus DC socket and thermistor (rear). Deburr the holes with a larger drill bit and in particular, make sure that the area around the hole inside the box for Q1 to mount on is smooth so that the insulating washer will not be punctured. Holes need to be drilled in the side of the box as shown in Fig.5. These holes are for the three LEDs and power switch on one side and the DC socket and TH2 thermistor mount on the other side. The end of the box adjacent to Q1’s hole needs a 9.5mm hole for the cable grommet (our photos in fact show a 12.5mm grommet – because we had one – but a 9.5mm grommet would be better). TH2 is mounted on a spade terminal using a 4mm length of heatshrink tubing. This then mounts on the box to detect heatsink temperature. First, cut the thermistor leads to 5mm length and solder two 50mm lengths of light-duty insulated wire to it. Insulate the joints with 1.5mm heatshrink tubing. Now attach the two free wire ends to the 2-pin header connector. The thermistor can be attached to the spade terminal with the heatshrink tubing. While you are about it, cut, solder and insulate a similar pair of wires for switch S1. These wires should be roughly 70mm long. One connects to Beware sheep in wolf’s clothing! Be careful if you buy NiMH batteries over the ’net – you might not quite get what you think you’re getting. We’ve seen several warnings about the ratings of rechargeable batteries coming from suppliers in Hong Kong and China (among other places) and readily available on eBay, for example. It seems some of Asia’s inscrutable manufacturers or distributors simply print whatever they think will sell their cells without too much angst. If that means labelling a 1500mAh cell (which of course is much cheaper to produce), as a 2500mAh, then so be it. Another source has warned about ‘C’ and ‘D’ cells which are actually ‘AA’ cells inside a ‘C’ or ‘D’ case. Even if you do pay a little more to buy your NiMH or NiCad cells from retailers in Australia (and that’s not always the case anyway), you have the availability of recourse if your purchase isn’t what it appeared to be or what you thought it should be. Try doing that with an email address in, well, where? The old maxim applies: if it looks too good to be true, it probably is! 40  Silicon Chip The heatsink and PC board are screwed to the bottom of the box, which (when completed) is then turned over and becomes the top side. The lid then becomes the base. Before mounting the heatsink, apply a thin smear of heatsink compound to its base. Then attach the heatsink to the box bottom with the M3 x 10mm screws and 6.3mm threaded plastic standoffs. Next, secure Q1 to the base of the box, along with its silicone insulating washer and insulating bush. We used an M3 x 10mm from the inside and a 6mm M3 tapped spacer on the outside. You could use just an M3 nut here but the exposed screw thread does not look as neat as the spacer – and besides, the spacer is easier to grip when tightening it up! The PC board is secured to the Nylon spacers using four M3 x 5mm screws. Before going any further, check to make sure that the metal tab of Q1 is in fact isolated from the metal box. With your multimeter on a mid-range ohms scale, connect one lead to the box and the other to Q1’s tab (or the cathode [striped end] of diode D1). The reading should be above 1MW. If it is low ohms, check that the insulating washer and bush are installed correctly and that the washer is not punctured. If you get a low reading, correct the problem before proceeding. Attach the side panel label to the box and bend the LED leads over to just protrude through the holes in the side of the box. The previously-prepared thermistor (TH2) attaches to the side of the box with an M3 x 5mm screw and nut. Its wires connect to the PC board as shown. The roughly-70mm-long wires from switch S1 (which sits directly over IC2) connect to the appropriate PC stakes and both the switch itself and the PC stakes are insulated with heatshrink tubing. Wire the terminals on the PC board for the battery and thermistor (TH1) using medium-duty wire. We used red siliconchip.com.au Parts List 1 PC board, code 14109071, 98 x 53mm 1 diecast box, 111 x 60 x 30mm (HB-5062) 1 fan type heatsink, 55 x 105 x 25.5mm 1 mini SPDT toggle switch (S1) 2 2-way PC-mount screw terminals 1 PC-mount 2.5mm DC socket 1 18-pin IC socket 5 2-way headers 1 3-way header 3 jumper shunts 4 PC stakes 1 2-way jumper connector 2 NTC thermistors (10kW <at> 25°C (TH1, TH2) (Jaycar RN-3440 or equivalent) 1 4-way (or 6-way) automotive connector 1 9.5mm grommet 4 small adhesive rubber feet 1 50mm length of 1.5mm diameter heatshrink tubing 1 50mm length of 4mm diameter heatshrink tubing 4 6.35mm Nylon M3 tapped spacers 6 M3 x 5mm screws 5 M3 x 10mm screws 1 M3 x 6mm tapped spacer 2 M3 nuts 1 6.4mm spade lug chassis hole mounting 1 TO-220 silicone insulating washer 1 3mm TO-220 insulating bush 1 battery holder to suit cells to be charged 2 cable ties 30mm length of 0.8mm tinned copper wire 120mm lengths of red, black, green and yellow medium-duty hookup wire 120mm lengths of red and black light-duty hookup wire Heatsink compound for battery positive, black for battery negative and yellow and green wires for the thermistor wiring. These pass through the cable grommet and into the terminals. Because we wanted to make the charger adaptable to other batteries, the other ends of the wire connect to an automotive connector plug and socket which then connects to the battery holder and thermistor. For a permanent connection, the connector could be omitted, with the battery holder/thermistor wires going straight to the appropriate places on the battery holder. Ensure the connections to the thermistor are sleeved with heatshrink tubing to prevent any shorts to the battery holder terminals. The thermistor needs to be mounted in the battery holder so it contacts at least one of the cells under charge. We drilled a hole in a 4xAA cell holder so that the thermistor is sandwiched between the cells in the holder (see photo). Depending on the type of battery holder you use (or none at all) your cells may need to have the thermistor mounted with some hook and loop tape (eg, Velcro) around the cell body. siliconchip.com.au Semiconductors 1 PIC16F88-I/P microcontroller programmed with NiMHCharger.hex (IC1) 1 LM358 dual op amp (IC2) 1 IRF540 Mosfet (Q1) 1 LM317T adjustable 3-terminal regulator (REG1) Setup With IC1 still out of its socket, connect your plugpack to the DC socket (positive to the centre of the plug) and turn on. The power LED should light. Connect a multimeter between TP +5V and TP GND and adjust VR6 for a reading of 5.0V. Now check that there is 5V between pin 14 and pin 5 of the IC1 socket. If this is correct, switch off power, wait a short time and then install IC1. Adjustments The thermistor is adjusted using VR5, so that the voltage between TP5 and TP GND is 2.5V when the thermistor is at 25°C (ie, if the ambient temperature is 25°C, adjust VR5 so that the voltage between TP5 and TP GND is 2.5V). If the ambient is 20°C, set it for 2.8V or to 2.2V for 30°C. Both the timeout and dT/dt values are adjusted using trimpots VR1 and 2 3mm green LEDs (LED1,LED2) 1 3mm red LED (LED3) 1 16V 1W zener diode (ZD1) 1 1N5822 3A Schottky diode (D1) 1 1N4004 1A diode (D2) Capacitors 1 220mF 50V PC electrolytic 1 100mF 16V PC electrolytic 2 10mF 16V PC electrolytic 1 220nF MKT polyester (code 0.22mF, 220n or 224) 2 100nF MKT polyester (code 0.1mF, 100n or 104) Resistors (0.25W, 1%) 1 10kW 3 470W 1 9.1kW 1 120W 1 8.2kW 1 47W 3 1kW 1 0.22W 5W Trimpots 1 500W horizontal trimpot (code 501) (VR6) 4 10kW horizontal trimpots (code 103) (VR1-VR4) 1 20kW horizontal trimpot (code 203) (VR5) VR2. Test points have been included to allow easy measurement. The timeout can be set from 0-25 hours. In its simplest arrangement, the voltage at TP1 gives the timeout in hours. So, for example, if the VR1 setting gives 5V between TP1 and TP GND, the timeout is 5 hours. If you need longer than this time period, then you can install LK1. This acts as a x5 multiplier so that the time period is increased. So, for example, with LK1 installed and VR1 set so that TP1 is 5V, the timeout will be 25 hours. Similarly, if TP1 is 1.2V, then the timeout will be six hours (5 x 1.2). Refer to the “NiMH charger settings” section to work out the timer value required. The table at the end of this article also shows typical settings for various capacity cells. Temperature rise detection (dT/dt) can be adjusted from between 0.5°C per minute rise to 5°C per minute rise. This is done using VR2 and measuring between TP2 and TP GND. There is a direct correlation between the voltage and the setting: a setting of 2.5V at TP2 will set the dT/dt value September 2007  41 10mm LONG M3 MACHINE SCREW 4 x 5mm LONG M3 SCREWS PC BOARD INSULATING BUSH 4 x 6.35mm M3 TAPPED SPACERS Q1 SILICONE WASHER 6.35mm LONG M3 TAPPED SPACER 4 x 10mm LONG M3 SCREWS FAN TYPE HEATSINK Q1 MOUNTING HOLE HOLE FOR CABLE GROMMET 9.5mm DIAM 3.0mm DIAMETER 6.0mm DIAMETER 10.5 6 37 LID SIDE 48 CL LID SIDE 59 Q1 MOUNTING HOLE 70 Q1 MOUNTING HOLE 3.0mm DIAMETER 6.35mm DIAMETER ALL DIMENSIONS IN MILLIMETRES Fig.4 (top) shows the way the PC board assembly and Q1 are mounted in the box, while Fig.5 (above and right) gives you all the drilling details for the case. to 2.5°C per minute rise. Initially set VR2 so that the voltage at TP2 is 2.5V. Option Installing links LK2 and LK3 enable top-up and trickle charge respectively. If you want top-up only, install LK2; if you want both top-up and trickle charge install LK2 and LK3; if you want trickle without top-up, install LK3 only. If any of these two links are selected, 3.0mm DIAMETER 9.0 8.0 LID SIDE you will need to set the trickle charge rate. The top-up charge is fixed at four times the trickle charge. Trickle charge, trimpot VR3 allows adjustment from 500mA down to less than 20mA. Note that some battery packs have a thermistor already installed. This should not be used unless it has the same resistance characteristics as the one specified. The thermistor should measure about 10kW at 25°C and the 32 71 resistance should fall with increasing temperature. NiMH charger settings Before setting up the charge, timeout and trickle settings you need some extra snippets of information. You will need to know the Ah rating (or mAh) of the cells or the battery – this will normally be printed on the side of the cells or battery. You also need to know the nominal Resistor Colour Codes o o o o o o o o No. 1 1 1 3 3 1 1 42  Silicon Chip Value 10kW 9.1kW 8.2kW 1kW 470W 120W 47W 4-Band Code (1%) brown black orange brown white brown red brown grey red red brown brown black red brown yellow violet brown brown brown red brown brown yellow violet black brown 5-Band Code (1%) brown black black red brown white brown black brown brown grey red black brown brown brown black black brown brown yellow violet black black brown brown red black black brown yellow violet black gold brown siliconchip.com.au Battery or cell capacity Trickle Current (LK3 in) Top up with LK2 will be 4 x trickle setting Slow Charge (15h) Standard Charge (5h) Fast Charge (1.5h*) (* at or below 2.5A) (VR1 <at> 1.5V, LK1 out) (VR1 <at> 3V, LK1 in) (Do not select top-up) (VR1 <at> 5V, LK1 out) (Top-up not recommended) 200mAh 10mA (VR3 <at> 100mV) 20mA (VR4 <at> 20mV) 60mA (VR4 <at> 60mV) 200mA (VR4 <at> 200mV) 400mAh 20mA (VR3 <at> 200mV) 40mA (VR4 <at> 40mV) 120mA (VR4 <at> 120mV) 400mA (VR4 <at> 400mV) 700mAh 35mA (VR3 <at> 350mV) 70mA (VR4 <at> 70mV) 210mA (VR4 <at> 210mV) 700mA (VR4 <at> 700mV) 900mAh 45mA (VR3 <at> 450mV) 90mA (VR4 <at> 90mV) 270mA (VR4 <at> 270mV) 900mA (VR4 <at> 900mV) 1000mAh 50mA (VR3 <at> 500mV) 100mA (VR4 <at> 100mV) 300mA (VR4 <at> 300mV) 1.0A (VR4 <at> 1.0V) 1500mAh 75mA (VR3 <at> 750mV) 150mA (VR4 <at> 150mV) 450mA (VR4 <at> 450mV) 1.5A (VR4 <at> 1.5V) 2000mAh 100mA (VR3 <at> 1.0V) 200mA (VR4 <at> 200mV) 600mA (VR4 <at> 600mV) 2.0A (VR4 <at> 2.0V) 2400mAh 120mA (VR3 <at> 1.2V) 240mA (VR4 <at> 240mV) 720mA (VR4 <at> 720mV) 2.4A (VR4 <at> 2.4V) 2500mAh 125mA (VR3 <at> 1.25V) 250mA (VR4 <at> 250mV) 750mA (VR4 <at> 750mV) 2.5A (VR4 <at> 2.5V) 2700mAh 135mA 270mA 810mA (VR3 <at> 1.35V) (VR4 <at> 270mV) (VR4 <at> 810mV) 2.5A (1.6h) (VR4 <at> 2.5V) (VR1 <at> 1.6V, LK1 out) 3000mAh 150mA 300mA 900mA (VR3 <at> 1.50V) (VR4 <at> 300mV) (VR4 <at> 900mV) 2.5A (1.8h) (VR4 <at> 2.5V) (VR1 <at> 1.8V, LK1 out) 3300mAh 165mA 330mA) 990mA (VR3 <at> 1.65V) (VR4 <at> 330mV (VR4 <at> 990mV) 2.5A (2h) (VR4 <at> 2.5V) (VR1 <at> 2.0V, LK1 out) 4000mAh 200mA 400mA 1.2A (VR3 <at> 2.0mV) (VR4 <at> 400mV) (VR4 <at> 1.2V) 2.5A (2.4h) (VR4 <at> 2.5V) (VR1 <at> 2.4V, LK1 out) 4500mAh 225mA 450mA 1.35A (VR3 <at> 2.25V) (VR4 <at> 450mV) (VR4 <at> 1.35V) 2.5A (2.7h) (VR4 <at> 2.5V) (VR1 <at> 2.7V, LK1 out) 250mA 500mA 1.5A (VR3 <at>2.5V) (VR4 <at> 500mV) (VR4 <at> 1.5V) 2.5A (3h) (VR4 <at> 2.5V) (VR1 <at> 3.0V, LK1 out) 5000mAh 450mA 900mA (VR3 <at>4.5V) (VR4 <at> 900mV) 9000mAh 2.5A (5.4h) (VR4 <at> 2.5V) (VR1 <at> 1.08V, LK1 in for x5) 2.5A (5.4h) (VR4 <at> 2.5V) (VR1 <at> 1.08V, LK1 in [x5]) This table shows typical settings of our Fast NiMH Charger for a range of cell capacities. battery voltage or the number of cells connected in series, the plugpack voltage and the plugpack current rating. Note that when using slow charging rates (eg, charging over 15 hours) the top-up current will be greater than the charge rate. In this case, do not enable top-up. At faster rates (eg, charging over five hours) the top-up may be similar to the charge rate and again top-up is not recommended. the charge current. So a 2500Ah battery charged at 1A should be charged after 2.5 hours, which means that the timeout is set to 3.75h. This would be a 3.75V setting at TP1. Any changes to the timeout value when charging will not take effect until power is switched off and on again. This includes changes to the LK1 setting. Any changes to other settings will be incorporated in the charging. Timeout Trickle Timeout should be set to 1.5 times the Ah rating of the battery divided by siliconchip.com.au The trickle charge requirement is calculated by dividing the amp hour rating of the cells by 20. So, for example, if the cells are 2400mAh, then the trickle current should be 120mA. When testing, the charger may stop before full charge or it may tend to overcharge the batteries. Under-charge will be evident if the charging period is too short and the batteries do not deliver power for the expected period. In this case, turn VR2 further clockwise to increase the dT/dt value. If the battery pack appears to get hot after full charge has been reached, turn VR2 back anticlockwise for a lower SC dT/dt value. September 2007  43