Making wired electric screwdriver from batteried

[AndreyS]

I've got simple electric screwdriver, which runs on 3 NiMH 1.2V batteries. The batteries are dead now.
Basically, even with batteries the screwdriver is quiet slow.
When I connect it to 5V PC psu, it runs great. I measured the current drawn, which is ~2A continious current @ 5V without load (without unscrewing anything) and about 3A @ load (though can be higher, if a screw is stuck).
Firstly, I've bought cheap chinese 1A 5V power adapter, but the screwdriver couldn't start, OCP is triggering. Then I changed feedback resistor (I know I shouldn't do that, but I reall wanted anyways). And it worked fine until load was applied, after which magic smoke pop out. MOSFET in controller IC has blown.

Then I've bought good quality 5V 3A cisco power adapter. I tested it with 1 Ohm load, probably a bit higer, because current drawn was 4.1 A. And it working just fine.
But, unfortunatelly, it wasn't enough to start screwdriver, OCP was triggering. The power adapter is well glued, therefore I would like to avoid taking it appart and playing around with resistors.

I've put in series 0.33Ohm resistor, which wasn't still enough to start screwdriver. Then I've put 1Ohm, and it worked great.

The question is 'is there a better way to start the screwdriver?, but without that resistor (it's getting hot quiet quick), maybe usage of low resistance MOSFET would solve the problem, but I cannot figure out the circuitry'

[AndreyS]

I forgot to mention: I omitted charging circuitry. The 5.1mm socket is directly connected to the DC motor and switch

[HackedFridgeMagnet]

I think your investigation is close to a resolution and you are on the right track, but I think you've made a mistake.
The 10ohm resistor and it works great?

Bypassing the charging circuitry is right. But make sure it's entirely disconnected.

If you can get it going with a bench psu then you can get it going with an appropriately rated ( current and voltage)  fixed power supply( plug pack)

ps. I did similar on my chain saw sharpener. But I had to put 2 plug packs in parallel to get the 2 amps. It looked pretty silly.

[AndreyS]

Yes I totally omitted charging circuitry by desoldering limiting resistor (which was in place for limiting the current going into batteries).
I'm pretty sure the problem lies in high current spike when I just turning on the screwdriver.
I also forgot to mention I soldered 3x 10000uF (30mF in total) caps and it didn't help.
And yes, the resistor is indeed 1Ohm

ps. I did similar on my chain saw sharpener. But I had to put 2 plug packs in parallel to get the 2 amps. It looked pretty silly.

that's great, I could do similar thing if I'd had another 5V power supply

[Boschi]

just a thought, why you didnt just swap out the old batteries with a new 18650 lithium cell? they can supply 20 amps easily, and with a tp4056 module from china you can also charge it with a usb port, basically back to new.
(a 18650 cell is 3.7v nominal, very close to the 3.6V from the 3 old cells)

[AndreyS]

just a thought, why you didnt just swap out the old batteries with a new 18650 lithium cell? they can supply 20 amps easily, and with a tp4056 module from china you can also charge it with a usb port, basically back to new.
(a 18650 cell is 3.7v nominal, very close to the 3.6V from the 3 old cells)

That's could be a nice workaround, but I would like to have wired screwdriver. Also I don't have 18650 battery currently.

I've bought 4.4A 5V power adapter and soldered two 0.33R 5W resistors. Works okay. But still don't like those resistors. Any improvements are welcome.

[james_s]

Isn't a wired screwdriver more hassle than just using a regular old manual screwdriver? 18650 batteries are dirt cheap.

[KL27x]

Maybe you can add some capacitors to get through the spike without tripping OCP. (They have to go on the PSU side of the power switch). Basically adding more filter/output capacitance to the PSU.