Instructables

Why Cordless Drill (Bosch 7,2volt) do not work using Laptop Adapter

Hello all..
I got problem with my cordless drill. It's battery is leak and i need to replace it using Laptop Adapter power suply.
I have HP laptop adapter with Voltage Output 18,5Volt 3.8Amp.
I just connected the + and - of laptop adapter to the cordless drill, BUT it do not want to turn and ONLY MOVE SEQUENCE (move-stop-move-stop-move-stop....).
Its move is so strange for me, it's like pulse voltage because the drill just move for a moment then its stop, then its move and stop and always like that...
I don't know is it DC or AC, and I've try to make circuit to make its voltage become DC using 4 diode and capasitor, But the result is same (move-stop-move-stop...).
Then, I try to connecting the cable adapter to the drill and then pull off and connecting again with rapid sequence. Then the drill want to turn fastly...
Please Help me, how to solve this, i need this adapter work for my cordless drill....

I've make a test to, I used AC dinamo (not DC dinamo motor) and connect it with the Laptop adapter, it's turn normally and fast...

Please anyone here help me...






scraptopower10 months ago
When the adapter pulses like that, it means it's overloaded. Cordless drills, even small ones, need a heck of a lot of current to get them started, a laptop charger like that will not supply enough current to start the motor. As you're running it at over twice it's voltage, the current requirements will be even higher. I have a little 7V cordless drill, it can easily draw 7 amps running at 14V, The motors look small, but they take a lot of power to run.

The resistance of motor when it is not turning is very low, so the motor looks like a short circuit to the power supply, which is why it's pulsing; It turns on, sees a short circuit and then turns off. Once a motor is up-to speed it "resistance" reduces due to the inductance of the coils. The solution is a better power supply that generates the right voltage.
Absolutely right on the power requirements thing.

Its "resistance" INCREASES as it speeds up, because the terminal voltage of a DC motor E= IR +backemf - Back emf is the voltage GENERATED by the motor as its coils spin in the magnetic field. The faster it goes, the higher the back EMF, until back EMF = terminal volts- IR
It has nothing to do with inductance !
ddeeen (author)  steveastrouk10 months ago
Hmm...
I'm not talent in electronic, But what should i do with this laptop adapter??

Laptop adapter is very different with ordinary wall adapter. Laptop adapter is SMPS one which it's current can be improved than the wall adapter. AND smps adapter is using pulse system on their circuit..
I've read it on some sources..
So, what do you thing about this SMPS adapter and is it caused by smps system??
The adapter cannot supply anything like enough current for a drill. The drill may need 30 or 40A
ddeeen (author)  scraptopower10 months ago
If it looks like a short circuit and become pulsing, why do if i use AC DYNAMO can turn normally?.
AC and DC dynamo is different, DC will work only on DC voltages and AC only work on AC voltages..
I've open the drill dynamo, it's belong to DC dynamo.

I've guessed the laptop adapter is AC voltages, then I make it become DC voltage using common circuit and it still move pulsing...

Help please..