My english is awful, but I'll do my best...
This is a schematic to build an AC power supply to your cordless power tool. It will work with any 18V Ni-CD battery-powered tool. It will NOT work with any another voltage.
You'll need some experience with electronics, the internal inrush currents that we'll use are dangerous, also you can explode your battery and tool if something is connected wrong.
Advantages:
-Up to 1.5Ah. of extra power to your tool giving extra torque
-less stress and longer lifetime to the battery
-extended worktime
-lasts for all day if used intermittently
Disadvantages:
-your cordless drill becomes corded, but can still be used without AC power
-a box following you
How it works:
Once you plug this power supply into AC mains and in the battery tool the PS will charge the battery while the trigger is not pressed at a rate of ~140mA, which is safe to any Ni-CD battery from 1.2Ah to 1.8Ah and good enough to charge a Lead-Acid battery. When the trigger is pressed the PS unit will stop charging the battery and act as a auxiliary power, giving up to 1.5A extra to your tool. Automatically it will charge the battery again when the trigger is released. This charge mode, plus 1.5A of auxiliary power will result in a longer battery lifetime, as the battery will be less stressed overtime. Also, you can use as a overnight, it will take 14hs-16hs to get fully charged, depending the state of charge.
The components are cheap and easy to find in a electronic store.
You'll have to make a box for this components, it can be made from wood or plastic.
Parts:
1x 30V 2A Transformer (15+15V center tapped works too*) T1
1x 50V/10A Diode bridge (KBPC1010 or similar) BR1
1x 50V/1A Diode bridge BR2
4x 35V/2200uF electrolytic capacitors C1-C4
1x 25V/3300uF electrolytic capacitors C5
1x 16V/100uF electrolytic capacitor C6
1x 35V/47uF electrolytic capacitor C7
2x 0.1-ohms 3W resistor R1/R2
1x 1.2K-ohms 1/4W resistor R3
9x 18-ohms 2W resistor R4**
10x 120-ohms 2W resistor R5**
1x 270-ohms 1W resistor R6
1x 2K potentiometer VR1
1x TIP41C TO-220 transistor (NOT TIP42C !!!) Q1
1x 7824 linear voltage regulator Q2
1x 7812 linear voltage regulator Q3
1x Green LED 5mm LED
1x General purpose 24V >5A DC relay S1
1x 10A fuse F1
3x Small heatsink
1x Large heatsink with fan (CPU cooler sold in any computer store)
*If you're using a center-tapped transformer then it should be 3A instead of 2A. Heatsink may be necessary.
Battery:
Most of you will prefer to attach a wire to the battery plugged in the tool to when using this power supply, so it can be easily disconnected when AC is not required or is not available. But if you prefer the tool lighter like me, or if you got a bad battery and need to buy another, or even doesn't have a charger because you bought a bare tool, consider use a Lead-Acid battery inside the supply unit. Lead-Acid batteries are heavier than Ni-CD, but are very powerful and much more cheaper than Ni-CD. For example:
3x 6V Lead-Acid Gel-Cells battery which can supply up to 4.5Ah, almost 4 times more than a 1.2Ah Ni-CD and costs about $10.00 ea., so, $30.00 to get 18V.
15x 1.2V Ni-CD C-Size 1.2Ah costs $3.00 ea., $45.00 for less capacity AND memory effect. Lead-Acid batteries DO NOT suffer memory effect. Only thing required is a 1 charge/month for 10-12hs to avoid sulphation.
This power supply works with both Lead-Acid and Ni-CD batteries with a simple potentiometer adjust. Ni-MH batteries may works too, but since they can't be left on the charger the cells can be damaged if left on the charger for too long. Lithium-based batteries are NOT suitable and may explode if used with this charger. If you got a Makita that uses Li-Ion and are interested in this guide, get some PB-Acid or Ni-CD ones, the tool to be modded is not the problem.
Tools:
-thermal grease
-silicone
-teflon tape
-masking tape
-screws
-A powerful glue
-A good multimeter
-Weld and iron
-wires AWG 20 to internal components and AWG 10 to the tool (if your battery is remote, otherwise, AWG 14)
Schematic:
Power resistors:
**R4 and R5 must be arranged in a parallel mode, just like R1 and R2:
Heatsinks:
This is the large heatsink from an AMD with small heatsinks attached to the 7824/7812 (middle-right) regulators and the transistor TIP41C (left). The small heatsinks were stuck with a mix of 50/50 silicone and thermal grease:
Cooling resistors:
Resistors have less resistance when hot making the maths a nightmare, this can compromise the precision of the charger, so I've attached them at front of fan:
VERY IMPORTANT:
The body of transistor and the heatsink MUST NOT carry any current between them. It must be insulated and tested before use, otherwise it will burn the relay:
All-In-Box:
Almost done, notice that I've use another fan in the transformer, that's because I got a bad quality transformer which is working overloaded at more than 50%. You shouldn't need this.
LET THE BATTERY TO BE PLUGGED LATER
When everything is OK, set the potentiometer to about 1000 ohms and power on. Measure the voltage between any negative and the hot wire to be attached on the battery, adjust your pot to reach between 22.2V to 22.4V for Ni-CD batteries and 22.1V to 22.3V for PB-Acid batteries. Let it for testing for about 30 min. and check voltages again. If everything is OK, plug off and connect the battery.
Safety:
Since we're working with DC voltages means that can't be inverted, or you can damage your drill. To plug the power supply with your tool/battery you can use those type of connectors, I don't know if they are easy to find in U.S., but any 2-pin or more plug that can't be inverted can be used:
enjoy
