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Cordless Power T Charger Contro Protect your investment and extend the life of your power tool rechargeable batteries. Add this Power Tool Charger Controller and never cook a Nicad again! W ELL, WE’VE SEEN how simple it required to reach full charge depends To see what happens when a batis to resuscitate the batteries in on the state of charge for the battery tery charges take a look at Fig.1. This your “cordless” power tools elsewhere pack at the start of charging. shows the typical voltage, temperature in this issue. Now it’s time to ensure Overcharging can destroy the batand internal pressure rise with charge. you don’t kill them all over again by tery pack because of the characterOnce charging goes past the 100% overcharging them. istics of the cells that make up the charge point (also known as the endIn short, while those battery-powbattery pack, which are usually Nicad point) the temperature and internal ered tools have many virtues, we are (Nickel Cadmium (NiCd)) or NiMH pressure rapidly rise and the voltage not so enthusiastic about their battery (Nickel Metal Hydride) chemistry. The initially rises and then falls. charging systems. two types tend to have fairly similar Continual overcharging will damAs we discussed, most low-cost characteristics and overcharging will age the cells due to the elevated power tools include a very basic chargtemperature. This accelerer: a plugpack to supply ates chemical reactions that MAIN FEATURES power and a resistor to contribute to the ageing limit the current flow into process. In extreme cases ng ori • Charging timeout nit mo ximum temperature the battery pack. There during overcharging, the • Minimum and ma tion tec de d ge ar ch is nothing to prevent internal pressure can cause /dt dT • perature detection overcharging: no timer to the cells to open their Ds LE n • Over and under tem tio d thermistor out indica switch off charging when safety vents to release the • Power, charging an limit the time has elapsed and pressure. The vents should • Adjustable timeout ting set /dt dT le tab no full-charge detection. re-close after the pressure g jus gin Ad ar • le ch table top-up and trick At best, this type of basic is released but sometimes • Optional and adjus charger will shorten the the cells are deformed by • Start switch after blackout battery pack life so that it the heat and permanent • Charging resumes will require replacing after damage occurs. only relatively few charges. severely shorten the life of both. What should happen? At worst, the basic charger can cause One of the main differences between destruction of the battery pack the the two as far as users are concerned is A well-designed charger will not very first time it is used! that Nicads can develop a “memory”, allow overcharging; in fact it will Destruction of the battery pack can where if they are only partially disswitch off the main charge when the happen if the charger is left on for too charged then charged again, eventucells reach their end point. long after the battery pack has reached ally they will “remember” this as Some chargers will just include full charge. And it is all too easy to their entire charge/discharge limit a timer to switch off charge after a forget to switch the charger off at the and therefore significantly reduce the certain period has elapsed. This is required time. The result is serious amount of power available. NiMH batnot ideal for the reasons already menovercharging teries do not have this characteristic. tioned and the timer should really You cannot even rely on the fact that However, Nicads are more suited to only be included as a fail-safe device; charging requires a certain time period the heavy discharge currents of power a backup to stop charging should the and the charger can be switched off tools and are usually supplied instead “detection of full charge” fail. after that because the time period of NiMH cells. Full charge of the battery pack can 32  Silicon Chip siliconchip.com.au Tool oller by JOHN CLARKE be determined in one of two ways. One way is to monitor the voltage 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, or the change in voltage with respect to time. In practice, this voltage change can be difficult to detect, especially with NiMH cells which do not show a marked voltage change at full charge. The second (and more reliable) method is to detect the temperature rise of one or two cells within the battery pack. When charging, the incom- ing electrical power is converted into stored energy via chemical reactions within each cell. These reactions are reversible – when an electrical load is connected they deliver electrical power. While charging at normal rates, the cells do not rise much in temperature because most of the incoming power is converted into useful stored energy. However, once the cells become fully charged, no more useful chemical reactions can occur. But if the charger stays connected, 100 65 1.46 80 CELL VOLTAGE 1.50 1.42 60 1.38 40 35 1.34 20 25 1.30 55 45 0 0 50 100 STATE OF CHARGE (%) Fig. 1: typical charging curves for Nicad batteries, as supplied in most cordless power tools. Cell temperature (green) and voltage (red) are most often used to detect the “end point” or 100% charge. siliconchip.com.au PRESSURE TEMPERATURE (°C) 75 power is still being forced in and this energy is converted to heat. Therefore the cells rise quickly in temperature. Detection of this change at the charging end point is called dT/dt or the change in temperature over time. The temperature rise is in the order of 2°C per minute. At the end point (where the cells are fully charged), charging is normally switched off to prevent the cells overcharging. Some chargers include a top-up charge after the endpoint to deliver a lower current to the cells to ensure they are fully charged. After top up, the cells are trickle-charged to maintain their full charge. The trickle charge can be maintained indefinitely because the cells are safely able to dissipate the small amount of heat generated. Our charger controller The S ILICON C HIP Power Tool Charger Controller uses the tool’s existing plugpack and battery charging unit/base. It simply connects in series between the two and therefore can control the charging process. Note that because the Charger Controller does not connect directly to the battery pack, it cannot measure the battery voltage. Instead it utilises dT/dt detection to stop charging at the end point. For this temperature measurement, the charger controller requires that December 2006  33 3.5mm JACK PLUG* POWER TOOL BATTERY PACK PLUG PACK CHARGING CONTROLLER CHARGING UNIT * MATES WITH 3.5mm JACK SOCKET ADDED TO BATTERY PACK FOR THERMISTOR Fig.2: the Charging Controller connects between the original plugpack’s lowvoltage output and the power tool’s charging base. A separate thermistor connection is also required, with the thermistor mounted on one of the new battery cells. a small NTC thermistor be installed within the power tool battery pack, with the two leads brought out to a 3.5mm jack socket. We discussed fitting this thermistor in the earlier article on repacking cells. As a backup we have included a timer that will switch off charging after a preset period should the thermistor end point detection fail. More safeguards Further safeguards to protect the cells are also included. Charging is initiated with the start switch (S1). However, charging will not start if the NTC thermistor is disconnected or if there is an over-temperature or undertemperature detection. The over-temperature setting is at 70°C while the under temperature setting is at 0°C. If the NTC thermistor is connected and the cell temperature is within the 0-70°C range, then charging will start. Charging will halt should the temperature fall below 0°C or if the thermistor is disconnected. Charging will resume when the temperature range is correct or the NTC thermistor is re-connected. However, if the temperature goes over 70°C, full charging will cease and will not automatically resume. If a blackout occurs during charging, charging will resume with return of power. The timeout period will also resume from where charging was interrupted. Charging will cease at the point where the dT/dt value is exceeded or if the timeout period expires. Pressing the start switch will resume charging from the start of the timeout period. You can also stop the charging process at any time by pressing the start switch. Reduced charge In its simplest form, the Charger Controller includes just the features mentioned above. However, you can also enable top-up and trickle charging if you wish. The top- up feature provides a reduced charge (typically at 400mA) for an hour to ensure full charge is reached after the main charge cycle. The trickle charge (at typically 100mA) continues after the top-up to maintain battery charge. As mentioned earlier, trickle charging does not generate a lot of heat so the battery can be left on trickle charge, ready for use at a moment’s notice. There’s nothing worse than picking up a drill to find that the battery has self-discharged (which they can do!). Adjustments Both the timeout period and dT/dt values are adjustable. Timeout can be set up to 25 hours while dT/dt can be selected between 0.5°C rise per minute to 5°C per minute. The trickle charge rate must be adjusted if the top-up and trickle charge option is selected. More details concerning the adjustments are included later in the setting-up section. Indication Three LEDs indicate the status of the charger controller: power, thermistor and charging. The power LED is lit whenever power is applied to the charger controller. The NTC thermistor LED lights whenever the thermistor is disconnected. When connected, the LED will be off unless there is an over-temperature or under-temperature condition. In these cases, the NTC thermistor LED flashes at a 1Hz rate when it measures over temperature and at a 0.5Hz (once every two seconds) rate when it measures under temperature. The charging LED is continuously lit during the main charging cycle and switches off when charging is complete. If top-up and trickle charging is selected, the charging LED will flash at a 1Hz rate during top-up charge and will flash at a 0.5Hz (once every two seconds) rate during trickle charge. When the thermistor LED is lit or flashing, the charging LED will be off. How it works The circuit for the Power Tool Charger Controller (Fig.3) is based around a PIC16F88 microcontroller. This performs all the logical decisions required to control the charging and Specifications Maximum Current ..............................5A Timeout adjustment ..........................From 0-5 hours, corresponding to 0-5V from VR1 at TP1. 0-25 hours with x5 link installed (LK1) dT/dt adjustment ...............................From 0.5°C-5°C rise/minute, corresponding to 0.5V to 5V from VR2 at TP2. Top up and Trickle Charge .................Available when LK2 is installed Trickle Charge adjustment. ...............From 100% to 1/50th of main charge current corresponding to 0-5V from VR3 at TP3. ............................................................100% to 1/250th with x5 link installed (LK3) Top up charge ....................................4 x trickle setting for 1 hour Topup and trickle switching rate .......30Hz. Over temperature cutout ...................70°C Under temperature detection ............0°C Current consumption .........................20-26mA depending on status LEDs 34  Silicon Chip siliconchip.com.au PLUGPACK INPUT D1 1N4004 A + TP5 REG1 LM317T K OUT IN – ADJ CON1 120Ω 10 µF 16V +5V 4 6 S1 +5V VR1 10k TIMEOUT 17 (5V = 5h) +5V VR2 10k ∆T/T (5V = 5°C/min) RB0 RB3 – λ LED1 CON2 AN2 IC1 RB7 PIC16F88P RB4 18 +5V RB6 RB1 TP4 47Ω 3 100nF CON3 AN1 TP3 VR4 20k Q1 STP45NF06L IRF540 S G LEDS TP1 1 D 10Ω 9 AN0 TP2 (5V = 1/50 of full charge rate) 2006 100 µF 16V 100nF 14 Vdd MCLR START SC  1k POWER K VR5 500Ω THERMISTOR INPUT + A 220 µF 50V +5V VR3 TRICKLE SET 10k TO CHARGER 470Ω AN4 RB2 Vss 5 TP GND K 13 LK1 TIMEOUT X5 10 LK2 TOPUP TRICKLE ENABLE 12 LK3 TOPUP TRICKLE X5 7 8 470Ω ADJ A LED2 THERMISTOR A λ K 1N4004 A LM317T IN OUT 470Ω POWER TOOL CHARGING CONTROLLER A K CHARGING λ LED3 Q1 D K G D S Fig.3: the PIC microcontroller analyses the charge state of the battery, turning the charger on and off by means of Mosfet Q1. Various parameters can be set by means of the links and trimpots. runs a software program specifically for this charger controller application. Apart from the PIC, there is not really much else to the circuit. When the thermistor is plugged into its socket, it forms a voltage divider in conjunction with trimpot VR4 across the 5V supply. VR4 is set so that the voltage across the thermistor is 2.5V at 25°C. The voltage across the thermistor is monitored by the PIC’s AN4 input (pin 3) via a 47W stopper resistor and 100nF filter capacitor. These are included to remove any RF signals and noise that could be present due to the ∗ UNDER ELECTRO thermistor being connected remotely from the circuit. The voltage at the AN4 input is converted into a digital value by the software and this is compared against the over- and under-temperature values and for dT/dt changes. VR1, VR2 and VR3 set the timeout, dT/dt and trickle charge current respectively. They each comprise a 10kW trimpot connected across the 5V supply. The wiper of each trimpot connects to one of the PIC’s AN0, AN2 or AN1 inputs. The voltages are converted to a digital value within IC1 so that the LED2 LED1 LED3 LK2 LK3 LK1 TP5 120Ω REG1 LM317T VR5 100 µF IC1 PIC16F88P S1 D1 TP4 TP2 100nF 220 µF 10Ω∗ 10Ω 50V 1k 100nF VR4 VR2 470Ω 470Ω∗ 470Ω∗ REQ1 LL ORT N O C RE GRA H C 16021141 TP1 TP3 VR3 PLUGPACK INPUT CON1 CON2 OUTPUT TO CHARGER CON3 47Ω TP GND FROM THERMISTOR settings can be processed in software. Note that the trimpots can be monitored via test points (TP1, TP2 & TP3) using a multimeter. For example, setting VR1 to give 4V at TP1 will set the timeout to four hours. The timeout value can be multiplied by a factor of five if jumper link LK1 is installed. With LK1 out, pin 13 is pulled to 5V via an internal pull-up resistor within IC1 With LK1 in, pin 13 is tied to ground. Links LK2 and LK3 work in a similar manner, with LK2 enabling the top-up and trickle charge when inserted. LK3 VR1 10 µF Fig.4: with the exception of the thermistor (which must be installed in the battery pack) everything fits onto one small PC board. The photo at right is reproduced same size to match the component overlay at left. siliconchip.com.au December 2006  35 Fig.5: the PC board sits 11mm up from the bottom of the case with the tops of the LEDs 17mm above the board so they just poke through the lid. The “start” pushbutton switch (not shown here) is below the lid surface, accessed through a hole in the lid. LID LEDS 17mm BOX 10mm LONG M3 TAPPED SPACERS PC BOARD 1mm THICK WASHERS 6mm LONG M3 CSK HEAD SCREWS increases the top-up and trickle current setting by a factor of 5 when inserted. Switch S1 is a normally-open pushbutton type. When open, input RB0 is pulled high via an internal pull-up resistor. When the switch is pressed, RB0 is taken to 0V and the charge timing begins its cycle. The software code provides switch debouncing, mainly to prevent a false initiation of the charge cycle. When RB0 is taken low, there is a short delay before the port is checked again. If it is still low, then the software waits for a further delay and rechecks. If it is still low the software assumes that the switch has been pressed. If RB0 is at 5V after any of the delay periods, it is assumed that the switch was not pressed. Outputs RB1 and RB2 drive the charging and thermistor LEDs respectively via 470W resistors. The Power LED is driven directly from the 5V supply via its 470W resistor. Mosfet Q1 is driven from IC1’s RB3 This view shows the completed Charge Controller unit before the lid is attached. 36  Silicon Chip 6mm LONG M3 SCREWS output via a 10W gate resistor. When Q1 is on, then the cells can be charged because the negative side of the charger is effectively connected to ground. During the main charge, RB3 is taken to 5V and Q1 is always switched on. However, during the Top-up and Trickle charge modes, RB3 can provide a PWM (pulse width modulation) signal with a reduced duty cycle, so that the Mosfet is only switched on for a small proportion of the time so that the average current is reduced. RB3 is pulsed at about 30Hz. The specified STP45NF06L Mosfet is a logic-level device that is fully switched on with a 5V gate voltage (standard Mosfets require around 10V of gate voltage in order to fully switch on). An IRF540 Mosfet could also be used because it switches on fully for gate voltages over 4.5V. Power for the circuit is taken directly from the original plugpack supply for the charger via diode D1. This provides reverse polarity protection for the fol- lowing 220mF capacitor and for regulator REG1. Note that diode D1 does not protect against reverse charging of the battery – therefore the original charger (ie, as supplied with the cordless tool) should be used. As shown on Fig.3, the plug­pack’s output is connected to the plugpack input socket of the Charge Controller and the “to charger” output socket is connected to the charger base. In this way, power for the Charger Controller is taken from the plugpack. If the connections are reversed, the Charger Controller will still operate but the battery will be discharged over time because it will be supplying power to the Charger Controller. An LM317T (REG1) is used to supply a regulated 5V supply. This was chosen in preference to a standard 5V regulator for two reasons. Firstly, this adjustable regulator can be adjusted to supply a precise 5V to make the settings of VR2 and VR3 more accurate. Secondly, the LM317T can accept a 45V input (when the output is 5V) compared to 35V for a standard 5V regulator. The extra input voltage that the LM317T can accept may be needed for an 18V battery pack. A high voltage is also specified for the 220mF capacitor at the IN terminal of REG1. In operation, REG1 has a nominal 1.25V between its OUT and ADJ (adjust) pins. If a 120W resistor is connected between these pins then there will be a current flow of about 10.42mA. This current flows in VR5 and will raise the output voltage to 5V when VR5 is set at 360W. This is because 10.42mA x 360W = 3.74V. When we add this voltage to our original 1.25V between the OUT and ADJ terminals, we get 5V. Note that the tolerance of the regu- You need to drill holes in the end of the case to mount the jack socket and provide access to the two DC sockets. siliconchip.com.au 3.5mm MONO JACK PLUG SINGLE CORE SHIELDED CABLE WIRE CONNECTS TO TIP, SHIELD BRAID TO SLEEVE 3.5mm MONO JACK PLUG WIRE CONNECTS TO TIP, SHIELD BRAID TO SLEEVE Fig.6: this diagram shows how to make the 3.5mm jack plug to 3.5mm jack plug lead for the thermistor connection. lator output to adjust pin voltage is between 1.2V and 1.3V. As well, a nominal 50mA current flows out of the adjust pin and this can affect the output voltage. These factors can be trimmed out with VR5 to set the output to precisely 5.00V. they lie flat on the PC board, with their leads bent down by 90° so that they go through their matching holes. During installation in the box, they are secured to the PC board with an M3 screw. Finally, install the DC sockets and the 3.5mm jack socket. Construction Installing it in a case The Power Tool Charger Controller is built on a PC board coded 14112061 and measuring 78 x 46mm. Begin construction by checking the PC board for any defects such as shorted tracks, breaks in the copper and incorrect hole sizes. Enlarge the holes for the DC sockets and the 3.5mm jack socket if necessary. Install the resistors first. The resistor colour code table can be used as a guide to finding each value but you should also use a digital multimeter to check each resistor before inserting it into the PC board. Solder each lead and cut the leads short against the underside of the PC board. Now solder in the diode and IC socket, taking care to orient them with the correct polarity. The capacitors can go in next. Note that the electrolytic types must be oriented with the polarity shown and that the large 220mF capacitor is mounted on its side (see photo) so that the assembled board will fit inside the box. LEDs 1-3 mount so that the top of each LED is 17mm above the surface of the PC board. Orient each LED with its anode (longer lead) towards the left of the PC board. LED1 is green while LED2 and LED3 are both red. Switch S1 must mount with its flat side towards IC1. When placing the trimpots, make sure the correct values are in each position. The link headers can also be installed for LK1, LK2 & LK3. REG1 and Q1 are installed so that The completed PC board is housed in a small translucent plastic case. The first job is to drill a hole for the 3.5mm jack socket. That done, clip the PC board into the integral side pillars of the box and mark out the positions for the screw holes in the base of the case for the Q1 and REG1 mounting supports – see Fig.5. Drill these holes to 3mm diameter and countersink the holes on the underside of the box. Next, install the two 10mm tapped standoffs and the 1mm spacers as shown in Fig.5 and secure the PC board in place. You can then mark out the positions for the DC socket holes in the side of the box and for the three LED holes switch S1 in the box lid. Drill these holes out. The switch surface will be slightly below the panel lid, so its hole will need to be large enough for your finger to reach in and push. siliconchip.com.au Setup Initially, leave IC1 out of its socket. Apply power from the plugpack to the DC input socket (positive to the centre of the plug) and check that the power LED lights. If it does, connect a multimeter between TP5 and TP GND and adjust VR5 for a reading of 5.0V. Now check that there is 5V between pin 14 and pin 5 of IC1’s socket. If this is correct, switch off and install IC1. Adjustments Trimpot VR4 is adjusted so that the voltage between TP4 and TP GND is 2.5V when the thermistor is at 25°C. Parts List – Power Tool Charger Controller 1 PC board, code 14112061, 78 x 46mm 1 plastic utility box, 83 x 54 x 31mm 1 momentary pushbutton PC mounting switch (S1) 1 3.5mm PC-mount stereo socket 1 3.5mm panel mount mono socket (installed within power tool for the NTC thermistor) 2 3.5mm mono line jack plugs 2 2.5mm DC line plugs 2 2.5mm DC sockets, PC mounting 1 18-pin IC socket 1 3-way DIL header 3 jumper shunts 1 NTC thermistor (10kW <at> 25°C) Jaycar RN-3440 or equivalent (installed in battery pack) 2 10mm M3 tapped brass spacers 2 5mm M3 countersunk screws 2 M3 screws 2 1mm spacers (washers) 1 1m length of single-core shielded cable 1 1m light-duty figure-8 cable Semiconductors 1 PIC16F88P microcontroller (IC1) programmed with CHRGCONT.ASM 1 STP45NF06L logic-level Mosfet (Q1) (or IRF540 – see text) 1 LM317T adjustable 3-terminal regulator (REG1) 2 3mm red LEDs (LED1 & LED2) 1 3mm green LED (LED3) 1 1N4004 1A diode (D1) Capacitors 1 220mF 50V PC electrolytic 1 100mF 16V PC electrolytic 1 10mF 16V PC electrolytic 2 100nF MKT polyester (0.1mF) (code 104 or 100n) Resistors (0.25W, 1%) 1 1kW 3 470W 1 120W 1 47W 1 10W 1 500W horizontal trimpot (VR5) 3 10kW horizontal trimpots (VR1-VR3) 1 20kW horizontal trimpot (VR4) Alternatively set the trimpot for 2.2V at 30°C or 2.8V at 20°C. Both the timeout and dT/dt values are adjustable and these are changed using trimpots VR1 and VR2. Test points have been included to allow easy measurement of these trimpot December 2006  37 Fig.7: this is the fullsize artwork for the front panel label. It can be cut out and attached to the case lid. CHARGING THERMISTOR POWER THERMISTOR INPUT SILICON CHIP www.siliconchip.com.au OUTPUT TO CHARGER PLUGPACK IN START CORDLESS TOOL CHARGER CONTROLLER settings. The timeout is easily set anywhere from 0-25 hours. In its simplest arrangement, the voltage at TP1 gives the timeout in hours. So, for example, if VR1 is set to provide 5V at TP1, then the timeout is five hours. If you need a timeout longer than this, install LK1. This link acts as a x5 multiplier. So, for example, with LK1 installed and with VR1 set so that TP1 is at 5V, the timeout will be 25 hours. Similarly, if TP1 is 1.2V then the timeout will be six hours (5 x 1.2). Most chargers that come with batterypowered tools will state the required charge time. Temperature rise detection (dT/dt) can be adjusted from between 0.5°C per minute to 5°C per minute. This is adjusted using VR2 and by measuring at TP2. The negative connection of your multimeter connects to TP GND. There is a direct correlation between the voltage and the setting. So, for example, a setting of 2.5V at TP2 will set the dT/dt value to a 2.5°C per minute rise. Initially, set VR2 so that the voltage at TP2 is 2.5V. Charging options Top-up and trickle charge is enabled by installing link LK2. If this option is selected you will need to set the trickle charge rate. The top-up charge is fixed at four times the trickle charge and the trickle charge is set using VR3 Connection USING THIS CONTROLLER (1) This controller can be used with an NiMH or Nicad battery pack of up to 15 cells (18V). (2) The minimum rated voltage battery pack that the unit can control is 6V (five cells). (3) This controller MUST NOT be used to control the 240VAC input to any drill charger. As indicated in the article, it must only be used to control low-voltage circuits (ie, is connects in series between the low-voltage output of the power tool’s original plugpack and the charging base). and link LK3. If LK3 is not installed, then VR3 allows the trickle charge to be set from unity to 1/50 of the main charge current. With link LK3 installed, the ratio is multiplied by a factor of 5. The trickle charge requirement is calculated by dividing the amp hour rating of the cells by 20. If the cells are 2400mAh, then the trickle current should be 120mA. To set VR3 you need to know the charge current of your charger. This is usually quoted on the charger. It can also be measured with a multimeter connected in line between the plugpack and charger when the battery pack is charging. VR3 (and link LK3) Resistor Colour Codes o   No. o 1 o 3 o 1 o 1 o 1   Value   1kW   470W   120W   47W   10W 38  Silicon Chip 4-Band Code (1%) brown black red brown yellow violet brown brown brown red brown brown yellow violet black brown brown black black brown provide the division ratio required to reduce the charge current down to the trickle value. For example, if the main charge current is 3A and we want a 120mA trickle charge, the division required is 3/0.12 or 25. So VR3 should be set to 2.5V. If a ratio of more than 50 is required, link LK3 can be installed to allow the value to be increased by 5 to a maximum of 250. 5-Band Code (1%) brown black black brown brown yellow violet black black brown brown red black black brown yellow violet black gold brown brown black black gold brown As shown in Fig.2, the Power Tool Charger Controller simply connects in-line between the charger’s plugpack and the charging base. This means that the plugpack supplied with the cordless tool connects directly to the Power Tool Charger Controller. A separate lead connects between the Power Tool Charger Controller and the original charger. You will need to make up this lead using a length of 2-way wire (figure-8 wire) and two DC plugs. Similarly you will need a 3.5mm jack plug to 3.5mm jack plug lead for the thermistor connection. Fig.6 shows how to do this for the 3.5mm jack lead. The wiring is similar for the DC socket lead. As explained in the earlier article, the power tool must have a battery pack that has the thermistor installed and a 3.5mm jack socket added to the battery pack for connection to the Power Tool Charger Controller. Note that some battery packs have a thermistor already installed. This should be replaced because it may not have the same resistance characteristics as the one we specify. It may also connect the negative side of the battery pack to one side of the NTC thermistor. Our circuit requires an isolated thermistor connection to avoid bypassing the Mosfet. Setting up Depending on settings, the charger may stop before full charge or it may tend to overcharge the batteries. Undercharge will be evident if the charging period appears to be too short and the power tool does not run for the usual period before charging is required. In this case, turn VR2 further clockwise to increase the dT/dt value. Alternatively, if the battery pack appears to get hot after full charge has been reached, adjust VR2 anticlockSC wise for a lower dT/dt value. siliconchip.com.au